Monday, June 14, 1993
                                SEARCH PROGRAMS
                                  8:30 - 12:00

Chair(s):  E. Bowell
           A. H. Delsemme


Marsden B. G.*
   Search Programs for Comets

Although there are currently no professional search programs specifically
designed for comets, comet discoveries are a significant by-product of the
current search programs for near-earth asteroids. Some comets are also found
in more general photographic sky patrols. With their emphasis on the
opposition point and the ecliptic, the near-earth-asteroid patrols are clearly
biased toward discovering short-period comets. During the past decade, for
example, fully half of the new short-period comets have been discovered with
the two Schmidt telescopes at Palomar, while the fraction of long-period
comets discovered at Palomar is more like one out of four. Discoveries with
the U.K. Schmidt at Siding Spring follow the same pattern but with about one-
third the level of success. More than one comet out of three is a new short-
period comet, a substantial increase from the one out of four or five that
prevailed rather consistently for almost two centuries. The recent overall
discovery rate of comets is considerably greater than in the years around
1960, after the first Palomar Sky Survey was completed, and even in the case
of visual discoveries of long-period comets this increase is by a factor of
two. Excluding the Kreutz sungrazing comets discovered with the SOLWIND and
SMM coronagraphs, visual detections by amateur astronomers dominate among the
comets with the smaller perihelion distances and consistently account for
almost 90 percent of discoveries with perihelia inside the orbit of the earth.
Although visual detections may seem rather haphazard, many amateur astronomers
do conduct their searches systematically, special consideration being given to
the optimal phase of the moon for morning versus evening discoveries. Reliance
on visual detections means that only one long-period comet out of seven or
eight is being found more than three months before perihelion passage, even
for perihelia out to the orbit of Mars. In addition to the obvious point of
the short notice one has in order to prepare for making interesting new
physical observations of a potentially bright comet, the limited time span
usually makes it impossible to decide whether a comet is likely to be "new" or
"old" in the Oort sense, a factor that affects how bright the object is likely
to become. Furthermore, even if there is a long observed arc after perihelion,
the presence of nongravitational forces can adversely influence the decision.
It is therefore highly desirable to discover long-period comets of small
perihelion distance when they are far enough away that a true determination of
the "original" reciprocal of the semimajor axis can be reliably determined.
Thought should therefore be given to an appropriate extension of the process
by which professional discoveries of comets are made.


Scotti J. V.*   Gehrels T.   Rabinowitz D. L.
   Computer Aided Near Earth Object Detection -- A Spacewatch Perspective

Abstract: The Spacewatch program at the University of Arizona has pioneered
automatic methods of detecting Near Earth Objects (NEOs) (Gehrels, 1991). The
software presently in use is described by Rabinowitz (1991) and includes three
modes of object detection: automatic motion identification; automatic streak
identification; and visual streak identification. The present image scale,
using a thinned backside-illuminated Tektronix 2048 x 2048 pixel CCD with 24
micron pixels, is 1.076 arcseconds per pixel. For automatic motion detection,
with an integration time on the equator of 147 seconds, having the telescope
drive off, the 4 sigma detection threshold is near magnitude V=21.2 for nearly
stellar asteroid images. The automatic streak detection is able to locate
streaks whose peak signal is above ~4 sigma and whose length is longer than
about 10 pixels. Some visually detected streaks have had peak signals near ~1
sigma. Specific examples of each mode of detection will be presented here,
along with a discussion of future detailed improvements.

Between 1990 September 25 and 1993 March 4, 36 new Near-Earth asteroids, two
comets and a Saturn crossing Centaur, (5145) Pholus, have been discovered with
the system. An additional four comets, six NEOs, and (2060) Chiron were also
"re-discovered". Each month, ~2,000 main belt asteroids are also detected;
their positions are available electronically to whomever wants them. Of the 36
new NEOs, 8 of them were found by visual identification of their long trailed
images (Scotti et al., 1991; Rabinowitz, 1993) These include the faintest
objects ever detected outside the Earth's atmosphere and the closest observed
approaches of any NEO. Additionally, 1991 VG, which was detected
automatically, has the lowest relative velocity to the orbit of the Earth and
is the third faintest NEO yet detected. The population of the smallest objects
detected by Spacewatch is overabundant when compared to an extrapolated size-
frequency relation for NEOs as determined from crater size-frequencies on the
moon and from the large NEOs found by Spacewatch (Rabinowitz, 1992;
Rabinowitz, 1993). Compared to the distribution of orbits of the large NEOs,
the orbits found for these small objects include an excess of low
eccentricities with perihelion near the orbit of the Earth indicating a
possible distinct subclass of NEOs which we have called the "Arjunas"
(Rabinowitz, et al. 1993).

Future upgrades in both hardware, software, and telescope aperture may allow
an order of magnitude increase in the rate of discovery of NEOs in the next
several years. Several of the techniques proposed for the Spaceguard Survey
(Morrison, 1992) have already been tested by Spacewatch, and others will need
to be tested in the near future before such a survey can be implemented.

References: Gehrels, T. 1991. Scanning with Charge-Coupled Devices. Space Sci.
Rev. 58, 347-375. Morrison, D. (Ed.) 1992. The Spaceguard Survey: Report of
the NASA International Near-Earth-Object Detection Workshop. Jet Propulsion
Laboratory/California Insititute of Technology, Pasadena, California.
Rabinowitz, D.L. 1991. Detection of Earth-Approaching Asteroids in Near Real
Time. Astron. J. 101, 1518-1529. Rabinowitz, D.L. 1992. The Flux of Small
Asteroids Near the Earth. In Asteroids, Comets, Meteors 1991 (A.W. Harris and
E. Bowell, Eds.), pp 481-485. Rabinowitz, D.L. 1993. The Size Distribution of
the Earth-Approaching Asteroids. Astrophys. J. 407, in press. Rabinowitz,
D.L., T. Gehrels, J.V. Scotti, R.S. McMillan, M.L. Perry, W. Wisniewski, S.
Larson, E. Howell, and B.E.A. Mueller 1993. The Terrestrial Asteroid Belt: A
New Population of Near-Earth Asteroids. Submitted to Nature. Scotti, J.V.,
D.L. Rabinowitz, and B.G. Marsden 1991 Near miss of the Earth by a Small
Asteroid. Nature 354, 287-289.


Shoemaker E. M.*   Shoemaker C. S.   Levison H. F.
   Survey of the Jupiter Trojans

Six photographic surveys have been done that provide the bulk of our present
knowledge about populations and dynamical structure of the L4 and L5 Trojan
swarms associated with Jupiter. These include the Palomar-Leiden (PLS), T-1,
T-2, and T-3 surveys, all carried out by C.J. van Houtlen and I. van Houtlen-
Groenveld and their colleagues with the plates taken with the 1.2-m Schmidt at
Palomar in the 1960's and 1970's, a survey by S.J. Bus and colleagues with
plates taken on the 0.6-m Schmidt at Cerro Tololo in 1988 and 1989, and a
survey of bright Trojans carried out by C.S. and E.M. Shoemaker and colleagues
with the Palomar 0.46-m Schmidt between 1985 and present. The results from
these surveys combined with the efforts of other observers has led to the
discovery of 278 Trojans; 180 of these now have multiple opposition orbits and
109 are numbered. On the basis of the survey for the bright Trojans by the
Shoemakers, discovery is now thought to be essentially complete to H = 9.5.
Extrapolation to H = 13.5 from the surveys for faint Trojans suggests that the
population of Trojans larger than 15 km in diameter is about 2000 in the L4
swarm and about 1400 in the large swarm.

In an attempt to relate the current structure of the Trojan swarms to their
formation and subsequent dynamical evolution, we have undertaken to map the
stability field of Trojans in the coordinates proper eccentricity, ep, and
libration amplitude, D. To accomplish this mapping, we integrated numerically
the orbits of 110 particles with ep in the range 0 to 0.8 and D in the range 0
to 140 degrees for a billion years. We find that with increasing time the
stability field shrinks in both ep and D. A surprising result is that a
significant number of Trojans lie outside the region of one billion year
stability. About 8% of the Trojans are unstable on this timescale. The Trojan
swarms are dissipating with half-life of the order of the age of the solar
system. Particularly unstable Trojans were lost early in solar system history,
and the early Trojan swarms must have been many times more massive than the
present swarms.


Luu J. X.*
   The Kuiper Belt Comets

The existence of a belt of comets in the outer solar system, called the Kuiper
Belt, has often been postulated to explain the origin of the short-period
comets. The reality of the belt seems likely following the discovery of the
first Kuiper Belt candidate, 1992 QB1. Crude lower limits on the population of
the Kuiper Belt can be established from the discovery that there are at least
N ~ 1440 objects similar to 1992 QB1 in the belt, giving rise to a minimum mass
of 2 x 10^-3 M(sub)Earth. Searches for other members of the Kuiper Belt are in
progress. Stronger constraints on the structure and population of the belt can
only be obtained by the detection of other belt members.

The Kuiper Belt comets are likely to be primordial remnants of the planetesimal
disk from which the solar system accreted, and as such should provide a wealth
of information on early accretion processes.  According to the current theories
of evolution of cometary nuclei, the Kuiper Belt comets are expected to possess
mantles ("irradiation mantles") that are different from mantles of comets that
have been heated by the Sun ("rubble mantles").  Kuiper Belt candidate 1992 QB1
and another newly found distant "asteroid," 5145 Pholus, display a distinctive
red color unseen in other small bodies of the solar system, which supports the
existence of the irradiation mantle.


Babadzhanov P. B.*
   Meteoroids

A brief review of available data on the structure and composition of meteoroids,
obtained mainly from photographic observations of meteors, is presented.

The fact of meteoroids' fragmentation in the Earth's atmosphere was registered
repeatedly by means of different observational methods and especially by the
photographic method of instantaneous exposure. Four principal forms of
fragmentation is established, among them the quasi-continuous fragmentation,
i.e. a gradual release of small fragments from the surface of the parent
meteoroid and their subsequent evaporation, is most spread.

The analysis of photographic observations of meteors shows that about 30% of
meteoroids undergo a quasi-continuous fragmentation. Not only porous and crumbly
meteoroids are subjected to fragmentation in the atmosphere but more dense stony
and iron-stony meteoroids, as well. Taking into account the quasi-continuous
fragmentation, by mathematical simulation of photometric light curves of meteors
and by that of meteoroids' decelerations in the atmosphere the densities of more
than 300 meteoroids have been determined by present, which vary in the range
from 0.2 to 8 g/cm^3.

It is of special interest the meteors registered at anomaly high altitudes
because it was proposed that such meteors were produced by meteoroids, which
contain the organic compounds.





                             Monday, June 14, 1993
                                SPACE MISSIONS
                                 14:00 - 16:00

Chair(s):  M. Coradini
           J. Rahe


Rahe J.*
   The Clementine Mission to the Moon and Geographos

   No Abstract Available


Farquhar R.   Cheng A.   Coughlin T.   Dunham D.
   Near-Earth Asteroid Rendezvous (NEAR)

NASA is planning to initiate a new program of low-cost planetary missions
called the "Discovery" Program. Discovery missions will have focused
scientific objectives and strict limits on project costs and development time.
In 1991, the Discovery Science Working Group recommended that the first
mission of the Discovery Program should be a rendezvous with a near-Earth
asteroid. The selected target for the Near-Earth Asteroid Rendezvous (NEAR)
mission is the small near-Earth asteroid 4660 Nereus (formerly known as 1982
DB). NEAR is currently scheduled to be launched in January 1998 with a
rendezvous at Nereus in January 2000. It is planned to carry out a flyby of
the main-belt asteroid 2019 van Albada prior to the Nereus rendezvous, as well
as flybys of several near-Earth objects during a proposed extended-mission
phase. A simple, robust spacecraft has been designed that can accomplish the
NEAR mission within the Discovery Program cost guidelines. The science payload
weighs about 50 kg and includes a visible imager, an imaging spectrograph, a
gamma-ray spectrometer, and a magnetometer.


Shimizu M.*
   SOCCER Mission -- Scientific Objectives and Strategy

Since 1987 ISAS and NASA have been discussing the possibilities of a cometary
mission called SOCCER (Sample of Comet Coma Earth Return) using ISASs new M-V
rocket. Four workshops have been held at ISAS for this. The scientific
objectives of a comet mission can be classified into three categories:

1.  Refractories. An essential element of solar system material is waste.
Redox states of various terrestrial planets may be determined by the
interaction of water with dust particles in the solar nebula. Two important
isotopic anomalies, those of H and O, could be correlated with separation of
various isotopically different water molecules also in the nebula. The
recently proposed LID (light-induced drift) effect [1] might explain these
phenomena simultaneously. Oxygen isotopic measurements of cometary dust
particles may contribute to this type of discussion.

2.  Organics. The recent establishment of the importance of the riboxyme (RNA
enzyme) [2] clearly suggests that the final targets for the organic study of
solar system materials are not amino acids but nucleotides. RNA (a sort of
PAH) is an unstable molecule, but could be retained in the icy materials. Even
poly- (dioxy) nucleotides are all right. Of course, amino acids are also
interesting targets that could be detected with high sensitivity by capillary
electrophoresis with laser techniques [3].

3.  Volatiles. The collection of volatiles is difficult with a SOCCER-type
mission. A Rosetta-type mission is absolutely necessary.  I shall discuss
these sciences, and provide a description of the SOCCER mission.

References: [1] Bloemink H. I. et al. (1993) Phys. Rev. Lett., 70, 742.
[2] Watson J. et al. (1987) Molecular Biology of the Gene, Vol. 2, chapter 28.
Benjamin/Cummins. [3] Cheng Y. and Dovichi N. (1988) Science, 242, 562.


Belton M. J. S.*
   SMACS: Small Missions to Asteroids and Comets. A Fast, Low Cost Approach to
   the Space Reconnaissance of Near-Earth Objects

SMACS (Small Missions to Asteroids and Comets) is a NASA/DISCOVERY-class
mission candidate that will rapidly and efficiently expand the scientific
reconnaissance of asteroids and cometary nuclei begun by the Galileo, Giotto,
and VEGA missions. The concept is to target a series of small spacecraft to
carefully selected taxonomic types of near-Earth asteroids (NEA's) and
cometary nuclei (CN) to perform a comparative remote sensing reconnaissance of
their physical and chemical properties. The intent is to perform a
reconnaissance of the full range of known types and evolutionary states in as
few missions as possible and at the minimum expense. SMACS also includes
international participation to extend the concept to as wide a range of
measurement techniques as is feasible.

The SMACS mission will initiate this effort with separate launches of four
small spacecraft on low-cost Pegasus XL boosters. The first is to 2100 Ra-
Shalom (a primitive C-type asteroid); the second to 1986 DA (an M-type
asteroid thought to be a fragment of the exposed core of a differentiated
parent body); the third to P/Finlay (a CN probably in an intermediate stage of
evolution); and a fourth to 3200 Phaethon (an F-type and a strong candidate
for an extinct comet in the final stage of CN evolution). Successful
encounters with these targets should immediately yield any distinctive
physical and chemical differences that exist between each of them and with the
range of S-types observed from Galileo (951 Gaspra and 243 Ida) and the active
CN, P/Halley, observed from Giotto/VEGA. After completing its prime mission,
each spacecraft may be available for an extended mission to another near-Earth
object(NEO). Possible extended missions include one to a rare V-type (3361
Orpheus). Each encounter is in the flyby mode and is accomplished with a low-
cost, 3-axis stabilized spacecraft with an advanced technology, high-
resolution (5-14 rad/pixel), large-format (2048 x 2048), charge-coupled device
(CCD) camera fully integrated into the attitude control system. A second
instrument will be considered for each flight depending on an analysis of mass
and cost margins and the possibility for international cooperation. The
encounters will be largely autonomous and self-sequenced. Spatially resolved
images in nine spectral bands (350-1000 run) will be acquired over a full
rotation period and a range of phase angles, giving an approximately global
view of the surface morphology, data on mineralogical and physical
heterogeneity, stereo information on localized topography and spin, and
photometric information on regolith or surface microstructure characteristics.
Surface resolutions up to 8 m/pixel will be obtained.

Single-string systems will be used to reduce cost. Redundancy against mission
failure and cost overruns is provided by separate launches. Redundancy against
mass growth is provided by the ability to move to lower C3 targets. Launch
support, communications, and mission planning will be the responsibility of
the Jet Propulsion Laboratory; spacecraft development will be the
responsibility of Ball Aerospace; and the camera development and integration
will be the responsibility of Malin Space Science Systems. The total
development cost for the four flights is estimated at $139M including
reserves. The first mission costs $64M with the additional three missions
averaging $25M each.


Coradini M.*
   The Planetary Cornerstone of ESA

No Abstract Available


Laurance R. J.*   Schwehm G. H.   Hechler M.
   Rosetta Mission Scenario

   No Abstract Available


Langevin Y.*   Rosetta Science Team
   Science Objectives of Rosetta, the Planetary Cornerstone of ESA

   No Abstract Available


Eberhardt P.*   Grun E.
   The In Situ Analysis Capabilities of the Rosetta Mission

No Abstract Available


Coradini A.*
   Remote Sensing of a Cometary Nucleus by Rosetta

   No Abstract Available


Kissel J.*   Beck P.
   Analysis of Cometary Dust: Halley Results and Option for the Future

Space Missions VEGA 1+2 and GIOTTO to comet p/Halley provided the only mass-
spectrometric data on cometary organics available so far. The analysis of the
minor constituents on the mass spectra by Kissel and Krueger (1987) allowed a
characterization of the nature of the organic material as mostly unsaturated
hydrocarbons with a low fraction containing Oxygen or Nitrogen as heteroatoms.
On this basis Krueger and Kissel (1989) developed a scenario for a possible role
of comets in the origin of life on earth. Later a comparison with the PICCA data
from on board GIOTTO allowed to estimate the average molecular weight of the
refractory part of the organics to be some 150 to 250 Da.

The Comet Rendezvous and Asteroid Flyby (CRAF) Mission of NASA would have
provided a next opportunity for in situ analysis of organic cometary material
with more sophisticated instruments like a time-of-flight SIMS, a gas
chromatograph, and a neutral gas mass spectrometer. This mission has, however,
been cancelled in January 1992.

Since then two agencies are preparing the selection process for new space-
missions: NASA and ESA. While in ESA a comet rendezvous mission ROSETTA is
competing for being the third so-called 'cornerstone' of the agency, within NASA
the DISCOVERY program for small mission has currently selected three cometary
mission proposals for further study: One CONTOUR providing a flyby at three
different comets, carrying a camera and a dust mass spectrometer, C4 MISSION
(Cometary Coma Chemical Composition) carrying a camera, and a gas chromatograph
to a comet rendezvous, and SMACS even smaller crafts to asteroids and comets
carrying a camera and maybe one additional instrument.

All these concept would start their hardware phase around 1998, be ready for
launch in 2002 and returning data between 2005 and 2012. This is a clear
indication of the big cut, cometary research had to take, when CRAF was
cancelled. It is, however, encouraging that interest in comets is still alive.
Supporting laboratory work should be continued, as it greatly enhances the value
of the in-situ data once they become available.

References: Kissel, J. and Krueger, F.R, NATURE: 1987, 326, 755-760 Krueger,
F.R. and Kissel, J., Origin of Life: 1989, 19, 87-94 Krueger, F.R., Korth, A.
and Kissel, J., Space Science Reviews: 1991, 56, 167-175


Vallerani E.*   Bevilacqua F.
   The Participation of Alenia Spazio in the Programs for Solar System
   Exploration

   No Abstract Available





                             Monday, June 14, 1993
                              DYNAMICS OF COMETS
                                 16:30 - 18:00

Chair(s):  A. Sokolsky


Benest D.*   Gonczi R.
   Stochasticity of Comet P/GE-Wang

Three comets are now known to be at or near the 1/1 resonance with Jupiter:
P/Slaughter-Burnham, P/Boethin, and P/Ge-Wang. Although details of the
individual orbits differ, the three comets have very similar general dynamical
behaviour: Their orbits show many transitions between the different types of
resonant motion (satellite and anti-satellite librations, and circulation).

The stochastic character of such cometary orbits, mainly due to encounters with
Jupiter, is investigated using Lyapunov Characteristic Indicators. For each
comet of the group, we study the influences of initial mean anomaly,
eccentricity and inclination on the stochasticity of the orbit.

P/Slaughter-Burnham and P/Boethin have been previously studied. We present here
our first results for P/Ge-Wang.


Emel'yanenko V. V.*
   Dynamical Evolution of Long-Period Comets and Meteor Streams

The dynamical evolution of long-period comets and meteor streams is considered
by using the mappings describing planetary perturbations in an analytical form
for any nearly-parabolic orbit [1]. The comparison with numerical integration
has shown the great efficiency of this method. The formulae that permit us to
take into account close encounters and nongravitational effects have been
obtained.

A distribution like the Oort cloud is a typical stage of the evolution of
comets in nearly-parabolic orbits under planetary perturbations. The general
picture of the evolution of the distribution function over w = 1/a (a is the
semimajor axis) does not vary significantly with the initial value of w.
Nongravitational forces are important aspects of the transformation from long-
period orbits into short-period orbits. For the typical nongravitational
forces, w5-10% of initial nearly parabolic comets with perihelion distances w1
AU evolve quickly to orbits with aphelia inside Neptunes orbit. It is quite
possible that the observed distribution of comets has arisen by the action of
planetary perturbations for 3-5 Myr. For the orbits of classes I,II from [2],
the observed ratio of the number of "new" comets to that of "old" comets is
obtained, when a mean observable lifetime is less than 10 revolutions.
However, there is a difficulty of explaining the number of long-period comets
with w r 0.006 (1/AU).

The above-mentioned method has allowed us to study the evolution of long-
period meteor streams as well. Models of the ecliptic cross sections of the
Perseid, Eta-Aquarid, Orionid, and Lyrid meteor streams have been constructed.
Most particles of the Perseid stream cross the ecliptic plane in the range
0.9-1.1 AU from the Sun. The Orionids and Eta-Aquarids are distributed in the
ecliptic cross section near a certain line that may reach to the orbit of
Neptune. The position of this line is hardly dependent upon the initial
conditions of modelling. The possibility of a short-term outburst in the
activity of the Lyrids in 1994 is noticed.

References:   [1] Emelyanenko V. V. (1992) Celest. Mech. Dyn. Astron., 54,
91-110. [2] Marsden B. G. et al. (1978).


Levison H. F.*   Duncan M. J.
   The Origin of Jupiter-Family Comets

We investigate the hypothesis that a trans-Neptunian belt of comets, the
Kuiper Belt, is the source of the known Jupiter-family comets (periods <= 20
years). We present the results of numerical orbit integrations that track the
dynamical evolution of objects initially in nearly circular orbits in the
Kuiper Belt to visible Jupiter-family comets. Unlike all previous work in this
area, the orbits of the comets are followed by direct integrations that
include the effects of mutually interacting Jovian planets with the correct
planetary masses. We employ a technique that effectively regularizes close
encounters between comets and planets. This study is divided into two parts:
the dynamical evolution of objects in the Kuiper Belt from nearly circular
orbits until they become Neptune-crossers; and the subsequent evolution to
visible comets.

We first present the results of billion year numerical integrations of the
orbital evolution of more than one thousand test particles on initially low-
inclination, low-eccentricity orbits within the Kuiper Belt. Particles which
eventually crossed Neptune's orbit typically showed long periods (up to
several billion years) of relatively low-eccentricity oscillations punctuated
by a very rapid jump to Neptune-crossing eccentricity. We discuss the
subsequent evolution of these particles below. An intricate structure in the
region between 35 and 45 AU is found at the end of the billion year
simulation.

We next follow the dynamical evolution of Neptune-crossing objects until they
either become visible comets or are ejected from the solar system. We use the
results of our first integration as a model for the initial conditions of this
calculation. We compare the resulting orbital element distributions to those
of the real Jupiter-family and find good agreement. We use these calculations
to estimate the current number of objects in the active regions of the Kuiper
Belt. We also estimate the number of invisible objects currently in the armada
of comets moving through the outer solar system.


Lindgren M.*
   The Definition Dependency of the Duration of Jupiter Family Visits

The population of what is generally called the Jupiter family is dynamically
chaotic, and is characterized by objects jumping in and out of the family (i.e.,
visits) on timescales of 10^3-10^4 years. This, together with the fact that the
observed population most probably does not represent the orbital distribution of
the whole population of these objects, makes it necessary to make theoretical
models of the dynamical transfer from different source populations.

As part of a larger project where the dynamical timescales of short-period
comets are studied, some aspects of the problems concerning the definition of a
visit of a comet to the Jupiter family will be discussed.

The presented results are based on a large set of numerically integrated orbital
evolutions of fictitious comets with initial orbits in the Jupiter-Saturn
region.


Shoemaker E. M.*   Shoemaker C. S.
   The Flux of Periodic Comets Near Earth

A total of 25 active Earth-crossing periodic comets has been discovered; 12
are in the Jupiter family (J-f) and 13 in the Halley family (H-f). The Earth-
crossing H-f comets constitute 62% of the discovered comets of the Halley
family. The large majority of the Earth-crossing J-f comets and nearly all the
Earth-crossing H-f comets have been discovered visually at small to moderate
solar elongations. This is the region where techniques of visual search tend
to minimize bias against discovery of high-inclination comets. No H-r comets
have been discovered with q >1.574, whereas more than 90% of J-f comets have q
>1.0AU; many are now being found photographically with q >2.0 AU. The average
period of H-f comets is much longer than for J-f comets, which results in many
fewer perihelion passages and fewer opportunities for discovery for a given
period of observations. Moreover, the very broad distribution of inclinations
of H-f comets results in a strong bias against discovery by photographic
observations, which are concentrated along the ecliptic and near opposition.
Because of these selection effects, it is inappropriate to compare the raw
statistics for total discovered H-f and J-f comets with theoretically
predicted distributions.

Present understanding of the history of activity of J-f comets and discoveries
of "extinct" comets (bodies of asteroidal appearance on Jupiter-crossing
orbits) lead us to expect that the activity of most J-f comets has been
quenched by repeated passage substantially nearer the Sun than their current
q. To date, 12 candidate extinct J-f comets have been discovered; one of them
is Earth-crossing. Shoemaker et al. (1986) estimated from the set of objects
discovered through 1985 that the total population of extinct J-f comets
brighter than B(1,0)=18 is 1460 +- 650. This number is about 9 times higher
than the estimated population of active J-f comet nuclei. However, the mean i
of discovered extinct J-f comets is about twice that of the known active
Earth-crossing J-f comets. The true mean i of extinct J-f comets must be much
higher that that observed, due to observational bias against discovery of
high-i objects. The true ratio of extinct to active comets may be roughly
20:1. The current number of active Earth-crossing J-f comet nuclei larger than
1 km in diameter is estimated to be about 40 plus or minus 20, and the number
of extinct J-f comet nuclei >1 km in diameter is very roughly estimated at
800. The abundance of extinct J-f comets may be about 40% that of the ordinary
Earth- crossing asteroids, and their collision with Earth may account for 10%
to 15% of terrestrial impact craters larger than 20 km in diameter.

The number of active Earth-crossing H-f comets must be much greater than the
number discovered; completeness of discovery of comets is proportional to the
frequency of opportunity of discovery. As the mean period of Earth-crossing H-
f comets (89 years) is 16 times longer than the mean period of Earth-crossing
J-f comets (5.56 years), discovery of H-f comets should be roughly 16 times
less complete than discovery of J-f comets. Mean H for the H-f comets (7.6) is
2.7 magnitudes brighter than the mean for J-f comets (10.3); this difference
in mean magnitude is equivalent to a difference in cumulative frequency by a
factor of 12. To the same level of completeness of discovery of Earth-crossing
J-f comets, there must be roughly 12 to 16 times as many active Earth-crossing
H-f comets as have been discovered. Interestingly, this yields a ratio of H-f
to J-f comets among the active Earth crossers that is similar to the ratio
predicted by Quinn et al. (1990) for periodic comets derived from the Oort
cloud, when account is taken of dynamical lifetimes. If the two estimates of
completeness of discovery are averaged, the number of active Earth-crossing H-
f comets corresponding to the 12 known active J-f comets is very roughly 200.

The ratio of extinct to active H-f comets cannot be determined from
observations at the present time, as only one extinct H-f comet, 1991 DA, has
been discovered. Thus, we are forced to fall back on theoretical arguments to
estimate the number of extinct Earth-crossing H-f comets. Probably nearly all
J-f comets, in the course of their dynamical evolution to short period, have
passed through a stage of intermediate-period orbits corresponding to Halley-
family orbits. The steady-state number of H-f comets should exceed the number
of J-f comets by a factor equal to the ratio of the dynamical lifetimes of the
two families. This factor is roughly equal to the ratio of the mean periods of
the two families or about 16:1. This is only a crude estimate, however because
we must also understand the evolution to low q of both H-f and J-f comets. If
the ratio of extinct to active H-f comets is of the order of 16:1, the
contribution of extinct H-f comets to impact cratering on Earth must be very
roughly comparable with that of extinct J-f comets. Together, periodic comets
may account for about one-fourth of the production of terrestrial impact
craters larger than 20 km in diameter, a fraction about equal to the
contribution of long-period comets. Periodic and long-period comets combined
probably are responsible for most of the largest impact craters on Earth,
including those that may be associated with mass extinctions.


Whipple F. L.*
   The Dilemma of the New-Comet Flux

Abstract: The question of the constancy of the flux rate of "new" comets from
the Oort cloud is critical in studies of the origin of comets. The large
numbers may be explained by our being in a "comet shower or else by a loss of
intrinsic brightness by new comets during their first passage through the
inner solar system--hence the dilemma. Improvements in the quality and
quantity of reliable orbits now make possible fairly precise evaluation of the
data involved in the dilemma and a realistic comparison of the two
alternatives.

If we are in a comet shower the increase with respect to the last several tens
of thousands of years is not more than a factor of 1.8 nor magnitude loss of
more than 0^m.85. This applies if new comets are defined as those with periods
greater than 10^6 years. If the limit is set at 2.8 x 10^6 years, we may not
be really in a comet shower and a good fraction of the comets with periods in
the range 10^6 < P < 2.8 x 10^6 yr. may be returning comets. The Oort cloud
would then be fairly stable to nearly l0.000 AU.


Dermott S. F.*   Liou J. C.
   Where to Look for Low Inclination Objects in the Kuiper Belt

The recent search in the plane of the ecliptic for objects in the outer solar
system resulted in the discovery of 1992QB1 (Jewitt and Luu, 1992) in a low
inclination orbit. We point out that the plane of the ecliptic may not be the
best place to search for objects in very low inclination orbits, particularly
if very small fields of view are used, and that it would be better to conduct
such searches in the Laplacian plane.





                             Monday, June 14, 1993
                             TOUTATIS, IDA, GASPRA
                                 16:30 - 18:00

Chair(s):  M. J. S. Belton


Weissman P. R.*   Carlson R. W.   Smythe W. D.   Byrne L. C.   Ocampo A. C.
Kamp L.   Kieffer H. H.   Soderblom L. A.   Fanale F. P.   Granahan J. C.
McCord T. B.
   Galileo NIMS Thermal Observations of Asteroid 951 Gaspra

The Near Infrared Mapping Spectrometer (NIMS) onboard the Galileo spacecraft
performed spectral and thermal imaging of asteroid 951 Gaspra during the
spacecraft flyby on October 29, 1991. Analysis of the spatially resolved
thermal images give peak brightness temperatures of 225 to 235 K, depending on
the emissivity assumed. Estimated color temperatures from a 3-wavelength
(4.43, 4.71, and 4.99 micrometer) least-squares fit are similar. These results
have been compared with a thermal model of asteroid 951 Gaspra at the time of
the 1991 Galileo encounter, constructed using the KRC program (Kieffer et al.,
J. Geophys. Res. 82, 4249, 1977). The observed peak temperatures suggest that
the thermal behavior of the surface of Gaspra can not be simulated with either
a "bare rock" or "deep regolith" model. The implied thermal inertia is ~0.0025
cal cm^-2 sec^-1/2 K^-1 , somewhat more than twice the value typical for the
Moon.  These results suggest the existence of a modest regolith on Gaspra,
which could be a thin dust layer, a combination of dusty patches and rock
outcrops, or a rubble pile mixture of fine and coarse material. Additional
NIMS measurements of the hemispherically averaged temperature on Gaspra as the
spacecraft approached the asteroid will also be reported. Differences
resulting from other choices for thermo-physical parameters in the model will
be explored. Similar measurements are currently being planned for the Galileo
encounter with asteroid 243 Ida, which will occur in late August of 1993. This
work was supported by NASA through the Galileo Project at JPL.


Thomas P.*   Veverka J.   Belton M. J. S.   Galileo SSI Team
   The Shape and Surface of Gaspra

Gaspra is a highly irregular object with principal diameters of 18.2, 10.5,
and 8.9 km, and a mean radius of 6.1 km. The mean radius, axial ratios, and
pole orientation derived from the Galileo data agree closely with values
inferred from groundbased observations. Assuming that the mass distribution is
homogeneous, the rotation axis (RA = 9.47 degrees, Dec = +26.7 degrees) is
aligned with the maximum moment of inertia. Gaspra's shape is defined in part
by three extensive flat areas and two large, shallow concavities some 9-10 km
wide. One of the flat areas is about 6 km across, and defines a plane within
+- 0.2 km. In terms of limb roughness and the size of concavities, Gaspra's
shape does not differ markedly from those of some small satellites.

Grooves, linear depressions and aligned rows of pits occur widely on the
region of Gaspra seen at the highest resolution (54 m/pxl). These features
strongly suggest disturbance of loose material of at least several 10's of
meters depth, most likely by fracturing. Ridges, some of which parallel
grooves, and one of which transects an impact crater, also suggest some
faulting or deformation along preferred directions in Gaspra.

Refined analysis of imaging spectral data (400 to 1100 nm) reveals small but
significant color variations over the asteroid's surface, variations that
indicate a subtle heterogeneity in surface composition. The average albedo of
Gaspra's surface is 0.23 near 500 nm. The heterogeneity may result from
downslope movement of surficial material, but Gaspra does not show the
evidence for long distance movement of materials over the surface as does
Deimos. The interpretation of grooves being formed in loose material implies
that regolith is present in both high and low regions of Gaspra.

Craters on Gaspra can be divided into two categories: a very subdued,
apparently older population, and a population of much crisper (younger?)
craters. The latter is characterized by an index (exponent of the differential
power law describing the distribution of crater diameters) of -4.3 +- 0.3,
appreciably steeper than the theoretically derived value of -3.5 expected for
collisional equilibrium.

The continuity of structural features such as flat surfaces, ridges, and
grooves over the length of the asteroid suggests a body-wide structural grain.
This continuity of structure is consistent with Gaspra being a single object
derived from a substantially larger precursor body by collisions; it is
difficult to reconcile with hypotheses that Gaspra is a binary or a collection
of smaller bodies.


Festou M. C.*   Stern S. A.   Na C.
   Near-UV Observations of 4179 Toutatis

We observed the fast moving asteroid 4179 Toutatis using the International
Ultraviolet Explorer (IUE) spacecraft at the time of its closest approach to
the Earth on 10-11 December 1992 (Delta = 0.28-0.32 AU). Four long wavelength
(Lambda Lambda ~ 2400-3300 Angstroms) UV spectra were obtained that allow the
determination of the albedo and the UV color of this small, slowly rotating
object. The middle of the first and last spectra were exposed 22 hours apart,
thus allowing a limited study of the UV rotational variation of 4179 Toutatis'
lightcurve. The IUE data permit us to extend the spectrum of this unusually
well observed asteroid significantly below the atmospheric cut-off.
Additionally, these data permit comparison of the spectral properties of
Toutatis with those of asteroids previously observed with the IUE
spectrographs, in particular the large asteroid 4 Vesta.


Spencer J. R.*   Emerson J. P.   Tholen D. J.   Skinner C.   Meixner M.
Graham J.   Walther D.   Ghez A.   Campins H.   Osip D.   Rieke G.   Rieke M.
   The Thermal Emission of 4179 Toutatis from 150 to 6 Degrees Phase Angle

The Toutatis flyby of Earth in December 1992 provided a unique opportunity to
observe the thermal emission from an asteroid over a very large range of phase
angles. The high-resolution radar observations of Ostro et al. also provided
unique "ground truth" to assist in interpretation of the telescopic data.
Emerson, Skinner, Meixner, Graham, Walther, and Ghez observed Toutatis from
the U.K. Infrared Telescope on Mauna Kea using the 10-micron Berkeley Infrared
Camera. First observations were at 12 micrometers at 17:45 UT on Dec. 6, at an
unprecedented phase angle of 150.5 degrees, and on subsequent days Toutatis
was observed at phase angles down to 56 degrees at wavelengths between 8.5 and
11.8 micrometers. Campins, Osip, Rieke, and Rieke obtained a 10 micrometer
flux on December 17 UT at 50 degree phase from the Steward Observatory 2.3m
telescope on Kitt Peak. On January 11 and 12 UT Spencer and Tholen observed
Toutatis at 6.7-5.8 degree phase angle at 8.7--20 micrometer wavelengths from
the NASA Infrared Telescope Facility on Mauna Kea, with near-simultaneous
visible-wavelength photometry. The preliminary thermal phase curve is shown
below.

Interpretation of these observations is still preliminary because as of March
1993 the sidereal rotation period, pole orientation, and dimensions of
Toutatis are still poorly known. However, the low phase angle January
observations are expected to be fairly well described by the Standard Thermal
Model (STM), which assumes surface temperatures in equilibrium with sunlight
and includes an empirical correction for surface roughness. Because of the
several day rotation period, large deviations from equilibrium temperatures
are unlikely for the sunlit regions seen at these low phase angles, even for a
very high thermal inertia surface. Also, though the STM assumes a spherical
body, shape effects such as shadows should be minimised at low phase angle and
the STM should yield a reasonable estimate of the projected surface area.
However, interpretation of the January thermal and visible data with the STM
gives an effective diameter of 2.0 km on 1/11, and 2.5 km on 1/12, with a
geometric albedo of 0.24. This is smaller than the preliminary radar diameter
estimates of about 4 km, and we do not yet know the cause of this discrepancy.

We have attempted preliminary interpretation of the higher phase angle
observations with a spherical rough-surface thermophysical model with a
variety of thermal inertias. Despite the long rotation period, the high phase
angle data, which samples thermal radiation from the night side, is quite
sensitive to thermal inertia. Attempted model fits are shown in the figure.
Clearly more sophisticated models are needed, but these must await better data
on Toutatis' shape and rotation. Current models suggest a relatively high
thermal inertia, however.

Figure 1, which appears here in the hard copy, shows a comparison of multi-
wavelength Toutatis thermal phase curve with rough-surface thermophysical
models. Observations at slightly different wavelengths have been combined. The
models assume a sphere of diameter of 2.2 km with the sun and earth in the
equatorial plane, a rotation rate of 10 days, a bolometric albedo of 0.1, and
a surface roughness comparable to main belt asteroids. An approximate
correction for the changing heliocentric distance has been applied. The
``Rocky'' model has a thermal inertia of 2 x 10^6 erg cm^-2 s^-1/2 K^-1, and
the ``Dusty'' model thermal inertia is 5 x 10^4.


Barucci M. A.*   Fulchignoni M.   Angeli C.   Colas F.   De Sanctis C.
Dotto E.   Lazzaro D.   Lecacheux J.
   4179 Toutatis

An international campaign has been organized around the word to observe the
asteroid 4179 Toutatis (1989 AC) during December 1992 (closest approach Dec.
8th within 0.024 A.U. of the Earth) and January 1993. Our group participated
to observe it with continuous runs starting December 7th, until the end of
January 1993. We obrained CCD photometric data for 30 nights taken at Haute
Provence, Pic du Midi and Asiago Observatories.

Our first observations, obtained at ESO during last summer, seem to indicate a
very slow rotator asteroid. The results from first radar observations made by
S. Ostro from Arecibo and spectrophotometric observations made by D. Tholen
from Hawaii Observatory, seem to indicate an S type object having a "contact
binary" shape with component diameters of 4 and 2.5 Km respectively.

The aim of our work is to have a precise determination of the rotational
period, variations of the lightcurve shape as it moves acrosss the sky,
determination of the inclination of the rotation axis and the shape of the
asteroid. In addition our aim is to performe a photometric observation at
different phase angle to obtain a good phase-function. Toutatis passes through
a minimum solar phase angle of less than 0.25 degrees, so the phase function
can exibit the "opposition spike" and the the detailed shape of the phase
function may give information on surface morphology and regolith.

As done for the asteroid 951 Gaspra (Barucci et al. 1992) we will use the
results of the observations to obtain a model of 4179 Toutatis inverting the
available lightcurves. The comparison of the obtained model with the radar
images will represent the second check on the quality of the inversion
methods.

Barucci M.A., Cellino A., De Sanctis C., Fulchignoni M., Lumme K., Zappala V.
and Magnusson P.: 1992. Ground-based Gaspra modelling comparison with the
first Galileo image. Astron. Astrophys. 266, 394.


Mukai T.*   Iwata T.   Kikuchi S.   Mukai S.
   Polarimetry of Asteroids

Why do asteroids, comets and other airless bodies in the solar system show
similar phase angle alpha dependences of linear polarization? That is, in
alpha <- 20 degree - 25 degree, the polarization takes the negative branch
with the minimum depth at about alpha = 10 degree. Beyond the inversion angle
of alpha = 20 degree - 25 degree, the polarization increases with a constant
slope and reaches the maximum value at nearly alpha = 90 degrees.

Taking into account the scattering of light by large rough particles and small
aggregates in the regolith on the surface of asteroids, a phase function of
linear polarization is examined. Our calculated results will be compared with
the recent data of visible polarimetry of 4179 Toutatis observed at 7 channels
between the wavelengths of 3600 Angstroms and 7600 Angstroms . Toutatis was
favourable to observe at higher phase angles (see figure 1 on hard copy). From
a quick look of our measurements, it is found that a peak of polarization
expected at alpha = 90 degrees cannot be seen from the data at the available
phase angles less than 101 degrees. Further details of our calculated results,
as well as polarimetric data, will be reported.

Figure 1, on the hard copy, shows a relation of a phase angle and sun-asteroid
distance for 4179 Toutatis. Each circle indicates the date.


Blanco C.   Riccioli D.*
   Photoelectric Observations of 4179 Toutatis

The results of B and V photoelectric observations obtained at Serra La Nave
stellar station of Catane Astrophysical Observatory over five nights during the
recent passage of 4179 Toutatis are reported. From the analysis of our data and
those from "Toutatis news" some tentative values of the rotational period have
been deduced.


Binzel R. P.*   Slivan S. M.
   Asteroid 243 IDA as a Member of the Koronis Family: Predictions and
   Implications for the Galileo Encounter

The Galileo spacecraft encounter target 243 Ida is a member of the Koronis
family, one of the first families recognized by Hirayama early this century.
As a member of one of the most firmly established families, in situ
measurements of Ida will provide a valuable insight into the outcomes of
catastrophic disruption events. The benefit of Ida's membership in a
well-defined family is that with reasonable confidence we can identify other
asteroids that were created at the same time and location as Ida. Thus results
from in situ measurements for the formation age and collisional history of Ida
can be readily extended to a whole family of other bodies.

We are conducting a complementary study of the Koronis family in order to
provide the best context for interpreting the Galileo Ida results.
Specifically, we are continuing observations to study the distribution of
rotation rates, inferred shapes, and spin vector orientations of a large
sample of Koronis family members. Work to date [1] has suggested that the
Koronis family may have formed relatively recently compared to the age of the
solar system, perhaps within less than 10^9 years. We are investigating
whether a possible preferential orientation of Koronis family spin vectors,
which would imply a recent formation, proves to be significant when the sample
size is increased.

In addition to physical studies of Koronis family members to investigate their
collisional history, we can also estimate the relative collisional history of
Ida with respect to 951 Gaspra, based on the known population statistics
within the main belt. The mean intrinsic collisional probability for Ida is
estimated to be 5.0 x 10^-18 km^-2 yr^-1, comparable to Gaspra [2]. The
impactor velocity distribution for Ida, however, shows a lower velocity tail
than for Gaspra, owing to its more planar and circular orbit. Potential Ida
impactors have a mean encounter velocity of 4.3 km sec^-1. In addition, the
number density of potential impacting asteroids in Ida's vicinity is about
150% greater than for Gaspra.

References:  [1] Binzel R. P. (1988) Icarus, 73, 303. [2] Namiki N. et al.
(1991) Geophys. Res. Lett., 18, 1155 (1991).


Magnusson P.*
   A Spin and Shape Model of 243 Ida

Earth-based photometric observations have been used to derive a reliable model
of the asteroid 243 Ida prior to the Galileo flyby on August 28, 1993. The spin
vector (with a two-fold ambiguity) and the absolute rotational phases of the
principal axes at the time of the encounter have been determined. A triaxial
ellipsoid shape model has also been derived. Work on more detailed shape and
albedo variegation maps are in progress at the time of abstract submission. The
results are partly based on unpublished photometric observations by R. P.
Binzel, M. Di Martino, Frueh, M. Gonano-Beurer, S. Mottola, D. J. Tholen, and
W. Z. Wisniewski.


Dotto E.*   Barucci M. A.   Fulchignoni M.   De Sanctis M. C.   Angeli C.
Lazzaro D.
   Observational Results and Model of 243 Ida

During last year we carried out an observational campaign on the asteroid 243
Ida in order to achieve the maximum possible information from ground-based
observations before its Galileo fly-by.

We have carried out our observations at the Pic du Midi, (December, 26-28
1992; January, 16-21 1993) and Haute Provence (December, 25-29 1992; January
24-February 1, 1993) and Granada (November, 14-24 1992) observatories and our
observational program continues at the European Southern Observatory (April,
12-20 1993) with the aim to obtain reflectance spectra, UBV photometric
lightcurves and IR data. These data will be used to study the surface
composition and to propose a model of large scale surface features of this
asteroid.

The model of 243 Ida is obtained numerically using an algorithm developed by
Barucci et al. (1992) that, by using as baseline model a regular triaxial
ellipsoid, produces synthetic lightcurves. The comparison between the
observational lightcurves and the synthetic ones allow us to hypothize the
local morphological variations which, modifing the baseline shape of the
asteroid, cause the peculiar lightcurve features.

We will present the observational results. The models, obtained using as pole
orientation both that determined by Magnusson et al. and the one by T.
Michalowski (private comunications), will be discussed.

Barucci M.A., Cellino A., De Sanctis C., Fulchignoni M., Lumme K., Zappala` V.
and Magnusson P.: 1992 Ground-based Gaspra modelling: comparison with the
first Galileo image. Astron. Astrophys. 266, 385-394.





                             Tuesday, June 15, 1993
                             EXISTING POPULATIONS
                                  8:30 - 12:00

Chair(s):  B. A. Lindblad
           P. R. Weissman


Tedesco E. F.*
   Main Belt Asteroids (Present Population)

Main belt asteroids are those with orbits wholly within the
orbits of Mars and Jupiter. It is here where the vast
majority of the asteroids are located. During the past three
years our knowledge of these objects has increased through
detailed studies of several of the largest asteroids as well
as survey-type investigations of significant numbers of,
mostly smaller, asteroids. I have restricted the scope of
this review to summarizing and synthesizing investigations
directed primarily toward understanding the physical
properties of main-belt asteroids.

Studies on large main-belt asteroids published between early
1990 through 1992 include a definitive determination of the
size and shape of 2 Pallas by Dunham et al. (1990, Astron.
J. 99, 1636), lightcurve studies of 4 Vesta by Rock and
Hollis (1990, J. Brit. Astron. Assoc. 100, 17), 14 Irene and
51 Nemausa by Belskaya and Dovgopol (1992, ACM 91, 45), and
of 34 Circe, 98 Ianthe, 115 Thyra, 174 Phaedra, 270 Anahita,
389 Industria, 419 Aurelia, and 804 Hispania by Magnusson
and Lagerkvist (1991, Astron. & Astrophys. Suppl. Ser. 87,
269). A new determination of the mass of 1 Ceres was
published by Williams (1992, ACM 91, 641). A paper
presenting millimeter and sub-millimeter observations, and
their interpretation, of 4 Vesta by Redman et al. (1992,
Astron. J. 104, 405).

Photometric studies of small asteroids (i.e., diameters <-
50 km) were published by Schober and Stadler (1990, Astron.
Astrophys. 230, 233), Wisniewski (1991, Icarus 90, 117), and
Barucci et al. (1992, Astron. J. 103, 1679). Investigations
of spin vectors and (usually) shapes for several dozen
asteroids were also published during this period (Magnusson,
1990, Icarus 85, 229; Drummond et al., 1991, Icarus 89, 44;
Kwiatkowski and Michalowski, 1992, ACM 91, 333; Michalowski,
1992, ACM 91, 417).

Two second-generation updates to major data catalogs of,
mostly main-belt, asteroids were published in this time
frame: The Asteroid Photometric Catalogue (Lagerkvist et
al., 1992, Uppsala Astronomical Observatory) and the IRAS
Minor Planet Surve Catalog (1992, Tedesco et al., National
Space Science Data Center).

My review will show how these studies, together with
previous work, provide a reasonably complete picture of the
distributions, sizes, and general shapes and compositions,
of large main-belt asteroids. At the same time, they also
offer tantalizing glimpses into possible differences in the
physical properties of smaller main-belt asteroids.


Kresak L.*
   Comets (Existing Populations)

The huge system of comets, filling the whole interplanetary space and
surrounding it to more than 1000-times the distance of Pluto, is unique not
only by its size but, in particular, by entirely different evolutionary scales
of its individual subsystems. The members of the Oort cloud are apparently the
most primitive objects of the solar system. Only rare star passages reduce the
heliocentric velocities of a small fraction of these comets from about 100 m/s
to less than 2 m/s, which is a prerequisite of the injection into the vicinity
of the Sun, starting their activity and making them detectable as "new"
comets. This transfer takes typically 2 to 3 x 10^6 years. After the first
passage near the Sun, almost one half of these objects escape from the solar
system, and the revolution periods of the others drop mostly to less than 10%
of the original value, changing them into "old" comets.

Another persistent source is the Kuiper belt, located at the boundary of the
planetary system, about which we have only indirect evidence. It is the
necessity of progressive replenishment of the system of short-lived
short-period comets, and incompatibility of its strong flattening with the
structure of the Oort cloud. There are different views on the long-term
stability of the outer Oort cloud, in connection with the encounters with
giant molecular clouds; and comets from the Kuiper belt can also provide its
replenishment and maintenance.

Only recently the first probable member of the Kuiper belt - 1992 QB1 - was
discovered. Its size, as well as that of 2060 Chiron and
P/Schwassmann-Wachmann 1, lend support to the opinion that progressive
disintegration of larger objects may play here even a more important role than
individual dynamical captures. As a result, the cometary population of the
inner solar system can undergo considerable non-random variations with time.
Through the comets of Halley type, forming the inner boundary of the system of
long-period comets, we come to the highly flattened Jupiter family. Our
knowledge of this subsystem is much more complete than that of any other. It
is of basic importance for our understanding of the whole comet population
because it evolves on very short time scales covered by astronomical
observations, the survival time of its active members being only about 10^-6
of the age of the solar system. The variable population and structure of the
Jupiter family of comets is governed by the frequency and efficiency of
planetary encounters, the course and impact of orbital resonances,
nongravitational accelerations, splits, outbursts, and transient dormant
phases. The process of physical aging makes the dynamical evolution
asymmetric; the injections into the inner planetary system are only to about
10% compensated by the ejections, and for a majority of objects the final fate
is a total extinction or disintegration. This implies direct relationships to
other systems of interplanetary objects: formation of dust trails, their
dispersion into meteor streams, contribution to the maintenance of the
zodiacal cloud, and mixing of the population of extinct comets and their
fragments with similar objects of asteroidal origin.


McFadden L.-A.*
   The Comet-Asteroid Transition

The Near-Earth asteroids are in orbits that are dynamically unstable over the
age of the solar system. There must be a steady source or sources supplying
this population which are expected to be the main asteroid belt and the short
period comets.  Dynamical studies and observations of the physical properties
of Near-earth asteroids combined contribute to advancing our understanding of
the nature and evolution of the small bodies in the near-Earth environment and
ultimately the compositional differences in the initial reservoirs from which
the asteroids and comets formed.

Some objects are discovered and designated as asteroids only to reveal that
they are indeed comets as evidenced by their dust and gas comae, e.g. 1986 TF
= Comet P/Parker-Hartley (IAUC 4752). Conversely, asteroid 4015 1979 VA is the
same object as Comet P/Wilson-Harrington (1949 III) (IAUC 5585), but no
evidence of a gas or dust coma was observed when it was discovered in 1979.
Other objects are found in very unstable, Jupiter-crossing orbits with no
evidence of cometary activity, e.g. 5335 1991 DA and 5145 Pholus, 944 Hidalgo,
3552 Don Quixote.

Physical observations of planet-crossing asteroids to determine whether they
have weak dust and/or gas comae are necessary to determine which asteroids
evolved from comets. If no evidence of a coma is observed, it does not
eliminate the possiblity that the object evolved from a comet, it could mean
that it is currently dormant. If an object has a high albedo and strong
absorption bands in the UV, visible and near-infrared spectral regions, it is
probably derived from evolution of orbits from the inner regions of the main
asteroid belt, as such characteristics are a reasonably reliable signature of
the inner asteroid belt.

It is important to continue to make observations of known cometary nuclei to
understand their range of physical properties, to understand their evolution
as low activity, dormant or extinct nuclei, and to search for a diagnostic
physical signature of a cometary remnant. Physical observations coupled with
dynamical studies will reveal the compositional differences between the
different reservoirs in the solar nebula from which these bodies formed.


Steel D. I.*
   Meteoroid Streams

Meteoroid streams, producing meteor showers if some part of the stream has a
node near 1 AU, have complex structures that are only just beginning to be
understood. The old simplistic idea of a narrow loop being formed about the
orbit of a parent comet with one, or possibly two, terrestrial intersection(s)
is now being replaced by the recognition that their dynamical evolution may
render convoluted and distorted ribbon shapes with up to eight distinct
showers being generated. As such, the streams are excellent tracers of the
sorts of orbital evolution that may be undergone by larger objects (asteroids
and comets) in the inner solar system; indeed it is now known that objects now
observed as Apollo-type asteroids may also be the progenitors of meteoroid
streams.

Searches for showers associated with newly discovered possible parent objects
may be carried out either via the calculation of theoretical meteor radiants
(which have hitherto been derived using an untenable method) or through
searches of catalogs of individual meteor orbits. In order to accomplish the
latter, about 68,000 radar, photographic, and TV meteor orbits from various
programs in the U.S.A., the former Soviet Union, Canada, and Australia are
available from the IAU Meteor Data Center, and more than 250,000 orbits of
very faint meteors have been determined over the past three years using a new
facility in New Zealand.

The discovery among IRAS data of dust trails lagging behind comets has opened
up a new way in which meteoroid streams may be investigated, although the
relationship between these trails and the streams observed as meteor showers
at the Earth is by no means clear at this stage. Similarly radar, radio, and
spacecraft impact observations of meteoroids near cometary nuclei have added
to our knowledge.

In spite of the improvement in our understanding of meteoroid streams over the
past few years it is clear that there is much still to be done. The words of
W. F. Denning in 1923 are still pertinent: "Few astronomers occupy themselves
with the observation and investigation of meteors, and yet it is an attractive
field of work offering inviting prospects of new discoveries."


Dermott S. F.*
   Zodiacal Dust Bands

   No Abstract Available


Ceplecha Z.*
   Meteoroid Properties from Atmospheric Interactions

When a meteoroid collides with the Earth's atmosphere, most of the mutual kineti
in interaction processes. The meteor phenomenon is thus very much dependent on c
and structure of the meteoroid. Even the preheating is a sensitive indicator and
heights of the luminous trajectories can thus be used to classify meteoroids (A,
The ablation part of the trajectory, which is practically identical with the lum
reflects the composition and structure of larger bodies even better. Especially
abilities of meteoroids (terminal heights), when normalized to the same velocity
mass, and to the same inclination of the trajectory to horizon, differs by 1:100
air densities (groups I, II, IIIA, IIIB). Enormous variety of compositions and s
our "surface experience" from studying meteorites is evident: meteorites are jus
strongest part of interplanetary matter coming to the Earth, which survived atmo
(groups I and II).

Statistical criteria and studies based on the single-body theory enabled the dis
composition groups in the past. The biggest disadvantage of these procedures was
impossibility of separating the value of ablation coefficient from the bulk dens
this was done only statistical way.

Several years ago a new single-body model applied directly to individual well-ob
meteoroids (multistation photographs) yielded the possibility of separating the
from the bulk density. This method was used to all available data on PN and EN f
result of better values of ablation coefficients. At the same time a new phenome
gross fragmentation. This seemed to be important for about 25% of all observed f
the time dependences of residuals received from single-body solutions were inspe
graphical means.

Recently a new model of gross fragmentation was proposed and checked on several
the fragmentation is directly visible as splitting trails on photographs. The gr
model gives the distance flown by the meteoroid along its trajectory, l , as fun
l(t). This distance is also directly derived as l(sub)obs from measured values o
meteor photographs, i.e., from apparent distances along the trail measured for e
(velocity is an indirectly derived value and moreover it is the first derivative
distances). Together with height, h, for each time mark, t, the distance, l, is
determine the ablation coefficient, the shape-density coefficient, position of t
fragmentation point and the amount of fragmented material relatively to the main
by means of the least squares fit of l to l(sub)obs.

A computer program for automatic search of gross fragmentation points was prepar
applied to a sample of 80 records of PN fireballs with precise values of the mea
and heights. From them 21 proved to be without gross-fragmentation (category NF)
point of overwhelming amount of fragmentation (category 1F), 11 with many points
of comparable amount of fragmentation (category MF), 29 with too low accuracy to
among the above categories. Thus we have a new tool for classification of such m
which we have very precise data available, i.e., classification according to the
continuous fragmentation and their ability to be then crushed suddenly in the at
point. The classification becomes thus two dimensional in separating at least pa
of composition from the influence of structure of the body.

The dynamic pressure rho v^2 (rho = the air density) at the fragmentation points
categories tends to cluster at three separate values: 2.5, 5.3, and 8.0 Mdyn/cm^
and 20% of cases, respectively. The average of the rest is 0.8~Mdyn/cm^2.

The amount of fragmentation for 1F category is typically 60%, which corresponds
mass into approximately two halves with some accompanying small fragments. The s
important value is between 95% and 99%, which corresponds to almost complete dis
the body. The distribution of the amount of fragmentation for the MF category is
than for the 1F category: all values are between 35% and 95%, but a 65% peak is
Thus the typical sudden fragmentation for almost half of all fragmented meteoroi
to stripping away slightly more than half of the mass.

The resulting ablation coefficients from our model differ from the values derive
statistical separation of sigma from K. Type I: sigma = 0.012 s^2/km^2 (previous
0.017); type II: 0.037 (0.041); type IIIA: 0.13 (0.10); type IIIB: 0.6 (0.21). T
densities of meteoroids from our model are about the same as the previous statis





                             Tuesday, June 15, 1993
                              INTERRELATIONSHIPS
                                 14:00 - 16:30

Chair(s):  I. Williams
           M. Greenberg


Stohl J.   Porubcan V.
   Complexes of the Associated Meteoroids, Asteroids and Comets

(This paper was presented by I. Williams)
Search for associations between meteoroids and asteroids based on orbital
comparison has led to the conclusion about the existence of closely associated
complexes consisting of one or more asteroids on the one hand and of several
meteoroids concentrated into one or more streams on the other hand. As a
prototype of this kind of complex is the Taurid complex consisting of comet
P/Encke, a few asteroids (2201 Oljato, 5025 P-L, 1984 KB and 1991 BA at least)
and several meteor streams (Northern and Southern Taurids, Northern Piscids,
Southern Arietids, Northern and Southern Chi Orionids, Ro Geminids). Evidence
is presented in favour of the reality of some other complexes of meteoroids,
asteroids and comets, with the asteroids 4179 Toutatis, 1980 AA, 1989 VB,
together with 4015 (1979 VA), 3671 Dionysius and some others as their
prominent representatives. Conclusion about these complexes are in perfect
agreement with Drummond's asteroid streams, and to some degree also with
Obrubov's minor body complexes. The consequences of the results and the nature
of the comet-asterois-meteoroid complexes are discussed.


Safranov V. S.*
   Origin of Asteroids and a Cometary Cloud

The asteroid belt and the cometary cloud are by-products of the process of
formation of the planets and their origin is tightly connected with the
latter. At the present time we have a more or less satisfactory theory of a
final stage of accumulation of bodies (orderly growth of the largest planet
embryos with the timescale of 10^8 yr in the terrestrial region) as well as
numerical simulations of an early stage of this process (rapid runaway growth
of larger planetesimals into planet embryos with the timescale of 10^5 yr). An
intermediate stage has not yet been studied quantitatively. At this stage
relative velocities (eccentricities) of bodies increased much faster than
before and planetesimals of the Jupiter's zone began to penetrate the asteroid
zone and somewhat later the transplutonian region. Several mechanisms took
part in preventing formation of a normal-sized planet in the asteroidal zone
(AZ) and in removing most of bodies from the zone: a) Sweeping of AZ bodies by
more massive Jupiter zone bodies (JZB) penetrating AZ due to their larger
orbital eccentricities. b) Increase of random velocities by mean-motion and
secular resonancec. Resonant perturbations of AZB could be effective when the
resonances scanned through the asteroid belt due to variation of Jupiter's
distance from the sun while it accreted the gas and ejected bodies from the
solar system. c) Smaller bodies up to a few km could be removed from the AZ by
gas drag in the radial direction toward the higher gas pressure. Relative role
of these mechanisms is not quite clear.

Hypotheses of origin of the cometary cloud are discussed. The only plausible
mechanism of its formation is the growth of planetesimals in the region of
giant planets, increase of semimajor axes of their orbits by the perturbations
of these planets to high values a > 5000 AU and then decoupling of the
perihelia from orbits of outer planets by tidal forces of the Galaxy and
stellar perturbations. Continuous dissipation of comets from the outer Oort
cloud implies its replenishment by comets diffusing from an invisible more
dense inner cometary cloud.


Wetherill G. W.*
   Growth of Planetesimals in the Asteroid Belt

Calculations of runaway growth of ~10^18g planetesimals at 1 AU (1) have been
extended (2) to include:
1.  3-body effects in the low velocity regime (3,4).
2.  Independent variation of e and i (5).
3.  Fragmentation (both catastrophic and cratering).
4.  Formation of neighboring runaway bodies.Runaway is initiated by a brief
excursion of larger bodies into the low velocity regime after ~10^4 years,
leading to rapid merger of these bodies because of their low inclinations and
large gravitational cross-sections.  Formation of ~10^25g runaway bodies then
increases the velocities between these bodies and the smaller planetesimals,
tending to slow runaway growth.  This is offset by the self-regulating effect
of fragmentation of the smaller bodies.  Collision fragments are slowed by gas
drag, facilitating their capture by runaway bodies.  Runaway growth thus
occurs at a nearly uniform rate, leading to ~10^26g bodies in ~10^5 years at 1
AU.

These calculations have been extended to the asteroid belt (2.5 AU).  This
introduces differences as a result of the a^3/2 dependence of the orbital
period, differences in the low velocity "gravitational range" of the bodies,
and differences in the boundary of the low velocity regime for a given
relative velocity.  For the same surface density, Ceres-size bodies are formed
after ~80000 years, and continue their growth to form ~10^26g bodies after
~1.7x10^5 years.  Growth rate is approximately proportional to surface
density.

Formation of "asteroid size" final bodies, rather than objects the size of
small planets, depends on whether or not external perturbations, presumably by
Jupiter, can quench the runaway at ~10^5 years.  If so, the evolution of the
asteroid belt can proceed collisionally in the way described earlier (6).  If
not, an alternative model (7) in which large runaways, together with Jupiter
perturbations, gravitationally clear the asteroid belt on a ~2x10^8 year time
scale, will be more appropriate.  Distinction between these models will
require better understanding of Jupiter formation, and of evidence relevant to
collisional vs. gravitational clearing of the asteroid belt.

REFERENCES (1) Wetherill G.W. and Stewart G.R.(1989)Icarus 100,307-325.(2)
Wetherill G.W. (1993) Submitted to Icarus.(3) Greenzweig Y. and Lissauer
J.J.(1990) Icarus 87, 40-77/(4) Ida S. (1990) Icarus 88, 129-145.(5) Stewart
G.R.(1993) Submitted to Icarus.(6) Wetherill G.W.(1989) Asteroids II, 661-680
U. of Ariz.(7) Wetherill G.W.(1992) Icarus 100, 307-325.


Peale S. J.*
   Trojan Precursors in the Primordial Solar Nebula

If a 13 M(sub)Earth mass Jupiter core is assumed not to disturb the mass
distribution in the solar nebula, the dissipation of orbital energy of
planetesimals librating about the L4 (leading) and L5 (trailing) Trojan points
due to nebular drag does not generally result in  rapid growth in the libration
amplitude and loss from the region. Dissipation of orbital energy while the
planetesimal is further from the Sun than the stationary point in the frame
rotating with Jupiter (or the elliptic fixed point in a surface of section if
Jupiter's orbit is eccentric) causes the amplitude of libration to be reduced,
whereas dissipation while it is closer to the Sun than this point increases the
amplitude of libration. The effects of dissipation outside the stationary point
nearly cancel the effects of the dissipation inside, resulting in either a very
slow increase or decrease in the amplitude of libration depending on the details
of the nebular model. Those models of the nebula that have a larger ratio of the
midplane gas density a small distance outside Jupiter's orbital distance to the
gas density an equal distance inside Jupiter's distance give the librating
planetesimals a higher stability. By higher stability we mean that the
planetesimal will more likely remain permanently in the region with decreasing
amplitude of libration, or if the amplitude is increasing, the rate of increase
is lower. The L4 stationary points of Trojan precursors experiencing nebular
drag are located more than 60 degrees in front of Jupiter, whereas the L5
stationary points are located less than 60 degrees behind Jupiter. The
separation of these stationary points from +-60 degrees increases as the
particle diameter d decreases, reaching at the L5 equivalent a maximum of about
108 degrees in front of Jupiter for d ~ 30 m (smaller planetesimals do not
librate in front of Jupiter), but only a few degrees behind Jupiter for d ~ 1 m.
If Jupiter has a nonzero orbital eccentricity, the stability of the trailing, L5
planetesimals is increased, but the stability of the leading, L4 planetesimals
is decreased. Even if libration amplitudes are increasing slowly, the
planetesimals librate around their particular stationary points in the frame
rotating with Jupiter for times that are significant fractions of the nebular
lifetime. This means that as Jupiter's core grows, it will induce the growth of
larger Trojans as planetesimals are concentrated in the librating regions by
collisional scattering.

When Jupiter's mass approaches its current value, an annular gap in the
distribution of nebular material is believed to form. The reversal of the radial
pressure gradient on the outside of the gap means that the planetesimals are
accelerated from behind when on the outside of their libration trajectories.
This reverses the trend toward smaller libration amplitudes on this part of the
trajectory and the libration amplitude is increased on all parts of the
libration trajectory. Small planetesimals are quickly lost, although for d >
3 km lifetimes are probably longer than the remaining life of the nebula. The
inference is that much of the material trapped into libration when Jupiter was
small accumulated to very large bodies before Jupiter got large enough to create
a gap. The larger Trojans would survive the dispersing effects of the gap.

The striking asymmetry in the response of the planetesimals to the nebular drag
between the L4 and L5 points implies that there should be an asymmetry in the
numbers or total mass of Trojans remaining trapped near each stability point
after the nebula is  dissipated. However, these results favor more material
being trapped at the trailing Trojan point, which is apparently contrary to
observation. Still, the collisional evolution of the Trojans over 4.6 x
10^9 years may have skewed the mass distributions in such a way as to mask what
was there initially.


Delsemme A. H.*
   Cometary Origin of Lunar Craters

During the first billion years, the early intense cratering of the Moon was
almost entirely due to the orbital diffusion of comets induced by the
formation of the giant planets. This is demonstrated by the fit of the lunar
cratering record with the diffusion model of the cometary orbits from the
giant planets' zones. This diffusion starts immediately after the runaway
growth of the giant planets' embryos, required by other considerations [1].
The fact that the observed cratering rate can be duplicated by the sum of four
exponential rates that are in proportion to the periods of the four giant
planets is particularly striking.

The model proposed earlier [2] was developed recently [3,4,5]. In my most
recent revision, Hubbard's (1984) empirical data have convinced me to use
larger masses for the giant planets'embryos. Aimed at predicting the comet
contribution to the Earth's final stages of accretion, my model brings 1 order
of magnitude more water and 3 orders of magnitude more gases than what remains
now in the oceans and in the atmosphere. These amounts are welcome to justify
the considerable losses of volatiles due to the late impacts of large
planetesimals predicted by theory [6]. There is now a consensus [7,8] on the
order of magnitude of cometary matter brought to Earth by this mechanism.

The present paper represents my first effort to introduce a chronology in the
diffusion of the cometary orbits, in order to understand how this bombardment
interfered with the final accumulation of the Earth. I call timescale for
orbital diffusion, the time for the number of scattered bodies to diminish by
a factor of 1/e. In 1972 I used 60 m.y. for that of Jupiter's zone. Numerical
experiments done by Everhart [9] have convinced me to revise it to 70 m.y. The
other time constants are in proportion to the giant planets periods. The
early mass in the asteroid belt is assumed to be 10 Earth masses. This leads
to the followinq data:

Relevant zone:                 Asteroids   Jupiter   Saturn   Uranus   Neptune
Epoch for onset (10^6 yr)         10         10        20       50        80
Time scale (1/e)(10^6 yr)         28         70       175      410       820
Mass rate (10^23g/10^6 yr)        8.8       17.0      1.12     0.060    0.008

The figure, which in the hard copy appears here, shows the same results
graphically. On this logarithmic diagram, the exponential timescales are
straight lines. The sum of all contributions is the dashed line. The data for
the Moon cratering rates [10] have been normalized to the same units and
represented by circles with a central dot. Extrapolating the model to the
present predicts that the Neptune contribution is not quite finished. This can
be interpreted as the short-period comets, coming from Neptune's period
resonances in the Kuiper belt. Our model predicts one comet still hitting the
Earth every 20,000 yr. This is not too far-fetched for such a simple-minded
model. One conclusion seems inescapable: practically all lunar craters come
from cometary stuff.

References:  [1] Safronov V. S. (199l) Icarus, 94, 260-271. [2] Delsemme A. H.
(1981) in Comets and the Origins of Life (Ponnamperuma, ed.), pp. 141-159,
Reidel Dordrecht. [3] Delsemme A. H. (199l) in Comets in the Post-Halley Era,
Vol I (Newburn et al., eds.), pp. 377-428, Kluwer. [4] Delsemme A. H. (1992)
Origins of Life, 21, 279-298. [5] Delsemme A. H. (1993) Adv. Space Res., in
press. [6] Wetherill G. W. (199l) Science, 253, 535-538. [7] Ip W. H. and
Fernandez J. A. (1988) Icarus, 74, 47-61. [8] Chyba C. F. (1987) Nature, 330,
632-636. [9] Everhart E. (1977) Comets, Asteroids, Meteorites (Delsemme, ed.)
pp. 99-104 , U. of Toledo. [10] Carr M. H. et al. (1984) Geology of
Terrestrial Planets, NASA SP-469.


Helin E. F.*
   (4015) 1979 VA: 'Missing Link' Discovered

Apollo Asteroid (4015) 1979 VA was discovered in November of 1979 by Helin at
Palomar with the 0.46m Schmidt Telescope. Its' orbital elements immediately
indicated a possible cometary origin. With an extremely eccentric orbit, it
approaches the orbit of Jupiter (at the time, the largest "Q", aphelion, of
any known near-Earth asteroid). Physical observations acquired during the
discovery apparition suggested that it was carbonaceous in nature.

Research into prediscovery observations of Near-Earth Asteroids (Bowell et.
al., 1992) has located Palomar Sky Survey photographic plates taken in 1949
observations of (4015) 1979 VA, not as an asteroid, but rather a small
cometary image (IAU Circular Nos. 5585 and 5586, August 13, 1992). In 30
years, we have witnessed photographically, the physical transition of a comet
to an asteroid.

(4015) 1979 VA was observed last year in order to see if any cometary activity
was present during this approach to perihelion. No obvious activity was
detected from ground based telescopes. As mentioned earlier, the observations
from 1949 show the object as an asteroidal like image, but with a small tail.
In 1979, when discovered as a bright asteroid, it showed no apparent coma or
tail. Curiously, however, there were some unexplainable irregularities in the
data. Review of this data leads to the theory that it may have been degassing
intermittently (Harris, personal communication). Review of the large amount of
data is now underway to better understand this tantalizing object.

As a consequence of identifying (4015) 1979 VA as a former comet, it has
become a priority small body target for a Discovery Mission. The selection of
(4015), a defunct comet, is an exciting target considering its recent
evolution into an asteroid. NASA and ESA are both considering missions to
(4015). This evidence of the generic relationship between comets and asteroids
has been long-awaited.

This research was conducted at the Jet Propulsion Laboratory and at Palomar
Observatory, California Institute of Technology, under contract with the
National Aeronautics and Space Administration (NASA).


Schulz R.*   A'Hearn M. F.   McFadden L. A.   Yeomans D. K.   Haken M. E.
Chamberlin A.
   2201 Oljato and 1566 Icarus: Comets or Asteroids?  A Comparison with Comet
   Wilson-Harrington Also Known as Asteroid (4015) 1979 VA

We report on the results of spectrophotometric observations of the possible
cometary candidates 2201 Oljato and 1566 Icarus as well as of the object
classified as asteroid (4015) 1979 VA, which was discovered to be in fact
Comet P/Wilson-Harrington (1949III) (IAU Circular 5585). Spatially resolved
spectra of 2201 Oljato and P/Wilson-Harrington taken with the Ohio State
University CCD Spectrograph on the 1.8-m Perkins reflector of the Ohio
Wesleyan and Ohio State University at Lowell Observatory on Oct. 7th and 8th,
1992, will be compared. The spectrograph was equipped with a 272 by 528 pixel
CCD with a spatial scale of 0.95 arc sec per pixel. The spectra cover the
region between 3240 Angstroms and 6160 Angstroms, which includes the
significant emission bands of CN, C2, and C3. In addition, bias and flat-field
frames as well as exposures of a FeNe comparison lamp for wavelength
calibration and observations of several spectrophotometric standard stars and
solar analogs were obtained to allow the flux calibration and the
approximation of the continuum. The images were flux calibrated and upper
limits for the production rates of the three radicals in 2201 Oljato and
P/Wilson-Harrington were calculated and compared. Additionally, the spectral
reflectance of the continuum compared to a solar analog in the region between
3240 Angstroms and 6160 Angstroms will be discussed for both objects.

The ultraviolet spectra of 2201 Oljato (taken on Oct. 14th to 15th ,Oct. 23th
to 24th, and Nov. 2nd, 1992), 1566 Icarus (Sept. 30, 1991), and
P/Wilson-Harrington (Sept. 30th, Oct. 1st to Oct. 2nd, 1992) observed with the
International Ultraviolet Explorer (IUE) satellite observatory have also been
searched for evidence of gaseous emissions. The upper limits for the
production rate of the OH radical deterrnined for all three objects will be
compared and discussed. Unfortunately we cannot include an analysis of the
continuum in the ultraviolet, because the apparent magnitudes of the objects
have only been between 18^m and 17^m and the limiting magnitude for detection
of continuum with the IUE is of the order of 10^m.

The comparison of 2201 Oljato and 1566 Icarus with Comet Wilson-Harrington, of
which we know that it is at least a dormant comet, shows promise with regard
to clues to the question whether 2201 Oljato and 1566 Icarus might be extinct
or dormant cometary nuclei. The results will be also discussed in view to
necessary future observations.


Chamberlin A. B.*   McFadden L. A.   Schulz R.   Schleicher D. G.
   2201 Oljato and 4015 Comet P/Wilson-Harrington: Search for CN (0-0) Band
   Emission

2201 Oljato is a planet-crossing asteroid which may have evolved from either
the main asteroid belt or the reservoir of short-period comets into its
current, dynamically unstable, planet crossing orbit. There is both
observational evidence of the asteroid's physical properties (McFadden et al.,
1993) and dynamical evidence (e.g. Milani et al. 1989) that this asteroid
might be an inactive cometary remnant. Asteroid 4015 1979 VA was discovered to
be the same object as comet P/Wilson-Harrington 1949 III by Bowell (IAUC
5585). With this knowledge, we made measurements of these two objects to
search for CN emission, which is the most easily observed volatile species in
comets.

Ground-based observations were made in October 1992 using Lowell Observatory's
1.8m telescope with the Ohio State University CCD spectrograph in order to
measure the gas production rate of CN. The CCD spectrograph was configured to
give a wavelength range of approximately 3240A to 6160A. 2201 Olajto was
observed on the nights of 04, 06, 07, 08, and 26 October 1992 pre perihelion
with a heliocentric velocity (r-dot) of ~20 km/s and heliocentric (r) and
geocentric (D) distances of ~1.28 and ~1.10 AU respectively. A total of 16
exposures (20 minutes each) were taken of 2201 Oljato. 4015 Comet P/Wilson-
Harrington was observed on 04, 06, 07, and 08 October 1992 post perihelion
with r-dot=~11 km/s, r=~1.17 AU, and D=~0.76 AU. A total of 11 exposures (20
minutes each) were taken of 4015 Comet P/Wilson-Harrington. Extinction, flux
calibration, and solar analog stars were observed on each night.

The column density of CN is calculated from the observed flux at the CN (0-0)
band taking into account the fluorescence efficiency at the geocentric
distance, heliocentric distance, and heliocentric velocity of each object.
Finally, the column densities are converted to CN production rates on the
basis of the Haser (1957) model. The results will be compared with production
rates observed in comets. Using flux calibrated spectra of 2201 Oljato and
4015 Comet P/Wilson-Harrington, we will divide by a solar analog spectrum to
produce relative reflectance spectra. Comparison to previously obtained
reflectance spectra will be made.

REFERENCES: McFadden, L.A., Cochran, A.L., Barker, E.S., Cruikshank, D.P.,
Hartman, W.K. 1993. J. Geophys. Res. 98, No. E2, pp. 3031-3041, . Milani, A.,
Carpino, M., Hahn, G., Nobili, A.M. 1989. Icarus, 78:212-269.


Meech K. J.   Weaver H. A.*   Zellner B.   Noll K. S.
   Mass Loss Limits from 5145 Pholus

Both ground-based and Hubble Space Telescope (HST) images of the Chiron-like
asteroid 5145 Pholus were obtained in an effort to search for cometary
activity just past the perihelion time in 1991 September. Ground-based
observations were made with the University of Hawaii 2.2m telescope on UT 1992
Jan 31 (R filter; 3200 sec) and with the Cerro Tololo Interamerican
Observatory 4m telescope on 1992 March 7 (BVRI filters; 800 sec) at
heliocentric distances of 8.71 and 8.72 AU respectively, under conditions of
good seeing. HST data were obtained on 1992 April 28 with the Planetary Camera
(F555W, F785LP filters; 2400 sec total exposure time through the F785LP filter
and 900 sec total through F555W). Neither the ground-based nor HST images show
signs of coma. We will discuss the restoration of the HST images using the
Lucy-Richardson technique and discuss how the data were compared to stellar
profiles for both the ground-based and HST images. We will present models for
limits on steady-state dust production and possible bound coma on Pholus and
compare this to dust production on Chiron and limits for other extinct comet
candidates which have been observed.

^1Currently on leave at {\em The Johns Hopkins University} \end{document}


Campins H.*   Osip D.   Gustafson B. A. S.   Rieke G.   Rieke M.   Larson S.
Schleicher D.
   A Multi-Wavelength Study of the Potentially Meteorite-producing Comet
   P/Wilson-Harrington (4015 1979VA)

The identification of object 4015, 1979 VA, as Comet P/Wilson-Harrington, P/W-
H, (IAUC 5585, 1992) supports the notion that extinct or dormant nuclei of
periodic comets can have asteroidal appearance, and that some fraction of the
Earth-crossing asteroids have a cometary origin. Furthermore, orbital
characteristics make P/W-H particularly interesting even among the comet-
asteroid transition objects. It has a low inclination orbit, perihelion just
inside 1 AU, and aphelion in the middle of the asteroid belt. This orbit has
the potential of delivering meteorites to Earth at relatively low velocities.
The figure, which appears here in the hard copy, plots the orbital
eccentricity versus reciprocal semimajor axis of meteorite producing
meteoroids (crosses) listed by Halliday et al., (J. Roy. Astron. Soc. Can.,
83, 49--80. 1989) and of P/W-H (open circle). P/W-H plots close to several
meteoroids; we are in the process of doing orbital integrations to determine
under what circumstances the comet may have produced any of these objects.

We also obtained time series CCD images (red filter lambda(sub)c=7000 Angstrom
delta lambda=2600 Angstrom) on UT 1992 Dec. 01 and 02 at Lowell Observatory
and in the Cousins red bandpass on UT 1992 Dec. 21 and 1993 Jan. 26 at U. of
Arizona. These images are being used to determine the rotational
characteristics of the comet as well as to search for any detectable coma.
Simultaneous reflected (1.26, 1.60, and 2.22 micrometers) and thermal (10.6
micrometers) infrared observations were obtained on UT 1992 Dec. 17 at U. of
Arizona. Thermal modelling utilizing these observations will provide estimates
of the radius and albedo. However, at the time of this writing, the rotational
characteristics have not been determined, hence the surface temperature is
uncertain. If slow rotation is assumed (standard thermal model) a preliminary
estimate of the radius is 1.5 km.


Stern S. A.*   Stewart G. R.
   Collisions in the Kuiper Disk

We have constructed models of the collision rates between cometary bodies in
the present-day Kuiper Disk (KD) under various assumptions about the total
mass and the distribution of orbital (a,e,i) elements in the Disk. These
calculations demonstrate that significant collisional evolution can take place
in the Kuiper Disk. By computing the optical depth of debris created from
collisions in the KD, our calculations reveal that the present-day radial mass
distribution of the solar system undergoes a sharp discontinuity (or edge) at
Neptune. That is, we can demonstrate that the Kuiper Disk is not massive
compared to Neptune. Based on this result, we conclude that either the
primordial solar system's mass distribution was similarly truncated near 30
AU, or subsequent collisional/dynamical evolution has depleted the 30-100 AU
zone by several orders of magnitude. We will report on these results and a
model predicting the detectability of thermal-IR emission from collisionally-
produced dust in the Kuiper Disk.


Napier W. M.*
   Near-Earth Asteroid Groups

A statistical examination of fine structure in the near-Earth object system
was carried out and revealed the presence of several asteroid groups, in each
of which the members had closely similar orbits. Two strong groups (each with
>12 known asteroids) appear to be associated with Comets Giacobini-Zinner and
Encke respectively. Statistical testing established the reality of these
groupings at a high confidence level. They have short dynamical lifetimes and
may represent an intermediate stage in the fragmentation of very large comets;
asteroids within these groups may in fact be dormant comets with lifetimes
>10^5 yr.





                             Tuesday, June 15, 1993
                PHYSICAL PROCESSES IN COMETARY COMAE AND TAILS
                                 17:00 - 18:00

Chair(s):  C. Arpigny


Huebner W. F.*   Boice D. C.
   Gas and Dust Velocities in Comets Determined from a Dusty Coma Model

Multifluid, hydrodynamic simulations for the gas, dust, and plasma flow
appropriate for the spacecraft encounters with comets P/Halley, P/Giacobini-
Zinner, and P/Grigg-Skjellerup allow a comparitive study of how features of
cometary comae scale with gas production (Q) and dust-to-gas mass ratio. The
simulations are based on a spherically symmetric, neutral coma model with
detailed photo and gas-phase chemistry and dust entrainment by the gas. The
model includes a separate energy balance for the electrons, separate flow of
the neutral gas, fast neutral atomic and molecular hydrogen, dust size
distributions with fragmentation, distributed coma sources of gas-phase
species related to the dust, and a smooth transition to free molecular flow in
the outer coma.

These simulations show that the terminal gas velocity is constant at about 1
km s^-1 for gas production rates less than approximately 1.0^29 molecules s^-
1. At higher gas production rates, the terminal gas velocity steadily
increases to about 2 km s^-1 at Q = 2.10^30 molecules s^-1. This behavior is
caused by the photodissociation of water coupled to the extent of the
collision zone in the inner coma. It is consistent with water velocities
determined from OH observations and the in situ measurements of the gas
velocity of P/Halley by the Giotto NMS instrument. The terminal dust
velocities for sizes less than about 10 micrometers appear to follow a similar
relation with an increase beginning at about Q = 4.10^28 molecules s^-1. In
comparison with observations, the model can be used to deduce gas and dust
velocity laws within the coma.


Crifo J. F.*
   A General Physicochemical Model of the Inner Coma of Active Comets: 1. Gas
   and Dust Outflow in the Case of Dominantly Distributed Gas and Dust
   Production

A new physicochemical model of the inner coma of active comets
has been developed. The main goal of the model is to
systematically explore which gas and dust production schemes are
compatible with the very incomplete experimental information
available on the inner coma and nucleus of comets.

The method consists of computing the outflow (under solar
illumination) of a multiphase gas and dust mixture with full
allowance for interphase transfers between two reference
surfaces. The outer surface is the closest surface to the nucleus
where significant information on the coma composition is
available; any given solution is considered acceptable or
unacceptable by reference to this available information. At the
inner surface (the nucleus surface), the boundary conditions are
fully free, i.e., the size of the active area, the ratio between
solid mass emission rate and gas mass emission rate, and the
properties of the solids emitted (mass spectrum, relative
volatile-to-refractory content of the solids, mineralic
composition of the refractory part) can be prescribed freely. In
other words, at the nucleus (1) the gas phase composition and
mass flux and (2) the solid grain total mass flux, mass spectrum,
and internal grain chemical composition can differ radically from
the observed quantities at the outer surface (only the total flux
of each chemical species is preserved). The driving process that
introduces these differences is solar illumination (visible and
UV).

Great care is taken to make use only of state-of-the art gas
dynamic methods reliably used by many groups to model laboratory
and industrial multiphase flows of condensible vapors, and to
exclude the numerous unphysical assumptions that weaken many
existing coma models [1], so that the solutions obtained could
also (regardless of cost) be tested by simulation experiments and
by appropriate in situ instrumentation.

As an example of the model capabilities, we present one solution
adjusted to P/Halley flyby gas and dust data, in which 95% of the
gas and 95% of the measured dust distribution are produced
between 0 and 500 km from nucleus surface by large (centimeter-
size) fragments of the nucleus surface (i.e., made up of ice,
silicate, and carbonaceous material) emitted from very small
areas of the nucleus and accelerated away by the 5% of the gas
that issues directly from these areas. We show that solutions of
this kind are free from the objections to the classical nucleus
production models, and thus are likely to represent the structure
of the inner coma of active comets much more realistically than
classical coma models. We conclude that the design of future in
situ inner coma probing instrumentation onboard a rendezvous
spacecraft should not be based on predictions from such classical
models.

References:  [1] Crifo J. F. (1991) in Comets in the Post-Halley
Era, Vol. 2 (R. L. Newburn and J. Rahe, eds.), 937-989, Kluwer,
Dordrecht.


Colom P.*   Bockelee-Morvan D.   Bourgois G.   Crovisier J.   Gerard E.
Jorda L.   Despois D.   Paubert G.
   Radio Observations of Anisotropic Outgassing in Comets

We observed comets P/Halley during one year, Austin (1990 V) from mid-February
to mid-June 1990, Levy (1990 XX) from mid-June to the end of September 1990
and P/Swift-Tuttle 1992t from mid-October 1992 to the 12th of January 1993 at
the Nancay Radio Telescope.

Often, the OH radical 18-cm lines observed at Nancay were blueshifted with
respect to the nucleus velocity. Redshifts were also observed a few times.
Greenstein effect can cause line asymmetry. After modelling and substracting
this effect, a significant line shift of -0.16 km.s^-1 is still found for
P/Halley. In comets Austin and Levy, similar values were obtained. Comet
P/Swift-Tuttle produced a much stronger systematic blueshift of 0.5 to 1.0
km.s^-1.

The IRAM 30-m Telescope observed the millimetre (1 to 3 mm) rotational lines
of several molecules in these comets: HCN in all four comets, H2CO, CH3OH and
H2S in comets Austin, Levy and P/Swift-Tuttle. Blueshifts were also observed,
in close agreement with the residual line shifts of the OH line. This shows
that the Greenstein effect in the OH lines is correctly understood, and that
no other secondary effect is affecting this dissociation product.

The case for P/Swift-Tuttle is peculiarly interesting, because of the high
values of the line shifts and of the high signal-to-noise ratios, which allows
a good determination of the unusual asymmetric line shape. This latter is
probably related to the strong dust jets observed in optical images.

All these blueshifts are the line-of-sight projections of the gas bulk
velocities, and reflect the presence of asymmetries in the gas expansion
responsable for the non-gravitational forces acting onto the nucleus of these
comets. Since all observations were made at acute phase angle, this
corresponds to an excess outgassing towards the Sun. These observations,
combined with the non-gravitational parameters A1 and A2, should allow us to
constrain the nuclei masses.


Shulman L. M.*
   CHON-Particles as a Possible Spread Source of Molecules in the Inner Coma of
   Comet Halley

There is evidence that a spread source of volatiles exists in a comet coma
except the surface of its nucleus, which is the usual source of parent
molecules. Sometimes the anomalous distribution of brightness is observed in
the inner comas. It rarely looks like the ring structure, because most
volatiles are vaporized from the nuclear surface and the additional source may
be too weak to make sufficient contrast on the image of the comet. Recently,
in the VEGA-GIOTTO missions to Comet Halley, cyanogen jets were detected and
these jets accompanied the dust ones so as dust produced CN.

Nazarchuk [1] has found in the Comet Halley spectra the inverse dependence of
the intensities of different emission on the nucleocentric distance. It may be
caused by the jets, but it is strange that this phenomenon occurs on different
dates approximately at the same nucleocentric distances and those distances
are different for different species.

A possible reason of the effect is proposed and analyzed. To explain the
inversion of the space distribution of a comet gas one needs an additional
source of this gas, which is most intense at some distance from the nucleus.
Dust particles of the appropriate chemical composition can be proposed as this
source. The question is, what can delay their sublimation until they reach the
definite nucleocentric distance?

The hypothesis is discussed that the evaporation of the grain should be
enhanced abruptly when surface tension takes the participation in the process
of sublimation of the grains. The equation of sublimation of small particles
really has an "explosive" solution that gives the spread source of volatiles.
This source would be localized at some sphere around the comet nucleus and
very close to it. The necessary conditions to explain the observed facts are
discussed.

References:   [1] Nazarchuk H. K. (1993), this volume.


Cosmovici C. B.*   Schwarz G.   Ip W. H.   Fink U.
   H2O+ Jets in the Coma of Comet Halley?

Previous investigations applying the "ring n1asking" technique to CCD images
obtained at the South African Observatory in the period March 13-18, 1986, led
to the detection of CN, C2, C3, and ([OI] + NH2) jet features in the coma of
Comet Halley (1). In complement to these results we report here the probable
detection of jet features in water ion emission at 6193 Angstrom that may be
used as a tracer for the time-dependent anisotropic emission profile of the
water vapour from the cometary nucleus.

We compared the South Africa images with images taken at Catalina Observatory
with the same filter on January 20 and March 7, 8, and 24, 1986, in order to
exclude instrumental errors. A clear H20+ jet on March 1 in the Arizona images
has no continuum counterpart.

Because of the narrow bandwidth of the filter (6193 +- 21 Angstrom) other
significant molecular contaminations could be excluded.

In this paper we will discuss the mechanism that could lead to H20+ jets
formation and compare the results with other H20 distribution measurements in
the Coma of Comet Halley.

References: (1) C.B.Cosmovici, G.Schwarz, W.H.Ip and P.Mack (1988) Nature,
332, 705.

The figure, which appears in the hard copy, shows a CCD image of H2O+ jets on
March 14, 1986, 03:28 U.T.


Brandt J. C.*   Caputo F. M.   Niedner M. B. Jr.   Yi Y.
   Disconnection Events (DEs) and Sector Boundaries: The Evidence from Comet
   Halley 1985-1986

Cometary and solar wind data for the interval December 1985
through April 1986 are presented for the purpose of determining
the solar-wind conditions associated with DEs. This paper adds 3
DEs to the work of Yi et al. (1993) and now includes all 19 DEs
believed to be "obvious" in Comet Halley.

The 3 DEs have disconnection times of January 16.9, January 19.9,
and March 11.4, 1986. Currently all these DEs have only a single
image. The disconnection times are inferred from average
kinematics for other DEs.

The solar-wind data come from IMP-8, ICE, PVO, and Vega-1; these
spacecraft have been used to establish approximately 50 sector-
boundary crossings and the velocity for this entire period came
primarily from IMP-8. Sector boundaries at the sun (Hoeksema
1989) have been extrapolated into heliospheric space using
measured solar-wind speeds and different source- surface radii to
determine the 'best-fit' to the spacecraft data. This best-fit
boundary is compared with the times and locations of DEs as are
the times of high-speed streams.

DEs show a clear association with sector boundaries, but not with
high-speed solar-wind streams. For example, the IMP-8 data for
this time interval shows only 9 streams where the speed is 600
km/sec or greater. Thus, the DE mechanisms favored include those
of Niedner and Brandt (1978) which invokes sunward magnetic
reconnection and of Russell, Saunders, Phillips and Fedder
(1986), if sector-boundary crossings can trigger tailward
magnetic reconnection. If the DE locations are shifted by an
amount corresponding to a 1.0-day delay between crossing the
sector boundary and the actual disconnection, the dispersion of
the DEs around the best-fit sector boundary is reasonably close
to the dispersion of the spacecraft crossings around the best-fit
boundary.

If essentially all DEs have a common physical cause and the DEs
observed in comet Halley are representative, modeling efforts
should focus on conditions at sector boundaries. Final acceptance
of any mechanism requires a detailed simulation that produces
DEs. _






                             Tuesday, June 15, 1993
                                  METEORITES
                                 17:00 - 18:00

Chair(s):  D. W. Hughes


Longo G.*   Serra R.   Cecchini S.   Galli M.
   Search for Microremnants of the Tunguska Cosmic Body

On June 30, 1908, at 07^h 14^m 28^s, Local Time, the explosion of a cosmic body
over a point (60 degrees 53' 09" N, 101 degrees 53' 40" E) in the basin of the
Podkamennaja Tunguska River resulted in 2200 km^2 of forest fall and in a great
number of trees and branches burned in a large part of the explosion area. The
nature and composition of the Tunguska Cosmic body (TCB) is still controversial.
The two more plausible explanations consider the explosion in the atmosphere at
an altitude of 5-10 km of a comet or an asteroid. In the last few years the
cometary hypothesis was favored on the basis that a low-density object was
needed to explain the Tunguska catastrophe. Recently, it has been shown [1]
that, taking into account the effect of aerodynamic forces that can fracture the
object, the TCB can be a normal density object and the Tunguska event is fully
compatible with the catastrophic disruption of a roughly 30-m-diameter meteorite
of the common stony class. However, the same calculations do not exclude the
possibility that the TCB was a high-velocity iron object, nor rule out a
carbonaceous asteroid as an explanation of the event. Taking into account that
the input parameters of calculations are poorly known, the cometary impactor
hypothesis cannot be considered definitely dismantled. The only way to reach the
certainty about the nature and composition of the TCB remains the search for
some of its remnants. Up to now, no impact craters or meteorite fragments have
been found in an area of 15,000 km^2. A bed-by-bed chemical analysis of the
peat, in three different places of the Tunguska site, has given uncertain
results. A great part of the spherules from soil samples, collected several tens
of kilometers from the epicentre, have a terrestrial or micrometeoritic origin,
though some of them are possible relic candidates for the TCB.

In the present work a new method is used to obtain experimental data on the
nature of the TCB. The leading hypothesis is that in the forest conifers, such
as Picea obovata, which have survived the Tunguska catastrophe, the fluid resin
freshly emitted for some physiological effect could have acted as a trap, as it
happens in amber, for aerial particles present at the moment of the event. In
that type of tree, some resin is actually produced inside sapwood and partially
emitted year by year to the exterior, in a fluid state, during the most active
vegetation period. Part of it, deposited around little dead branches on the
outer bark of the tree, solidifies and becomes gradually included from outside,
by the growing wood. So if the fresh resin can act like a trap for aerial
microparticles, the portion of them embedded in the tree can eventually be found
in the resin layer along the limit between the growth rings and the withered
branch. The growth rings, being rather easily datable, can give information on
the age of the resin.

To collect the necessary wood samples from trees that have survived the
catastrophe, the authors of the present work participated in the summer of 1991
in an international expedition led by the academician N. V. Vasiljev and the
astronomer G. V. Andreev from Tomsk [2]. The samples collected have the form of
thin transversal stem slices, sawn at different heights, or of cylindrical cores
extracted from living trees and containing the embedded withered branches
covered by resin. After cutting the samples, in order to put in evidence the
branches, the interstice branch-stems, containing the resin layers, were opened.
Thus it has been possible to observe with a scanning electron microscope (1) the
surface of the resin that was adjacent to the dead branches and (2) the
particles extracted after dissolving the resin inside filters placed in capsules
of a soxelet. A Philips SEM 515, equipped with an EDAX PV9100 windowed X-ray
spectrometer, has been employed to investigate the morphology, dimensions, and
chemical composition of the particles contained in the samples. The samples were
mounted on SEM specimen stubs and subsequently coated with a 30-nm-thick carbon
film to prevent the accumulation of the electrostatic charge deposited by the
electron beam. The SEM was operating at 30 kV with a specimen probe current of
5 nA. A computerized electron detector has been used for determining the
concentration of components with atomic numbers greater than 10. The resin
corresponding to the tree rings of the years 1890-1930 has been searched on 12
branches of 7 trees situated in different directions within a radius of 8 km
from the epicenter of the Tunguska catastrophe. For comparison, six other
branches of a tree growing at more than 1100 km from the epicenter and the resin
deposited on the roots of a tree uprooted by the explosion of 1908 have also
been examined. Up to now a total of 6309 particles (average size 5 micrometers x
3 micrometers) have been observed. Those that have nothing to do with the
Tunguska event are discussed elsewhere. For certain elements clear abundance
peaks have been found in correspondence with the period of the Tunguska event.
These results will be presented for the first time at the ACM '93 conference.

References:  [1] Chyba F. et al. (1993) Nature, 361, 40. [2] Cecchini S. et al.
(1992) Sapere, 944, 9.


Andreev G. V.*   Goldin V. V.   Vasilyev N. V.
   1908 Tunguska Event: Energy, Orbit, Composition, Origin

This paper reviews the Russian published and unpublished data on the Tunguska
1908 explosion.

On June 30 at 00 h:14 m UT a catastrophic collision of a small cosmic body
with the Earth took place over Central Siberia in the interriver of
Podkamennaya and Nizhnyaya Tunguskas (60.9 grad to N; 101.9 grad to E). The
Tunguska cosmic body exploded at an altitude of 5-8 km, with an energy of ~20-
50 Mtn TNT. The object responsible had a radius of ~40-100 m. The forest,
which is 2150 m^2 in area, was felled radially from the epicenter. A special
field program of the Tunguska Expedition was carried out over 15 years to
investigate the inner structures of the directions of the fallen trees. The
seismic waves were fixed in Jena, Irkutsk, Tashkent, and Tiflis. These
fundamental data provide a good opportunity for the reconstruction of the
picture of atmospheric disruption and explosion of the Tunguska body.

The coordinates of the visible radiant were determined with help of the eye-
witness testimonies catalogue. This apparent geocentric radiant (A = 300-310,
H = 12-22 grad) corresponds to the heliocentric orbit of Apollo-type asteroids
or "burnt-out" comets.

For determination of the element and isotope composition of the Tunguska
object, an area of 15,000 km^2 was covered by the cosmochemical survey. This
paper analyzes the more-than-30-year history of the Tunguska Expedition
Cosmochemical Program.

Data on the Tunguska geomagnetic effect, on the local remagnetization of
soils, on the changes of thermoluminescent properties of rocks, on the
atmospheric optical and polarization anomalies, on the biological (including
genetic) after-effects are discussed.

This paper reports on the aims and first results of the International Tunguska
Expedition and International Research Program.


Korlevic K.*
   Powerful Bolide Explosion Over the North Italy

On the night of January 19, 1993, at 00:33:20 +-15 sec. UT, many people from
north Italy, Switzerland, Slovenia, and Croatia witnessed a very bright flash
in the sky. The fireball entered the atmosphere over the Adriatic sea from
azimuth of 310 +-15 (SE-NW), inclination lower than 20, and the mass greater
than 50 tons. The velocity was greater then 20 km/s. The luminosity in the
first part of the flight was M ~ -13 with incrementing intensity and on the
end a very intense flash.

The complete destruction of the body was over the Italian region of Emilia,
between the cities of Bologna, Faenza, and Lugo, at an altitude of 20-25 km
from the Earths surface. During the flash that lasted from one to two seconds
the intensity of light varied, having two distinct peaks. At the peaks of the
luminosity, the globe emitted from 1 to 5, and possibly some tenths of KT of
energy was liberated as light.

Because the flash occurred near a cloud layer, it was intensively illuminated
and distant observers described this effect as baffles or horizontal jets.

In the region of Emilia the low-frequency roar (thunder) was also heard, some
80 seconds after the flash. The sound was very deep and strong. For 20 seconds
the windows and, in the city of Faenza, walls, also vibrated.

A possible change in the intensity of Earths magnetic field was also recorded
in central Italy from 0:56 and 1:00 UT. Unfortunately the direction of flight
of the body was outside the region monitored by the fireball European Network.
Near the city of Porec (Croatia ) one house started to burn for the reasons
other than the seen fireball, but some eyewitnesses connected the flash in the
sky with the house fire, and from that begun a newspaper and TV story that
wrongly divulged the story of a fall of a meteorite.

The observed event is very remarkable and the presented data are the best
possible to have been collected till now. This is one more case that very
massive meteoroids or small asteroids of group (type) I are completely
destroyed by aerodynamic pressure. This is also further confirmation that
during Earth collision with small bodies that maintain high velocity depth in
the atmosphere and have lower resistance to pressure, these bodies are
destroyed before they reach the ground, leaving no geological consequences.

Now the search for possible downfall of condensed meteoritic material from the
plasma globe sizing micrometers, in the region of epicentre is in progress.
The possibility to have more detailed data exists if some of the satellites
for early warning bearing infrared radiometer, registered the emitted infrared
radiation of the fireball.


Gallino R.*   Raiteri C. M.   Busso M.
   Correlated Isotope Anomalies in Meteoritic Silicon Carbide Grains

   No Abstract Available


Clark B. E.*   Fanale F. P.   Robinson M. S.
   Regolith Optical Alteration Experiments on the Murchison Carbonaceous
   Chondrite: Spectral Effects

INTRODUCTION:  When using telescopic and remote sensing observations to derive
mineralogical information, the importance of "space weathering" to the optical
surfaces of target objects should be considered. As on the Moon, the optical
properties of the regolith may differ from those of the bedrock, leading to
erroneous interpretation. We present the results of an experimental simulation
of the melting and recrystallization that may occur to the surface of a
carbonaceous chondrite meteorite parent body due to impact heating or other
processes. We have performed similar experiments on ordinary chondrite
meteorites to specifically study the changes in spectral reflectance properties.
For this paper we report the mineralogical changes that occur from simulated
regolith alteration on Murchison, and we discuss the resulting changes in
spectral properties.

METHOD:  The meteorite sample was ground to powder with grain sizes of <180 and
<90 microneters. The reflectance spectra of these powders were measured from 0.3
to 2.5 micrometers. The sample was then melted and quenched in a fusion furnace
at a temperature of 1700 degrees C and reground to a powder for measurement of
its reflectance spectrum. Optical microscopy and electron microprobe analyses
were performed on the samples before and after alteration.

RESULTS:  The spectral effects of simulated regolith alteration by melting and
comminution are significant. Following alteration the fused sample shows a
dramatic increase in reflectance of about 40% as well as a band center
wavelength shift of about 0.25 micrometers, from 0.75 to 1.0 micrometers. This
would indicate a correspondingly significant change to the mineralogy of the
altered sample. In fact, photomicrographs of polished grain mounts of Murchison,
before and after alteration, do indeed indicate a mineralogical change.

Silicate alteration:  Olivines present in chondrules have varying low-fayalite
compositions (0-1) in the before sample, whereas in the after sample olivines
have much higher fayalitic compositions (~20). The altered sample also shows
skeletal quench textures in olivines and an increase in average crystal size.
Whereas matrix composes roughly 80% of the unaltered Murchison, there is a
complete absence of fine-grained dark matrix material in the altered sample.

Metal-troilite alteration:  In addition, metallic components are about a factor
of 3 times more abundant in the altered sample (8.3%) than they are in the
unaltered sample (3.0%). The metal and troilite phases that occurred as separate
grains in the unaltered sample are now intergrown and more numerous in the
altered sample.

CONCLUSIONS:  The simultaneous increase in fayalitic olivine and Fe,Ni and
troilite in the altered sample indicates that destruction and reduction of the
iron-rich matrix must have occurred. Carbon reduction of the hydrated silicates
composing 80% of Murchison's matrix is thus responsible for the wavelength shift
of the mafic absorption band. Whereas in the unaltered sample the band position
at 0.75 micrometers indicates lower Fe2+/(Fe2+ + Fe3+), the after alteration
band position at 1.0 micrometers indicates higher Fe2+/(Fe2+ + Fe3+), or lower
total Fe3+. Destruction and recrystallization of the dark matrix material is
also likely to be the dominant cause of the increase in reflectance seen in the
altered sample. Finally, it is interesting to note that the normalized spectral
reflectance curves of ordinary and carbonaceous chondrites are quite similar
after alteration, but are quite dissimilar before alteration.

We are in the process of performing similar experiments with other petrologic
types of the carbonaceous chondrites. Changes in spectral properties will be
used to analyze spectral variations that possibly occur due to surface
processses among the low-albedo C-, B-, G-, and F-type asteroids as well as the
moons of Mars, Phobos, and Deimos, and the irregular satellites of Jupiter.

ACKNOWLEDGMENTS: We are especially indebted to Tim McCoy (Univ. of Hawaii).


Padevet V.*
   Comets and Meteorites

By testing fireballs, associated with cometary fragments, in the Earth's
atmosphere, their depth of penetration, and by comparing their theoretical
residual masses with meteorite falls, it is argued that there are also
chondrites, besides volatiles, in cometary nuclei. Ordinary chondrites mostly
orbit in Apollo- Amor- and Athenae-type orbits, and carbonaceous chondrites
probably predominate in more eccentric orbits whose aphelia are beyond
Jupiter. Chondrites probably originate in bodies larger than comets are today.
The history of the relations between the parental bodies of chondrites and
comets should be studied in greater detail.





                            Wednesday, June 16, 1993
                                   DYNAMICS
                                  8:30 - 12:00

Chair(s):  Cl. Froeschle
           F. L. Whipple


Knezevic Z.*   Milani A.
   Asteroid Proper Elements: A Big Picture

Determination of the asteroid proper elements, that serve as parameters for
classification of asteroids into families, has dramatically improved in the
recent years both from the point of view of the accuracy of the results and of
the number of objects for which they can be and have been derived. Depending
on the dynamical characteristics of the different groups of asteroids (main
belt, high eccentricity/inclination, resonant, etc.), different methods and
theories have been used for that purpose - analytical, semianalytical and
numerical. This presentation is intended to provide a review of the current
status-of-the-art in the field, to emphasize the achievements and to discuss
the open problems.

The theory by Milani and Knezevic (1990, 1992) provides the most accurate
results for the bulk of the known asteroids.  It is an analytic theory, based
on the truncated development of the perturbing Hamiltonian, and thus is
particularly suitable for most of the main belt objects of low-to-moderate
inclination and/or eccentricity. The short and the long periodic perturbations
up to degree four in the first order with respect to perturbing mass and these
of degree two in the second order are averaged out in this theory by means of
the two canonical transformations. Theory accounts for the perturbations by
all the major planets, with different levels of truncation; an iterative
algorithm copes with the variable frequency problem; the software monitors the
occurrence of the secular resonances, switching if necessary to an adapted
theory; a quality code indicates the reliability of the results.  The
algorithm is quite efficient and applicable for mass production of proper
elements; the typical accuracy is about 0.001 or better, except close to the
mean motion and secular resonances, and in the high eccentricity/inclination
regions, where it is seriously deteriorated.

One of the most recent achievements in the field is the semianalytic theory by
Lemaitre and Morbidelli (1993), which does not use the expansion in
eccentricity and inclination of the asteroid; conversely, it makes use of only
linear planetary terms.  All the terms proportional to e' and i' are
considered as a perturbation, so that the integrable part becomes a function
of the asteroid's argument of perihelion. The theory is particularly suitable
for the high eccentricity/inclination objects. It provides proper elements
with about the same accuracy throughout the asteroid belt, presently with an
error of approximately 0.003-0.004 in proper eccentricity, and somewhat less
in proper inclination. This theory also involves an iterative procedure, it
includes some second order perturbations and could account for the effects of
more planets.

Proper elements for Trojan asteroids have been computed by Milani (1993). The
procedure consisted in numerical integration, under the perturbation by the
four major planets, for 1 Myr, and in digital filtering and compressing of the
output into a "synthetic theory".  The proper modes of oscillation of the
variables related to eccentricity, inclination, perihelion and node defined
the proper elements; the remaining proper element is defined as the amplitude
of the oscillation of the semimajor axis associated with the libration period.
The stability of the proper eccentricity and inclination in the covered time
span was within 0.0025 with only a few exceptions.

Similar numerical procedure has been used by Schubart (1991) to derive the
proper parameters for Hilda group of asteroids. However, in this case the
numerical integration covered much shorter time spans (100,000 years at best),
and no accuracy estimates were given.

One can state, in conclusion, that the efforts to produce the asteroid proper
elements of an accuracy which will enable a reliable identification of
asteroid families are giving the results; we have, in many cases, already
reached the required accuracy, in some more complicated cases we are very
close to it, and we are now even able to tackle this problem for some
asteroids for which, quite recently, this seemed to be practically impossible.

References:  Lemaitre A. and Morbidelli A. (1993) Celestial Mechanics,
submitted.  Milani A. (1993) Celestial Mechanics, in press.  Milani A. and
Knezevic Z. (1990) Celestial Mechanics, 49, 347.  Milani A. and Knezevic Z.
(1992) Icarus, 98, 211.  Schubart J. (1991) Astron. Astrophys., 241, 297.


Milani A.*
   Trojans

   No Abstract Available


Ferraz-Mello S.*
   Kirkwood Gaps and Groups

   No Abstract Available


Froeschle Ch.*
   Secular Resonances

   No Abstract Available


Farinella P.*
   Meteorite Transport

   No Abstract Available


Fernandez J. A.*
   Dynamics of Comets: Recent Developments and New Challenges

There is a broad consensus that long-period comets come from
a huge reservoir surrounding the solar system, as proposed
originally by Oort. Yet, the classical picture of the Oort
cloud has substantially changed during the last decade. In
addition to passing stars, giant molecular clouds and the
tidal force of the galactic disk have been identified as
major perturbers of the Oort cloud. In particular, giant
molecular clouds may be responsible for limiting the size of
the stable Oort cloud to no more than ~3 times 10^4 AU, i.e.
about one third of the classical Oort's radius.

Most comets are injected into the planetary region by the
quasi-steady action of the tidal force of the galactic disk.
The concentration of aphelion points of new and young long-
period comets toward mid-galactic latitudes is a consequence
of its dominant influence. Some clusterings of aphelion
points not associated with the galactic structure may be
signatures of close stellar passages in the recent past.
Their invoked association with the solar antapex remains as
a very controversial issue.

The frequency of comet passages by the inner planetary
region could experience significant fluctuations with time
as the Oort cloud meets random strong perturbers. In
particular penetrating encounters with giant molecular
clouds and stellar passages at distances of a few 10^3 AU
may trigger comet showers as intense as 10^2 - 10^3 times
the background comet flux at average intervals of a few 10^8
years. The observed ordered pattern of most comet aphelia
associated with the galactic structure argues against a
recent strong perturber of the Oort cloud able to blur it,
thus suggesting that the current frequency of comet passages
is near its quiescent level.

Comets covering a wide range of dynamical ages (from near
parabolic orbits down to periods P ~- 20 years) can be
satisfactorily explained as a multiple-step capture process
by Jupiter. This mechanism cannot be applied to the Jupiter
family comets (periods P < 20 years) because of the
difficulty -if not impossibility- to explain in this way
some of their properties, in particular the lack of
retrograde orbits. This suggests another dynamical path for
the Jupiter family comets that goes through the transjovian
region. Objects like P/Schwassmann-Wachmann 1 and Chiron may
be suitable precursors of the Jupiter family. The problem is
to identify possible sources of the outer solar system
objects. So far, two appear as the most promising ones: 1)
The capture of Oort cloud comets by Saturn, Uranus and
Neptune. For this mechanism to work properly it is required
that the capture of low-inclination orbits be strongly
favored and that it provides the right number of comets to
explain the population size of the Jupiter family. 2) The
transfer of comets from a transneptunian comet belt, the so
called Kuiper belt. The question here is to identify the
dynamical mechanism(s) responsible for transferring belt
comets to Neptune-crossing orbits. It is possible that
secular perturbations by the giant planets may be able to
remove belt comets over the age of the solar system.

A new picture of the comet reservoir is emerging in which it
stretches from just beyond Neptune to several 10^4 AU,
passing from a flat structure to a spherical one. The Kuiper
belt can become accessible to large telescopes, as shown by
the recent discovery of 1992QB1, possibly a belt member. The
major challenge will be to explore the region usually
unaccessible to external perturbers that goes from several
tens AU to a few thousands AU. A significant mass may have
been locked there from the beginnings of the solar system,
giving rise to an inner core that feeds the outer or
classical Oort cloud. Our aim will be to briefly discuss
some of the topics summarized here.


Yeomans D. K.*
   Nongravitational Effects on Comets

The cometary nongravitational accelerations arise when vaporizing ices impart
a rocket-like thrust to the cometary nucleus. Although there have been several
attempts to account for these nongravitational effects, the model introduced
by Marsden, Sekanina, and Yeomans in 1973 is still the most widely used. Using
observational data and this latter model, one solves for parameters in each of
three orthogonal components that represent the thrust acceleration components,
in AU/(day), when the comet is one AU from the sun. In general, only the
radial (A1) and transverse (A2) components in the comet's orbit plane are
included in the orbit determination because a solution for the component (A3)
normal to the comet's orbit plane rarely produces a meaningful result.
Implicit in this model are the assumptions that the cometary outgassing is due
to vaporizirlg water ice but the model does not include assumptions concerning
the thermal lag angle (the angle between the subsolar point and the region
where the outgassing reaches a maximum) nor are there any assumptions on the
size or location of the active region(s). During each return to the sun's
neighborhood, the model computes the average radial and transverse thrust
accelerations which are assumed to act symmetrically with respect to
perihelion. In the pre-Halley era, the magnitude, sign, and time variation of
these computed nongravitational parameters were often interpreted in terms of
the nucleus' rotation direction, its thermal lag angle, and the precession of
the rotation axis with time. In these simpler times, the observed acceleration
(deceleration) of a comet's mean motion would be explained in terms of an
outgassing nucleus with a small thermal lag angle in retrograde (direct)
rotation; the radial nongravitational parameter (A1) would be positive and
larger in magnitude than the corresponding transverse parameter (A2). Prompted
by the discrete active areas evident in the spacecraft images of comet
Halley's nucleus, this simple and straight forward physical interpretation of
nongravitational effects is giving way to the more complex realities of
nature. Whatever its limitations, the existing model for nongravitational
effects has substantially improved the orbital solutions and ephemerides for
most active comets and nongravitational parameters already exist for a large
number of comets. One must ask how these parameters can be correctly
interpreted to extract useful information on the physical characteristics of
cometary nuclei.

Through the recent work of M. Festou, H. Rickman, L. Kamel, D. Yeomans, P.
Chodas and others, it has become apparent that much of the observed
nongravitational effects are due to a radial nongravitational acceleration
acting asymmetrically with respect to perihelion rather than being due to the
transverse component acting symmetrically. Largely as a result of a continuing
series of papers by Z. Sekanina, the critical role of small localized active
areas is being defined. The location of these active areas on the cometary
nucleus and the orientation of the nucleus rotation axis determine where in a
comet's orbit a particular source will be exposed to sunlight and become
active. The acceleration or deceleration of a comet's mean motion can then be
understood in terms of when a particular source (or sources) becomes active.
In this new physical model, thermal lag angles are not necessary, the rotation
direction is not necessarily related to the determined nongravitational
parameters, and the sign of the radial parameter (A1) can be positive or
negative. The normal nongravitational parameter (A3) is only meaningful in
very specialized circumstances and discontinuous values for the parameters
over time can result when old sources cease or new sources become active.





                            Wednesday, June 16, 1993
                  THE COSMOGONIC MESSAGE FROM COMETARY NUCLEI
                                 14:00 - 17:30

Chair(s):  H. Rickman


Luu J.*
   Physical Properties of Cometary Nuclei

   No Abstract Available


Crovisier J.*
   Molecular Abundances in Comets

The molecular composition of cometary volatiles is a basic information on the
nature of comets and an important clue to their formation mechanisms. For a
long time, the only informations on this composition were coming from the
signatures of dissociation products in the visible spectrum. It is only
recently that we obtained direct identifications of cometary volatiles through
in in situ exploration as well as from remote sensing at UV, IR and radio
wavelengths.

We will present an inventory of known cometary volatiles, with a critical
review of the evaluation of their abundances. Water prevails, but CO compounds
(CO and CO2) and CHO compounds (CH3OH, H2CO) are relatively abundant. No
hydrocarbon has yet been definitely identified. It has often been argued that
nitrogen is deficient, but this may be due to the unability of cometary nuclei
to retain N2.

Some abundance determinations are still controversial, especially for species
(like H2CO) which may come either from the nucleus, or from a distributed
source (grains), or from both. Important variations from comet to comet may be
present.

There are still many unidentified compounds, some of them possibly relatively
abundant. The progenitors of the C2 and C3 radicals are still to be found. The
important infrared emission around 3.4 microns cannot be fully explained by
CH3OH and H2CO, and other CHO species may be present in abundance. Aromatic
compounds are revealed by the presence of a noticeable feature at 3.28
microns.

It must be pointed out that this inventory of abundances pertains to volatiles
sublimed at about 1 AU from the Sun. Because of fractionation processes, it
may greatly differ from the true volatile abundances within the nucleus. The
activity of comets at large heliocentric distances testifies to the importance
of highly volatile species.

All cometary volatiles are important gas and condensed phase interstellar
constituants. A detailed comparison of cometary and interstellar abundances is
not easy, however, because our knowledge of gas-phase interstellar molecules
is biased towards polar species, and the abundances of solid-phase
interstellar species is very poorly known. It results that one cannot yet
conclude whether comets formed form unaltered insterstellar matter.


Prialnik D.*
   Modelling the Structure and Activity of Comet Nuclei

   No Abstract Available


Yamamoto T.*   Kouchi A.   Kozasa T.   Kuroda T.   Greenberg J. M.
   Crystallinity of the Ice of a Cometary Nucleus

Crystallinity of the ice of a cometary nucleus serves as one
of the physical clues to revealing the formation conditions
of cometary matter and the history thereafter. Yamamoto [1]
pointed out that whether cometary ice is a condensate in the
primordial solar nebula or in the interstellar cloud can be
determined by revealing whether ice in the nucleus is
crystal or amorphous. In this paper we study conditions for
condensation of amorphous ice on a cold substrate and its
preservation both theoretically and experimentally.

Conditions for formation and preservation of amorphous ice
are presented. It is shown that the formation conditions
depend not only on the temperature but also on the flux of
water vapor onto the substrate at the condensation.  A
critical flux F(sub)c above which amorphous ice is formed is
given by F(sub)c = D(sub)s/a^4, where D(sub)s is the surface
diffusion coefficient of water molecules on the substrate
and a is the lattice constant of crystalline ice. The time
scale t(sub)c for crystallization of amorphous ice is
investigated on the basis of the theories of crystalline
nucleation and growth. A theoretical expression of t(sub)c
is obtained, which has the same form as the empirical
formula of t(sub)c = A exp(E/T) [e.g., 2] used in the study
of the thermal evolution of comets, where A and E are the
parameters that have been determined from experiments so
far.

We made measurement of F(sub)c of water vapor on a substrate
of polycrystalline cubic ice in the following way. Water
vapor was deposited on the substrate at a constant flux
while gradually lowering the substrate temperature. During
deposition the structural change of the ice film was
observed by reflection electron diffraction. At a
temperature T the structure was observed to change from
cubic ice to amorphous ice. Thus the critical flux F(sub)c
at temperature T was determined. By changing the flux and
making the same observations, we obtained F(sub)c as a
function of T. We found that the measured F(sub)c is
consistent with the theoretical prediction.

Crystallinity of ice formed in various astrophysical
environments is investigated. It is shown that the ice
condensed in the primordial solar nebula conditions should
be crystalline, whereas that condensed in the interstellar
cloud conditions should be amorphous. Implications of the
results for the origin of cometary ice are discussed.

References:  [1] Yamamoto T. (1991) In Comets in the Post-
Halley Era, Vol. 1 (R.L.~Newburn et al., eds.), 361-376. [2]
Schmitt B. et al. (1989) In ESA SP-302, 65-69.


Greenberg J. M.*
   The Chemistry of Comets and Interstellar Clouds

   No Abstract Available


Meech K. J.   Belton M. J. S.*   Mueller B. E. A.   Dicksion M. W.   Li H. R.
   Nucleus Properties of P/Schwassmann-Wachmann 1

Time series photometric measurements are presented of comet P/Schwassmann-
Wachmann I at a heliocentric distance of 5.886 AU when the comet possessed an
extensive coma. The lightcurve shows a modulation caused by the rotation of
the nucleus. The rotation period is considerably shorter than the 5 day period
found by Whipple (1980) and we find substantial evidence that the nucleus may
be in a complex spin state characterized by two periods 14.0 and 32.3 hrs.
Models of the rate at which the rotational lightcurve range decreases as a
function of the amount of coma in the aperture has determined that the
projected maximum to minimum axis ratio of the comet is 2.6 and that the
product of the albedo times the rotationally averaged nucleus radius size,
p(sub)r.R(sub)n^2 , is 9.5 +- 0.3 km^2. Assuming a minimum geometric albedo of
p(sub)r = 0.04, the maximum projected average nucleus radius is R(sub)n = 15.4
+- 0.2 km, which is only 44% of the size estimated by Roemer (1966). However,
using the albedo determined by Cruikshank and Brown (1983) of p(sub)r = 0.13,
the nucleus radius is only R(sub)n = 8.6 +- 0.1 km. Because of the unknown
nucleus orientation, these will be upper limits to the nuclear size. It
appears that the nucleus of P/Schwassmann-Wachmann I is not the large nucleus
that it has been believed to be for nearly 40 years.

The full text of this contribution has been submitted (22 February, 1993) to
The Astronomical Journal for publication.

References: Cruikshank, D.P., and R.H. Brown (1983). Icarus, 56, 377 Roemer,
E. (1966). Mem. Soc. Roy. Sci. Liege, 13, Ser. 5: 23. Whipple, F.L. (1980).
Astronomical J., 85, 305.


Fulle M.*
   Spin Axis Orientation of 2060 Chiron from Dust Coma Modeling

The numerical inverse model of comet dust tails (Fulle 1989) is improved in
order to take into account possible effects of the nucleus rotation on the
dust shell axis orientation, and is applied to the asymmetric dust coma image
of 2060 Chiron (West 1991). Tests of the model allow to prove that the dust
shell axis orientation (i) strongly affects the coma image fits, (ii) is
uniquely determined and (iii) introduces changes of the second order in the
solutions (loss rates and size distribution) with respect to other non linear
free parameters (time and size dependence of the dust velocity and ejection
anisotropy). The comparison of our results with those of coma photometry
allows to infer the most probable spin axis orientation: retrograde rotation
and obliquity of about 145 degrees. The strongest activity occurs about 10
minutes after the Sun culmination. The ejection velocity of grains of 40
micron diameter is about 5 m/s, much lower than the probable gas escaping
velocity, thus suggesting a strong deceleration due to the nucleus gravity.
The correlation between nucleus rotation and coma shape leads us to exclude
significant orbital escaping from a possible bound coma close to the nucleus
surface (Meech and Belton 1990). The mass loss rate is much higher than the
estimate of Luu and Jewitt (1990), reaching about 20 kg/s between 1987 and
1990, and is dominated by grains of 0.1 mm size, as confirmed by the time
averaged size distribution, the power index of which is -3.2 for diameters
between 1 micron and 1 mm. As in the case of comet P/Scwassmann-Wachmann 1
(Fulle 1992), the persistent coma of Chiron has no memory of the outbursts
observed by means of coma photometry. These bursts are dominated by micron
sized grains which are ejected at high velocities and therefore leave the coma
in short times, further dragged out by solar radiation pressure.

References

Fulle M., 1989, A&A 217, 283. Fulle M., 1992, Nature 359, 42. Luu J.X., Jewitt
D.C., 1990 AJ 100, 913. Meech K.J, Belton M.J.S., 1990, AJ 100, 1323. West
R.M., 1991, A&A 241, 635.


Mumma M. J.*   Hoban S.   Reuter D. C.   DiSanti M.
   Methanol in Recent Comets: Evidence for Two Distinct Cometary Populations

The abundance of methanol has now been measured for six comets:  two are
dynamically new, three are short-period comets, and one is a long-period
comet. The sampled comets exhibit two distinct values for the relative
abundance CH3OH/H2O (Table 1). The simplest interpretation is that the two
values are cosmogonic, i.e., that the comets represent two populations that
formed in different regions of the solar nebula, or accumulated at different
times during the collapse phase.

Current models for the dynamical evolution of cometary orbits argue that two
reservoirs contribute to the current ensemble of short-period comets [1,2].
The first reservoir has two parts: an outer Oort cloud (~10^12 comets at ~10^5
AU) and an inner Oort cloud (~10^13 comets at ~10^4 AU). The inner Oort cloud
consists of comets formed in the Uranus-Neptune region of the solar nebula and
ejected from the inner solar system by the growing protoplanets. The outer
Oort cloud is populated from the inner Oort cloud by galactic perturbations,
thus we expect these comets to be identical in nature. The second reservoir,
the so-called Kuiper Belt (~10^8-10^9 comets at ~100 AU), consists of comets
that formed beyond the orbit of Pluto and have remained there ever since. The
existence of the Kuiper Belt is invoked to explain the observed excess number
of low inclination short-period comets [1]; however, Shoemaker has argued that
high-inclination short-period comets may only appear to be deficient in number
because of systematically larger perihelion distances, owing to the Tisserand
invariant. The orbital inclination of a given short-period comet is not a
definitive indicator of its origin.

Since two of the three comets with high (~5%) methanol abundance are
dynamically new, we associate high methanol abundance (Population I comets)
with formation in the Uranus-Neptune region of the solar nebula. The third is
P/Swift-Tuttle, which is now a short-period comet. However, its methanol
abundance suggests that it once was an Oort cloud comet that must have been
captured long ago since it has resided in the inner solar system long enough
to generate a meteor stream. The three comets with low (~1%) methanol
abundance (Population II) may have originated elsewhere, possibly in the
hypothetical Kuiper Belt. The observed compositional dichotomy is also
consistent with a single formation region, if radiation processing of
precometary ices varied strongly with time in the nebula. Then late-forming
comets may contain less methanol than those formed earlier in the nebular
collapse phase. Measurements of methanol and related species in additional
comets would test the composition and relative numbers of type I and II
comets, for comparison with radiation chemistry models and predicted orbital
statistics [1,2].

Table 1. The Methanol Abundance in Recent Comets.

Comet   Period  Inclination     Methanol/water(%)
        Ref

Wilson 1987VII  New     147     6 +- 1  [3]
Austin 1990V    New      59     5 +- 2  [4,5]
P/Swift-Tuttle  130 y   114     6 +- 1  [6,7,8]
P/Halley        76 y    162     1 +- 1  [3,9,10]
P/Brorsen-Metcalf       70 y    19      1 +- 1  [4]
Levy 1990XX     15,000 y ?      132     1 +- 1  [4,5]

References: [1] Duncan M. et al. (1988) Ap. J., 328, L69-L73. [2] Mumma M. J.
et al. (1993) in Protostars and Planets III, pp. 1177-1252. [3] Reuter D. C
(1992) Ap. J., 386, 330-335. [4] Hoban S. et al. (1991) Icarus, 93, 122-134.
[5] Bockelee-Morvan et al. (1990) in ESA SP-315, pp. 143-148; Nature, 350,
318-320. [6] Davies J. et al. (1992) IAUC 5659; Mon. Not. R. Astr. Soc.,
submitted. [7] Hoban S. et al. (1993) Icarus, submitted. [8] Paubert G. et al.
(1992). IAUC, 5653; IAUC, 5664. [9] Geiss J. et al. (1991) Astron. Astrophys.,
247, 226-234. [10]. Eberhardt P. et al. (1991). Bull. Am. Astron. Soc., 23,
1161.


Colangeli L.*   Mennella V.   Merluzzi P.   Rotundi A.   Bussoletti E.
   Cometary Dust Simulation in the Laboratory: Experimental Results

Many astronomical observations available today on a wide variety of comets have
allowed better characterization of both physical and chemical properties of this
class of "primordial" objects. In addition, space missions toward P/Halley in
1986 and P/Grigg-Skjellerup in 1992 have provided a wealth of data obtained by
"in situ" measurements. Nevertheless, many problems concerning the actual nature
of comets remain and need further investigation to be better understood. In this
context, the laboratory simulation of processes active in the cometary
environment can provide valuable information for comparison with astronomical
data. An important subject to be clarified is the relation between the present
and the original composition of comets. In particular, can the comets be
considered the reservoir of unaltered interstellar dust? The systematic study of
morphological, structural, and chemical properties of "analog" materials either
produced in the laboratory or originating from terrestrial samples can
complement astronomical observations on both comets and interstellar medium to
try to answer the previous question. From several years, our group has been
involved in the field of laboratory simulation of cosmic dust. Recent laboratory
data, obtained on both carbonaceous and silicatic micrometer and/or
submicrometer grains, are applied to discuss some hypotheses that have been
formulated about the chemical and structural composition of these components.


Baratta G. A.   Leto G.*   Palumbo M. E.   Spinella F.   Strazzulla G.
   Ion Irradiation Experiments Relevant to the Physics of Minor Objects in the
   Solar System

Ion irradiation experiments on astrophysically relevant targets have been
conducted for more than one decade in our laboratory. The relevance of the
results for the cometary physics [e.g., 1] and the reflectance properties of
asteroids and other small objects [e.g., 2] have been assessed.

In this paper we present new experimental results concerning (1) production of
new species (including CO, CO2, and CH4) from irradiated frosts (among which are
CH3OH+H2O mixtures) and (2) changes in the sublimation properties of irradiated
ice mixtures (including CO and H2O).

The results are discussed in the light of their potential ability to predict
molecular ratios (e.g., CH4/CO) on cometary nuclei and contribute to a better
knowledge of cometary formation processes and subsequent evolution under the
influence of external agents such as cosmic ion irradiation and thermal cycling.

This research has been supported by the Italian Space Agency (ASI).

References:  [1] Strazzulla G. and Johnson R. E. (1991) in Comets in the Post-
Halley Era (R. Newburn et al., eds.), 243, Kluwer, London. [2] Andronico G. et
al. (1987) A&A, 184, 333.


Haruyama J.*   Yamamoto T.   Mizutani H.   Greenberg J. M.
   How Pristine is a Cometary Nucleus?

We have studied thermal history of cometary nuclei during residence in the Oort
cloud, using the low thermal conductivity of amorphous ice recently obtained by
Kouchi et al. [1].

Since a cometary nucleus is a porous aggregate of grains with an individual
grain being composed of a refractory core and an icy mantle, the bulk thermal
conductivity of a cometary nucleus is assumed to be expressed by the product of
the thermal conductivity of individual grains and a reduction factor resulting
from the porous structure of the nucleus. The initial state of H2O ice is
assumed to be amorphous.

The results show that, before crystallization from amorphous ice to crystalline
ice occurred, the bulk thermal conductivity of comets was very low and the
nucleus was effectively heated up by radioactive nuclides 40K, 232Th, 235U, and
238U with the chondritic abundances. It is shown that the thermal histories are
clearly classified into two distinct types depending mainly on the nucleus
thermal conductivity kappa: (1) The comets of small kappa experience internal
temperature higher than 125 K during residence in the Oort cloud, and most of
the ice in the nucleus crystallizes. (2) For comets with sufficiently large
kappa, on the other hand, the temperature does not become high (<100 K) so that
the initial amorphous ice is almost completely preserved. These results imply
that the ice in Halley-sized comets of about 5 km radius crystallized during
residence in the Oort cloud.

For the case of (2), we present analytic expressions of a maximum temperature
T(sub)max and the total fraction of crystallization xi(sub)max on the basis of
the analysis of the physical processes of the crystallization. In most cases,
T(sub)max is around 80 K, implying that CO or CO2 trapped in H2O ice is
preserved.

A criterion of the crystallization is derived in an analytic expression, which
makes it possible to examine the dependence on the physical quantities relevant
to the crystallization. Discussion is given on the properties of comets that
preserve pristine amorphous ice.

References:  [1] Kouchi A. et al. (1992) Astrophys. J., 388, L73-L76.





                            Wednesday, June 16, 1993
                             PHYSICS OF ASTEROIDS
                                 14:00 - 16:00

Chair(s):  A. W. Harris


Velichko F. P.*   Vdovichenko V. D.   Gaisin S. M.   Mosina S. A.
   Asteroid 4 Vesta: Simultaneous UV and IR Spectrophotometry

For eight nights in October and November 1990 a new detailed spectrophotometry
in the 360-1050-nm wavelength region of the asteroid 4 Vesta was carried out
(in particular the ultraviolet and infrared parts were measured). The
observations show that the ultraviolet part of the spectrum and the pyroxene
band profile (800-1000 nm) varies with asteroid rotation. The nature of the
variation is correlated with small peculiarities of the V-lightcurve.


Binzel R. P.*
   Chips Off of 4 Vesta: A Newly Confirmed Asteroid Family and Link to Basaltic
   Achondrite Meteorites

Because of its unique spectroscopic signature, asteroid 4 Vesta makes an ideal
test case for studying major impact events which may excavate large fragments
from planetary surfaces. Assuming that fragments from Vesta retain a basaltic
spectral signature (deep pyroxene absolption band near 9000A) similar to their
parent body, they may be readily distinguished from other asteroids even if
they happen to be located far from Vesta. Predictions for an asteroid family
related to Vesta date back to work by Williams in 1979 and 1989 [1,2].
However, predictions for a large membership within a possible Vesta family
were first published by Zappala et al. 1990 [3]. Continuing work by Williams
[4] also predicted many members within a potential Vesta family.

The prediction of this large Vesta family membership was coincident with the
establishment of the first capability to routinely examine the spectral
properties of such small D < 10 km asteroids, having V magnitudes typically
ranging from 16 to 18. The Small MainBelt Asteroid Spectroscopic Survey
(SMASS) is being conducted with a CCD spectrograph on the 2.4-m Hiltner
telescope at the Michigan-Dartmouth-MIT Observatory located at Kitt Peak,
Arizona. An examination of the spectral properties of predicted Vesta family
members became one of the initial objectives of the SMASS program.

Over the last two years, we have examined 15 predicted Vesta family members
[1-4] and found that 12 display basaltic achondrite spectral properties
similar to Vesta itself [5]. Given their spectral similarities and predicted
dynamical association, we believe these D <= 10 km asteroids represent
fragments genetically related to Vesta itself. Thus these 12 asteroids, 1906,
1929, 1933, 2590, 3155, 3268, 3657, 3944, 3968, 4038, 4147, and 4546 represent
the first physically confirmed members which conclusively establish the
reality of a Vesta family.

An additional 8 small (D <= 10 km) basaltic achondrite asteroids have been
discovered between Vesta and the 3:1 resonance. We believe these asteroids
(1273, 2011, 2113, 2442, 3153, 3869, 4005, and 4215) are also genetically
related to Vesta and thus constitute additional family members. Their
locations, bridging the space between Vesta and the 3:1 resonance, has other
significant implications. Vesta has been at the center of the debate over the
source of the howardite-eucrite-diogenite (HED) meteorites since the early
1970s [6,7] and the 3:1 resonance is a major dynamical transport region to the
inner solar system. Despite its unique compositional match to HED basaltic
achondrite meteorites, substantial dynamical difficulties in delivering
fragments from Vesta to the resonance [7] have precluded any conclusive HED
parent body link. However, the current orbits for these additionally
discovered Vesta fragments imply ejection velocities in excess of 700 m/sec.
Smaller (~1 km) fragments can therefore be expected to have been ejected with
velocities >=1 km/sec, sufficient to reach the 3:1 and V6 resonances. Thus it
now appears to be dynamically viable for Vesta to be linked as the HED
meteorite parent body.

References:  [1] Williams J. G. (1979) in Asteroids (T. Gehrels, ed.), 1040,
Univ. of Arizona, Tucson. [2] Williams J. G. (1989) in Asteroids Il (Binzel et
al., eds.), 1034, Univ. of Arizona, Tucson. [3] Zappala V. et al. (1990)
Astron. J., 100, 2030. [4] Williams J. G. (1992) Icarus, 96, 251. [5] Binzel
R. P. and Xu S. (1993) Science, in press. [6] Drake M. J. (1979) in Asteroids
(T. Gehrels, ed.), 765, Univ. of Arizona, Tucson. [7] Wetherill G. W. (1987)
Phil. Trans. R. Soc. London, A323, 323.


Michalowski T.*   Kwiatkowski T.   Di Martino M.   Krugly Yu. N.
Shevchenko V. G.   Velichko F. P.
   532 Herculina: Photometry, Polarimetry and Model

The lightcurves of the asteroid 532 Herculina obtained at Torino Observatory (19
(1988), and Kharkov Observatory (1992) are presented. On 5 June 1992 the variati
polarization was also observed.

Herculina has very unusual lightcurves. They exhibit one pair of extrema during
and two pairs of extrema during the rest. These variations are within the rotati
9.4 hours. We present the model that explains such strange behaviour of this ast


Goidet B.*   Renard J. B.   Levasseur-Regourd A. C.
   Polarization of Asteroids, Synthetic Curves and Characteristic Parameters

New asteroidal polarimetric measurements have been performed at various phase
angles for asteroids Toro, 1992 AC, Vesta and Egeria with the Pic du Midi 2
meters telescope. These data points, together with those already found in the
literature, have been used to draw synthetic polarization curves for different
asteroids.

The data corresponding to asteroids of the same taxonomic types are
comparable. Some preliminary trigonometric fits already allow us to estimate
and compare the characteristic parameters of the polarization curves
(inversion angle alpha(sub)0, slope at inversion h, minimum of polarization
P(sub)min at phase angle alpha(sub)min) for the different types and to
determine the taxonomic type of newly observed asteroids.


Degtjarev V. S.*   Kolokolova L. O.
   Some Diagnostic Possibilities for Studying Asteroids by Circular
   Polarization

Attempts to use circular polarization V for studying atmosphereless bodies
were undertaken at the end of the 1960s and were discontinued at the start of
the 1970s because of its negligible values: V < 0.01% for the Moon, Mercury
and Mars [1]. But our laboratory measurements of circular polarization [2]
showed that low values of V are characteristic of materials with small
absorption (dielectrics) and can be great for metals. Then atmosphereless
bodies with expected great abundance of free metal in their material, namely S
and M asteroids, can demonstrate appreciable circular polarization.

To establish possible values of circular polarization for these objects we
made measurements of the Stokes vectors for different samples simulating
asteroidal regolith. The laboratory Stokes polarimeter [2] designed and
constructed at our Observatory was used. The basic material for the samples
was a pyroxene powder (hyperstene 70%, enstatite 30%). Iron and nickel grains
were added to pyroxene to produce samples with different ratio metal/pyroxene.
Both metallic and pyroxene grains were less than 75 microns.

The circular polarization is found very small (V < 0.05%) for pyroxene powder.
But its values increase linearly with metal content and can exhibit V > 1% for
pure metal powders. Circular polarization depends on phase angle, changes in
the trend of curve V vs.phase angle are due to surface structure.

Our measurements were made for homogeneous mixture of metallic and silicate
grains to simulate the surface structure of undifferentiated asteroidal
material. On the base of data for pure pyroxene and pure metals we calculated
values of circular polarization for differentiated material. For the same
ratio metal/pyroxene the calculated results fit to the measured results.

From the results it is concluded that
- circular polarization can be used for finding metals in asteroidal regolith
and for evaluation of metal abundance in it;
- dependence of circular polarization on phase angle can give information
about surface structure;
- unlike spectral characteristics [3], circular polarization does not depend
on differentiation of material therefore joint use of spectral and
polarimetric results could give information about differentiation of asteroid
material.

We should note that circular polarization can show observable values only for
bodies of irregular shape or for separate regions of them and for phase angles
more than 20 degrees. Thus the study of circular polarization can be realized
mainly at space missions or for Near-Earth asteroids.

1. Kemp J.C., Wolstencroft R.D., Swedlund J.B. Nature, v.232, 165, 1971.
2. Degtjarev V.S., Kolokolova L.O. Earth, Moon and Planets, v.34, 45, 1992
3. Feierberg M.A., Larson H.P., Chapman C.R. Astrophys.J., v.257, 361, 1982.


Jamsa S.*   Peltoniemi J.   Lumme K.
   Thermal Emission from Rotating Asteroids: Effects of Surface Roughness

We calculate thermal emission from rotating asteroids at various wavelengths.
The observed intensity depends on surface roughness, viewing, and illumination
directions and angular velocity of the asteroid. When a rough surface is
heated by radiation, its temperature is not constant but varies because of
shadows and varying orientation of surface normal. It is the temperature
distribution visible to the detector that determines the emitted flux at that
direction. Instead of using the simple relation

I(lambda) = A sigma (lambda, T(sub)o)B(lambda, T(sub)o)             (1)

(sigma is emissivity, B is Planck's function and A is the projected area
normal to the line of sight), which holds for a surface at a uniform
temperature To, one must use the more general relation

I(lambda) =  dA(T)sigma (lambda,T)B(lambda,T),                      (2)

where dA(T) is the differential projected area in the detector's view that is
at temperature T.

The shape of the surface at mm- or larger scale is assumed to be characterized
by Gaussian statistics. This yields a Gaussian distribution also for the
slopes of the facets the surface is thought to consist of. We solve a
diffusion equation to calculate the temperature of such facets whose normals'
orientation varies, and which thus receive sunlight in different manners
during the revolution of the asteroid. It is here that its angular velocity
comes into play. Lunar-like material properties are assumed in the
calculations. It is then calculated which of the facets are visible to the
detector and what their projected area is. Radiation is assumed to be emitted
as well as absorbed in proportion to the projected area, and thus the total
flux in the given direction can be calculated using Eq. 2.

Ray optics is used: it is assumed that the dominating wavelengths are much
smaller than the roughness scale, which must therefore be at least
millimeters. Multiple scattering is approximately accounted for by using our
earlier result (A & A, in press).

The results for smooth and rough surfaces are very different, especially in
predictions of disk-resolved intensities. Such results as well as integrated
results will be presented.


Belskaya I. N.*   Lagerkvist C.-I.
   Physical Properties of M-type Asteroids

The number of asteroids claimed to be of type M is presently 61 objects but
for about ten of them the classification is uncertain. We have compared the
mean values and dispersions of the main physical characteristics of the M-type
asteroids with the corresponding properties of C and S asteroids in order to
understand their real differences and homogeneity. All chosen C and S
asteroids have diameters and orbital semi-major axes in the same range as the
considered M asteroids (30-260 km; 2.3-3.2 AU). The following physical
parameters were considered: UBV and JHK colours, ECAS colours, albedo, the
depth of the negative polarization branch P(sub)min, rotation period,
lightcurve amplitude and the Fourier coefficients of well-defined lightcurves.
Statistically meaningful differences in the mean values of M-type objects as
compared to the C and S populations were found in the ultraviolet (U-B, u-v,
s-v) and red (v-w, v-x) colours, albedo and P(sub)min. These are the most
informative characteristics distinguishing M-type asteroids from others. One
way to enlarge the population of the M-type could be to observe any of these
colours for the about 300 unclassified asteroids with moderate IRAS albedos.
The fast rotation and large lightcurve amplitudes of the M-type asteroids were
confirmed. Variances of all parameters were compared between different
asteroid classes. The population of M-type asteroids turned out to be more
homogeneous than the S class.


Durda D. D.*   Dermott S. F.
   The Collisional History of the Mainbelt Asteroid Population and Hirayama
   Asteroid Families

A self-consistent model of the collisional evolution of the mainbelt asteroids
is presented, with emphasis on the collisional history of asteroid families.
Using a runaway growth initial size distribution based upon the work of
Wetherill (1992, private communication) and a modified size-strengh scaling
law, we have been able to match in detail the observed size-frequency
distribution of the mainbelt asteroids. After 4.5 billion model years of
collisional evolution we are able to fit: (1) the excess of asteroids at
~100 km diameter, (2) the transition to an equilibrium distribution for
diameters <~30 km, (3) the absolute number of asteroids in the equilibrium
population, and (4) the extended "tail" of asteroids larger than 350 km.
During a model run we also follow the collisional evolution of families of
fragments resulting from single breakup events representing the formation of
Hirayama asteroid families. Thus, for the first time, we examine the
collisional evolution of asteroid families due to impacts by a projectile
population which is itself collisionally evolving toward the observed
distribution.


Lindblad B. A.*
   The Statistical Significance of Small Asteroid Families

Following the study of Hirayama (1918) a number of researchers have published
lists of asteroid dynamical families in three-dimensional proper a, e, i-
space. In these studies the major Hirayama families are identified and, for
similarly sized samples, have approximately the same members. Numerous small
families with only a few members appear in most lists. In fact more than 60
different minor asteroid families have been proposed in the literature. An
inspection of the data shows that the agreement between the classifications of
various authors is poor. Several of the proposed minor families are various
Flora subgroups. These apparently arise when the investigator has decided to
use a stricter rejection level for the Flora family than for the other
families in his study. The present paper briefly discusses these results.

The paper next studies the probability that an asteroid family consisting of n
members may have arisen by pure chance in a sample of N asteroids (n << N).
This is done by constructing a large number of random asteroid populations
each with N members. These populations are then searched for families using
the D criterion at the same rejection level as was used in the original family
search (Lindblad 1992). Numerical values are given for this probability in a
sample consiting of N = 4100 proper elements.

References

Lindblad B. A. (1992) in Asteroids, Comets, Meteors 91, (A. W. Harris and E.
Bowell, eds.), pp. 645-647, LPI, Houston.


Shoemaker C. S.*   Holt H. E.   Shoemaker E. M.   Bowell E.   Levy D. H.
   The Palomar Asteroid and Comet Survey (PACS), 1983-1993

The Palomar Asteroid and Comet Survey (PACS) was begun in 1983 as a long-term
project to evaluate the populations and fluxes of Earth-crossing and other
planet-crossing asteroids and comets. It is a collaborative project between
the PACS observing team and Edward Bowell of Lowell Observatory. Observations
have been carried out on the 46-cm Schmidt camera at Palomar Observatory by
E.M. and C.S. Shoemaker, H.E. Holt, and D.H. Levy with the occasional
assistance of students. Well over 10,000 unknown asteroids and about 100
comets have been detected in a total of about 7,000 search fields
photographed. Limitations of manpower have precluded measuring and reporting
all the objects detected; hence the effort has been focused mainly on the
planet-crossing bodies and on asteroids with unusual motion. Astrometric
positions have been obtained for about 3,000 unnumbered asteroids. About 7% of
these are now numbered; discovery of about 140 of the newly numbered objects
is credited to the PACS team. Altogether, 43 Earth-approaching asteroids have
been discovered, and about 30 others have been independently detected. Sixty
Mars-crossers and about 150 high-inclination asteroids, chiefly Phocaeas and
Hungarias, have been discovered. Beginning in 1985, special attention was paid
to Trojan asteroids, and 64 Trojans have been found, 47 of which are credited
to PACS. Forty-five of the new Trojans have been numbered, a total that
exceeds the full set of numbered Trojans at the outset of the survey. Of the
comets detected in PACS, 29 are credited as discoveries by members of the
survey team; 14 of these are short-period comets. In addition to active
comets, several Jupiter-crossing or Jupiter-approaching objects of asteroidal
appearance have been found that are almost certainly extinct comets. Although
the prime aim of PACS is discovery, astrometric measurement of known objects
has led to the improvement of hundreds of orbits, and has accelerated the
numbering of many asteroids.

The frequency of discovery and independent detection of planet-crossing bodies
as a function of the area of sky covered in the systematic PACS survey has
provided a foundation for refined estimation of the population of Earth-
crossing asteroids larger than about 1 km in diameter, of their flux near the
Earth, and of the present cratering rate by asteroid impact on Earth [1].
Collision of asteroids produces most impact craters smaller than 20 km in
diameter; the cratering rate estimated from PACS observations is consistent
with the geologic record of cratering for the past 120 million years. The PACS
discoveries also provided the basis for calculating the asteroid impact and
cratering rate on Venus and for estimating the mean age of the Venusian
surface revealed in the Magellan radar images [2]. The average age of the
surface of Venus was shown to be less than 500 million years.

Henry Holt's discovery of Apollo asteroid (4581) Asclepius = 1989 FC, which
missed the Earth by about 1-1/2 times the distance to the Moon on March 23,
1989, substantially increased public awareness of the hazard of asteroid
impact. Another discovery made in the course of PACS, by David Levy and Holt,
was (5261) Eureka = 1990 MB. This asteroid turned out to be a Mars Trojan [3],
thus establishing the existence of a new dynamical class.
References: [1] Shoemaker, E.M., Wolfe, R.F., and Shoemaker, C.S., 1990,
Asteroid and comet flux in the neighborhood of the Earth, in V.L. Sharpton and
P.D. Ward, eds, Global Catastrophes in Earth History; An Interdisciplinary
Conference on Impacts, Volcanism and Mass Mortality: Geological Society of
America Special Paper 247, p. 155-170. [2] Shoemaker, E.M., Wolfe, R.F., and
Shoemaker, C.S., 1991, Asteroid flux and impact cratering rate on Venus, in
Abstracts of papers submitted to the Twenty-second Lunar and Planetary Science
Conference, part 3, Houston, Lunary and Planetary Institute, p. 1253-1254. [3]
Bowell, E., Holt, H.E., Levy, D.H., Innanen, K.A., Mikkola, S. and Shoemaker,
E.M., 1990, 1990 MB: A Mars Trojan: American Astronomical Society Bulletin, v.
22, p. 1357. Mikkola, S., Innanen, K., Muinonen, K., and Bowell, E., 1993, A
preliminary analysis of the orbit of the Mars Trojan (5261) Eureka: Celestial
Mechanics, in press.





                            Wednesday, June 16, 1993
                        THE INTERPLANETARY DUST COMPLEX
                                 16:30 - 17:30

Chair(s):  H. Scholl


Jayaraman S.*   Dermott S. F.   Liou J.-C.
   Near-Earth Resonance Structure of the Zodiacal Cloud

Dust particles created due to collisions in the asteroid belt spiral in toward
the Sun due to Poynting-Robertson drag. A significant fraction of these
particles are trapped in mean motion resonances outside the orbits of Earth
and Venus. When the particle encounters a resonance, capture into the
resonance can be treated as a probabilistic event. The probability of trapping
depends on the size of the dust particle and its orbital eccentricity, as well
as the order of the resonance. Once capture occurs, the orbital eccentricity
of the particle increases, eventually allowing it to escape. This phenomenon
leads to a bottleneck effect in the flow of dust particles toward the Sun,
creating a particle number density enhancement in the Zodiacal Cloud.
Preliminary results from numerical integrations yield the fraction of
particles trapped in first-order resonances outside the Earth. Further
analysis determines the variation of the number of trapped particles as a
function of the particle size. Eventually, for a given particle size
distribution the existence of any azimuthal pattem related to a given
resonance is obtained. This phenomenon involves particles near the Earth (that
are blighter as they are closer to the Sun), and is particularly important for
it results in the time variability of the Zodiacal Cloud.


Taylor A. D.*   McBride N.   Grun E.   McDonnell J. A. M.
   The Evolution of Particles Away from the IRAS Dust Bands

The IRAS observations of dust bands in the asteroid belt have been explained
by dust in the orbits of the Eos, Koronis and Themis asteroid families.
Assuming this is the source of dust in the bands we consider the subsequent
evolution of dust Sunward. When the dust is first released radiation pressure
will insert the new meteoroid into an orbit with its semimajor axis larger
than the source asteroid. Poynting-Robertson drag will act to shrink (and
slowly circularise) the orbit until the orbital period matches that of the
next Jupiter resonance. The low eccentricity of the meteoroid orbit at this
interior resonance may lead to a rapid increase in eccentricity thereby
increasing the effect of Poynting-Robertson drag while shifting the aphelion
toward larger heliocentric radii. A meteoroid that makes this resonance jump
(with its increased eccentricity) has a much reduced probability of reacting
to a subsequent resonance and could therefore continue an uninterupted
Poynting-Robertson infall toward the Sun. The distribution of spatial density
associated with this jump will tend to concentrate particles at heliocentric
radii just outside the circular equivalent of the resonance. Results from the
Ulysses dust detector through the asteroid belt suggest such concentrations
may exist; a tentative in situ detection of the IRAS dust bands by Ulysses.


Xu Y.-L.*   Dermott S. F.   Gustafson B. A. S.   Liou J. C.
   Models of the Zodiacal Cloud and the Solar System Dust Bands for a Spectrum
   of Wavebands

The zodiacal emission (ZE) observed by the Infrared Astronomical Satellite
(IRAS; Hauser et al., 1984) holds vital clues to the origin and physical
properties of interplanetary dust. The high resolution zodiacal history file
prepared by the Infrared Processing and Analysis Center at Caltech has been
used to derive a complete structure of the smooth, large-scale zodiacal
background and the narrower solar system dust bands in wavebands of 12, 25 and
60 micrometers. In 1984, Dermott et al. (Nature, 312, 505-509) suggested that
the solar system dust bands discovered by IRAS are produced by the gradual
comminution of the asteroids in the major Hirayama asteroid families. We have
developed a threedimensional numerical model, SIMUL, to simulate the
interplanetary dust cloud and to confirm the asteroidal origin hypothesis. The
SIMUL model allows for the calculation of thermal flux produced by any
particular distribution of dust particle orbits and reproduces the exact
viewing geometry of the IRAS telescope. SIMUL takes into account the orbital
evolution due to PR drag, light pressure and planetary perturbations, the
size-frequency distribution due to collision, the thermal properties of dust
particles and their variation with size. SIMUL establishes first a spatial
density distribution for the cross-sectional area of the particles, then
performs line-of-sight integration to obtain thermal brightness at any
direction and any observing time. The simulation results by SIMUL in different
wavelengths are compared with corresponding IRAS measurements. The model
reveals that (1) the individual families are responsible for the individual
peaks in the fine structure of observed therrnal brightness; (2) the
combinational contribution from major asteroid families accounts for the
overall shape of the thermal brightness distribution; (3) the deviation of the
symmetry plane of the dust bands from the ecliptic is due to gravitational
perturbations. The modeling supports strongly the asteroidal origin hypothesis
for the solar system dust bands. However, the overall shape of the background
emission cannot be fully simulated by asteroidal sources alone.


Mann I.*   Grun E.
   Dust Particles Beyond the Asteroid Belt - A Study Based on Recent Results of
   the Ulysses Dust Experiment

We present and discuss dust fluxes detected by the Ulysses dust experiment
beyond the asteroid belt. Whereas dust fluxes detected inside about 2 AU are
compatible with a population of interplanetary dust particles moving on low
eccentric and low inclination orbits, different populations of particles are
required in order to explain the data obtained beyond this region. A dust
population on randomly inclined orbits can explain the rather flat and
slightly increasing flux beyond approximately 2.5 AU. Within a distance of
1 AU around Jupiter periodic bursts of submicron sized dust grains were
detected. An additional flow of high speed micron-sized particles on
retrograde orbits is most probably of interstellar origin. We will concentrate
our analysis on the discrimination of interstellar and interplanetary dust.


Flynn G. J.*
   Identification of Cometary Dust Among the Interplanetary Dust Collected from
   the Earth's Stratosphere

The distribution of peak temperatures reached on atmospheric entry suggests a
large fraction of the interplanetary dust particles (IDPs) collected from the
Earth's stratosphere are derived from main-belt asteroidal parent bodies (1).
This is consistent with inferences from IRAS data and orbital modeling
suggesting that catastrophic collisions in the main-belt provide a sufficient
amount of dust to resupply the zodiacal cloud (2, 3). However, comets are
observed to contribute dust to the interplanetary medium, and some of the IDPs
collected from the stratosphere can be identified as cometary by thermal
criteria.

Main-belt asteroidal IDPs generally have geocentric velocities <-6 km/s (1, 4)
at Earth encounter. This limits their atmospheric entry velocities to <-12.6
km/s, restricting the peak temperatures reached on atmospheric entry. The peak
temperatures reached by main-belt asteroidal IDPs under the most extreme entry
ccnditions (highest velocity and normal incidence), calculated using the model
Whipple model (5), are shown in the figure

Internal indicators of the peak temperature reached on atmospheric entry, such
as the presence of volatile elements, low-temperature mineral phases,
radiation damage tracks, and/or solar wind implanted noble gases, can be used
to separate the IDPs collected from the stratosphere into two groups: one
consisting of particles heated more extremely than is possible for main-belt
asteroidal particles of that size and density, and a second consisting of
particles which have experienced less extreme heating. The first group
contains only comet particleR and the rare particles from asteroids in highly
eliptical or highly inclined orbits. The second group contains a mixture of
main-belt asteroidal particles and cometary particles which encountered the
Earth at shallow incidence angles.

Individual stratospheric IDPs which have a high probability of cometary origin
have been identified, and their chemical, physical, and mineralogical
properties will be described.

REFERENCES:
1) Flynn, G. J. (1989) Icarus, 77, 287-310. 2) Dermott, S. F. et al. (1992) in
Asteroids, Comets, and Meteors 1991, 153-156. 3) Reach, W. T. (1991) in Origin
and Evolution of Interplanetary Dust, 211-214. 4) Jackson A. A., and Zook H.
A. (1992) Icarus, 97, 70-84. 5) Whipple, F. L. (1950) proc. Nat Acad. Sci.
USA, 36, 687-695.

Figure 1, which appears here in the hard copy, shows peak temperature (Tmax)
reached by main-belt asteroidal particles during Earth atmospheric entry
versus particle diameter and density (p). The modeling assumed normal
incidence and an entry velocity of 12.6 km/s.


Lumme K.*   Rahola J.
   Light Scattering by Solar System Dust Particles in the Discrete-Dipole
   Approximation

We present photometric measurements of snowball fields carried out by one of
us in cold, dry outdoor conditions in the winters of 1979 and 1980 at Aarne
Karjalainen Observatory, Kiiminki, Finland. The snowballs consisted of varying
proportions of pure snow (typical particle size 100 m) and extremely black,
fine-grained boron carbide (B4C, particle size 10 to 50 micrometers). The
geometric albedos ranged from 5% for the darkest to 44% for the pure snow
samples. The smallest phase angle was 0.3 deg, and the measurements were
carried out in white, unpolarized light using a photometer designed for
asteroid observations.

Almost all the measured phase curves show the kind of sharp opposition effect
observed in Saturn's rings, many planetary satellites, and the asteroids (44)
Nysa and (64) Angelina. The opposition effect sets in at phase angles between
1 and 2 deg and, depending on the purity of the snowballs, amounts to as much
as a 0.3 mag brightening at a phse angle of 0.3 deg. It is intriguing that the
measured opposition effect is enhanced with an increased proportion of boron
carbide; that is, in darker samples.

Can the measurements be explained by the coherent multiple backscattering
mechanism? For pure snow with large particle sizes, the typical free paths for
multiply scattered light can be large compared to the wavelength, and can
theoretically result in an extremely narrow, unobservable opposition effect.
When boron carbide is added, the free path becomes shorter, which can broaden
the oppositon effect to observable widths. However, shadow-hiding due to the
dark boron carbide particles may also affect the oppositon effect.





                            Thursday, June 17, 1993
                      PHYSICAL OBSERVATIONS AND MODELING
                                  8:30 - 12:00

Chair(s):  D. F. Lupishko
           A. C. Levasseur-Regourd


Chapman C. R.*
   Galileo Encounters

No Abstract Available


Schwehm G. H.*
   Giotto Extended Mission

No Abstract Available


Grun E.*
   Dust Measurements in the Outer Solar System

Remote observations of zodiacal light and in situ detection of micrometeoroids
provide information on the spatial distribution of interplanetary dust and its
dynamical properties, sources, and sinks. Pioneers 10 and 11 and the Galileo and
Ulysses spaceprobes took measurements of interplanetary dust from a distance of
0.7 to 18 AU from the Sun. Distinctly different populations of dust particles
exist in the inner and outer solar system. In the inner solar system, out to
about 3 AU, zodiacal dust particles are recognized by their scattered light,
their thermal emission, and in situ detection from spaceprobes. These particles
orbit the Sun on low inclination (i <= 30 degrees) and moderate eccentricity
(e <= 0.6) orbits. Their spatial density falls off with approximately the
inverse of the solar distance. Dust particles on high inclination or even
retrograde orbits dominate the dust population outside about 4 AU. The dust
detector onboard the Ulysses spaceprobe identified interstellar dust sweeping
through the outer solar system on hyperbolic trajectories. Within about 1 AU
from Jupiter, Ulysses discovered periodic streams of dust particles originating
from within the jovian system.


Tholen D. J.*
   Asteroid Spectra and Taxonomy

No Abstract Available


Muinonen K. O.*   Lupishko D. F.
   Light Scattering by Solar System Dust: Does the Theory Explain the
   Observations?

We review recent theoretical, observational, and experimental developments in
the optical photometry and polarimetry of small solar system bodies and
interplanetary dust, paying special attention to the opposition effect
(nonlinear brightening at small phase angles) and the reversal of linear
polarization (a change in the predominant plane) toward opposition. Coherent
backscattering has been invoked to explain both of the aforementioned
phenomena, though shadow-hiding also contributes to the opposition surge. In
analyzing the observations and experiments, we subject the coherent
backscattering mechanism to critical examination. The mechanism is based on
Maxwell's electromagnetic equations, but it is not currentlly possible to
solve the equations rigorously for arbitrary dust particles or regoliths. We
are therefore limited to qualitative interpretation of the observations and
experiments, and must leave quantitative interpretation as a future goal.


Rickman H.*
   Comet Nuclei

No Abstract Available


A'Hearn M. F.*
   Gas and Plasma in Comae

No Abstract Available





                             Friday, June 18, 1993
                             ORIGIN AND EVOLUTION
                                  8:30 - 12:00

Chair(s):  V. S. Safranov
           G. W. Wetherill


Weidenschilling S.*
   Formation of the Inner Solar System Bodies

   No Abstract Available


Bailey M.*
   Formation of the Outer Solar System Bodies

   No Abstract Available


Bell J.*
   Thermal and Collisional Evolution of Asteoroids

   No Abstract Available


Zappala V.*
   Asteroid Families

   No Abstract Available


Schleicher D. G.*
   Comet Taxonomy and Evolution

Comets are widely believed to be the most pristine remnants from the epoch of
solar system formation and, as such, can provide important clues to the
physical and chemical processes which took place at that time. For this
reason, much of cometary research is aimed at obtaining an understanding of
the origin, basic compositional and physical characteristics, and fate of
cometary nuclei.

Numerous papers in recent years have explored the dynamical evolution of
cometary orbits and the evidence that there are two reservoirs of comets - the
Oort cloud and the Kuiper belt - which are the sources of dynamically new
comets and short-period comets, respectively. If this scenario is true, then
compositional differences might be expected because the comets in these two
groups would have formed at different locations and, thus, under different
conditions within the proto-solar nebula. While not wholly immune from
evolutionary processes while "stored" in either the Oort Cloud or Kuiper Belt,
one would expect that any compositional differences would still be evident
today. Thus far, quantitative studies of the chemical composition of comets -
based on abundances and production rates of species within cometary comae - do
not show clear evidence for this type of trend with orbital dynamics. However,
substantial evidence of compositional differences among comets does exist.
While the majority of comets studied have the same basic composition,
significant depletions or enhancements of certain species have also been
ohserved in many comets. For instance, CO has been reported by Feldman (in ACM
II, Lagerkvist et al., eds., pp. 263-267, 1986) to vary from 1% to 27% with
respect to water. Our own taxonomic studies indicate that about 30% of comets
are depleted in C2 and C3 (but not CN) by various amounts (up to a factor of
20) (Schleicher et al., Bull. Amer. Astron. Soc. 24, 1004, 1992), while Comet
Yanaka (1988r) was observed by Fink (Science 257, 1926-1929, 1992) to
apparently have a "normal" abundance of NH2 with respect to water but with CN
and C2 depleted by factors of 25 and 100, respectively. These and other
compositional data sets will be the primary focus of this paper.

The evolution of comets is another important area of study, and a variety of
evidence for evolutionary effects also exists. For instance, dynamically new
comets are observed to be preferentially brighter prior to perihelion than
after, possibly due to the higher volatility of the original surface compared
with material deeper within the nucleus. There are well-established cases of
short-period comets fading with successive apparitions and eventually
disappearing, while other comets have been mis-identified as asteroids due to
their asteroidal appearance on some occasions. While the data available for
investigating evolution based on chemical composition is much more limited, I
will present what evidence there is and discuss our current state of
knowledge.


Schleicher D. J.*
   Comet Taxonomy and Evolution

   No Abstract Available


Sicardy B.*
   Small Bodies Around Other Stars

   No Abstract Available





                             Friday, June 18, 1993
                                 COMET NUCLEI
                                 14:00 - 15:30

Chair(s):  M. Festou


Jorda L.*   Colas F.   Lecacheux J.
   The Dust Jets of Comet P/Swift-Tuttle 1992t

The periodic comet Swift-Tuttle is known to be an "atypical" comet, producing
nice and strong dust jets and fan shapes easily observable. We observed this
comet 12 evenings between November 20 and December 20, 1992 with the 1m
telescope at the Pic du Midi Observatory using a CCD frame and broadband
filters. During these observations, the seeing was between 0.5 and 1
arcsecond, leading to high quality images: details of less than 500 km can be
seen on some of them. On the November images, a strong helicoidal dust jet
rotating with time was clearly visible until a distance of 30,000 km from the
nucleus, whereas another fainter jet appeared on some images until 10,000 km.
The December images revealed the apparition of two new jets after the
perihelion passage, one of them being visible in the dust tail, more than
100,000 km away from the nucleus. Due to the rotation of the nucleus, the
movement of the jets is visible from hour to hour, and from one night to
another. On the 9 nights of observation in November, the patterns were almost
the same every three nights.

The series of images obtained allow us to derive the parameters that describe
the rotation of the nucleus: the rotation period, the orientation of the spin
axis and the location of the active zones on the nucleus. We compare these
parameters with those deduced by Sekanina from drawings made during the 1862
return of P/Swift-Tuttle.


Gebhard J.*   Grun E.   Kochan H.   Lammerzahl P.
   Gas-Dust Interaction in Comet Simulation Experiments

The comet simulation experiment KOSI-9 was performed in December 1990. It was
intended to study processes that could occur on a cometary nucleus resulting
from variable irradiation by light. According to observations of comets from the
Earth, the buildup of a dust mantle and its increase respective to decrease in
thickness was expected in coincidence with the varying insolation. A sample was
placed in a vacuum chamber at a pressure of 10^-6 mbar and consisted of 90% (by
weight) water ice and 10% olivine grains (maximum grain size = 10 micrometers).
It has been insolated repeatedly at variable intensities ranging from 200 to
1900 W/m^2 (1.4 solar constants). During the first insolation cycle at highest
irradiation levels strong gas, ice, and dust particle emissions were observed.
The rapid growth of a dust mantle was deduced from the measured suface
temperatures of up to 350 K, while the sublimation temperature of water ice was
210 K. During the second insolation period when the H2O gas flux had reached a
critical value of a few 10^21 molecules m^-2 s^-1, avalanches within the mantle
material occurred on the inclined sample surface. The mantle broke up locally
and opened up a fresh icy surface. The trajectories of the emitted ice/dust
particles are detected by a video camera and their ice content was determined by
an ice particle detector. These data were compared with a model for the particle
trajectories to better unterstand the coupling mechanism of ice/dust particles
in a molecular gas flow.


Mueller B. E. A.*   Belton M. J. S.
   Is P/Schwassmann-Wachmann 2 Really a Simple Rotator?

Abstract

P/Halley is the only comet known with a well established complex rotation [1].
Extensive light-curve coverage of activc comets far away from tlle Sun when
their activity is not dominant has only been done for a few comets. Karen
Meech (Meech et al. [2]) used a new method to account for coma contribution to
the reflected light of the nucleus of P/SW 1 and the analysis of her
light-curve data strongly suggests that its nucleus might also be in an
excited state of energy. If these two comets show complex spin states, why do
other comets not show the same behavior? Is it due to different behavior of
outgassing from the nucleus (e.g. due to structure of the crust, number of
vents, dynamical age) or is it just due to the poor light-curve coverage of
comets and/or our methods of periodicity analysis? Some of these questions
will be addressed in this pAper, in particular using the recent observations
of P/SW 2. The photometric data of P/SW 2 near aphelion by Luu and Jewitt [3]
were re-analyzed with special emphasis on multiple period search. Results of
this analysis will be presented at the meeting.

References

[1] Belton, M.J.S., W.H. Julian, A.J. Anderson, and B.E.A. Mueller, 1991:
Icarus 93,183.

[2] Meech, K.J., M.J.S. Belton, B.E.A. Mueller, M.W. Dicksion, and H.R. Li,
1993: submitted to A..J..

[3] Luu, J.X. and D.C. Jewitt, 1992: ?p.J. 104, 22?13.


Keller H. U.*   Thomas N.
   Active Areas on Cometary Nuclei

Evidence is accumulating that the nuclei of short period comets are typically
5 km in radius, of low albedo (~4%), and exhibit activity over only a small
fraction of their surfaces. Their densities could be as low as 300 kg m^-3
(although this parameter remains very uncertain) indicating a highly porous
structure. The imaging experiments on the flyby missions to comet P/Halley
revealed little or no difference in surface reflectivity between active and
inactive regions. A Moon-like phase function for the surface was also derived.
Active areas were around 3 km in diameter although observable features on both
the inactive surface and within the active regions were typically 500 to 1000
m in dimension. The low density and high porosity suggest that up to 30 m
depth of material can be lost per apparition from an active region on a comet
like P/Halley. A depression such as the Crater on P/Halley must therefore be
at least a few hundred years old (if created by sublimation). The emission
from P/Halley appears not to have changed significantly since records began.
These conclusions suggest that active areas are very stable over long
timescales.

Fine structures in the observed dust emission (also ~500 m in dimension) have
been modelled in two different ways. They may result from variations in
surface topography (craters) or from hydrodynamics interactions between gases
emitted from active areas surrounding inactive areas. An alternative mechanism
not yet thoroughly investigated is whether the fine structures could arise
from inhomogeneous emission. On a short timescale, monitoring of the comet's
activity over the three hours of the Giotto flyby showed no significant
changes. Several centimetres could therefore be eroded from an active area
without affecting the magnitude of the emission.

The presentation will summarise our knowledge of active areas on comets and
suggest that cometary nuclei should no longer be considered as "dirty
snowballs" but as "icy dirtballs".


Gustafson B. A. S.*   Palm T.   Andersson M. W.   Adolfsson L. G.   Xu Y. L.
Zerull R. H.
   Optical and Physical Properties of Comet Dust Models; Numerical and
   Laboratory Results

Clusters generated by ballistic spheres condensing on a nucleus, the Vold-
Sutherland model, are known to generate tenuous structures of apparently
fractal dimension. Even sparser structures, that may be relevant to comet dust
and meteoroids, are formed by using small clusters of spheres as building
blocks to generate balistic aggregates. In our models, each set of two to four
spheres represents individual interstellar grains that may have aggregated to
form comets. Larger building blocks are used to simulate a possible
precometary stage of aggregation in the presolar nebula.

We calculate optical properties of the aggregates consisting of up to a few
hundred unit spheres using three separate implementations of the Discrete
Dipole Approximation. The numerical results are compared with precise
microwave analog measurements to illustrate the advantages and disadvantages
of each implementation.

We also calculated the mean free path of a gas molecule inside the clusters
for use in modeling of physical stress and in thermal modeling of cometary
meteoroid material under a variety of conditions including atmospheric entry.
The calculations may also be relevant to some modeling of gas diffusion in
comet nuclei. The mean free path l(sub)m is a function of the packing factor p
and the unit particle radii r. We find that l(sub)m may be approximated by 1.
The diffusion coefficient may be approximated by D ~pi kT/72mmD(sub)o(p^-1 -1)
where D(sub)o ~2 and the exact value depends on the detailed structure.


Kochan H.*   Gebhard J.   Lammerzahl P.   Lorenz E.   Thiel K.
   Mantle Dynamics and Dust Emission Phenomena at Cometary Analogous
   Ice-/Mineral-Mixtures

During the last six years the "Comet-Simulation Program (KOSI)" was funded by
the Deutsche Forschungsgemeinschaft. This program will be closed in 1993 and
so it seems worthwhile to review the results in the view of findings gained
during the last experiments dating in the end of 1992. [Grun et al., 1987 and
Kochan et al. 1988].

In more than 10 large scale experiments performed in the Space Simulator of
DLR Koln a lot of physical processes within the model comets, at their surface
and shortly above, for the first time have been investigated "in situ". DLR
developed different video camera systems to monitor the ice sample's surface
from different aspects, the total view of the surface with zooming possibility
as well, as views in azimuth and in elevation.

In general it came out, that the ice-loaded dust particles are not loosely
deposited on the insolated surface. They glue together by sintering processes.
[Kochan et al., 1990, 1991]. Before these particles can entrain into the gas
jet, their connecting bonds to the neighbors have to be eroded. Sintering
takes place within the complete ice-/mineral-matrix [Ratke, et al., 1992].
Just below the surface the porous material is hardened in addition to the
sintering process by recondensation of the sublimated water vapor.

In a series of experiments with different ice-/mineral mixtures we found that
dust mantles preferably are formed during insolation periods with low
irradiation intensity. Intensifying the insolation, avalanches within the dust
mantle occurred, directly correlated with bursts of emitted particles. The IR
cameras also monitoring the sample surface, immediately show a decrease in
temperature within the avalanche area. This indicates an exposure of Iresh
ice-/mineral surface. The avalanches seem to be an important mechanism of a
local and a time variation of the "cometary" emission activity.

To get a better understanding of the gas-particle interaction, physically
defined by the drag force of the gas-flow exerted on the particle, the
particle emission was observed in a combined effort from different sites of
diagnostics. To get a synoptic view of all processes, all monitoring systems
used the same radio transmitted time reference. The particle dynamics was
monitored by the different video cameras (velocity) as well as by
piezoelectric impact detectors (momentum) and ice-collectors. Fluffy
aggregates with sizes of 500 micrometers and masses ranging from 0.2
micrograms to around 23 micrograms were recorded respectively collected. In
addition to the investigation of these emitted particles, the entrainment of
fluffy aggregates ranging from 400 micrometers to 1000 micrometers in size,
injected from above into the gas jet is also observed. The reaction of these
"well defined" particles with the jet is another clue to the gas particle
interaction processes. The evaluation is rather promising to deliver drag
coefficient and gas velocity data. The "synoptic" view of the mantle dynamics
and the particle emission will be given in the presentation.

References:  E. Grun, H. Kochan, K. Roessler and D. Stoffler, Simulation of
cometary nuclei, in Proc. Symposium on Diversity and Similarity of Comets,
eds. E.J. Rolfe and B. Battrick, ESA-SP 278, 501508 (1987)  H. Kochan, J.
Benkhoff, A. Bischoff, H. Fechtig, B. Feuerbacher, E. Grun, F. Joo, J.
Klinger, H. Kohl, D. Krankowsky, K. Roessler, W. Seboldt, K. Thiel, G. Schwehm
and U. Weishaupt, Laboratory simulation of a cometary nucleus: Experimental
setup and first results, Proc. 19th Lunar and Planetary Sci. Conf., 487-492
(1988)  H. Kochan, L. Ratke, K. Thiel, E. Grun. Partlcle Emisslon from
Artificial Cometary Surfaces: Material Science Aspects, Proc. of the 20th
Lunar and Planetary Science Conf., Houston, 401411 (1990)  H. Kochan, W.J.
Markiewicz, and H.U. Keller, KOSI: Gas drag derived from particle
trajectories, Geophys. Res. Lett., Vol. 18, No. 2, 273-276, (1991) L. Ratke,
H. Kochan, and H. Thomas, Laboratory Studies on Cometary Crust Formation: The
importance of Sintering, Asteroids, Comets, Meteors, Flagstaff 1991, pp.
497-500 Lunar and Planetary Institute, Houston, 1992.





                             Friday, June 18, 1993
                                    METEORS
                                 14:00 - 15:30

Chair(s):  P. Pecina


ReVelle D. O.*   Ceplecha Z.
   Analysis of Identified Iron Meteoroids: Relationships with M Type
   Earth-Crossing Asteroids

Using the approach of ReVelle and Rajan(1988; 1989) and a new technique
developed by Ceplecha(1991; 1992), we have re-analyzed fireballs from the U.S.
Prairie Network data whose behavior was similar to that expected of relatively
large Nickel-Iron meteoroids entering the atmosphere. ReVelle and Rajan
originally examined the fireball data and found six objects that satisfied
criteria similar to those developed by Wetherill and ReVelle(1981) for
locating chondritic meteoroids among the fireballs. The criteria used included
a near-matching of the theoretical and the corrected, observed end heights and
a photometric to dynamic mass ratio near unity. In this work we have re-
examined these six cases using a fragmentation analysis technique developed by
Ceplecha. The analysis uses the observed distances flown by the body along its
trajectory and fits them to theoretical values expressed as a function of
time. The resulting values contain also the place and degree of fragmentation
which the individual bodies experienced during entry. As a part of this
process the mean ablation coefficient is also determined. After re-analysis of
these six cases, we can say with confidence that five of the six bodies
identified had no fragmentation at all and that four of the six also had
exceptionally large ablation coefficients, a feature that is characteristic of
such bodies according to ReVelle and Rajan. In this paper we will present a
detailed analysis of these meteoroids both from a dynamics approach and from
an orbital viewpoint. Thus, we hope to be able to link these fireballs more
substantially to bodies that have already been shown to be M class, earth-
crossing asteroids(Tedesco and Gradie, 1987). The identified objects and some
of their preliminary physical properties are shown in the table on the hard
copy.


Borovicka J.*
   Two Components in Meteor Spectra

Analyzing a spectrum of a bright fireball, it was found that the brightness of
about 300 lines can be explained assuming the temperature of 4400 K and
thermal equilibrium. However, observed lines of Si II and Mg II should not be
visible under such conditions at all. Their presence is an evidence for
another radiating region in the meteor. Indeed, assuming two regions with
identical chemical composition and temperatures 4400 K and 10,000 K
respectively, the spectrum can be completely explained.

The high temperature component will be called "the second spectrum". I presume
its origin in the shock wave. For the 17 km/s fireball the mass ratio of low
temperature to high temperature gas is approximately 6000:1, but it was shown
that the importance of the second spectrum raises with increasing meteor
velocity. Both regions must be strictly divided, though they cannot be
resolved spatially from observations (as e.g. wake can).

The figure, which appears here in the hard copy, shows typical lines of the
meteor second spectrum. Synthetic spectrum of a gas containing 95% of air and
5% of meteoritic material under the temperature of 10,000K is presented.


Rajchl J.*
   From Thermodynamics and Algebra to Meteoroid Structure

The interaction of meteoroids with a planetary atmosphere formulated
previously by the author [1] using entropy production, i.e., the ratio of the
familiar ablation and dynamics equations of meteor physics, led to a cross-
complementarity relation between bright and faint meteors, which was also
confirmed observationally [2]. This nonequilibrium thermodynamical formulation
enables its further, nonlinear extention via an earlier, interaction layer
concept [3]. All such results, in combination with a cubic-equation-
bifurcation solution, following from algebra and composed of the sum of the
ablation and dynamics equations, compared with some observational
consequencies, forces the following conclusion: a distinction between compact
and fractal meteoroid material structure is more fundamental in respect to the
normal and anomal behaviour of both the faint and bright meteors, than
composition connected with the cometary and carbonaceous nature of bodies. A
new hypothesis about the various structures origins in relation to different
meteor velocity groups is offered.

References:  [1] Rajchl J. (1979) Bull. Astr. Inst. Czechosl., 70. [2] Rajchl
J. (1987) Bull. Astr. Inst. Czechosl. 67, 217. [3] Rajchl J. (1969) Bull.
Astr. Inst. Czechosl., 20, 363.


Pecina P.*
   On the Determination of Meteoroid Orbital Elements

The classical method of determination of orbital elements of meteoroids is
based on a sequence of corrections applied to velocity vector of observed
meteoroid. The main purpose of such approach consists in the determination of
meteoroid motion under the gravitational influence of the Sun while we are
able to observe this motion under the influence of the Earth. The classical
method treats this switch from Earth's gravitational center to the solar one
as jump-like process assuming tacitly the size of the activity sphere of Earth
is negligible as compared with the size of this sphere of the Sun. Using other
words, the time the meteoroid needs to pass the Earth's activity sphere is
considered to be negligible in comparison with its orbital period. Assuming on
the contrary to classics the time of the passage is finite we should treat the
meteoroid motion as continuous with respect to its the time evolution from the
time moment of its apparition to the moment the Sun's gravitational influence
surely dominates. As a consequence, we have numericaly integrated the
equations of motion of meteoroid under the simultaneous action both of the Sun
and the Earth from the time of the appearance of particular body back to the
moment the meteoroid - Earth distance reached at least 10 millions km to
ensure the body was well inside the sphere of activity of the Sun. In order to
visualize the differences we have used precise data of 17 fireballs
photographed by European fireball network in the past. It is shown that the
orbital elements following from the new method differ in general from
classical those. We can draw the general conclusion that the slower the
meteoroid the lesser its eccentricity as well as semi-major axis as compared
with their classical values.


Lindblad B. A.*   Porubcan V.
   The Orbit of the Perseid Meteor Stream as Determined from Photographic Data

In view of the increased interest in the Perseids, we have computed a precise
orbit for the Perseid meteor stream based on two-station photographic
recordings. In the study all precisely reduced photographic orbits available
at the IAU meteor data center in Lund (Lindblad, 1991) were searched for
Perseids. The Perseid orbits found in the search were checked for internal
consistency of the orbital elements. These checks revealed some minor
inconsistencies and it was necessary to reject a few orbits. The derived mean
Perseid orbit is compared with that of previos determinations and with the
orbit of the parent comet (Swift-Tuttle).

References: Lindblad B. A. (1991) in Origin and Evolution of Interplanetary
Dust (A. C. Levasseur-Regourd and H. Hasegawa, eds.), pp. 311-314, Kluwer
Acad. Publ.


Harris N. W.   Hughes D. W.*
   The Relationship Between the Semi-Major Axis and the Mass of Perseid
   Meteoroids

Meteoroid streams are produced by decaying comets. The cometary ices are
sublimated by the absorbed solar radiation and momentum transfer from the
resulting flux of gas pushes loosely bound meteoroid dust away from the
cometary nucleus. The meteoroid ejection velocity is a function of, among
other things, the inverse square root of the effective radius of the dust
particle. Hence the width of the resulting meteoroid stream and concomitantly
the stream orbital semi-major axis distribution should vary as a function of
the mass of the meteoroids. An investigation of this function has been carried
out using photographic Perseid stream meteoroids, of which there are an
abundance of in meteoroid orbit catalogues. We expect the Poynting-Robertson
effect to have only a limited influence on the orbital parameters of the
meteoroids being considered, as their estimated masses are in excess of
0.001 g. Both parent comet (P/Swift-Tuttle) and stream have high orbital
inclinations so it is also expected that neither have suffered much from
gravitational perturbations due to the major jovian planets.

It is shown that the most massive meteoroids are concentrated at the central
core of the stream with the meteoroids of lesser masses having a much more
diffuse distribution of orbits. This is consistent with the meteoroid emission
velocity from the cometary nucleus decreasing with mass. This finding is
confirmed by the fact that plots of meteoroid mass as a function of semi-major
axis show that, on the whole, the spread in the distribution of meteoroid
orbital semi-major axis decreases with meteoroid mass. There is, however, one
complication. The spread in the semi-major axis distribution becomes narrower
for meteoroids in the mass range 1 > m > 0.1 g. Further investigation using
Geminid meteoroids reveals that this is not a unique characteristic.

The general Perseid meteoroid semi-major axis distribution is modelled using a
simple meteoroid ejection function. It is shown that the observed distribution
cannot be easily reproduced using the "Whipple Formula" because meteoroid
ejection velocities predicted by that formula are too small. The "Whipple
Formula" does, however, produce a curve of relative ejection velocity as a
function of mass which has a similar form to the observed curve of orbital
semi-major axis as a function of mass.


Williams I. P.   Wu Z.*
   Comet Giacobini-Zinner and the Draconid Meteor Shower

The most spectacular meteor showers in the present century were probably the
Draconid showers of 1946 and 1933. The Draconids gave lesser showers in 1926,
1952, and 1985 but very weak showers or none in other years. It is well known
that this is because the meteoroid stream is young and the showers occurred
when the parent comet (Giacobini-Zinner ~period 6.5 years) was close to the
Earth. An interesting question is why the showers only occurred in some of the
years, and the strongest showers (or storms) only in 1933 and 1946, and not
with every return of Comet Giacobini-Zinner to the Earth.

It is our purpose to investigate the dynamical behaviour of the Draconid
meteoroid stream and the meteor shower. We discuss the emission mechanism of
the particles from the parent body and assume that two sizes of the particles
were ejected from the cometary nucleus at the perihelion passage. 500
particles of each kind were ejected at each return and their equations of
motion integrated to 1985. The cross-section of the stream in the ecliptic and
the profile of the stream are analyzed. We find that both the ejection
mechanism and planetary gravitation interact to cause the observed showers,
but the ejection mechanism plays a more important role. The grains contribute
almost nothing to the shower if emitted more than three returns ago, and the
strongest showers arise from the very fresh meteoroids not more than one year
old.


Taylor A. D.*   Baggaley W. J.   Bennett R. G. T.   Steel D. I.
   Radar Measurements of Very High Velocity Meteors by AMOR

Approximately one percent of the meteors detected by the Christchurch meteor
radar, AMOR occur almost simultaneously on all three receiver sites indicating
velocities on the order of hundreds of kilometers per second. Previous meteor
radars relying on Fresnel diffraction methods for velocity determination are
precluded from measuring such high values by aliasing problems associated with
the sampling rate. There is considerable evidence for this aliasing within the
recorded AMOR echo amplitude profiles.

The AMOR high velocity sample has a mean ionisation height 7 km above that for
more typical speeds indicating a mean speed of 140 km/s. Other velocity
estimation methods also indicate very high speeds. These include: The shift in
detected elevation angle as the meteor approaches its specular reflection
point, the rate of change in amplitude as the meteor moves past the specular
point and the shift in elevation angle for echoes from the rapidly decaying
high altitude meteors.

Particle diameters are typically in the range 20-50 micrometers for these very
high velocity meteors. The radiant distribution in right ascension is
consistent with particles from an interstellar origin, however the
geographical location and beam configuration of AMOR conspire so as not to
exclude the possibility of an interplanetary source.


Steel D. I.*   Clube S. V. M.
   The Small Comet Hypothesis: Support from Radar Meteors?

In a well-known and much-debated series of papers, Frank et al. have suggested
and defended their hypothesis that the Earth is struck every few seconds by
minicomets of mass ~100 tonnes, based upon satellite data apparently showing
brief UV obscurations of limited areas of the upper atmosphere. One of the
major pieces of supporting evidence that Frank et al. claim for their
phenomenon (if real and not an instrumental artifact) having an
extraterrestrial origin is the apparent agreement between the obscuration rate
and the diurnal and seasonal variation in radar meteor counts. This has not
been challenged in previous criticisms of the hypothesis (see A. J.Dessler,
Rev. Geophys., 29, 355-382, 1991) and indeed has been cited as being strong
evidence in favour of the small comet hypothesis. In this paper we show that
the particular radar data used by Frank et al. were not representative and
that an unbiased selection from available meteor data does not quantitatively
substantiate their claims. In spite of this we believe that they have indeed
identified a previously  unrecognized phenomenon with a source in the
meteoroidal complex, and we give an alternative model for its origin and
temporal variation.





                             Friday, June 18, 1993
                 COMETS:  COMPOSITION AND MATERIAL PROPERTIES
                                 16:00 - 17:30

Chair(s):  H. U. Keller


Samarasinha N. H.*   Belton M. J. S.
   CO in Comet Halley

The fact that CO is the second most abundant species observed in cometary
comae together with the low sublimation temperature of CO suggest that CO may
be the source behind cometary activity at large heliocentric distances. Thus,
CO is a very important tracer in understanding the chemistry and activity of
cometary nuclei.

An analysis of the CO distribution of the coma of Comet Halley along the path
of the Giotto spacecraft by Eberhardt et al. (1987, Astron. Astrophys. 187,
481-484) concluded that a distributed source is responsible for >50% of the CO
in the coma. However, they do not discuss the three-dimensional nature of the
source. On the other hand, a recent analysis of CN jets of Comet Halley by
Klavetter and A'Hearn (1993, Submitted to Icarus) indicates that >10-55% of CN
in the coma is due to jets and are produced outside the nucleus.

We have modelled the CO distribution due to gaseous jets and compared them to
the one observed by Giotto. We will discuss the results of modelling for the
jets formed by active areas in the rotational model of Belton et al. (1991,
Icarus 93, 183-193). We find that the active area number 1 in that model may
be responsible for a significant fraction of the observed CO distribution. The
relevant physical parameters for the jets as constrained by the observations
and comparisons with other studies will also be discussed.


Hoban S.*   Reuter D. C.   Mumma M. J.   DiSanti M. A.
   Infrared Measurements of Methanol in Comet P/Swift-Tuttle

Using the CRyogenic SPectrometer at the Kitt Peak National Observatory, we
obtained infrared spectroscopic measurements of fluorescent emission from
methanol in comet P/Swift-Tuttle on 1992 November 8, 9, and 12 and December 8
and 10. We retrieve production rates of methanol from the V3 band at 3.52
micrometers for each night (see table). We find our results to be consistent
with values reported by Paubert et al. (1992, IAU Circ. No. 5664) from
measurements of methanol with the IRAM millimeter telescope, and with infrared
spectroscopic measurements with CGS4 on the UKIRT (Davies et al., 1992, IAU
Circ. No 5659). For the lower-resolution spectrum from November 8, we are able
to extend the model to the nu-2 and nu-9 bands near 3.4 micrometers, and we
find that methanol contributes a significant fraction of 3.4-micrometer
"cometary organic feature" in this comet. An example of the lambda/delta
lambda = 88 measurements from November 8.1, the lambda/delta lambda = 476
measurements from November 9.1, and the methanol model are shown in the
figure, which appears in the hard copy. We find the relative abundance of
methanol with respect to water (mixing ratio) to be 8 +/- 3%. There may be
weak evidence for temporal variability in the methanol mixing ratio; however,
we have insufficent data to determine if there exists a correlation with
rotational phase. Formaldehyde was not detected; 3-sigma upper limits are
given in the table.

UT date QCH30H  M*      QH CO   M**
(1992)  (lO^28 s^-1)    (%)     (lO^27 s^-1)    (%)

Nov 8.058       2.9 +/- 0.7     12 +/- 5        <6.0    <2.4
Nov 9.066       2.0 +/- 0.5     8 +/- 3 <0.4    <0.2
Nov 12.102      4.2 +/- 1.9     14 +/- 7        <1.8    <0.6
Dec 8.076       6.0 +/- 1.6     9 +/- 4 <1.7    <0.3
Dec 10.069      3.1 +/- 1.0     5 +/- 2 <3.1    <0.5

*MCH30H: relative abundance of methanol with respect to water.
**MH2CO: relative abundance of formaldehyde with respect to water, 3-sigma
upper limits.


Bockelee-Morvan D.*   Padman R.   Davies J. K.   Crovisier J.
   Observations of Sub-millimetre Lines of CH3OH, HCN and H2CO in Comet
   P/Swift-Tuttle with the James Clerk Maxwell Telescope

Sub-millimetre molecular line observations of comet P/Swift-Tuttle were
undertaken on December 6, 7 and 8th 1992 with the James Clerk Maxwell
Telescope at Mauna Kea. Hydrogen cyanide (HCN), formaldehyde (H2CO) and
methanol (CH30H) were successfully observed.

HCN was detected through its J=4-3 rotational line at 354.505 GHz. The antenna
temperature reached 2.5 K and the signal-to-noise ratio was close to 90. To
our knowledge, this is the strongest radio signal ever recorded in a comet.
Formaldehyde was observed through its 5(1,5)-4(1,4) line at 351.77 GHz with a
signal-to-noise of about 15. Methanol was detected for the first time at
submillimetre wavelengths, through at least 7 of its J=7-6 rotational
transitions at 338 GHz and one line at 341.42 GHz. The analysis of the
relative intensities of the CH3OH lines will give stringent constraints on the
excitation conditions of methanol in the coma. A preliminary study suggests
that the J=7 rotational levels are populated according to a Boltzmannian
distribution at the temperature of 50 K.

As preliminary estimates the observed line intensities correspond to HCN, H2CO
and CH30H production rates of 1. 10^27, 5. 10^27 and 2. 10^28 s^-1
respectively, if direct release from the nucleus is assumed. Given the OH
production rate of 5. 10^29 s^-1 measured in the UV and the radio, we deduce
HCN and H2CO abundances relative to water of 0.2 and 1% respectively. The high
abundance measured for CH3OH (4%) confirms that this species is an important
constituent of cometary atmospheres whose abundance is variable from comet to
comet.

All the detected lines are highly asymmetric in shape and strongly shifted
towards the blue. Such an asymmetry is indicative of strong anisotropic
outgassing towards the Sun and can be correlated with the morphology of jets
of the inner coma observed in the visible.


Despois D.*   Paubert G.   Colom P.   Bockelee-Morvan D.   Crovisier J.
   Observations of Comet P/Swift-Tuttle 1992t at IRAM

The millimetre spectrum of P/Swift-Tuttle, the comet responsible for the
Perseid meteor stream, was observed with the IRAM 30-m telescope on November
11-14 and 21, 1992, and on January 6-7, 1993. These dates bracket the
perihelion, which took place on December 12. The distance to the Sun was about
1.1 AU and the distance to the Earth increased from 1.2 AU for the first
period to 2 AU for the last one.

The following molecules were detected, with signal-to-noise ratios ranging
from 7 to 30 (November 21): HCN (J(1-0) line at 89 GHz); CH3OH (12 lines
around 145 and 165 GHz); H2S (1(sub)10-1(sub)01 line at 168.8 GHz); H2CO
(3(sub)12-2(sub)11 line at 225.7 GHz). In January, HCN, CH3OH and H2CO were
also observed, which will allow a comparison of the molecular productions
before and after perihelion.

From the line intensities, we evaluate the molecular production rates relative
to water on November 21 to be 0.25 10^27 s^-1 for HCN, 2. 10^28 s^-1 for
CH3OH, 1.5 10^27 s^-1 for H2S and 2 10^27 s^-1  for H2CO (assuming a parent-
molecule distribution for this latter molecule). The water production rate at
that moment, estimated from OH observations, was of the order of 5 10^29
molecules s^-1.

The simultaneous observation of several rotational lines of methanol will
permit us to evaluate the rotational temperature of this molecule and to
constrain the physical conditions within the coma.

All line profiles are asymmetric with a cusp at negative velocities. They are
blueshifted by 0.5-0.7 km s^-1. It is the first time so important radio line
shifts are observed in a comet.

Figure 1, which appears here in the hard copy, shows P/Swift-Tuttle on
November 21, 1992: H2CO 3(sub)12-2(sub)11 line at 225.7 GHz.


Weaver H. A.*   Feldman P. D.   McPhate J. B.   A'Hearn M. F.   Arpigny C.
Smith T. E.
   Deriving CO2 and CO Abundances from Cometary Spectra Taken with the Hubble
   Space Telescope

Three comets have now been observed spectroscopically with the Hubble Space
Telescope (HST): periodic comet P/Hartley 2 (1991 XV) in 1991 September, the
dynamically new comet Shoemaker-Levy (1991a1 ) in 1992 July, and periodic
comet P/Schaumasse (1992x) in 1993 March. Several new cometary emissions were
clearly detected in the 1800 - 2300 Angstrom region in comets P/Hartley 2 and
Shoemaker-Levy and have been identified with the Cameron system of CO. We have
analyzed the Cameron band emission observed in the HST spectra considering
five possible excitation mechanisms: photodissociative excitation of CO2 ,
electron impact dissociative excitation of CO2^+ , dissociative recombination
of CO2^+  with electrons, electron impact excitation of CO, and resonance
fluorescence of CO in the Cameron system. In the case of P/Hartley 2, we find
that photodissociative excitation of CO2  is probably responsible for at least
60% of the emission, which means that these observations of the Cameron bands
can be used to derive the relative CO2  abundance in the nucleus. For
P/Hartley 2 we obtain CO2 /H2O ~4 - 9%, with the highest value being valid if
photodissociative excitation is responsible for all/ of the emission and the
lowest value (which we regard as much less likely) being valid when the
contributions from the other excitation processes are pushed to rather extreme
upper limits. Similar conclusions apparently apply to the case of comet
Shoemaker-Levy. We have searched for CO in these comets by looking for
emission in the Fourth Positive Group bands near 1500 Angstroms. In P/Hartley
2 we find that CO/H2O <-2% (3 sigma), while there is a tentative detection of
CO in Shoemaker-Levy at a relative abundance of a few percent. Emissions in
the (B - X) and (A - X) bands of CO2^+ were detected in comets P/Hartley 2 and
Shoemaker-Levy. In principle, these bands can also be used to probe the CO2
abundance, but our observations demonstrate that the excitation mechanisms for
these bands are poorly understood.

^1On leave from the Space Telescope Science Institute.


Feldman P. D.*
   The Abundance of Ammonia in Some Recent Comets

The abundance of ammonia, relative to water, in cometary ice remains an open
question. Values for comet P/Halley derived from a variety of remote and in
situ observations range over an order of magnitude from 0.1% to 1-2%.
Recently, we presented a derivation based on an analysis of the spatial
distribution of NH emission in the A^3II-X^3 Sigma^-(0,0) band at 3360
Angstrom as measured by the Russian ASTRON satellite (Feldman et al., Ap. J.
404, 348, 1993). The large spatial extent of the data permitted a novel method
of analysis that is largely independent of the details of a photodissociation
model, many of whose parameters are only poorly known. For 9 April 1986, this
analysis gives a ratio of NH3/H2O production rates in the range of 0.5-1.0%.
Simultaneous observations of NH on the same date by the International
Ultraviolet Explorer (IUE) allow us to infer NH3/H2O abundances on other dates
for comet P/Halley (including the time of the Giotto encounter) as well as
relative abundances in other moderately active comets for which the IUE data
has satisfactory signal/noise ratio. We have extended this analysis in two
aspects, accounting for the variability in gas production rate of P/Halley,
and allowing for the effects of differing geocentric distance (the effective
size of the spectrograph aperture projected onto the comet) for different IUE
observations. Where possible we present direct comparisons with relative
abundances derived from ground-based observations of NH2.


Hanner M. S.*   Lynch D. K.   Russell R. W.
   10 micrometer Spectra of Comets and the Composition of Silicate Grains

A 10-micrometer emission feature is seen in filter photometry of many comets,
but high signal/noise spectra exist for only a few. Spectra of Bradfield 1987
XXIX, Levy 1990 XX, both long period comets, and P/Halley show a strong,
double-peaked emission feature with maxima at 9.8 micrometers and 11.25
micrometers. They differ by ~ 0.4 micrometers in their width and 8-9
micrometers rise. No single silicate mineral can account for the observed
emission feature. Mg-rich olivine (forsterite) matches the 11.25-micrometer
peak, while enstatite and amorphous olivine have maxima at 9.8 micrometer. To
fit the 8-9-micrometer rise, amorphous enstatite, amorphous anorthite, or
hydrated silicates are possible. Based on the spectra, data from the Halley
flybys, and chondritic aggregate IDPs, a mix of Mg-rich olivine and pyroxenes
seems the most likely composition of the silicates.

The new comets Wilson 1987 VII, OLR 1989 XIX, and Austin 1990 V each have a
unique spectrum that differs from those above, implying differences in the
composition of the silicates. This research was carried out in part at the Jet
Propulsion Laboratory, California Institute of Technology, under contract with
NASA.


Levasseur-Regourd A. Ch.*   Renard J. B.   Goidet B.
   Polarimetric and Photometric Properties of Cometary Dust

The return of comet Halley in 1985-1986 had triggered numerous observations of
brightness and polarization of solar light scattered by cometary grains. The
Giotto flyby of comet Grigg-Skjellerup in 1992 has allowed the first inner
coma probing of a comet, down to a nucleus distance of about 200 km. Besides,
recent progresses in ground based imaging have allowed brightness and
polarization images of cometary comae to be obtained.

A significant data set has therefore been obtained for various comets, for
various phase angles (at least in a 1-degree to 110-degree range), and with a
significant spatial and temporal resolution. It is of interest to notice that
the phase angle dependence of the linear polarization degree in the outer coma
(nucleus distance greater than 2000 km) of a comet does not significantly
depend upon the comet, at least as long as its solar distance is below about 2
AU and its phase angle is smaller than 45 degrees. Possible wavelength
dependence will be discussed, and the significant spatial variations that
occur in the inner coma and more precisely in the near nucleus region and
along dust jets will be discussed.

These properties will be tentatively interpreted, while straightforward
interpretations with spherical grains which poorly represent the irregular
cometary grains will be avoided. Also, the optical properties of the cometary
grains will be compared to those of i) asteroidal surfaces obtained from
numerous asteroids observations, ii) interplanetary dust grains derived from
inversion of zodiacal light and zodiacal emission data. Various questions
about the composition, surface properties, porosity and evolution (e.g.
fragmentation, sublimation) of these small bodies will be addressed.





                             Friday, June 18, 1993
                             DYNAMICS OF ASTEROIDS
                                 16:00 - 17:30

Chair(s):  R. P. Binzel


Scholl H.*
   Different Mechanisms to Deplete the Kirkwood Gaps?

Numerical experiments covering some 10 Million years indicate three different
mechanisms for the depletion of the Kirkwood gaps, which we like to call the
planet-crossing, the secular resonance, and the threshold eccentricity
mechanism. It appears that the Kirkwood gaps have not formed in the same way.
The results can serve as a basis to explain why resonances are not always
associated with depleted regions.

Besides the well known mechanism to deplete the 3/1 resonance region due to
Mars- and Earth-crossing (planet-crossing), our new experiments show that
approaches to Jupiter can deplete resonance regions without the assistance of
Mars or of the Earth. For the latter case, two intrinsic parameters are
identified: the amplitude and the frequency of Jupiter's eccentricity. In the
latter case, the secular resonance case, gaps can be formed only in a model
including all the outer planets.

In the amplitude case, gaps can be formed in the model of the restricted
three-body problem with Jupiter on a fixed elliptic orbit. A minimum threshold
eccentricity for Jupiter is necessary for gap formation depending on the
individual resonance. This explains why the 3/2 and the 4/3 resonance in the
asteroid belt is not depleted: Jupiter's maximum eccentricity does not exceed
the corresponding threshold eccentricity.


Lemaitre A.*
   Large Proper Elements for Highly Inclined Orbits: Tests and Accuracy

The classical method used to identify families of asteroids is the calculation
of proper elements of all known asteroids followed by an analysis of the
shortest distances between them. These proper elements are not easy to
estimate; they have to be quasi-invariants of motion for very long periods of
time and they are obtained by double average over the short and long periodic
terms of the N body problem.

A very complete theory, based on series expansions in eccentricity and
inclination, has been developed by Milani and Knevzevic in the years 1988-
1990; a set of proper elements for numbered asteroids (1 to 1800), calculated
by Williams in 1979, numerically, is available too. We have developed a third
theory, mainly Williams's but expressed in action angle variables, to try to
identify new families of asteroids, eventually nonnumbered and with high
inclinations. Consistency, accuracy, comparisons, and results will be
presented and discussed.


Morbidelli A.*
   A Secular Integrator for Asteroid Fragments

This work is a byproduct of the proper-element program by A. Lemaitre and A.
Morbidelli. The Hamiltonian of the problem (massless asteroid perturbed by
Jupiter and Saturn) is averaged with respect to the mean longitudes of both
asteroid and planets, and is coded in a new grid directly in Arnold action-angle
variables. These are the most suitable ones in order to take into account the
strongly nonlinear dynamics related to the motion of the argument of perihelion,
which is dominant at large inclination. Only the three main terms, corresponding
to the three main secular resonances nu(sub)5, nu(sub)6, and nu(sub)16 are
retained in the perturbation. The averaged equations of motion are integrated
directly in action-angle variables. Therefore this secular integrator turns out
to be very fast (30 seconds for 1 million years on a HP710 workstation).
However, due to the simplifications of the model, the results are qualitative.
This secular integrator is therefore very suitable for statistical studies on
the behavior of thousands of fictitious objects, such as simulated fragments of
real steroids, in order to study the dynamical mechanisms of meteorite
transport. We plan to complete this secular integrator with a Monte Carlo
simulation of the effects of close approaches with the inner planets.


Bowell E.*   Muinonen K. O.
   Asteroid Bayesian Orbital Error Analysis: Applications

We summarize recent progress in our work on the determination of asteroid
orbits by applying Bayesian probabilities to optical astrometry. Although we
determine orbits in the historically accepted way (least-squares differential
correction), we regard the orbital elements as probabilistic rather than
deterministic. Our protocol has spawned a variety of applications, among which
are:

Orbit Refinement. We have discussed short-term (days), medium-term
(single-apparition), and long-term orbital and ephemeris accuracy, together
with optimizing future observing strategy, in a paper in press in Icarus.
Preprints will be available.

Orbital Element Database. We plan to recompute all asteroid orbits, and to
make available a public-domain orbit database via ftp (cf. Bowell and
Muinonen, this conference). Associated with each set of osculating
least-squares elements will be the orbital covariance matrix and metrics used
to characterize orbital quality. As part of this work we will develop an
automated method of orbit determination.

Criterion for Numbering Asteroids. We recommend use of the so-called
leak--longitude, eccentricity, and angular momentum (k)--metric to decide
whether an asteroid has been well enough observed for numbering. The leak
metric correlates with an asteroid's ephemeris accuracy over a given interval.

Proper Element Computation. We have devised a metric (termed the sleak, or
simplified leak metric) that helps one decide whether the osculating elements
of single-apparition asteroids are accurate enough to warrant computation of
proper elements (cf. Milani et al., op. cit.).

Recovery Strategy. By estimating an asteroid's sky-plane uncertainty as a
function of time, one can decide when observational opportunities will arise
for an asteroid's recoverv, and whether narrow- or wide-field searches are
appropriate. Searches for lost asteroids, such as (719) Albert, can be carried
out largely using archive plates by accumulating evidence that a lost asteroid
was not in various parts of its orbit at given times.

Planetary Collision Probability. It is particularly important to assess the
hazard due to near-Earth objects (NEOs). We start by defining the a posteriori
probability density, define the collision probability, and then use the change
in orbital x^2 as a measure of NEO-Earth proximity over a given interval.

Asteroid Mass Determination. Rather than use the dynamics of a particular
asteroidasteroid encounter, we will identify all such encounters that may have
produced observable effects, undertake astrometry using archive plates where
necessary, and then use the Bayesian method to solve simultaneously for the
masses of all the perturbers involved.


Hadjidemetriou J. D.*
   Averaged Hamiltonians at the 2:1 and 3:1 Resonances of the Asteroid Problem,
   Valid Globally

Abstract: The method of averaging is a very useful tool in the study of
resonant asteroid motion. The averaged Hamiltonian is expressed in the iorm of
a power series in a small parameter (the eccentricity of the asteroid), and
one can obtain in this way quite simple expressions. However, these
Hamiltonians are valid for small values of the parameter only and therefore
are not very useful for a complete understanding of the evolution of the
asteroid. More elaborate methods have been developed by several investigators,
who obtained averaged Hamiltonians valid for all values of the eccentricity,
but these latter expressions are very complicated and not readily expressible
in elementary functions. In the present study we propose a novel method to
obtain a realistic model for the averaged Hamiltonian near a resonance, which
is very simple and is valid globally. The main idea is to transform the simple
averaged Hamiltonian, which is originally valid for small values oI the
eccentricity, to a new Hamiltonian by adding a correction term. The criterion
is that the corrected Hamiltonian must have the same fixed points with the
correct stability indices as the real model (in this case the elliptic
restricted three body problem). Since the fixed points of the averaged
Hamiltonian correspond to the periodic orbits of the real model, a detailed
knowledge of the families of periodic orbits of the elliptic restricted
problem near the main resonances is essential.

The corrected model obtained in this way represents a dynamical system which
is similar to the real one, in the sense that the topology of its phase spase
is the same as that of the real system (obtained by the method of Poincare
mapping on a suitable surface oI section). Thus, the evolution of the asteroid
obtained by the above model should be essentially the same as that of the real
system, since similar dynamical systems have the same generic properties.

In the present study we have applied the above method to two main resonances
in the asteroid belt, the 2:1 and the 3:1 resonance. The model obtained
includes in both cases all the high eccentricity resonances (that are missing
in the simple expression). The model for the 2:1 resonance also includes the
high instability near the collision at the eccentricty e = 0.6 and the fine
structure of interchallge from stability to instability neal collision. The
evolution of the asteroid, as obtained by the corrected Hamiltonian compales
very well with similal studies carried out by different methods.

By the above described technique we can obtain simple and realistic models for
the averaged Hamiltonian near any other resonance in the asteroid problem.

References

Hadjidemetriou, J.D. (1992): Resonant Motion tn the Restricted Three Body
Problem, Celest. Mech. (to appear). Henrard, J. and Lemaitre, A. (1987): A
Perturbatiue Treatment of the 2/1 Jovian Resonance, Icarus 69, 266- 279.
Klafke, J.C., Ferraz-Mello, S., Michtchenko, T., (1992):
Very-high-eccentricity Librations at some higher order
resonances, I.A.U. Symposium 152 (to appear). Morbidelli, A. and Giorgilli, A.
(199Oa): On the Dymamics in the Asteroids Belt. Part I: General Theory,
Celest. Mech. 47, 145-172. Morbidelli, A. and Giorgilli, A. (199Ob): On the
Dynamics of the Asteroids Belt. Part II: Detailed Study of the  Main
Resonances, Celest. Mech. 47, 173-204.


Chauvineau B.*   Farinella P.
   On the Evolution of Binary Earth-approaching Asteroids

Radar and lightcurve data suggest that a significant fraction of the Earth-
crossing asteroids are contact or nearly-contact binary systems. These
binaries have the relative orbit of their components changed by tidal forces
when they experience close encounters with the Earth (and the other inner
planets). Farinella (1992, Icarus, 96, 284) showed that the time scale for a
binary to undergo important orbital energy changes is comparable to its
lifetime vs. impact against the planets. We have now refined this estimate, by
calculating orbital energy changes for a range of initial conditions by a
Monte-Carlo technique and including the effects caused by the hyperbolic
(rather than rectilinear) geocentric motion of the binary's center of mass.
The results show that the encounters are effective in changing the orbital
energy--i.e., the semimajor axis of the system--unless or until the components
are very close to each other or in physical contact. Also, the orbital angular
momentum of the binary is changed over the same time scale as the energy,
implying that the binary's eccentricity evolves at a similar pace as the
semimajor axis, the final end-product of this evolution being either escape of
the components or a collision between them. This may explain the origin of
contact or nearly-contact systems such as 4769 Castalia and 4179 Toutatis.


Walker R. G.*   Schlapfer M. F.
   A Search for Small Bodies in L4 and L5 of the Earth-Sun System

The existence of small bodies near the Lagrangian points of planets other than
Jupiter has long been a subject of speculation. The recent discovery of a
Trojan in the orbit of Mars tempts one to surmise that similar bodies may
exist in Earth's orbit. We have carried out a search for detections of small
bodies in the L4 and L5 libration regions of the Earth-Sun system using the
IRAS survey data. The sensitivity of IRAS to detect Earth Trojans by their
thermal emission exceeds that of previous photographic surveys, while the
nearly total sky coverage of IRAS permits a relatively uniform search
extending to high ecliptic latitudes.

We have extracted all (760,000) of the high reliability 12 micrometer point
source detections from the IRAS detections file, CN28, that were observed at
solar elongations between 60o and 88o. We have modeled the expected
distribution of particles trapped in L4 and L5, and bounded their apparent
motions in a coordinate system rotating with the Earth about the Sun. Using
this model we have searched the detections for sources that define
trajectories corresponding to Earth Trojans with osculating orbital elements
in the range of semimajor axis 0.994 < a < 1.006; eccentricity 0.0 to 0.034;
and inclination from 0 degrees to 30 degrees. A trajectory is defined by
linking three or more detections (with similar infrared colors) that share the
same sense of apparent motion.

We also created 1077 orbits of simulated Earth trojans, 500 that passed
through the region of L4 scanned by IRAS during the time of the IRAS mission,
and 577 that passed through L5. IRAS would have detected 311 of these in L4
and 549 in L5. Half of these were detected on at least 4 occasions in L4, and
at least 7 times in L5. The maximum number of detections of a single object
was 42. This is due to the high degree of redundancy of the IRAS scan
coverage. The simulated trojans were used to tune our linking algorithms, and
to provide information relating to the completeness and reliability of the
search.

A list of candidate Earth-trojans is presented and their physical properties
discussed. A statistical assessment of the survey is made in terms of its
coverage of the libration regions, its completeness, and its reliability.
These results are further discussed in the context of their implications about
the Earth-trojan population.


Brunini A.*
   Close Encounters Between the Terrestrial Planets and Planet-Crosser
   Asteroids

In this paper, we asses the importance of close encounters between the
terrestrial planets and planet-crossing asteroids. We analyse the potential
importance of time-dependence in the flux-rate of asteroids to planet-crosser
orbits, concluding that an early heavy bombardment of the planet's surfaces
should be less than the present cratering rate, by about 3 orders of
magnitude. This result is in complete concordance with previous
analysis of the lunar cratering chronology.






                                POSTER SESSION
                                   ASTEROIDS



Afonso G.   Gomes R.   Florczac M.
   Asteroid Fragments in Earth-Crossing Orbits

The dynamics of small bodies in the solar system is ruled also by
nongravitational forces as the Poynting-Robertson effect and the Yarkovsky
effect.

The easiest explanation for the Poynting-Robertson drag force is identifying it
with the phenomenon of light aberration. This effect is dissipative and makes
the particle spiral inwardly to the Sun. For meter-sized bodies, Poynting-
Robertson is very weak to cause any observable effect. Another transverse force,
also induced by radiation pressure, is dominant for this size range: the
Yarkovsky effect.

The idea behind Yarkovsky effect is that reemission of radiation from a rotating
body is higher in the dusk hemisphere than it is in the dawn hemisphere, thus
creating a force in the direction opposite to the hotter hemisphere. The
Yarkovsky force can be dissipative or antidissipative causing the body to spiral
inward or outwardly to the Sun, respectively.

The purpose of this work is that of better understanding the dynamics of meter-
sized bodies (for instance asteroid fragments) subject to Yarkovsky effect and
in resonance with the main planets. We make the deduction of the Yarkovsky
effect solving the time-dependent heat conduction equation. We present some
examples with the dissipative and the antidissipative forces.

The dissipative force may confirm that many of meteorites found on the surface
of the Earth have their origin in the combined effect of Yarkovsky force and
resonances with Jupiter.

With the antidissipative force some good examples of trappings in resonances
with Jupiter can be found, because in these cases the orbits are converging and
capture is more natural.


Aikman G. C. L.   Balam D. D.   Tatum J. B.
   Astrometry of Near-Earth Asteroids from Victoria

In order to expedite accurate orbital determination of Earth-approaching
asteroids, initial discoveries must be followed by astrometric tracking over
arcs long enough to enable reliable recovery of such objects at later
apparitions. Our astrometric program at Victoria combines CCD imaging with
1.8-m and 0.5-m reflectors and photographic imaging with a 0.25-m Schmidt
telescope, to allow tracking of newly recognized objects to faint magnitudes,
as well as wide-angle recovery of some brighter objects. Some of the more
interesting objects monitored in recent months will be discussed.


Alfimova A. V.*
   The Modeling of Stochastic Orbits of Minor Planets

The asteroidal belt is considered to be the stochastic dynamical system of many
objects. Integration of stochastic equation of the process of evolution of the
belt as Markoff's semistationary semiergodic process, in toroidal coordinates,
given about 80% of objects in inner part of the belt have stochastic orbits.
That is, the model of the process have the property of the absence of after-
action, than the finding of eigenvalues of state matrix of each object in some
appointed time moment give us the possibility of prognosing of more probable
trajectories of that object in the future. For example, there are results of
culculations for two asteroids from the NEA class.


Alfimova E. V.
   Stochasticity of the Process of Formation of the Asteroidal Belt in the
   Solar System

In this work the problem of investigating the process of formation of the
asteroidal belt in the solar system is qualitatively well-founded. This process
is stochastic semistationary Markoff's process, if the objects have a stochastic
dynamical system. There are basic equations of this process and characteristic
energing distribution of known minor planets.


Almar I.   Kelemen J.
   Recent Activity of the Konkoly Observatory in Cometary and Asteroid Research

New European Subnode of the PDS Small Bodies Node in Budapest

NASA has recently established a European Subnode of its Planetary Data System
in our Institute. It will provide distribution of NASA's data on  small bodies
in the Solar System within Europe and will also provide  expertise and
assistance in archiving the data from selected space missions.

ON LINE CAPABILITIES: Specially selected data files are available through
Internet (E-mail or remote login). CURRENTLY AVAILABLE DATA SETS: Complete IHW
CD-ROM set with corresponding manual and other CD-ROM-s. Further data sets
will be added soon. NODE CONTACT INFORMATION: Address: H-1525 Budapest POB
67., Hungary FAX: 36-1-1569640 E-mail: pds@astro.rmki.kfki.hu

VEGA TV SYSTEM Comet Halley imaging experiment

During the last decade the Central Research Institute of Physics (KFKI)
Budapest, and the Konkoly Observatory performed an extensive study of Comet
Halley using the VEGA TV System experiment:
1) Near nucleus region, jets:  Intensity profiles along the axes of jets
within 100 km of the nucleus of Comet Halley were analysed and a sharp
deviation from a 1/r dependence toward a less steep gradient was found.
2) Rotational motion of the nucleus: Using both the ground based and
spacecraft observations, the VEGA TVS team at the KFKI continued to analyse
the rotational motion of the nucleus.
3) International Halley Watch Archive: Spacecraft and ground based
observations were sent to the IHW Archive, namely VEGA-1 and VEGA-2 raw and
processed images and photographic observations carried out with the Schmidt
telescope of the Observatory.

Asteroid observations

Photoelectric photometric observations of selected asteorids were carried out
in order to determine their shape, rotational properties and surface
irregularities by analysing the light curves.


Angeli C.   Barucci M. A.   Chevreton M.   Herpe G.   De Sanctis C.
Fulchignoni M.
   CCD Observations of Small Asteroids

We are continuing the long-term observational program started by our group in
1984 to study rotational properties of small asteroids with a diameter less
than 50 km. These small asteroids put important constraints on the models of
asteroid collisional evolution; the knowledge of their rotational properties
help in understanding the formation and the evolution of the Solar System.

Asteroid rotation is the result of the angular momentum added by mutual
collisions to the initial angular momentum determined by the formation
processes. The knowledge of the actual distribution of the spin rates can help
to interpret the evolutionary scenario (collision, fragmentation, etc) of
these objects. Most of the current knowledge on asteroid rotational properties
is deduced by the analysis of the photoelectric and CCD photometry
lightcurves.

Up to now, the rotational periods of about 250 asteroids with D<=50 km are
known; this represents a very poor sample of a population that, even if we
limit our analysis to the kilometer size objects, contains several thousand of
objects. In this work we present the results obtained during 19 good nights in
the period spanning between July 27, 1992 and February 1, 1993. The
observations of asteroid CCD lightcurves were carried out at Haute-Provence
Observatory (France) using the 1.2m telescope, and at Calar Alto Observatory
(Spain) using the 1.5m telescope. The result of this work is the determination
of the rotational periods of the asteroids 2078 Nanking, 1441 Bolyai, 1520
Imatra, 44551988XA, 22411979WM, 3139 Shantou, 372S Valsecchi, 3485 Barucci,
4708 Polydoros, 4015 1979VA, 4140 1976VA, 3986 1985SF2 and 2557 Putnam.


Asphaug E.*   Melosh H. J.   Vickery A.
   High-velocity Ejection of Large Fragments from Asteroids in Non-Catastrophic
   Impact Events: Results for Vesta

The recent confirmation of large impact fragments from the asteroid 4 Vesta
(Binzel and Shu 1993) has forced a re-evaluation of the impacts of asteroids.
These "chips off Vesta" are smali asteroids (~4-7km) bearing spectral
signatures of basatic achondrites whose proper elements e and sini are very
close to Vesta's, and whose mean heliocentric distance a ranges from inside
Vesta out to the 3:1 Jovian resonance. This is the route which takes these
chips to Earth; Vesta (c.f. Drake 1979) is probably the ultimate Eucrite
parent body. This large (~570 km diameter) parent asteroid was not disrupted
by the impact which ejected these large and fast (~500 m/s) fragments; as such
it is probably unique in the present Solar System. It is apparent (Gaffey,
1984) that Vesta has preserved its basaltic crust except in the hemisphere of
a rather large impact event, indicated by a large region of exposed olivine.

The puzzling aspect of large, fast ejecta fragments from an undisrupted target
can be summarized in very general terms as follows: Fast ejection velocities
require a high impact velocity. High impact velocities result in large
pressures and high strain rates. Large pressures and strain rates lead to
small fragments. Furthermore, laboratory impact experiments seldom show large
fragments traveling at any appreciable fraction of the impact velocity. But
some fundamental aspects of the problem are ignored in this analysis: (1) a 4
km chip off Vesta is, to scale, only ~0.4 mm (smaller than an individual
basalt grain) in a laboratory impact experiment; (2) spallation can double the
ejection velocities while maintaining relatively low shock pressure (Melosh
1985), and (3) the impactor may have been too large for point-source ejecta
computations to be valid. Escape velocity on Vesta is ~350 m/s, such that
fragmentation of the target does not imply its disruption. Large spall plates
ejected at 50 m/s would, for example, fall back in nearly their onginal
onentation. We shall address this issue with some planetary data and with
hydrocode computations.

Vickery (1987) has shown, by measunng the diameters of secondary craters from
large impacts on Mercury and on Mars, that a quantity of sizable ejecta (>~2
km) leaves large craters at high velocities (>~500 m/s). Consider for instance
Lyot, a 200 km crater formed on Mars (transient diameter Dt ~ 150 km).
Assuming an impact speed of 8 km/s, gravity scaling (Schmidt and Housen 1987)
tells us that the impactor diameter was 15 km, assuming the same density (2.7
g/cc) for both target and impactor.

Gravity plays a considerable role in the formahon of this crater bowl on Mars,
yet the ejection of spall fragments is due solely to the immediate effects of
the passing shock and is not strongly effected by gravity. Thus, if Vesta can
withstand this same impactor without disrupting (and without the dispersal of
its basalt crust), then the range of fragment sizes and velocities which
formed at Lyot can be expected. To test this hypothesis, we conducted a
hydrocode experiment with this equivalent impactor at the same velocity. Vesta
survived this impact without any hint of catastrophic disassembly. We ran
experiments with impactors as large as 60 km diameter at 8 km/s impact speeds
without resulting in the breakup of Vesta.
Several hydrocode fragmentation models shall be presented, for a variety of
initial Vestas, including a solid body, a strongly heterogeneous target, and a
target with a strong crust and weak interior. In each case some large material
(~2 to 10 km) is ejected at escaping velocities (>~350 m/s).

REFERENCES: Binzel, R.P., and Shu, X., Icarus in press (1993). Drake, M.J., in
Astero'ds, U. of A. Press (1979). Gaffey, M., LPSC XIX (1984). Melosh, H.J.,
Geology 13 (1985). Schmidt, R., and Housen, K., Int. J. Imp. Eng. V (1987).
Vickery, A., Geophys. Res. Lett. 14 (1987).


Barbieri C.   Lazzarin M.   Marzari F.   Cremonese G.
   CCD Reflectance Spectra of Apollo Asteroid 4179 Toutatis

Low-resolution spectra were obtained for the Earth-crossing asteroid 4179
Toutatis during its 1992-93 close encounter with the Earth.

The latest Toutatis apparition has represented an extraordinary opportunity
for observations and investigations of a small body as this event has been the
closest approach of any known minor planet or comet between now and 2000. In
the campaign organized around the world to observe the asteroid Toutatis, our
group observed it spectroscopically in the visible on September 27th and 28th
1992 at ESO-La Silla (Chile) when it was at Delta = 0.410 AU from the Earth
and on January 4th and 5th 1993 (Delta = 0.185 AU) at the Asiago Observatory
(Italy) obtaining a total of 35 spectra. The reflectance spectra were acquired
using the 2.2-m telescope + CCD/EFOSC2 combination at La Silla and the 1.82-m
telescope + CCD/Boller & Chivens combination at Asiago. The EFOSC2
spectrograph was equipped with a 300 ll/mm grating covering a spectral range
3500-9000 angstroms; the B&C spectrograph with a 150 ll/mm grating covering a
spectral range 3700-7800 angstroms. Bias, flat-field, calibration lamp,
standard stars and solar analogs were secured in order to wavelength and flux
calibrate the asteroidal spectra and to eliminate the solar contribution.

The reduction is still in progress and at present data from the ESO 2.2-m
telescope have been reduced with the MIDAS package. The principal aim of the
analysis of these spectra is to investigate the surface composition of the
asteroid in an effort to learn about its origin and formation and, more
generally, of the AAA asteroids and their possible relation with cometary
nuclei.

The figure, which appears here in the hard copy, shows that the principal aim
of the analysis of these spectra is to investigate the surface composition of
the asteroid in an effort to learn about its origin and formation and, more
generally, of the AAA asteroids and their possible relation with cometary
nuclei.


Bec-Borsenberger A.
   Determination of Masses of Main Minor Planets

The equations of the motion of the minor planets are function not only of the
initial orbital elements but also of the masses of the disturbing bodies.
Therefore, the values of the masses of the main minor planets are determined
when the system of the equations of the motion for Hipparcos minor planets is
solved by using observations of minor planets, specially all the observations
made during the campaign of observations for Hipparcos.

References Bec-Borsenberger A. 1992, Solar system objects observed by
Hipparcos, A&A, vol. 258, 94-98. Landgraf W. 1988, The mass of Ceres, A&A,
vol. 191, 161-166. Schubart J. 1922, Positions of asteroids for use in mass
determinations, A&A Suppl. Ser., vol. 94, 259-263.


Bocsa G.   Birlan M.
   Astrometric Measurements of Minor Planets in 1991-1992

In this paper, minor planets precise positions observed in 1991-1992 are present

I.  INSTRUMENTAL TECHNIQUE

For performing the observations, a 360/6000 mm astrograph was used, with 2x2 squ
an ORWO ZU 21, 24x24 cm plate was exposed. The measurements of the plates were c
ASCORECORD Carl-Zeiss measuring maching. Computations were done with an IBM PC 3

II. THE PROCEDURE

The observations were made by two methods: Metcalf's method and blinking--double
The positions were calculated using five reference stars from STAR CATALOG (SAO)
CATALOGUE. The programme was conceived in the Astronomical Institute of the Roma

III.  RESULTS

The table gives a synthesis of the work.  For each minor planet the table includ
name of asteroid, date of observation, correct values for U.T., right ascension
reference stars catalogue and the method used.





Bowell E.   Muinonen K. O.   Wasserman L. H.
   A Public-Domain Asteroid Orbit File

We plan to construct a file of J2000.0 osculating elements for all asteroids
having reasonably calculable orbits. The file will be made accessible publicly
via anonymous ftp (we regret that we do not in general have the resources to
provide the file in other forms).

A precursor elements file has been set up for test purposes and distributed to
a limited number of users. It currently contains osculating elements
integrated to the nearest 100day Julian date (JDT 2449100.5 = 23 Apr 1993 TT)
of 5,493 numbered asteroids; 4,431 orbits of asteroids that have observational
arcs of 75 days or more or that are near-Earth approachers (q <- 1.3 AU); and
11,723 additional orbits of short-arc asteroids. In addition to
number/name/designation and osculating elements, the file contains information
such as perturbation scheme (e.g., Mercury through Neptune); publication
history/orbit computer; H, G magnitude parameters; and flags that indicate
Earth-crossing asteroid status, orbits for which uncertainly identified
observations were used, and whether an asteroid was observed during a survey
(PLS, T-1, T-2, T-3, and/or UCAS). We are updating the precursor file on an
almost daily basis as new orbits are computed. Our principal criteria for
orbit update are that no apparently satisfactory astrometric observations
should exhibit great-circle residuals exceeding 4 arcsec (a somewhat arbitrary
but computationally convenient threshold) and/or that the orbital arc be
significantly extended by new observations.

For the future file, we will recompute osculating elements for all orbits so
as to be able to incorporate orbital error parameters, such as the orbital
element covariance matrix and various metrics relating to orbital accuracy
that derive from the orbit determination method of Muinonen and Bowell (this
conference, for example). The error parameters, generally computed for orbits
derived from five or more observations, will allow users to estimate spatial
and sky-plane uncertainties as a function of time, to decide whether the
computation of proper elements is warranted, to design a recovery strategy,
and so on. In addition, we will provide more complete documentation of orbits
than is usually given. Following the publication of new observations, it will
usually be possible to update orbital error parameters without recomputing an
orbit. We expect the file itself to be updated every few days.

We invite potential users of the osculating element file to discuss its
proposed content with us and to make suggestions for its improvement.


Britals J.
   Development of Analytical Theories for Eccentric Orbits

In view of the difficulties caused by considerable orbital eccentricities, the
classical perturbation methods never have been applied to the asteroidal
orbits with an accuracy level comparable to the theories of major planets.
Using extended canonical transformations it is possible to change the
independent variable from time t to osculating eccentric anomaly E:

(L, G, H, 1, g, h; t, F) --> (L', G', H', 1', g', h'; E, F').

The resulting equations of motion remain canonical and are expressed in
action-angle-type Delaunay-like variables. The advantage of new equations is
the improved convergence of Hamiltonian, expanded in Fourier series over E
instead of t. That accelerates the application of Lie transforms (Deprit,
1969) to the new Hamiltonian F'. In the end we get the expressions of all our
variables as functions of osculating eccentric anomaly:

L(E),..., h(E); t(E)

Considering the immensely complex task of building these transformations, a
symbolic manipulation software is being developed that would enable us to work
with extremely large series expansions, encountered in various celestial
mechanics applications.

References: Britals J. (1992) Celest. Mech., 54, 305. Deprit A. (1969) Celest.
Mech., 1, 12.


Burbine T. H.   Bell J. F.
   Asteroid Taxonomy: Problems and Proposed Solutions

For the past 20 years, many different asteroid taxonomies have been developed to
classify asteroids according to different sets of observational parameters.  The
most widely used taxonomy, devised by Tholen [1,2], classifies approximately
1000 asteroids according to similarities in spectral reflectance from 0.3 to 1.1
micrometers and visual albedo.  No mineralogical information is used in forming
the classes, but the same spectral information that is used in classifying
asteroids in this wavelength region can give a rough interpretation of surface
composition. However, to understand the structure of the asteroid belt, these
taxonomies are only useful if the group together asteroids with somewhat similar
mineralogies and thermal histories.

In the last five years, observational datasets (e.g., 0.8- to 2.5-micrometer
spectra [3], CCD spectra [4], and 3-micrometer spectra [5]) have been compiled
for a much smaller number of asteroids. These "supplementary" datasets allow for
much better mineralogical interpretations of asteroids because they allow for
the determination of the presence or absence of features such as the 2-
micrometer feature due to pyroxene and/or spinel and a 3-micrometer feature due
to hydrated silicates. Results from the analyses of these datasets include the
creation of the K class [6], identification of anomalous S-type asteroids with
2-micrometer features (believed to be due to spinel) much stronger than their
weak to nonexistent 1-micrometer features [7], and the identification of
hydrated M-type asteroids [5]. These datasets show that many of the classes
contain members with vastly different compositions and/or thermal histories.

One proposed way of using these "supplementary" datasets to classify asteroids
would be the use of symbols to indicate the presence or absence of
mineralogically diagnostic absorption features. Classes (C, F, M, and T) that
contain members both with and without 3-micrometer features would use the symbol
"h" (e.g., Ch) for members with such a feature and the symbol "n" for asteroids
without such a feature. The S class would be broken down into S (olivine-
pyroxene mixtures), So (olivine-rich) [8,9], Sp (pyroxene-rich) [8,9], and Ss
(spinel-rich) subclasses. These new asteroid classes have been placed in a
revised Bell superclass table [10] (see Table 1) to try to better show the
diversity of the asteroid belt. Also included are two newly proposed classes, J
[11] and Z [12].  The mineralogical structure of Table 1 will be constantly
changing as the number of asteroids with these "supplementary" observations and
the resolution of the resulting spectra both increase. This will lead to the
identification of more absorption features and the creation of more asteroid
classes and subclasses with better mineralogical interpretations.

Table 1.

--------------------------------------------------------------------------------
Bell           Class      Inferred        Analogous                  Asteroid
Superclass                Minerals        Meteorites                 Example
--------------------------------------------------------------------------------

Primitive       D        organics           (none)                  721 Tabora
P        organics           (none)                   65 Cybele
Cn+Fn     anhydrous sil., C   (none)                31 Euphrosyne
(K)     ol, pyx, carbon     CO and CV chondrites       221 Eos
Ss         spinel           CO and CV chondrites?   387 Aquitania
(Z)       organics           (none)                  5145 Pholus
--------------------------------------------------------------------------------
Metamorphic     Th    clays, organics       (none)                   308 Polyxo
B+Ch+Fh+G  clays, opaques    altered carb. chondr.        10 Hygiea
Q     pyx, ol,grey NiFe    H, L, LL chondrites      1862 Apollo
Mh         clays         altered carb. chondr.        92 Undina
--------------------------------------------------------------------------------
Igneous         V        pyx, plag           eucrites                  4 Vesta
(J)          pyx             diogenites                3155 Lee
R         ol, pyx        ol-rich achondrites?     349 Dembowska
S       ol,pyx,red NiFe    lodranites,irons        18 Melpomene
So       ol,red NiFe         pallasites           354 Eleonora
Sp    pyx,plag,red NiFe    basaltic achondrites?   1036 Ganymed
A             ol             brachinites          446 Aeternitas
Mn           NiFe               irons                 16 Psyche
E         Fe-free pyx          aubrites             64 Angelina
--------------------------------------------------------------------------------
The superclass of the Tn subclass (e.g., 114 Kassandra) has been interpreted as
igneous [13] and metamorphic [14].

References:  [1] Tholen D. J. (1984) Ph.D. thesis, Univ. of Arizona, Tucson. [2]
Tholen D. J. (1989) In Asteroids II, 1139-1150. [3] Bell J. F. et al. (1988)
LPSC XIX, 57-58. [4] Sawyer S. R. (1991) Ph.D. thesis, Univ. of Texas, Austin.
[5] Jones T. D. et al. (1990) Icarus, 88, 172-192. [6] Bell J. F. (1988)
Meteoritics, 23, 256-257. [7] Burbine T. H. et al. (1992) Meteoritics, 27, 424-
434. [8] Chapman C. R. (1989) To be submitted. [9] Howell E. S. et al. (1993)
JGR, submitted. [10] Bell J. F. et al. (1989) In Asteroids II, 921-945. [11]
Binzel R. P. and Xu S. (1993) Science, submitted. [12] Mueller B. E. A. et al.
(1992) Icarus, 97, 150-154. [13] Britt D. T. et al. (1992) Meteoritics, 27, 207.
[14] Burbine T. H. Jr. (1991) M.S. thesis, Univ. of Pittsburgh.


Chauvineau B.*   Farinella P.   Mignard F.
   Planar Orbits About a Triaxial Body: Application to Asteroidal Satellites

We have studied the dynamical behaviour of a small satellite of a strongly
triaxial primary rotating about a principal axis of inertia, restricting
ourselves to the planar case (satellite orbits lying in the "equatorial"
symmetry plane of the primary). Since we were mainly interested in applying
the results to the dynamics of (natural and artificial) asteroidal satellites,
we have studied in detail the case of a triaxial ellipsoidal primary, with
axial ratios (square root of 2):1:1/(square root of 2) and rotation periods
ranging from 5 to 40 hours.  We have employed the classical method of
Poincare's surfaces of section to display the results of numerical integration
experiments, showing in particular how the triaxial shape of the primary
"perturbs" the dynamics of the Kepler problem seen in a rotating reference
frame where the primary is fixed. We find that a large chaotic zone appears
near the 1:1 resonance between the rotation period of the primary and the
orbital period of the satellite. Another small chaotic zone appears for
retrograde orbits passing close to the primary and corresponding to initial
velocities close to the escape velocity. We discuss the possibility of putting
an artificial satellite in orbit about a triaxial asteroid, providing examples
of orbits close to the asteroid but almost unaffected by its  a priori
unknown) mass distribution.


Chernova G. P.   Minikulov N. X.   Kiselev N. N.
   Photoelectric Photometry of the Asteroid 2078 Nanking

The asteroid 2078 Nanking was observed with the 1-m telescope at Sanglok
Observatory, Tadjikistan. The rotation period was determined to be 6.463
hours. The absolute magnitude V(1.0) was found to be 13.2 mag, the lightcurve
amplitude Delta V = 0.8 mag, and the colors U-B, B-V, V-R are 0.22 mag, 0.85
mag and 0.59 mag, respectively. Assuming that the geometric albedo P(sub)v = 0
12, the size of Nanking is 8.8 x 6.1 km.


Chernykh N. S.   Prokof'eva V. V.   Chernykh L. I.   Karachkina L. G.
Rumyantsev V. V.   Zhuravleva L. V.   Tarashchuk V. P.
   Observations of (4179) Toutatis in August 1992-March 1993 at the Crimean
   Astrophysical Observatory

The results of the astrometrical and photometrical observations of (4179)
Toutatis at the Crimean Astrophysical Observatory obtained in the time of the
approach of the asteroid are presented and discussed.

Forty-nine astrometrical positions were determined with the Zeiss double 40-cm
astrograph. Measured coordinates of the asteroid are referred to the PPM
Catalogue system. The precision of positions is about 0.5 arcsec. The
observations show that the orbital elements of Toutatis from MPC 21085 need
very little correction.

The photometrical observations of Toutatis in UBVRI system were carried out
using a high-sensitivity television complex attached to the 0.5-m meniscus
telescope. The observed brightness of the asteroid was in the range of 9-19
magnitudes. The exposure time changed from 6 sec up to 6 min. The color
variations during the night observations were recorded. The obtained
photometrical data may be used for determination of the rotation period.


Dahl O. C.   Aksnes K.
   Comet and Asteroid Astrometry with the Nordic Optical Telescope (NOT): A
   Progress Report

A program for astrometric charged-couple device (CCD) observations of comets
and asteroids has been initiated with the 2.5-m Nordic Optical Telescope on La
Palma. This paper reports on 92 observations of 2 comets and 23 asteroids and
assesses the astrometric accuracy of the observations.


Dahlgren M.
   An Orbital Evolution Study of the Hilda Asteroids

The orbital evolution of the Hilda asteroids, which is a group of asteroids
situated in the 3:2 mean motion resonance with Jupiter at 3.97 AU, have been
studied. Numerical integrations of all 52 numbered Hildas (as of January 1992)
have been performed. The 15:th order RADAU integrator (Everhart, 1985) and a
Jupiter to Neptune solar system modell have been used in the 600 000 years
forward and backward integration.

All 52 Hildas have very stable orbital evolution and no indication at all of
chaotic evolution is found. All Hildas except three are protected againt close
encounters with Jupiter by the libration of the critical argument around 0
degrees. The best protected asteroid, 3415 Danby, never comes closer to
Jupiter than 1.94 AU.

The three asteroids which have a circulating critical argument (334 Chicago,
1256 Normannia, 4196 Shuya) avoid close encounters with Jupiter because they
have lower eccentricity and the apsidal lines of Jupiter and the asteroid are
always aligned when the asteroid have its largest eccentricity. Eventhough,
334 Chicago comes as close as 1.06 AU from Jupiter, it do not come close
enough to have a chaotic evolution.

The result shows that the Hilda asteroids can reside for more than 10^6 years
in the 3:2 resonance, probably they are stable for much longer spans of time
because none of the Hildas show any sign of chaotic behavior.

References. Everhart, E., 1985, in Dynamics of Comets: Their Origin and
Evolution, Proceedings of the IAU Coll. 83, eds. A. Carusi. G. B. Valsecchi,
Reidel, Dordrecht, p. 185.


Dahlgren M.   Lagerkvist C.-I.   Lahulla J. F.
   CCD Photometry of Hilda Asteroids

The Hilda asteroids are an isolated group of asteroids in the outer parts of
the asteroid belt, they are situated in the stable 3:2 resonance with Jupiter
at 3.97 AU. We are performing an observational study of the Hilda asteroids
including both photometry and spectroscopy.

New results on the photometric part of the study will be presented.


Davis D. R.*   Ryan E. V.   Farinella P.
   Asteroid Collision Studies: Results Using Scaling Laws From Hydrocode Models
   and Dimensional Analysis

Recent development of a numerical hydrocode including finite strength effects
(Melosh et al., 1992) and the application of this code to problems of the
collisional fragmentation of asteroids has produced a new scaling law for the
specific energy needed to break up asteroids of different sizes. This work
predicts a more rapid weakening of asteroids with size in the strength regime
than did the earlier scaling theory of Housen and Holsapple (1990) which was
based on dimensional analysis but employed many of the same assumptions as did
the hydrocode algorithm. In Fig. 1, we present results from these two approaches
for basaltic targets. The dimensional analysis based law scales as D^-1/4, while
that based on the hydrocode calculations varies as D^-0.4, which predicts
significantly weaker (i.e., less collisional energy is required to fracture the
body) asteroids in the intermediate size range.

One important test for a scaling law for collisional disruption is whether it
produces results in collisional modeling programs that are consistent with
observations about the real asteroid population. For example, do plausible
collisional scenarios lead to the preservation of Vesta's basaltic crust over
the age of the solar system? Does the size distribution match that of the
present asteroid belt?

In order to test models generated using both the hydrocode calculation and the
Housen and Holsapple scaling law, we have incorporated each of them into our
program for studying asteroid collisional evolution. Cases were then run using
the same initial population of asteroids and collisional parameters, varying
only the choice of the scaling law between the runs. We have explored different
starting populations and choices of the critical collisional parameters,
including some results derived from the hydrocode calculations.

We will summarize these results and discuss implications of the different
scaling laws for the collisional history of asteroids based on our simulations
of asteroid collisional evolution.

References: Housen K. and Holsapple K. (1990) Icarus, 84, 226-253; Melosh J. H.
et al. (1992) JGR, 97, 14735-14759.

Figure 1, which appears here in the hard copy, shows hydrocode results for the
dependence of critical specific energy Q* on target size for basalt, and weak
and strong cement mortar. Housen and Holsapple's (1990) nominal scaling law for
basalt (solid curve) and energy scaling for basalt (solid line) are also shown.
Impact velocity is kept constant at 1.65 km/s.


De Angelis G.*
   Spin, Pole and Shape Determination for Sixty Asteroids

Introduction. The determination of spin axis and shape is well known to be
fundamentally important for studies about the rotational and physical
properties of asteroids. In particular, knowledge that the pole coordinate
distribution is random or not could indicate the probable non-Maxwellian
distribution of asteroid spin axes (see, e.g., [1]), while the distribution in
terms of size and shape could place important constraints on the theories
about the collisional history of some individual asteroids, of asteroid
families, and of the asteroid population as a whole (see, e.g., [2], [3], [4],
[5]). Many kinds of methods have been developed to determine pole coordinates.
In this paper a composite EA method is applied to asteroid lightcurve data
taken from literature.

Results for rotational and shape parameters (P(sub)sid the sidereal period,
lambda(sub)p and Beta(sub)p the pole coordinates, a/b and b/c the axial
ratios, Beta(sub)A the phase angle coefficient, and the sense of rotation)
have been obtained for sixty asteroids : for many of them it has been the
first pole and shape determination, for many of them the shape parameters or
the sidereal period and the sense of rotation have been obtained for the first
time, for some of them the solution parameters have been substantially
improved, for some of them the solution symmetry ambiguities present in
asteroid pole solutions have been removed for the first time, and the value of
the obtained parameters for already well-known asteroids are generally in
agreement with those obtained by other authors.

Method. The method used in the present work is characterized by the
combination of two complementary sources of information. First the lightcurve
amplitude is compared with the light variation of a smooth, featureless
triaxial ellipsoid rotating about its shortest axis, with geometrical
scattering and with a semiempirical phase angle law (see [6] and [7]). This
part of the analysis is known as the A method, and from this part alone the
shape and the pole coordinates can be derived. The other part of the analysis
is called the E method, and it is based on the fact that the changing relative
geometry of the Sun, Earth and the asteroid (see, e.g., [6]) as they move in
their orbits causes variations in the observed synodic period of rotation. The
actual size and sign of these variations depend on the orientation of the spin
axis and the sense of rotation, thus in principle the coordinates of the pole
can be deduced by studying the changes in the synodic period. The E method is
not based on any asteroid model, so from it no shape information can be
derived, but sidereal period and sense of rotation can be obtained. In the
present work, the E analysis has been performed with the "Photometric
Astrometry" method ([8] ; [9] ; [10]), in the way used by Michalowski [11].

With this composite EA method the solutions are obtained with a simultaneous
least square fit on both the E and A parts, with no trial pole solutions, so
bypassing the problems given by each method taken alone.

Results. Pole and shape solutions have been obtained with the method shown
above for 60 asteroids:
5 Astraea, 6 Hebe, 7 Iris, 8 Flora, 9 Metis, 15 Eunomia, 16 Psyche, 19
Fortuna, 21 Lutetia, 22 Kalliope, 29 Amphitrite, 32 Pomona, 39 Laetitia, 41
Daphne, 43 Ariadne, 44 Nysa, 45 Eugenia, 51 Nemausa, 55 Pandora, 63 Ausonia,
65 Cybele, 79 Eurynome, 83 Beatrix, 87 Sylvia, 88 Thisbe, 107 Camilla, 121
Hermione, 125 Liberatrix, 129 Antigone, 130 Elektra, 135 Hertha, 173 Ino, 196
Philomela, 201 Penelope, 216 Kleopatra, 238 Hypatia, 243 Ida, 250 Bettina, 281
Lucretia, 337 Devosa, 349 Dembowska, 354 Eleonora, 372 Palma, 389 Industria,
511 Davida, 516 Amherstia, 532 Herculina, 584 Semiramis, 624 Hektor, 683
Lanzia, 694 Ekard, 704 Interamnia, 747 Winchester, 776 Berbericia, 804
Hispania, 951 Gaspra, 1556 Icarus, 1620 Geographos, 1685 Toro and 1862 Apollo.

For many of these asteroids (83 Beatrix, 121 Hermione, 173 Ino, 238 Hypatia,
243 Ida, 281 Lucretia, 372 Palma, 389 Industria, 516 Amherstia, 683 Lanzia,
704 Interamnia, 747 Winchester and 776 Berbericia) this is the first spin,
pole and shape determination, for some of them (8 Flora, 51 Nemausa, 1566
Icarus, 1862 Apollo and in some aspects 1620 Geographos and 1685 Toro) this is
the first shape determination, for 8 Flora this is the first determination of
sidereal period and sense of rotation, for 1566 Icarus the sense of rotation
has been determined for the first time, and for many of them (8 Flora, 9
Metis, 21 Lutetia, 43 Ariadne, 87 Sylvia, 196 Philomela, 201 Penelope, 624
Hektor, 951 Gaspra, 1566 Icarus and in some aspects 532 Herkulina and 1862
Apollo) this one is the first pole solution resolved from ambiguities (see,
e.g. [12]). For sake of brevity it is not possible to describe the solutions
for each individual asteroid in detail. The values of the parameters obtained
for other already well-known asteroids are in close agreement with those by
other authors.

Conclusions. Solutions for parameters for asteroid rotation and shape have
been obtained for sixty asteroids. All the parameters (P(sub)sid,
lambda(sub)p, Beta(sub)p, a/b, b/c, Beta(sub)A and sense of rotation) to
describe the rotational and shape properties of an asteroid have been derived
with a technique linking together the well-established E and A methods. The
results have been strongly encouraging for further applications to a much
larger sample of objects, and for possible future developments of the
mathematical procedure.

References. [1] Harris A.W. and Burns J.A.,Icarus 40, 115-144 (1979).
[2] Tedesco E.F. and Zappala' V., Icarus 43, 33-50 (1980). [3] Dermott, S.F.,
Harris A.W. and Murray C.D., Icarus 57, 14-34 (1984). [4] Farinella P.,
Paolicchi P. and Zappala' V., Astron. Astrophys. 104, 159-165 (1981).
[5] Lagerkvist C.-I.. In Highlights of Astronomy, Vol.6 (R.M.West, Ed.),
pp.371-376. Kluwer, Boston, 1983. [6] Magnusson P., Icarus 68, 1-39 (1986).
[7] Zappala', V., Cellino A., Barucci M.A., Fulchignoni M. and Lupishko D.F.,
Astron Astropys. 231, 548-560 (1990). [8] Gehrels T., Astron. J. 72, 929-938
(1967). [9] Taylor R.C.. In Asteroids (T.Gehrels, Ed.), pp.480-493. Univ.of
Arizona Press, Tucson, 1979. [10] Taylor R.C. and Tedesco E.F., Icarus 54, 13-
22 (1983). [11] Michalowski T., Acta Astron. 38, 455-468 (1988). [12] Zappala'
V. and Knezevic Z., Icarus 59, 436-455 (1984).


Debehogne H.*
   Theorem of the Minimum

In verymuch problems, particularly in the reductions of astrometrical plates,
rectangular systems of equations (with more equations that unknowns) must be
solved. The effect (error-effect) of an error acting upon an independent term
(upon a star, in the reductions) is minimum at the center of gravity of the
independent terms (at the center of gravity of the stars, in the reductions)
only if the degree of the algebraic rectangular sytem (if the degree of the
bijection between sky and plate, in the reductions) is odd. If the degree is
even, the minimum is attained in all and each points of a circle centered in
this center of gravity, which is a relative maximum. The value of the radius
of this circle depends of the degree of the system (of the bijection, in the
reductions) and of the number of equations (of the number of stars, in the
reductions). Thus, the bijections of an odd degree must be taken (choosen) if
we want the maximum of accuracy at the center of the astrometrical plate.


Debehogne H.*
   Theorem of the Proper Direction of the Elements of an Orbit

We consider the effect upon the orbital elements of a systematical error
acting upon the observations. This error-effect is linear in function of the
absolute value of the error and sinusoidal in function of the direction of
this error. Thus, the zeros of the sinusoidal function give the direction with
an effect equal to zero (proper direction). This theorem must be taken into
account in the cases of the existence of a systematical error in the stars
catalogues, more accurately when the determination of such a systematical
error would have been performed.


Denchev P.   Shkodrov V.   Ivanova V.
   Light Curves of Some Asteroids

A new observational programme has been started at Department of Astronomy,
Bulgarian Academy of Sciences: photometry of asteroids, aimed determination of
their spin vectors. For this purpose and taking account possibilities of the
used telescope several asteroids were selected. First observations were
carried out with the electrophotometer at 60 cm Cassegrain telescope of
Belogradchik Observatory. In this paper we present the obtained light curves
of the observed asteriods.


Dikova S.*
   Statistics of Asteroids

A statistical investigation of asteroids has been carried out. The
distributions by orbital elements are presented. The structure of the
Asteroidal Belt is discussed.


Di Martino M.   Mottola S.
   Lightcurves and Rotational Periods of 1981 Midas and 1993 BX3: Two Large
   Amplitude Apollo Asteroids

We present photoelectric and CCD lightcurves of the Apollo asteroids 1981
Midas and 1993 BX3, a newly discovered object, obtained at the European
Southern Observatory during their apparitions in 1992 and 1993, respectively.

From the preliminary analysis of the collected data we have determined the
rotational period of these objects. The large lightcurve amplitude shown by
both asteroids is an indication of their elongated shape.


Dinev C.
   Hydrodynamical Approach to the Stability of the Asteroidal Belt

It is well known that the hydrodynamical equations are applied for
investigation of a given system, if in this system the Knudsen number (Kn) is
less than 0.2. In the present paper the Kn is calculated on the basis of the
commonly accepted estimations of the body's number in the asteroidal belt and
the mass of the asteroidal ring. It is shown that the asteroids satisfy the
above mentioned condition. The behaviour of the asteroids is treatted as a
thin gas disk. The hydrodynamical criteria of the secular stability are
developed. They are very simple and describe the Kirkwood's gaps and
asteroidal families.


Erikson A.   Lagerkvist C.-I.
   Pole Orientations and Shapes of Asteroids

Pole orientations and shapes of asteroids are fundamental parameters for
studying evolutionary and collisional processes in the asteroid belt. There are
reasons to expect that asteroid spin vectors and shapes are correlated to a
number of parameters such as size, heliocentric distance, taxonomic type, and
family membership and that an analysis of these relations will yield important
new cosmogonical information.

In order to study these relations a large number of spin vectors have to be
determined and we have an ongoing observational campaign, mainly conducted with
the ESO 1-m telescope, to obtain the necessary data. Some first results will be
presented.


Floria L.*
   A DS Formulation for Hyperbolic-type Motion Along Small-Pericentre-Distance
   Orbits Under Oblateness Perturbations

The present research was originally inspired by Hori's article "The Motion of a
Hyperbolic Artificial Satellite around the Oblate Earth" (1961, Astron. J., 66,
258-263) in which a canonical set of six Delaunay-like variables was derived and
which is applicable to the study of hyperbolic orbital motion; in that paper
this set was used to work out a first-order analytical solution to the zonal
oblateness problem of motion of a hyperbolic satellite. His way of proceeding
was based on an appropriately devised modification of the von Zeipel
perturbation method leading to the determination of the first-order terms of the
generating function for a near-identity canonical transformation with the help
of which the first-order variations due to the flattening of the central body
were calculated.

In a parallel way, this paper proposes a simple mathematical and dynamical model
for the approximate investigation of close-to-pericentre orbital motion of a
nonrecurrent small mass within the framework of positive-energy two-body
problems under the perturbing influence emanating from the major term of the
zonal gravitational potential due to an oblate spheroidal primary. The approach
taken in the present study resorts to the following main aspects of the question
of investigating some of the principal features about the same basic J(sub)2-
problem:

* True-anomaly DS-type elements are derived in the 8-dimensional, extended phase
space that are applicable to hyperbolic two-body motion

* The above dynamical problem is then formulated in such variables

* @ith the aim of obtaining an approximate analytical solution to the considered
problem by accomplishing a first-order integration of the new Hamiltonian, the
elimination of the first-order trigonometric terms occurring in the
corresponding, transformed Hamiltonian is undertaken in the extended phase space
by adapting and taking advantage of the ideas expounded in the aforementioned
paper by Hori.

To be precise, an infinitesimal contact transformation controlled by a
generating function is introduced whose first-order part is specified by
integrating the first-order equation of the von Zeipel procedure and replacing
the customary periodicity conditions on the unperturbed Kepler problem (holding
in bounded states of motion in the two-body problem) by the device of imposing
conditions at r = infinity.

To fully achieve the development and representation of the sought solution, the
first-order deviations from the Keplerian reference orbit due to the J(sub)2-
disturbing influence are derived with the help of the above generating function
of the transformation according to the standard procedure for setting up the
perturbation equations.

The approach taken in the present research allows one to reformulate the theory,
previously built up by Hori in employing hyperbolic Delaunay variables, in terms
of hyperbolic DS elements that use the true anomaly as the independent variable.


Gil Hutton R.   Licandro J.   Gallardo T.
   Pole Coordinates of the Asteroid 338 Budrosa: Implication for the Asteroidal
   Family 124

Using two standard methods we found that the possible north poles of the
asteroid 338 Budrosa are (152 degrees; +24) and (321 degrees; +33). The result
for this member of Family 124 agrees very well with the pole coordinates of
the biggest asteroid in the Family, 349 Dembowska, and if we consider the
agreement in their rotational periods, we have an important point to confirm
the physical reality of this family using a collisional scenario.


Harris A. W.*
   Empirical Models of Asteroid Phase Relations

The empirical magnitude model currently in use, the H-G system, is based on
the premise that the phase curve of any asteroid, Phi (alpha), can be
represented, in intensity units, as the linear sum of two intensity functions,
Phi(sub)1(alpha) and Phi(sub)2(alpha):

Phi(alpha)=a(sub)1Phi (sub)1(alpha)+a(sub)2Phi (sub)2(alpha),

where a(sub)1 and a(sub)2 are constants for a given object. In magnitude
units, we have:

V(alpha)=-2.5 log [a(sub)1Phi(sub)1(alpha)+a(sub)2Phi (sub)2(alpha)] = H-2.5
log [(1-G)Phi(sub)1(alpha)+GPhi(sub)2(alpha)],

where the second form is the H-G phase relation:
H=-2.5log(a(sub)1+a(sub)2); G=a(sub)2/(a(sub)1+a(sub)2).

In this form, the two reference functions, Phi(sub)1(alpha) and
Phi(sub)2(alpha), are normalized to be equal to 1.0 at zero phase angle.
Underlying this formalism is the hypothesis that singly scattered and
multiply-scattered components of the returned light from any surface obey the
same phase relations respectively, and that all that differs among phase
curves of actual objects is the ratio of one component to the other. While we
had in mind singly and multiply scattered light for Phi(sub)1(alpha) and Phi
(sub)2(alpha), the linear relations are equally valid for any arbitrarily
defined functions which are themselves a linear combination of
Phi(sub)1(alpha) and Phi(sub)2(alpha). Thus we can take for our reference
functions two observed phase relations of actual asteroids, and attempt to use
them to fit other asteroid phase data. We have done this using for Phi
(sub)1(alpha) and Phi(sub)2(alpha) smooth curves drawn through the data of
three very well-sampled phase curves of low, medium, and high albedo
asteroids: 47 Aglaja, p(sub)V=0.072; 113 Amalthea, p(sub)V=0.27; 44 Nysa,
p(sub)V=0.49. Initial attempts, using various pairs of the three curves, have
been discouraging, probably because they are in fact the set of curves which
clearly demonstrate the invalidity of the H-$ hypothesis. Indeed, when any two
are used to try to fit the data of the third object, the fit is poor.

The above formalism suggests a logical extension of the H-G system to three or
more parameters. One could consider the phase relation of an object to consist
of the linear sum of three (or more) components:

Phi(alpha)=a(sub)1Phi(sub)1(alpha)+a(sub)2Phi(sub)2(alpha)+a(sub)3Phi
(sub)3(alpha).

Since we already conveniently have three precisely defined empirical
functions, we can try fitting other phase data to a system of all three
intensity functions. I have made some preliminary investigations of this, and
found that for most data sets covering a limited phase angle range, the three-
parameter fit is good, but unstable because the three functions are so highly
correlated over limited phase ranges. What the exercise has demonstrated is
that it is not possible to predict magnitudes far outside the range of
observation, because of this high correlation.
Another variation is to consider linear sums in magnitude space, i.e.:

V(alpha)=a(sub)1V(sub)1(alpha)+a(sub)2V(sub)2(alpha)+...,

where V(sub)1(alpha),V(sub)2(alpha), ... are the same (empirical) functions as
Phi(sub)1(alpha), Phi(sub)2(alpha),..., but in magnitude units rather than
intensity units, that is, V(sub)i(alpha)=-2.5log [Phi(sub)i(alpha)]. Since
V(sub)1(0)=V(sub)2(0)=0, a(sub)1 and a(sub)2 are independent of H, so the
system can be a three-parameter system (4 parameters for a 3-function system);
or one can impose the constraint that a(sub)1+a(sub)2=1 reducing the system by
one parameter, but it is not fundamentally required. Some of these variations
have also been investigated in a preliminary way, using both two and three
function versions of the above formulation. Results of these fits for a number
of well sampled asteroid phase curves will be presented.


Howell E. S.   Merenyi E.   Lebofsky L. A.
   A Neural Network Asteroid Classification Based on Water of Hydration

The hydration state of surface minerals has proven to be an extremely useful
tracer of an asteroid's thermal history (Jones et al., 1990, Icarus, 88, and
Lebofsky et al., 1990, Icarus, 83). The distribution of hydrated minerals is a
probe of the metamorphic history of the asteroid belt, since the alteration
process producing hydrated minerals requires the presence of liquid water.
Over the last five years, we have obtained extensive asteroid observations in
the 3-micrometer spectral region to detect the presence of water. The strong
absorption band at 3 micrometers due to hydrated minerals is not easily masked
even by abundant opaque materials on low albedo objects. Using this
compositional information, we have obtained a cluster structure of those
asteroids for which data on the surface hydration state is available. The
technique for cluster structure determination is a Kohonen-type self-
organizing neural network. Previously, we have used this technique
successfully to extend the Tholen asteroid taxonomy (Tholen, 1981, PhD
dissertation), by increasing the spectral range from the visible (0.3-1.1
micrometers) to the near-infrared (0.3-2.5 micrometers). Now, we increase that
spectral window further to include the hydration feature at 3 micrometers.
With this additional compositional information, we hope to clarify the
relationship between taxonomic classes and asteroid surface composition in an
attempt to better understand the true compositional distribution of the
asteroids. Although the Tholen taxonomy has proven extremely valuable as a
guide to compositional distribution, there are many examples of compositional
diversity within asteroid classes. The S-type asteroids show a large range of
pyroxene and olivine compositions and relative abundances (Howell et al.,
1993, in press, and Gaffey et al., 1993, Icarus, submitted). Among the C-type
asteroids observed to date only about half are hydrated (Jones et al., 1990).
Based on their featureless, red-sloped spectra and moderately high albedo (A =
0.15-0.25), M-type asteroids have been associated with Fe-Ni metal. However,
at least two of these objects show water of hydration, inconsistent with an
Fe-Ni composition. E-type asteroids show a spectral similarity with the
enstatite achondrites or aubrites, but E-type asteroid 44 Nysa shows a
hydration band, inconsistent with this composition. Thus, we see a large
degree of compositional diversity within these taxonomic classes which are
spectrally uniform in visible and sometimes even near-infrared spectral
ranges. Extending the spectral range coverage for a larger sample of asteroids
will allow us to develop a more compositionally meaningful taxonomy. We can
then more fully explore the compositional diversity and spatial distribution
of the asteroids.


Ipatov S. I.
   Migration of Bodies to Earth

It is believed that bodies can migrate to the Earths orbit from the Kirkwood
gaps, from the inner part of the asteroid belt, from the belts beyond Neptune,
and from the Oort cloud. Our obtained results show [1,2] that even more bodies
from the 5:2 Kirkwood gap might reach the Earths orbit than from the 3:1 gap.
Contemporary bodies can migrate to the gaps due to the gravitational influence
of the largest asteroids and due to collisions with other asteroids.

The variations in orbital elements of three gravitationally interacting
objects with masses equal to that of Pluto showed that the eccentricity of one
of these objects could reach 0.4 during the existence of the solar system [3].
This result indicates the possibility of migration of some bodies from the
belt beyond Neptune to Neptune's orbit if the masses of the largest bodies in
the belt exceed the mass of Pluto. The results of computer simulations show
that more than 1% of the bodies can reach the orbit of Earth from the region
of Uranus and Neptune [4,5].

The orbital elements of Earth-crossing objects change significantly before
they impact planets. The obtained results show that the average time up to the
collision with Earth is equal to 10^8 years (but due to the ejection by
Jupiter the lifetime of these objects may be 10 times less). The mass
distribution of Earth-crossing objects may be the same as that for the main
asteroid belt. The average time when some object with a diameter d > D impacts
Earth may be equal to 0.5(D/D(sub)o)^1.8 where D(sub)o = 1 m.

We have made a series of calculations of evolving disks initially consisting
of planets and Mars-crossing objects. In some runs objects also were Earth-
and Jupiter-crossers. The gravitational influence of planets was taken into
account by the action spheres method, i.e., two two-body problems were
considered. We found that some objects migrated to Jupiter, which ejected them
into hyperbolic orbits, and some objects migrated inside the solar system. At
the initial eccentricities e(sub)o = 0.7$ and initial semimajor axes a(sub) =
3.1 (the objects coming from the belt beyond Neptune) or a(sub) = 2.82 (the
5:2 Jovian resonance) when half of the initial objects had been ejected from
the solar system, one-sixth of all initial objects were Earth-crossers but
were not Jupiter-crossers. More than 1% of all initial objects impacted Earth
during evolution. The number of objects that impacted Venus and Mercury was
not significantly less than that for Earth. At the last stages of disk
evolution, some objects were only Mercury- and Venus-crossers. At e(sub)o =
0.5 and a(sub)o = 1.7 the number of objects that impacted Earth was several
times larger than at e(sub)o = 0.7. If initial objects crossed only the orbit
of Mars, then the process of migration was much slower and more objects
impacted Mars than Earth.

References: [1] Ipatov S. I. (1992) Icarus, 95, 100-114. [2] Ipatov S. I.
(1992) Sol. Syst. Res., 26. [3] Ipatov S. I. (1988) Kinematics Phys. Celest.
Bodies, 4, 76-82. [4] Ipatov S. I. (1987) Earth, Moon, Planets, 39, 101-128.
[5] Ipatov S. I. (1993) Sol. Syst. Res., 27.


Krivova N. V.   Yagudina E. I.   Shor V. A.
   The Orbit of (4179) Toutatis from Optical and Radar Data

The technique used in the present investigation for processing both optical
and radar observations was described earlier [1]. The technique was there
applied to the data for asteroid (1862) Apollo and a comparison with the
results of other researchers [2] was made. In this paper the technique was
used for orbit determination of (4179) Toutatis on the base of all available
data (optical observations from 1934 to October 1992 and radar data, delay and
doppler, obtained in 1992 during asteroid's approach the Earth). The total
number of optical observations is equal to 320 and that of radar 28 (the
latter were kindly sent us by D.K. Yeomans).

The addition of 28 radar observations to optical ones reduced the errors of
orbital elements as follows: more then 20 times for the major semiaxis, 3
times for esin-pi and ecos-pi, and 3 times for the mean longitude. The
normalized RMS for different kind observations are: optical - 1.35, delay -
1.74, Doppler - 1.76, total - 1.35. An attempt to determine corrections to the
asteroid radius and to the zero-point of the reference catalogue was made.

References. 1. Krivova, N.V., Shor,V.A, Yagudina, E.I. -Orbit determination of
asteroids Apollo and Toutatis by optical and radar data. -In press. Workshop
on celestial mechanics and astrometry, Valencia, October, 1992.
2. Yeomans, D.K., et al., -Asteroid and comet orbits using radar data. Astron.
J., 103, 303-317, 1991.


Kunz J.   Trieloff M.   Jessberger E. K.
   The Collisional History of the Eucrites Asteroid

Since it was demonstrated that eucrites (achondrites, which are very similar
to terrestrial basalt) do not derive from the moon, the question remains,
which asteroid has suffered an extensive basaltic volcanism during an early
stage of the solar system and thus would be the parent body of eucrites and
the petrologically related howardites and diogenites. Spectrophotometric
analysis [1] suggests a link with Vesta 4 though there are some dynamical
problems to explain the material flux from Vesta to Earth. Recent results
demonstrate the existence of some small asteroids that are most probably
"chips off of Vesta" [2] and may be the intermediate source for the eucrites.
While Sm-Nd ages of ~4.5 Ga [3] most probably date the crystallisation of the
parent body's crust it is expected that by ^40Ar-^39Ar analysis the duration
of secondary processes like brecciation, reheating, and recrystallisation
caused by collisional events can be obtained as the K-Ar system can rather
easily be disturbed or reset.

Here we report on a high resolution ^40Ar-^39Ar study of matrix samples and
individual clasts from five Eucrites: Stannern, Pasamonte, ALH 76005, ALH
78132 and ALH 80102. Most age spectra are complex: In general the apparent
ages increase with increasing degassing temperature and in some cases are
additionally disturbed by ^39Ar recoil. So the low temperature ages provide
estimates of the time of the last thermal disturbing events while the high
temperature ages give lower limits for the last total degassing of the rocks.
Another general feature are the K/Ca spectra which for all samples are almost
constant for the first 70% of ^39Ar release and then level of by about one
order of magnitude.

Our results are in good agreement with those of other workers [4-7] and
support a period of cataclysmic bombardment on the HED parent body combined
with total resetting of the K-Ar system 4.4-3.9 billion years ago which was
followed by a period of less intense impact induced metamorphism causing
partial loss of radiogenic ^40Ar 3.9-2.8 billion years ago.

[1] McCord T.B., Adams J.B. and Johnson T.V. (1970) Science 168, 1445-1447 [2]
Binzel R.P. and Xu S. (1992) Bull. Am. Astron. Soc. 24, 939 [3] Warren P.H.,
Haack H. and Rasmussen K.L. (1991) J. Geophys. Res. 96,   5909-5923 [4]
Podosek F.A. and Huneke J.C. (1972) Geochim. Cosmochim. Acta 37, 667-684 [5]
Burghele A. (1980) Ph.D. thesis, Universitdt Mainz [6] Kaneoka I. (1981) Mem
.Nat. Inst. Pol. Res. Spec. Issue 17, 177-188 [7] Bogard D.D. and Garrison
D.H. (1991) Meteoritics 26, 320


Kuzmanoski M.   Knezevic Z.
   Perturbations of (1) Ceres on (3643) 1978 UN2

While systematically searching for the close encounters between asteroids in a
recent past, we found many cases of close encounters with the largest bodies.
In several such cases a significant change of motion of perturbed body
occurred after the approach. Here we would like to present the case of
encounter between (1) Ceres and (3643) 1978 UN(sub)2, in which the changes of
this latter asteroid's heliocentric equatorial coordinates, due to the
encounter, were quite large. The asteroids reached the minimum mutual distance
of 0.0083 AU on September 6, 1971, with a relative velocity of 2.76 km/sec. In
order to analyze this case in more detail, we performed a numerical
integration of perturbed asteroid, once under the perturbation of all the
major planets except Pluto, and the other time adding in the perturbing effect
of Ceres (assuming its mass to be 5.9 x 10^-10 solar mass), in a time span
starting 1000 days before the epoch of a close approach and ending up at the
year 2000.0.  The obtained differences of right ascensions are presented in
the figure below.  Providing the precise positions of (3643) are derived from
the past and the future observations (the next opposition is on December 1,
1993), the mass of (1) Ceres can be redetermined and hopefully improved.

The figure appears here in the hard copy.


Kwiatkowski T.
   Analysis of the Time-Shift Effect in Asteroid Lightcurves

A time shift, used in photometric astrometry, is an interval of time between
the moment at which the maximal cross-sectional area of the asteroid faces the
Earth and the moment at which the observer measures the maximal brightness of
the asteroid light variations.

Up till now two methods has been proposed to calculate time shifts. According
to Taylor (1979), time shifts can be obtained as half the difference in
asterocentric longitudes of the Earth and the Sun. Another approach, proposed
by Harris (1984) and Magnusson (1983), consists in calculating time shifts and
a difference in asterocentric longitudes of the Earth and the phase angle
bisector. In most cases both methods give the same values for time shifts.
There are geometries, however, at which significant differences appear.

To analyse this problem, a set of numerically modelled lightcurves was
obtained for different aspect, obliquity, and phase angles. The models used in
calculations had shapes of triaxial ellipsoids with different axis ratios. For
each lightcurve, a time shift was obtained, according to the general
definition given above. It was compared to the time shifts, calculated with
analytical formulae. This procedure was repeated with different laws of light
scattering (Lommel-Seeliger's, Bowell-Lumme's, and Hapke's). The results
showed that the time shifts obtained from numerically calculated lightcurves
are generally independent of the axis ratios of the models and of the
scattering laws. Differences between numerical and analytical time shifts
appeared when the rotation axis was in the region between the sub-Earth and
sub-Sun points.

The next step was to check what errors can be made by using the analytical
time shifts in case of more realistic models of asteroids. The most
significant results were obtained with the model of 433 Eros, some of which
are presented in the table below.

Y       M       D       Asp     Obl     PA      Max     Min     PAB     CPA

51      9       25.0    162     97      19      99.0    100.8   172.1   121.6
51      10      4.0     159     108     19      115.2   122.4   166.5   149.5
51      10      11.0    156     113     20      124.2   133.2   165.0   152.3
51      10      15.0    155     115     22      127.8   135.0   163.1   150.1
51      12      13.0    160     130     49      120.6   118.8   93.5    118.5
52      2       11.0    158     283     59      7.2     9.0     8.1     5.6
72      8       4.0     140     332     17      7.2     9.0     10.8    10.0
74      8       22.0    150     92      42      91.8    93.6    175.5   93.3
74      10      18.0    118     120     42      156.6   163.8   165.0   159.1
75      5       3.0     46      257     44      131.4   140.4   168.3   142.5
81      12      18.0    150     213     41      162.0   156.6   155.3   160.8

Explanations: Y,M,D--year, month, and day of one of observations of Eros,
taken from the Asteroid Photometric Catalogue (Lagerkvist et al., 1987).
Asp,Obl,PA--aspect, obliquity, and phase angle (in degrees); obliquity is
defined here as a dihedral angle between the plane of the aspect angle and the
plane, perpendicular to the phase angle; obliquity is measured counter-
clockwise.
Max,Min--time shifts (in degrees) obtained from numerically modelled
lightcurves; Max denotes the time shift of the maximal brightness, Min, of the
minimal; the accuracy of Max and Min is +/- 1 deg.
PAB,CPA--time-shifts (in degrees) obtained from analytical formulae, based on
definitions of Harris (1984) and Taylor (1979), respectively.

References: Harris A. W. et al. (1984) Icarus, 57, 251. Lagerkvist C.-I. et
al. (1987) Asteroid Photometric Catalogue, Consiglio Nazionale Ricerche, Roma.
Magnusson P. (1983) In Asteroids, Comets, Meteors (C.-I. Lagerkvist and H.
Rickman, eds.), pp.77-85, Astronomiska Observatoriet, Uppsala, Sweden. Taylor
R. C. (1979) In Asteroids (T. Gehrels, ed.), pp. 480-493, Univ. of Arizona
Press, Tucson


Kwiatkowski T.
   Determination of Physical Parameters of Near Earth Asteroids

There are several methods for obtaining, from the observed light variations,
physical parameters of asteroids (such as sidereal periods, spin orientations,
triaxial ellipsoid approximation of shapes, H,G parameters of phase curves).
Most of them are successfully used for main belt objects. Earth approachers,
however, are observed at quite different geometries (crucial for the following
disscussion are large phase angles and quick changes of aspect and obliquity
during a single apparition). This makes classical methods difficult to use--in
practice, only Photometric Astrometry can be applied to NEAs without bigger
changes. The other complementary approaches (amplitude-aspect and magnitude-
aspect methods) reveal many problems when used for NEAs. In fact, during the
last decade, only Drummond (1990) used full EAM method to obtain physical
parameters of two Earth approachers, 1580 Betulia and 3908.

In the present work, new elements are introduced into the classical EAM method
to avoid the above-mentioned problems. Amplitudes and magnitudes are
calculated by integration of one of the light scattering laws over the
visible, illuminated surface of the triaxial ellipsoid. As this is a time-
consuming process, only two points of the lightcurve (of maximal and minimal
brightness) are calculated. Rotation angles, which define these points, are
obtained from time shifts. The time shifts are also used for calculating
epochs of lightcurve extrema. This part of the algorithm does not differ from
the classical photometric astrometry, with one exception: there are
geometries, at which numerical time shifts are used instead of the ones
obtained from the analytical formula (see "Analysis of time-shift effect in
asteroid ligthcurves" in this volume for details).

References: Drummond J. D. and Wisniewski W. Z. (1990) Icarus, 83, 349.


Lagerkvist C.-I.   Lindgren M.   Tancredi G.
   A Search for Asteroids and Comets in the Direction of Jupiter

The ESO Schmidt telescope was used during March and April 1992 and 1993 to
search for asteroids and comets in a 16 x 16-degree field centered on Jupiter.
On the 1992 plates no comet brighter than the corresponding point source V
magnitude 21.4 was found, thus giving an upper limit on the number of comets in
the Jupiter family [1].

On the March 1992 plates a total of 4136 asteroid trails were identified coming
from some 1300 asteroids. On the plates from the follow-up observations in April
1992 more than 600 trails were found to be linked to the trails on the March
plates. A comparable number of new asteroids can be expected from the 1993
observations.

We will present final results on the upper limit on the Jupiter family as well
as statistics on the orbits of the asteroids and a comparison with results of
the Palomar-Leiden survey.

References:  [1] Tancredi and Lindgren, Icarus, submitted.


Lagerros J. S. V.   Lindgren M.   Lagerkvist C.-I.   Hahn G.
   Differential CCD Photometry of 1980 Tezcatlipoca, 2368 Beltrovata, 4769
   Castalia, 4953 1990 MU and 4954 Eric

Lightcurves of five near-Earth asteroids were obtained by CCD photometry in May
1992, with the Dutch 0.91 m telescope at ESO, La Silla, Chile. The weather
conditions allowed only differential photometry and a method was developed to
compare the different reference stars when an asteroid was moving too quick for
a single star to be used as a reference for a whole night. A rotational period
of 4^h.08 is determined for 4769 Castalia with a lightcurve amplitude of
approximately 0^m.5. The rotation periods are estimated for 1980 Tezcatlipoca
(~6^h) 2368 Beltrovata (~5^h) and 4954 Eric (~^12h), all of them with a
lightcurve amplitude of about 0^m.9. The 2.5 hours of observations of 4953 1990
MU did not give enough information to indicate any period, although the
amplitude is at least 0^m.3.


Lamberg L.   Lumme K.   Magnusson P.
   Inversion of Asteroid Photometry

Much of the theoretical basis for inversion of photometry to derive asteroid
shapes and/or albedo variegation maps was developed by Kaasalainen et al. (1992)
Astron. Astrophys., 259, 318-332. We have initiated a project aiming (1) to
adjust the method for optimum usage of available asteroid photometry, including
relative photometry and small lightcurve "pieces"; (2) to apply it to a survey
of as many asteroids as possible; and (3) to attempt to draw cosmogonically
interesting conclusions from the results, such as statistics on hemispherical
albedo variegation, departure from ellipsoidal shape, and a better separation of
original objects from collision fragments. We will present our basic
assumptions, inversion technique, test cases, and (hopefully) preliminary
results for some real asteroids.


Licandro J.
   Determination of the Spin Orientation of Asteroids Using the Free Albedo Map
   Method

Results for the pole orientation of the asteroids 31 EUPHROSYNE, 196
PHILOMENA, 338 BUDROSA AND 471 PAPAGENA using the Free Albedo Map Method (FAM)
(Hainaut et al., 1990) are presented and compared with previous poles obtained
by our group using two different amplitude aspect methods.

References Hainaut, O., Detal. A., Ibrahim-Denis, A. and Surdej J.: 1990, in
Asteroids, Comets, Meteors III, C.I. Lagerkvist, H. Rickman, B.A. Lindblad and
M. Lindgren eds., Uppsala Universitet, Sweden, p.87.


Lumme K.   Muinonen K. O.
   A Two-Parameter System for Linear Polarization of Some Solar System Objects

A two-parameter H, G magnitude system for asteroids has been adopted by IAU
Commission 20 in 1985. By this system it is possible to predict magnitudes of
asteroids at some phase angles for which there are no observations. The
ubiquitous feature of asteroid magnitudes is the opposition effect (excluding
the opposition spike) which seems to be well explained by the H, G system. We
are currently working on an analogous system for the linear polarization of
asteroids, planetary satellites and cometary comae. At least our preliminary
results are encouraging and suggest that such a system can be found. It is
effectively a combination of a few trigonometric functions multiplied by a
constant b. The other free parameter is the inversion angle alpha(sub)o of polar
and the other parameter b is clearly inversely proportional to the geometric
albedo. This function also seems to predict polarization with a reasonable
accuracy. We are careful in calculating all the errors and error envelopes
asssociated with the predictions. A preliminary form and fits will be
provided.


Martinez Gonzalez J. M.   Lopez Garcia A.*   Ortiz Gil A.   Chernetenko J.
   Orbits Determination and Star Catalogue Errors

We obtain Ceres orbit taking into account all perturbations and applying
correction of phase effect to observations. More than 3000 observations have
been used and the results agree with other determinations. We have
investigated also the influence of systematic errors of star catalogue on the
observations and its effect on final elements. To that aim, a simulation
procedure has been applied to Ceres observations, including different laws for
error of star catalogue versus spherical coordinates. The best relations have
been applied to real observations of Ceres and other asteroids, in order to
obtain elements and star catalogue corrections. Preliminary results of these
calculations are presented.


Michalowski T.   Krugly Yu. N.   Velichko F. P.
   Photometry, Phase Curve and Spin Vector of 675 Ludmilla

Asteroid 675 Ludmilla has been observed during two apparitions (1965 and 1973) a
synodical period is 7.717 hours. The lightcurves obtained at Kharkov Observatory
1993 oppositions are presented. These observations allowed us to obtain the phas
determine the H and G values of the HG-magnitude system. Using all available lig
have determined the sidereal period, sense of rotation, pole and triaxial ellips
Ludmilla.


Mohamed R. A.   Krugly Yu. N.   Velichko F. P.
   Periods of Two Near-Mars Asteroids: 626 Notburga and 2078 Nanking

Photoelectric observations of the near-Mars asteroids 626 Notburga and 2078
Nanking were carried out during 1992 oppositions. The synodic rotation periods
about 19 hr, 6.56+/-0.01 hr, and the lightcurve amplitudes of 0.20 mag and 1.0
mag were determined for Notburga and Nanking respectively. Observations show
that asteroid 2078 Nanking has a very elongated shape with a/b = 2.5.


Moons M.
   Secondary Resonances Inside Mean Motion Commensurabilities

We study the secondary resonances between the libration frequency of asteroids
at mean motion commensurabilities with Jupiter and one of the two other basic
frequencies of the Sun-Jupiter-asteroid system. For each of these two
fundamental frequencies, we locate the secondary resonances of low order that
appear at low values of the eccentricity of the asteroids. Integrating
numerically the trajectories of the averaged system, we also draw the shape of
the secondary resonances and show the extent of the chaotic zone on surfaces
of section.


Moons M.   Watanabe N.   Henrard J.
   Secular Resonances in the 2/1 Kirkwood Gap

We investigate the three-dimensional long-period dynamics inside the 2/1
Kirkwood gaps by means of a semianalytical perturbation method. Secular
resonances are identified and localized. In particular, a nodal secular
resonance with the frequency associated with the node of Uranus may provide an
explanation for the peculiar orbit found by Wisdom, which starts at low
eccentricity and inclination, meanders to high inclination and moderate
eccentricity, and ends up at high eccentricity and low inclination.


Muinonen K. O.   Bowell E.
   Orbital Uncertainties of Single-Apparition Asteroids

We have applied the two-body linear approximation of our Bayesian orbital
uncertainty analysis (Icarus, in press) to observations of more than 10,000
asteroids observed at one apparition only. We required five or more
observations, and assigned 1.0-arcsecond standard deviations to both R.A. and
Decl. All published high-precision observations were considered, without
regard to their quality, and the epochs of the orbital elements were chosen to
be midway between the first and last dates of observation. The entire analysis
took only 25 minutes on a 4.2-MFLOP computer, thanks to the simplicity and
efficiency of the two-body approximation. The final database included the
epoch, number of observations, observational arc, orbital elements and their
1-sigma uncertainties, several orbital quality metrics, and the covariances of
the orbital elements.

The cross-correlations among the miscellaneous database items were then
investigated. The so-called leak metric (an orbital quality metric: Muinonen
and Bowell, op. cit.) correlated slightly with the number of observations and
significantly with the observational arc. We found an empirical law of the
form theta ~ 5T(sub)arc^-2 (leak metric theta in radians, observational arc T(su
in days) for single-apparition main-belt asteroids. The law is typically
correct to an order of magnitude, which we consider to be intriguingly good,
though it fails to describe the quality of Earth-crossing and distant asteroid
orbits. Imitating Poisson statistics, we then included the number of
observations N(sub)obs, and empirically obtained theta 6T(sub)arc^-2/square root
of N(sub)obs-3 (units as above), valid for single-apparition main-belt
asteroids to within a factor of three.


Nakamura A. M.   Fujiwara A.
   Oblique Impact Disruption: Velocity and Spin Distribution of Fragments

Laboratory experiments of impact disruption have been performed to simulate
large scale collisions among asteroidal bodies. The distribution of velocity
and rotational frequency of fragments are the fundamental outcomes from
collisional disruptions. Recently, we made experiments of basalt and alumina
disruptions with incident angle of 60 degrees to the surface, and of gypsum
disruption with normal incidence (Nakamura and Fujiwara 1990, Nakamura,
Suguiyama, and Fujiwara 1991). It was shown that the fragment velocity in the
center of mass system (CMS) is proportional to -(1/2-1) power of the fragment
size. At a given size of the fragment, the CMS velocity ranges over an order
of magnitude. The velocity of basalt fragments from less oblique impact may
give systematically larger values than those of the others. We have performed
further analysis of the series of disruption experiments of the basalt target.
The incident velocity was fixed at about 3.3 km sec-1, whereas, the incident
angle was varied from 90 to 30 to the surface. The CMS velocity and rotational
frequency of fragments were determined. The overall feature of the size-
velocity and -frequency relation are similar to the previous ones; smaller
fragments have in general higher CMS velocity and frequency. However, it was
found that the largest fragments from highly oblique shots had velocity
smaller than those from nearly normal shots.

References:  Nakamura, A., and A. Fujiwara 1990. Velocity distribution of
fragments formed in a simulated collisional disruption. Icarus 92, 132-146.
Nakamura A., K. Suguiyama, and A. Fujiwara 1991. Velocity and spin of
fragments from impact disruptions: I. An experimental approach to a general
law between mass and velocity. Icarus 100, 127-135.


Naranjo O. A.   Stock J.
   Llano Del Hato Asteroid Survey, Preliminary Results

We have begun an asteroid survey using a 1-m Schmidt camera at Llano del Hato
National Observatory (Merida-Venezuela). The survey is been conducted on the
plane of the ecliptic. To find asteroids we used a blink comparator, to
obtaining positions a PSK-2 in its modality of 5 and 1 micrometers. Mainly we
have used IIa-O plates.

The maximun number of objects found on one plate, of one hour of exposition
with sidereal guiding, has been 28 asteroids. The data allow us to estimate a
distribution of velocities of objects up to 19-20 magnitudes. The positions
have been reported to the MPC. In this work we show preliminary results and a
comparation with other surveys done before.


Nolan M. C.*   Bottke W. F. Jr.   Kolvoord R. A.   Greenberg R.
   Numerical Simulation of Impacts on Small Asteroids: Application to 951
   Gaspra

A numerical hydrodynamic model of a projectile impacting a body, which includes
fracture (Asphaug et al., 1992, LPSC XXIII, 45), allows exploration of the
mechanics of impacts into asteroids. We have considered a spherical target with
the volume of 951 Gaspra (r = 6400 m) consisting either of solid basalt or of
fragmented basalt. In the solid basalt case, assuming a relative velocity of
5300 m/s, an impact deposits regolith over the entire surface, in events large
enough to produce craters 1 km or more in diameter. Any existing surface
regolith jumps ~100 m due to the "jolt" of the impact, which tends to obliterate
small craters, while preserving inhomogeneities in surface composition. The
results indicate that impacts that create larger craters also globally increase
the regolith thickness.

The largest impact modeled (284 m radius projectile) showed marginal
catastrophic disruption of the target: 20% of the target was accelerated to
escape velocity. A basin was formed nearly as big as the asteroid. The interior
of the body was significantly fractured: the largest remaining piece in the
interior rubble had a size of 1000 m. Thus this impact rubblized the
approximately 80% of the portion of the body which did not escape.

Examination of the results of the model suggests that the physical processes are
in the gravity-scaling regime (Housen et al., 1983, JGR, 88, 2485), rather than
the strength regime usually assumed for small targets, due to substantial,
shock-induced fragmentation of the rock before ejecta flow begins. This results
in the low ejecta velocities, which allow regolith deposition.

The steep size-frequency distribution of craters observed on Gaspra requires a
steep impactor size-frequency distribution. In addition, crater erasure due to
"jolting," combined with "cookie-cutter" erasure should cause noticeable
flattening of such a steep distribution ("saturation") in approximately 75 m.y.
The observed steep size distribution of craters requires either an impactor size
distribution steeper than an incremental distribution dN(d) is proportional to
d^-4, or a 500-million-year impact to have occurred approximately 50 m.y. ago.
The latter seems most plausible. This analysis predicts that such an event is
likely on 10% of Gaspra-size bodies. Asteroid 243 Ida (soon to be imaged by
Galileo) and other asteroids to be imaged in the future will improve the
statistics; most should have a flatter crater size-frequency distribution.


Noll K. S.   Zellner B.   Weaver H.   Storrs A.   Spencer J.   Whipple A.
Tholen D.
   Imaging Asteroid 4179 Toutatis with the Hubble Space Telescope

On December 10, 1992, we obtained four images of asteroid 4179 Toutatis when it
was 0.0291 AU from Earth and at 90 degrees solar phase angle using the Planetary
Camera on the Hubble Space Telescope. The images we obtained show evidence of
possible marginal resolution after deconvolution with both stellar and
theoretical point-spread functions, although the data are also consistent with
an unresolved source. We conclude that the maximum illuminated dimension at the
time of our observations was less than 2.6 km for a higly irregular object. A
near-spherical body would have a diameter of less than or equal to 1.9 km. These
results are at odds with groundbased observations of the lightcurve that were
near a local maximum at the time of our observations and with radar reflection
data that indicate two lobes with diameters as much as a factor of two greater
than our upper limit.


Ortiz Gil A.   Lopez Garcia A.*   Martinez Gonzalez J. M.   Yershov V.
   Automatic Measurement of Images in Astrometric Plates

We present our results on the process of automatic detection and measurement
of objects in astrometric plates. To this purpose, we have studied, compared
and developed classical and new algorithms to better determine the parameters
of the images we are working with. The main subjects of this work are: The
comparison of methods for star image enhancement. The use of algorithms for
removing anomalous images and noise. The determination of objects shape and
the discrimination between different kind of images. Application of models to
determine star positions with high accuracy. These methods are applied to real
images of stars and asteroids on astrometric plates and the results compared.


Paolicchi P.*   Verlicchi A.   Cellino A.
   A Semiempirical Model of Catastrophic Disruptions: Recent Improvements

Several improvements to the semiempirical approach to the physics of
catastrophic breakup events (see Paolicchi et al.,1989, Icarus 77,187) have
been recently carried out and are described in the present paper. The main
features of the model which have been significantly changed are concerned with
the problem of the derivation of a set of realistic, non-overlapping
fragments, as well as with a better treatment of the role played by
gravitational effects, when sizeable parent bodies are shattered by
catastrophic collisions. The main physical results obtained by means of the
improved model are discussed, and compared with the evidences coming from
laboratory experiments and astronomical observations. Some problems related to
the possibility of deriving reliable mass distributions of the fragments are
pointed out and extensively discussed.


Pechernikova G. V.   Vityazev A. V.
   Origin of Largest Asteroids' Rotation

The evolution of asteroids can be divided into three stages. In the first
stage, the growth of planetesimals takes place in the region between Jupiter
and Mars. During the second stage, the massive preplanetary bodies penetrate
into asteroids' zone from the region of Jupiter. As a consequence the removal
of a major part of the matter from the asteroids' zone keeps pace with the
increase of relative velocities of bodies, and the accumulation process stops.
The third stage is the one of the postaccumulation collisional evolution of
asteroids.

Is there any evidance of the first stage of evolution of their growth? In our
earlier works [1,2] we suggested a theory of the acquisition of angular
momentum by growing planets and evaluated the probabilities of the prograde
rotation at a given distance from the sun P(sub)+. This theory can be applied
to minor planets too. The comparison of theoretical estimates of the angular
momentum of the largest asteroids acquired during their growth and of the
observational data on rotation parameters of asteroids has shown that the
rotation of the largest of them should be a primordial one. It is obtained in
the accumulation process, and the consequent collisional evolution did not
contribute any essential changes to its value. Moreover, from the comparison
of P(sub)+, for the largest asteroids and the observed distribution of
obliquities of their spin axes (with respect to their orbital planes) it
follows that at the second stage of the evolution of asteroids the processes
of catastrophic disruption and of the removal of the matter from the
asteroidal zone prevailed. The largest asteroids which avoided the
catastrophic collisions with bodies from Jupiter's zone, continued to grow,
and the surface density of matter in their feeding zones effectively decreased
primarily due to the loss of larger bodies. The increase of the relative
velocities of the remaining asteroidal bodies had taken place when the major
part of the matter from the asteroidal zone was already lost.

References: [1] A.V.Vityazev and G. V. Pechernikova (1981) Sov. Astron., 25,
494-499. [2] A. V. Vityazev, G. V. Pechernikova and V. S. Safronov (1990)
Planets of the Earth Group: Origins and Early Evolution. Moscow, Nauka.



Piironen J. O.   Muinonen K. O.
   Backscattering of Light by Snow: Laboratory Experiments

We present photometric measurements of snowball fields carried out by one of us
conditions in the winters of 1979 and 1980 at Aarne Karjalainen Observatory, Kii
snowballs consisted of varying proportions of pure snow (typical particle size 1
extremely black, fine-grained boron carbide (B4C, particle size 10 to 50 microme
albedos ranged from 5% for the darkest to 44% for the pure snow samples. The sma
0.3 deg, and the measurements were carried out in white, unpolarized light using
for asteroid observations.

Almost all the measured phase curves show the kind of sharp opposition effect ob
many planetary satellites, and the asteroid (44) Nysa and (64) Angelina. The opp
phase angles between 1 and 2 deg and, depending on the purity of the snowballs,
0.3 mag brightening at a phase angle of 0.3 deg. It is intriguing that the measu
enhanced with an increased proportion of boron carbide; that is, in darker sampl

Can the measurements be explained by the coherent multiple backscattering mechan
with large particle sizes, the typical free paths for multiply scattered light c
wavelength, and can theoretically result in an extremely narrow, unobservable op
boron carbide is added, the free path becomes shorter, which can broaden the opp
observable widths. However, shadow-hiding due to the dark boron carbide particle
oppositon effect.





Riccioli D.   Blanco C.*   Di Martino M.   De Sanctis G.   Venditti C.
Venditti R.
   Lightcurves and Rotational Periods of Main Belt Asteroids. II

Within the collaborative program between the Istituto di Astronomia
dell'Universita' di Catania and the Osservatorio Astronomico di Torino, aiming
to enlarge the data set of the rotational properties of minor planets, the
photoelectric observations of 8 asteroids are presented.

The B and V lightcurves of 11 Parthenope, 121 Hermione, 136 Austria,  153 Hilda,
352 Gisela, 419 Aurelia, 665 Sabine and 914 Palisana are shown and the results
obtained from their analysis are discussed.


Solovaya N. A.   Pittich E. M.
   Orbital Evolution of Jupiter's 8th Satellite

The orbital evolution of Pasiphae, 8th satellite of Jupiter, has been
investigated. Its osculating orbit for an epoch 1938 October 29 was
numerically integrated within the interval 20,000 years. As commencing data
for the integration process the Grosch's jovicentric elements of the 8th
satellite were used. There are differences between the 8th satellite's motion
in a dynamical model of the solar system consisting of all planets, and in a
model with Jupiter and Sun only. Solar perturbations are presented on changes
of the satellite's osculating elements with time. The difference between the
three-body and n-body solution of the satellite motion is demonstrated by the
changes of the radius vector of the satellite.

The orbit of the satellite is continuously changed due to great Sun
perturbations. Within the investigated period the eccentricity of the
satellite's orbit changes periodically from 0.18 to 0.66, the inclination from
140 degrees to 156 degrees. The orbit is circular. Its perijove rotates with
the period of ~102 years, and the node with ~80 years.

Within the investigated time interval the question of the Hill's stability has
also been studied. The Jacobian constant calculated from positions and
velocities obtained by numerical integration changes less than the order of
1O^-8. It is known that the surface of zero velocity is closed around Jupiter
when the Jacobian constant of the satellite is greater than the Jacobian
constant of the inner point of the libration. In our case, the Jacobian
constant of the satellite is greater than the value of the Jacobian constant
for that outer point of libration, which is nearer to Jupiter. Therefore, the
motion of the satellite can be inside the close surface around Sun-Jupiter
region.

The presented dominant perturbations of Sun to the satellite allow the use of
the intermediate orbit of the averaged elliptic three-body problem for
investigation of the evolutionary changes in the motion of the satellite. The
mean motion of the node and the perijove of the satellite are defined by
analytical formulae, using the osculating non-Keplerian ellipse with moving
node and perijove, and changing eccentricity. The results obtained by this
theory are comparable with those calculated from the numerical integration.


Takagi Y.   Honda S.   Kato M.
   Scaling Law on Fragment Velocities of Catastrophic Impacts

I. Introduction.
Velocity distribution of fragments produced by catastrophic collisions of
planetesimals has significant effect on the protoplanet growth. Recently some
experimental researches to this problem have been performed (e.g., Nakamura
and Fujiwara, 1991; Nakamura et al., 1993; Takagi et al., 1991; 1992). In
these papers the point of discussion was the velocity dependence on the
fragment mass (Nakamura and Fujiwara (1991) suggested that the fragment
velocity is proportional to the reciprocal of sixth-root of mass, whereas
Takagi et al. (1991; 1992) indicated that the fragment velocity is independent
of the fragment mass). Although the clarification of the dependence may
provide some explanation on the fracturing process of catastrophic impacts,
general scaling law on the velocity distribution is required for the
elucidation of the planetary growth problem. In this paper we report some
results on the general law on velocities of fragments produced by catastrophic
impacts.

II. Experimental Methods.
Impact experiments were performed by a single-stage powder gun at Nagoya
University. Projectiles of cylindrical aluminums were impacted to targets of
cubic pyrophyllites horizontally in the velocity range of 428 to 908 m/sec.
Motions of fragments were recorded by a high-speed motion camera in 1500
frame/sec. Four mirrors were set in front of the lens to obtain the
stereographic picture of the fragment motions. The three-dimensional velocity
of each fragment was determined by the least- squares fitting of the three-
dimensional position in the successive frames. Target reconstructions were
performed using recovered fragments.  The distance from the impact point to
the initial position of each fragment was measured.

III. Results.
Result from a single run shows that the fragment velocity, v, negatively
correlates with the distance from the shot point to the initial position of
fragment, r; log (v/v*) proportional to -0.8 log(r/Lp) In this equation the
fragment velocity is normalized by the characteristic velocity of the target
material (Mizutani et al., 1990), v* = Y/Co d, where Y, Co, and d are the
strength, bulk sound velocity, and density of the target, respectively. The
distances are also normalized by the dimension of projectile, Lp. Results of
all runs indicate the correlation with almost the same coefficient (-0.74 to -
0.87). However the average velocity of each run correlates with the
Nondimensional Impact Stress (NDIS) as already shown by Takagi et al. (1991;
1992).

All the results show that the velocity is simply proportional to the shock
wave pressure at the initial position of fragment, P(r).  The least-squares
fitting gives the following relation; log (v/v*) = 0.094 + 0.27 log(P(r)/Y)
where Y is the compressive strength of target material. The pressure was
calculated by the equation; P(r) = Po (Lp/r)^alpha, where Po is the initial
shock pressure calculated by Rankine-Hugoniot equation. The decay coefficient
Cd was assumed to be 3.0.

IV. Summary.
The present experimental results show that fragment velocities are expressed
by a simple equation using the shock wave pressure at the initial position of
a fragment. This equation could not be derived straightforward from the
scaling law using the NDIS parameter (Mizutani et al., 1990). However this
result suggests that the shock wave pressure in a target is the key parameter
for the velocity distribution of impact fragmentation phenomena.

References. Mizutani, H., Y. Takagi, and S. Kawakami (1990). Icarus 87, 307--
326;  Nakamura, A. and A. Fujiwara (1991). Icarus 92, 132-146;  Nakamura, A.,
K. Sugiyama, and A. Fujiwara (1993). Icarus, in press;  Takagi, Y., M. Kato,
and H. Mizutani (1991). Proc. 24th. ISAS Lunar Planet. Symp., 34--38; Takagi,
Y., M. Kato, and H. Mizutani (1992).  Asteroids, Comets, Meteors 1991, 597-600


Vashkov'yak M. A.
   Qualitative and Numerical-Analytic Methods for Investigation of the
   Evolution of Asteroid Orbits

The present paper contains the short survey of the works, executed by author
in 80-th years at the Keldysh Institute of Applied Mathematics, the Russian
Academy of Scienses.

The qualitative investigation of the orbit evolution in restricted circular
three-body problem [1,2] is founded on the integratibility of twice-averged
scheme or Gauss problem. In these works the analysis of the orbit-crossing
conditions is carried out, the typical families of the integral curves are
constructed for different values of the parameters, and the extreme
characteristics of the evaluating orbits are calculated. The generalization of
the method on the concentric coplanar system of N perturbing bodies has
permitted to reveal the same qualitative pecularities of the orbit evolution
of the asteroids not belonging to the main belt [3].

More exact numerical-analytic method for investigation of the asteroid orbits
evolution [4] was created taking into account eccentricities, inclinations,
and the secular perturbations of the planet orbits. By the use of this method,
in particular, the posibility of qualitative change of the evolution of
perihelion argument for the orbit of asteroid 2335 (JAMES) was discovered [5].
It reduses to the transition from libration regime in circulation one and back
over interval of 10^5-10^6 years and gives the clear example of the motion in
the vicinity of the secular resonance. These results were confirmed
qualitatively in [6].

References: [1] Vashkov'yak M. A. (1981) Cosmic Research, 19, 1, 1-10 [2]
Vashkov'yak M. A. (1981) Cosmic Research, 19, 2, 99-109. [3] Vashkov'yak M. A.
(1981) Cosmic Research, 19, 4, 357-365. (Translated from Kosmicheskie
Issledovaniya).[4] Vashkov'yak M. A. (1985) Cosmic Research, 23, 3, 277-287.
(Translated from Kosmicheskie Issledovaniya). [5] Vashkov'yak M. A. (1986)
Cosmic Research, 24, 3, 255-267. (Translated from Kosmicheskie Issledovaniya).
[6] Froeschle Ch., Morbidelli A., Sholl H. (1991) Astron. and Asrophys., 249,
553-562.


Velichko F. P.   Mohamed R. A.   Michalowski T.
   Photometry, Polarimetry and Spin Vector of 10 Hygiea

Asteroid 10 Hygiea has been observed during several oppositions. First estimatio
synodical period gave a value of 18 hours. A paper by Michalowski et al. (1991),
period of 27.63 hours as well as pole coordinates and triaxial ellipsoid model o
Recently, new photometric and polarymetric observations of Hygiea were carried o
Observatory. They allowed us to get new physical parameters for this asteroid.

References:  MichalowskiT. et al. (1991) Astron. Astrophys. Suppl., 91, 53.


Yoshikawa M.   Nakamura T.
   Properties of Near-Miss Among Asteroids

We calculated the orbital motion of about 4500 numbered asteroids, taking into
account the effect of the gravitational attraction among asteroids as well as
the perturbation of nine planets. The calculation is carried out for about 130
years in the future, and such close encounters that distance between two
asteroids is less than 0.01 AU are obtained for this period.

About 12,000 near-misses are obtained by this calculation, and, as expected,
most of them occur in the main belt and near the ecliptic plain. However, this
calculation reveals that near-misses among asteroids take place in a rather
large region of the solar system. The range where near-misses take place is 0.99
< r(AU) < 4.89 and -1.12 < z(AU) < 1.15, where r is heliocentric distance and z
is distance from the ecliptic plain. The near-misses in the inner region of
solar system occur among Apollo-Amor type asteroids, and the outermost near-miss
occurs between two asteroids in the Hilda group. Near-misses among the Koronis,
Eos, and Themis families are also obtained. One of the interesting results is
that, at the inner edge of the main belt, the number of near-misses decreases
steeply, although, at the outer edge, it decreases gradually, and a relatively
large number of near-misses occur outside the main belt (r > 3.2). This result
suggests that collision of asteroids is possible even outside the main belt.

In addition, we analyzed the dependence of near-misses on orbital elements (a:
semimajor axis, e: eccentricity, i: inclination) and mass of asteroids, and we
obtained the following results: (1) The number of near-misses decreases as a
becomes large. This may be caused by the effect of the period of orbital
evolution. (2) The near-miss rate does not depend on the value of e. (3) The
near-miss rate becomes small as i becomes large. This is because the number
density of asteroids becomes smaller if z becomes larger. (4) It seems that
small asteroids are most likely to have near-misses. This is probably due to the
fact that small asteroids observed now have a tendency to have small major axes
and inclinations.


Yuasa M.
   Circular Least Square Approximation for Family-like Groups

A family-like group of asteroids distribute approximately on a circle in the
v-u plane and in the q-p plane, where the v-u plane is defined by the polar
coordinates with radius e (eccentricity) and angle omega (longitude of
perihelion), and the q-p plane is defined by the polar coordinates with radius
sinl (l=inclination) and angle Omega (longitude of node).

Then a new least square method for circularly distributed data is constructed
and is applied to Amor family-like group. By this method, we can obtain the
radius r and the rectangular coordinates (a, b) for the center of the circle,
which make d^2 = Sigma (r - r(sub)j)^2 to be minimized (r(sub)j denotes the
distance of each data from the center of the circle).


Zadunaisky P. E.   Giordano C. M.
   A Regularized, Universal Formulation for the Calculation of Ephemerides of
   the N-Body Problem

In a previous paper [1] we showed that the vectorial equation of motion of the
perturbed two-body problem

r-double dot + k^2(M +m)r/r^3 = P

where r(t(sub)o) = r(sub)o and r-dot(t(sub)o) = r-dot(sub)o are initial
conditions and P is a small perturbing function, can be solved by a process
where the final step has the form

r(t) = Fr(sub)o + Gr-dot(sub)o + Hg(sub)o
r-dot(t) = F-dotr(sub)o + G-dotr-dot(sub)o + H-dotg(sub)o

Here g(sub)o = r(sub)oxr-dot(sub)o is the angular momentum vector orthogonal
to the instantaneous plane of motion of the perturbed body and F, G, and H are
scalar functions of t and of r, r-dot, P, and their derivatives at the instant
t. If the perturbing function P is either null or coplanar with the
instantaneous plan of motion the function H is also null and our formulas
reduce to the classical Lagrangian form.

Our process involves a regularizing transformation of t for the case where the
perturbing function becomes close to a singularity. It involves also the
introduction of the so-called "universal functions" (the form of the series
that defines them is shown in the hard copy) that have a very fast convergence
and some interesting properties in this application [2-4].

In the present paper we extend our method to the N-body problem covering cases
where the perturbing forces can be either gravitational and nongravitationa--
like those acting on comets--or drag forces acting on artificial objects. Also
we demonstrate that the method is convergent and numerically stable and that
accurate estimations of local and global errors can be obtained. In our
numerical experiments we have included the Lagrangian Problem of Three Bodies
and the motion of the entire solar system. Our results show the advantages in
precision and stability of our approach as compared to some leading methods of
numerical integration of ordinary differential equations.

References:   [1] Zadunaisky P. E. and Giordano C. M. (1990) AIAA J. Guidance,
Control, Dynamics, 13, 1109-15. [2] Battin R. (1987) Introduction to the
Mathematics and Methods of Astrodynamics, AIAA, New York, Chapter 4. [3]
Stumpff K.(1959) Himmelsmechanik, VEB Deutscher Verlag, Berlin. [4] Stumpff K.
(1962) Calculation of Ephemerides from Initial Values, NASA Report D-1415.


Zaytsev A. L.   Vyshlov A. S.   Rzhiga O. N.   Shubin V. A.   Krivtsov A. P.
Zaytseva O. S.   Nabatov A. S.   Wielebinski R.   Altenhoff W. J.
Grishmanovsky V. A.   Kolyuka Yu. F.   Margorin O. K.   Sokolsky A. G.
Shor V. A.
   Six-centimeter Radar Observations of (4179) Toutatis

A radar signal was transmitted from Eupatorian Deep Space Station (Crimea) and
after reflection from asteroid was received at Effelsberg (Germany). The 70-m
antenna and 50 kW transmitter were used at Eupatoria, 100-m antenna and 90 K
noise temperature receiver at Effelsberg. When transmitting, the predicted
Doppler shift was compensated. Continuous wave or frequency-modulated radar
signals were used.

The observations were carried out on 8 and 9 December 1992. Spectrum analysis
of echo signals was made with 0.02-Hz frequency resolution. An echo bandwidth
B, which depends on synodic rotation period and aspect angle, was estimated by
three different methods. The values of B are 1.8-2.2 Hz. After correction for
fast apparent motion, preliminary estimations of proper rotation period of
Toutatis give values within interval from 9.9 to 11.7 days (if the rotation
axis was orthogonal to the line of sight and taking the asteroid diameter to
be equal to 6.0 km).

Two orthogonal circular polarizations have been received simultaneously.
Polarization ratios are equal to 0.24-0.28. This fact gives evidence of high
degree roughness of the asteroid surface at the wavelength scale. The most
interesting result is a bimodal distribution of power spectra for both
polarizations. The depth of a gap is as much as Pmax/2. It testifies that
(4179) Toutatis is a double object, like asteroid (4769) Castalia. The
preliminary estimation of radar cross section for 9 December gives the value
0.26 km2. The time delay and frequency shift data were obtained with the
precision that permits their use for more precise determination of the
Toutatis orbit.


Zhou H.
   The Effect of Collision in the Asteroid Belt

The collision of asteroids plays an important role in the origin and evolution
of the asteroid belt. In order to assess quantitatively the effect of asteroid
collision, we considered a Hill's problem consisted of the Sun, Jupiter and
asteroids in the asteroid belt. For the asteroid motion, in addition to the
graveties of the Sun and Jupiter, the gravitational interaction among
asteroids in the belt has to be taken into account. It can be calculated by
use of the annual mesh difference algorithm. The initial orbits of asteroid
belt are chosen at random, their orbital elements are obtained by the
numerical integral method.

Assuming that the asteroid collision is inelestic with the restitution
coefficient q (0 < q < 1), we calculated the collisional probability and
impact velocity in the asteroid belt. The evolutional processes of asteroid
belt are belt are presented for different initial conditions and various
collision parameters. The results show that the orbital elements and average
energies of asteroids are changed with the initial distribution and
parameters. Finally, the dynamical propeties of the asteroid belt are
discussed for the several cases.


Ziolkowski K.
   The Orbit of (944) Hidalgo and the Mass of Saturn

New orbital elements of the minor planet (944) Hidalgo have been
determined using 246 astrometric observations (made during 15
oppositions in the period 1920-1992) selected and weighted
according to mathematical criteria. A purely gravitational fit to
all observations yields a RMS residual of 1."32. The inclusion of
cometlike nongravitational effects in the equations of motion
reduces the RMS residual to the value of 1."23 and partly
flattens systematic residual trends. It is interesting that the
radial and transverse components of the nongravitational
acceleration (given by A1 and A2 parameters respectively) are
rather indeterminate, while the component normal to the orbit
plane (given by A3 parameter) seems to be quite resonable: A3 = -
(0.15 +/- 0.02) x 10^-8.

The motion of Hidalgo is clearly influenced by Saturn and
therefore can be used for the improvement of the mass of this
planet. The value of reciprocal mass of Saturn is determined to
be 3497.996 +/- 0.057.






                                POSTER SESSION
                                    COMETS



Akisawa H.   Sugawara K.   Tsumura M.   Watanabe J.
   On the Fan-shaped Coma and the Thermal History of Comet P/Gunn

We observed about the fan-shaped coma (Figure) and the visual magnitude of
comet P/Gunn during from February to September in 1989. We determined the
direction of rotation axis from the position angle of the fan-shaped coma by
Sekanina(1979)'s method. From the rotation axis, we calculated the thermal
history of each latitudes on the nucleus. This calculation indicated that the
northem hemisphere of the nucleus was heated before perihelion passage.
However, the light curve of visual magnitude of the comet was symmetric to
perihelion passage. These conclusions suggested the dust mantle was probably
developed on the nucleus of comet P/Gunn.


Arpigny C.   Weaver H. A.   A'Hearn M. F.   Feldman P. D.
   Observations of CH, NH, and NH2 in Comets P/Hartley 2 (1991 XV) and
   Shoemaker-Levy (1991a1) Using the Hubble Space Telescope

The study of minor species observed in cometary spectra is important as it may
allow constraints to be set on the relative abundances of their progenitors in
the cometary nucleus and thus, hopefully, provide information of cosmogonic
significance. For example, it would be interesting to obtain meaningful
values, or upper limits, for the CH4/H2O and NH3/H2O ratios, as these may give
some insight into the processes and conditions prevailing in the regions where
comets were formed. With this aim in mind, we have analysed a series of
spectra obtained with the Faint Object Spectrograph (FOS) on the Hubble Space
Telescope, showing, among others, the emissions due to the methyladyne
radical, CH, some of which may come from methane, and the imidogen, NH, and
amine, NH2, radicals, both of which are probably produced mainly by the
photolysis of ammonia.  These spectra have the following particularly
favorable characteristics :  (1) they refer to two comets of different
dynamical ages, P/Hartley 2 (1991 XV) and Shoemaker-Levy (1991a1), (2) they
were taken when these comets were at very similar distances from the sun and
from the earth (R and Delta both near 0.9 AU), and (3) they were secured with
the same instrument.  Using the fluxes measured in the ~1000 km X 3000 km
aperture of the FOS and recently re-evaluated excitation rates for the CH A-X
(0,0) band, the A-X (0-0) band of NH, and several bands of NH2, we obtain
average column densities for the radicals and derive production rates of their
parents. These production rates are referred to that of H2O, which itself is
estimated using emissions from OH and [OI] that were recorded either
simultaneously, or nearly so.  Spectral synthesis is used to separate the
forbidden oxygen line emissions from the superposed NH2 features, while
similarly, careful subtraction of the underlying continuum (using an
appropriately convolved solar spectrum) is carried out in order to extract the
relevant molecular fluxes. The influence of various model parameters (such as
scale-lengths, time-scales, and velocities) on the derived relative abundances
is discussed.


Batllo V.   Edelman C.
   Origin of Comets of the Jupiter Family

The hypothesis of complete capture of nearly parabolic comets into
short-period comets by a single quasi-collisional encounter with Jupiter is
considered. The shock would be treated as a single velocity impulse applied
tangentially to the initially open orbit at its perillelion becoming the
aphelion of the captured orbit.

C. Edelman has shown in a previous work that this ideal capture mechanism does
not contradict the distribution of orbital elements of the Jupiter family of
comets observed before 1984. In addition, the single captured orbit (modulo
the orientation) resulting from the testing procedure, suggests the break-up
due to Jupiter of some large incoming giant nearly parabolic comet.

The validity of the present assumption as regards capture with such a shock
with Jupiter is examined for comets observed after 1984 whose perihelia are
diversified. Once more the ideal capture scenario and the break-up hypothesis
are not contradicted by the observations.


Bockelee-Morvan D.   Bourgois G.   Colom P.   Crovisier J.   Gerard E.
Jorda L.
   Observations of OH in P/Swift-Tuttle and in Several Recent Weak Comets with
   the Nancay Radio Telescope

The OH radio lines at 18 cm were monitored in six comets with the Nancay radio
telescope in 1992 and early 1993: Zanotta-Brewington 1991g1, Bradfield 1992b,
Mueller 1991h1, Shoemaker-Levy 1991a1, P/Swift-Tuttle 1992t, and P/Schaumasse
1992x.

Neither Bradfield 1992b nor Mueller 1991h1 could be detected. Zanotta-
Brewington 1991g1, Shoemaker-Levy 1991a1 and P/Schaumasse 1992x were
relatively weak comets with OH production rates of the order of 10^28 s^-1 and
required several hours of integration to be detected.

P/Swift-Tuttle was observed and detected as soon as mid October 1992. The
signal was much stronger than for the other comets, especially at the
beginning of December 1992, when P/Swift-Tuttle was observed against galactic
background radiation. The OH production rate was several 10^29 s^-1. All along
the observations, the OH radio lines exhibited unusually high blueshifts,
corresponding to gas bulk motions of 0.5 to 1.0 km s^-1 towards the observer.
This undoubtedly corresponds to the strong jets observed in optical images and
to the important non-gravitational forces that affect the orbit of this comet.
P/Swift-Tuttle was still observed at Nancay at the beginning of January 1993
when the small solar elongation precluded optical observations.

A summary of these observations and of the estimations of the OH production
rates will be presented.


Bockelee-Morvan D.   Brooke T. Y.   Crovisier J.
   The Origin of the 3.2-3.6 Micron Emission Features in Comets: Gas or Dust

Using an infrared fluorescence model, we investigate the contribution of the
v(sub)2, v(sub)3 and v(sub)9 methanol bands to the 3.2-3.6 micrometer organic
emission feature observed in seven comets: P/Halley, Wilson 1987 VII, Bradfield
1987 XXIX, P/BrorsenMetcalf, Okazaki-Levy-Rudenko 1989 XIX, Austin 1990 V and
Levy 1990 XX. In 4 comets, the flux observed from 3.3 to 3.4 micrometers is
almost completely explained by methanol. The 7 comets present residual spectra
with a distinct emission feature extending from 3.38 to 3.5 micrometers and
centred on 3.428 micrometer (~2920 cm^-1). The flux of this 3.42 micrometer
feature is strongly correlated with the water production rate, suggesting a
gaseous origin with organics present at comparable abundances from comet to
comet. From an inspection of the laboratory spectra of small organic molecules,
no definite identification can be found. Four comets (P/Halley, Wilson 1987
VII, Bradfield 1987 XXIX and Levy 1990 XX) show a large emission excess in the
3.2 - 3.3 micrometer spectral region corresponding to the signature of
unsaturated aromatic organics. In the present sample, these comets are those
presenting high dust-to-gas ratios, as measured in the UV.


Bolatto A.   Carballo G.   Fernandez J. A.
   Dynamical Effects of Asymmetric Nongravitational Forces on Long-Period
   Comets

The model of asymmetric nongravitational forces developed by
Yeomans & Chodas (1989) is used to investigate their
perturbing effects on the orbital energy (1/$) of long-
period comets. Results are presented for different
combinations of light-curves asymmetries, lag angles, force
magnitudes and perihelion distances. It is found that the
change in 1/$ after a perihelion passage strongly decreases
with the perihelion distance q, and this perturbation turns
out to be significant only for comets approaching the Sun to
less than a few tenths AU.

Comparison of our results with the observations in the
scatter of the original values of 1/a of near parabolic
comets (-5 X 10^-4 AU^-1 < (1/a) *\(sub)orig < 5 X 10^-4
AU^-1) suggests that they are not significantly affected by
nongravitational forces and in general they may be
comparable to those found in periodic comets.


Bonev T.*   Jockers K.   Chernova G. P.
   Images of Comet Swift-Tuttle 1992t in the Light of H2O+ and CO+ Ions, and
   Dust

The ions of water and carbon monoxide are believed to originate
in the coma of comets by different mechanisms. The main channel
for generation of H2O^+ is photoionization of H2O. The main loss
mechanism for H2O is photodissociation. Therefore this mechanism
determines the extent of the H2O^+ source region. CO is much more
difficult to photodissociate than water. Therefore the CO^+
source is more extended. But CO^+ is not only produced by direct
photoionization of CO, but also by dissociative ionization of CO2
and by an unknown extended source, possibly related to the dust.
Ionization by charge exchange with solar wind protons is also
more important for CO^+ than for H2O^+. It is therefore of
interest to determine the extent of the H2O^+ and CO^+ comae in
comets.

Comet Swift-Tuttle was observed with the 2-m RC telescope of the
Bulgarian National Observatory and the focal reducer of the Max-
Planck-Institut for Aeronomy in the evenings of November 24, 25,
and 28, 1992, when the comet was at a heliocentric distance of
1.0 AU and a geocentric distance of 1.3 AU. We obtained images
through interference filters centered at 619.5 nm, 426 nm, and
642 nm (dust continuum window) with a width (FWHM) of ~3 nm. Some
images taken through a filter centered at 500 nm with the same
width show the C2 coma and dust. Using the images taken at 642
and 500 nm we make an attempt to remove dust and C2 coma
(emission at 618.9 nm) from the plasma images to reveal the
plasma distribution in the inner coma. The resulting cleaned
images will be presented and compared with the aim to detect
differences in the spatial distribution of CO^+ and H2O^+. As a
byproduct of this study we will also derive the dust distribution
and its color variation in the cometary coma.


Boswell J.   Hughes D. W.
   CCD Imaging of the Inner Coma of Comet P/Halley

One hundred ninety-five images of the inner coma of comet P/Halley were taken
between March 3rd and March 23rd, 1986, using a charge coupled device (CCD) at
the prime focus of the Anglo-Australian Telescope (Siding Springs, New South
Wales, latitude -31.277 degrees, longitude 149.066 degrees E, elevation 1164
m). The complete images subtended an area of coma 4.2 x 2.6 arcmins such that
each pixel in the 512 x 320 pixel array recorded an area of 0.49 arcsecs
square.

The library contains images taken using the eleven filters that were aboard
Giotto as part of the Halley Multicolour Camera. The exposure time for each
image was varied to ensure a good signal to noise ratio depending upon the
filter passband and the part of the coma under investigation. All the images
were then bias subtracted to eliminate any "thermal signal" and flat-field
corrected to ensure that there were no pixel to pixel sensitivity variations.
For the purpose of this investigation we selected 23 images that were taken
using either the red continuum or the C(sub)2 filter. The study concentrated
on the very innermost 59 x 59 block of pixels centred on the nucleus pixel.
After the average background had been removed from the images (see A'Hearn et
al., 1986), areas of higher than average brightness were analysed. In
particular, the position, dimensions and curvature of each feature were
measured.

By employing a simple one-dimensional rotating jet model we found that the
characteristics of the features observed on March 11th and 12th could not be
readily explained given a spin period of 7.4 days. However, results for T =
2.2 day were more encouraging and we discovered that the characteristics
could be emulated using the model if the emission latitude, lambda <=
30 degrees and the aspect angle (between the line of sight and the cometary
spin axis) theta was constrained such that 55 degrees <= theta <= 90 degrees.


Chen D.-H.   Zheng X.-T.
   On the Shape of Cometary Envelopes

It is shown by means of a modified Newton's formula that the outline of cometary
heads (the cometary envelope) is close to a catenary.


Chen D.-H.   Liu L.-Z.   Gilmore A.*
   Note on the Splitting of Comet Halley

In combination with our observation about the splitting of Comet Halley in March
1986, the events involving the sharp, straight feature in the antisolar
direction observed in the head of Comet Halley in 1910 (such as those occurred
on May 14, May 25, May 31 and June 2) are rediscussed. The analysis leads to the
following scenario: When Comet Halley explodes and splits, a fragment jettisoned
or thrown off from the nucleus will, after moving to the direction of its tail,
develop to a mini-comet. Although not well developed and permanent, it has its
own plasma tail and, sometimes, dust tail. If we assume as the definition
(Bobrovnikoff's) of a secondary nucleus its activity in the formation of jets
and emission fans, then the fragment should be considered as real secondary
nucleus.

It seems the current idea of tailward jet suggested by Sekanina and Larson is a
wrong explanation to the plasma tail of mini-comet and thereupon the rotation
period of 52-53 hr for Comet Halley is doubtful.


Chernova G. P.   Ahmedzyanov M.   Kiselev N. N.
   Photometry and Polarimetry of Comet Shoemaker-Levy 1991 a1

The photoelectric photometry and polarimetry of comet Shoemaker-Levy 1991 a1
was conducted with the 1-m telescope at Sanglok Observatory, Tadjikistan
during 1992 May 27- Aug 2. An estimate of production rates for species CN, C3,
and C2 and the dust particles have been made. The degree of polarization at
the phase angles 60 degrees - 70 degrees was somewhat smaller than the
corresponding polarization of P/Halley.


Churyumov K. I.   Filonenko V. S.
   Phase Dependencies of Cometary Light Curves

The analysis of phase dependencies of light curves of nine comets was
performed. Each light curve had a mathematical model based on regression
analysis. The model input characteristics are logr (r = heliocentric distance)
and alpha, the phase angle. For determination of the photometric parameters of
light curves the three-parametrical formula was used

m(sub)DELTA = H(sub)y + 2.5 logr + beta alpha

The F-criterion was used to test the hypothesis importance with 0.95
reliability. Also the value F = (S-S') * (N-3)/S was determined (here S' and S
are the sums of discrepancies of conditional equations under beta = 0 or beta
not = 0, N = numbers of equations. The F-distribution tables under 0.95
reliability gave values f(sub)0.95. With F > f(sub)0.95 the hypothesis beta
not = 0 was taken. In the contrary case beta = 0 was taken. The obtained
results are given in the Table.

Comets  beta    alpha   N

Tago-Sato-Kosaka(1969IX)        0       11-19   53
Abe (1970 XV)   0.019 +/- 0.006 23-53   70
Enke (1980 XI)  -0.0014 +/- 0.003       38-130  162
Churyumov-Gerasimenko   0.0031 +/- 0.004        12-38   333
(1982 VIII)
Tempel 1 (1983 XI)      0       11-43   238
Cernis (1983 XII)       0.06 +/- 0.01   7-18    236
Crommellin (1984IV)     -0.09 +/- 0.03  41-69   283
Halley (1986 III)       0       0-67    4140
Borrelly (1987 XXXIII)  0       28-42   505

The division of comets depending on the phase coefficient, beta, of the light
curve into two groups was suggested.

For group I, beta > 0. Comets = Abe (1970 XV), P/Churyumov-Gerasimenko (1982
VIII), Cernis (1983 XII). This group is characterized by great retardation of
maximum brightness after perihelion (the maximal retardation of 40-45 days
takes place for the comet P/Churyumov-Gerasimenko (1982 VIII)).

For group II, beta < 0. Comets = Crommellin (1984 IV), Enke (1980 XI). The
brightness maximum of lightcurve coincides with the momentum of the comet
passage through the perihelion.


Churyumov K. I.   Kleshchonok V. V.   Kravtsov F. I.
   Physical Conditions in Neutral Gas Atmosphere of P/Halley (1986 III) on the
   Basis of Spectral Observations

Comet P/Halley was observed by Churyumov on the 1-m Zeiss reflector with the
UAGS spectrograph and electrooptic camera in Assy (near Alma-Ata). During
three nights of observations May 8-10, 1986, 40 spectra of the comet and star
standards in the spectral range lambda lambda 370-630 nm were obtained. A
detailed analysis of the physical conditions in the comet's head spectra with
position angles 0 degrees, 30 degrees, 60 degrees, 90 degrees, 120 degrees,
150 degrees, 180 degrees, 210 degrees, 240 degrees, 270 degrees, 300 degrees,
and 330 degrees to the radius-vector of the comet respectively has been
performed. In accordance with the spectra measurements on the microphotometer
distribution of the surface brightness perpendicular to dispersion in
emissions CN (3883 Angstroms), C3 (4050 Angstroms), C2 (4737 Angstroms) and C2
(5165 Angstroms) were plotted. In accordance with the luminosity of each of
the lines the nucleus gas production for the molecules in question by Haser's
model was determined. In order to determine the outflow velocity of the gas
component, V, and the lifetime of the molecules, tau, monochromatic profiles
were processed by Shul'man's model. The two values vary substantially with
changes of the cometocentric distance, rho. The lifetime of the C3 molecules
decreases monotonously with the growth of rho. For CN and C2 molecules first
tau increases, and then after the maximum has been reached decreases at the
distance 10^4 -4 10^4 km. The outflow velocity, V, for all types of molecules
behaves similarly: it increases with rho up to the maximum, after that it does
not practically change. It appeared that it can be approximated with great
accuracy by the dependence represented by the equation, which, in the hard
copy, appears here.

The values of the determined physical parameters are given in the table,
which, in the hard copy, appears here.


Cochran A. L.   Trout A. P.
   Modeling the Variability of Gas Production from Comet Halley

We obtained spatially-resolved, moderate-resolution spectra of comet Halley
from McDonald Observatory during 1985 and 1986 in order to study the
distribution of gas in the coma. The data pre-perihelion were obtained with an
Intensified Dissector Scanner (IDS) while the post-perihelion data were
obtained with a long-slit CCD spectrograph (LCS). From these spectra, we
derived the distribution of CN, C(sub)3, and C(sub)2 in the coma. The pre-
perihelion data showed a reasonable amount of symmetry and constancy but the
profiles in the post-perihelion data are quite asymmetric and variable. This
is consistent with the range of activity seen by others. We used a gridding
version of the vectorial model (Festou, Astron. and Astrophys. 95, 69, 1981)
to derive fits to the pre-perihelion data assuming that the activity was
constant during timescales of days. We then used the derived lifetimes to try
to model the detailed shapes of a series of profiles from March 20-24, 1986.
For these models, we used the vectorial model and included variable production
rates. We were able to produce model results which are quite convincing fits
of the profiles. From this, we derive a ``light curve'' of gas production for
each model. We will present these light curves and compare them with the
observed photometrically derived light curve of Millis and Schleicher (Nature
324, 646, 1986). We show that we can derive a qualitatively similar light
curve with a lag in the production which is a function of the molecule under
study and the aperture size. These results demonstrate that a time series of
gas distribution profiles may be inverted to produce a light curve which
matches well with direct photometry.


Cochran W. D.   Schleicher D. G.   Cochran A. L.
   High Resolution Spectroscopy of CN in Comet Brorsen-Metcalf (1989 X)

We obtained a high resolution spectrum of the CN B-X Delta v = 0 band in Comet
Brorsen-Metcalf (1989 X) using the coude spectrograph of the McDonald
Observatory 2.7m telescope on 25~August~1989. At that time, the comet was at R
= 0.64 au, Delta = 0.77 au, with R-dot = -26.1 km/s and Delta-dot = 27.6 km/s.
The comet was approximately centered in a slit which was 61.5 arcsec long. The
resolving power of 60,000 allowed the individual rotational lines to be
resolved easily. These data will be used to test fluorescence models of
cometary CN emission by detailed comparison of the data with theoretical
calculations. We will also look for spatial variations of the spectrum which
may be diagnostic of the Greenstein Effect, or of collisional effects in the
inner coma. The signal/noise of the data is not high enough to measure the
13CN lines reliably.


Combi M. R.   Feldman P. D.
   IUE Observations of H Lyman-alpha in Comet P/Halley

Comet P/Halley was observed throughout the 1985-1986 apparition by the
International  Ultraviolet Observer (IUE) satellite. Although IUE observations
of other species are routinely analyzed, H Lyman-alpha observations have
generally not been because the complexities introduced by the small region of
the coma sampled by IUE compared with entire H coma and the fact that this
inner region of the H coma is often optically thick to solar Lyman-alpha
photons. We present here the results of the analysis of the IUE hydrogen
Lyman-alpha obselvations using the combination of the hybrid gasdynamic/Monte
Carlo model and a sphencal Lyman-alpha radiahve transfer model (Combi and
Feldman, 1992, Icarus, 97, 260). The observations include those with the large
aperture both centered on the position of the nucleus and offset by 40-64 arc
seconds. We have compared the water production rates inferred from the H
Lyman-alpha observations with those extracted from the IUE observations of OH
(Feldman et al. 1987, Astron. Astrophys., 187, 325) and analyzed with self-
consistent hybrid OH models (Combi et al., 1993, Astrophys. J., 408, in
press). The agreement is quite good even when the coma was highly optically
thick in early March.

Figure 1, which appears here in the hard copy, shows water production rates
determined by self-consistent analyses of H (circles) and OH (triangles) from
IUE observations of comet P/Halley. The filled symbols are postperihelion and
the open symbols are preperihelion. The error bars are fairly uniform owing to
the larger relative importance of the optical depth uncertainty at small
heliocentric distance but the larger relative uncertainty of the geocoronal
sky background subtraction at large heliocentric distance.


Cremonese G.   Fulle M.
   The Dust Environment of Comet Levy 1990c

We analyse wide field images of the dust perspectical antitail of Comet Levy
1990c taken on February 1991 with the UK Schmidt Telescope Unit. In this
poster we discuss the results provided by the inverse dust tail model, which
allows us to obtain information on the dust ejection velocity, loss rate and
size distribution for the time interval -180 < t < +100 days (related to
perihelion) and diameters bewteen 2 micron and 2 mm. The dust velocity
increases from 30+-10 m/s at t = -180+-20 to 100 m/s at perihelion, and then
decreases to 30+-10 m/s at t = +100. The mass loss rate reaches a broad
maximum of about 3 10**7 g/s ten days before perihelion. The power index of
the time-averaged size distribution is -3.2 +- 0.1.


Cremonese G.   Fulle M.
   The Neck-Line of Comet Okazaki-Levy-Rudenko 1989r

A wide field image of the dust tail of Comet Okazaki-Levy-Rudenko 1989r was
taken with the UK Schmidt Telescope Unit by A. Savage and C.A. Frost on
Decemebre 30.50, 1989. In this poster we interpret the complex features
recorded by the plate in terms of the Neck-Line model. Further, the inverse
dust tail model allows to obtain information on the dust environment of this
comet during four months before the observations.


Crifo J. F.*
   A General Physicochemical Model of the Inner Coma of Active Comets: 2. Gas
   Phase Van der Waals and Covalent Chemistry

A new physicochemical model of the inner coma of active comets has been
developed [1]. As one example of the model capabilities, the composition of the
inner coma of P/Halley at the time of the flyby missions has been computed,
assuming that 95% of the gas and 95% of the measured dust distribution are
produced between 0 and 500 km from nucleus surface by large (centimeter-size)
fragments of the nucleus surface (i.e., made up of ice, silicate, and
carbonaceous material) emitted from very small areas of the nucleus and
accelerated away by the 5% of the gas that issues directly from these areas [1].

We investigate here the consequences of these assumptions on the chemical
composition of the inner coma (neutrals and charged constituents). For that
purpose, we have incorporated in the model (1) the network of classsical (or
"covalent") photochemical reactions produced by UV illumination of a water vapor
atmosphere [2]; (2) the network of homogeneous recondensation reactions (or "Van
der Waals reactions") that develops in any expansion cooled water atmosphere
[2,3]; and (3) the network of water cluster charging reactions that was
previously shown [4] to result in the dominance of positively and negatively
charged heavy-water clusters in the inner coma under "classical" assumptions
with respect to gas and dust production (100% production at the surface of the
nucleus).

The net outcome of this gas phase chemistry under the present assumptions
exhibits a number of differences with the previous solutions. This is mainly due
to the different water supersaturation radial profile resulting from the radial
distribution of water vapor production, and to the increased UV transparency of
the inner coma also due to this effect. These results reinforce the conclusion
already arrived at in [1] that the design of future in situ inner coma probing
instrumentation onboard a rendezvous spacecraft should not be based on
predictions from "classical" coma models that were developed long before the
Halley flyby Era and did not address specifically the chemistry of the innermost
coma.

References:  [1] Crifo J. F., this volume.  [2] Crifo J. F. (1992) Astrophys.
J., 391, 336-352.  [3] Crifo J. F. (1990) Icarus, 84, 414-446. [4] Crifo J. F.
(1993) Adv. Space Res., in press.


Donnison J. R.
   Trends in Cometary Magnitude Data

The brightness distributions of long-period comets with P > 200 years,
intermediate-period comets with 15 < P < 200 years (Halley-type comets), and
short-period comets with P < 15 years (Jupiter family of comets) were modeled by
Pareto power law distributions. The models were fitted using maximum likelihood
methods and analyzed using a new trend analysis approach. The long-period comets
were divided into a number of physical parameter ranges, notably by perihelion
and discovery date, and examined for appropriate physical trends. Similarly, the
short-period and intermediate-period comets were examined for such trends,
particularly with increasing orbital period. Overwhelming evidence was found
that brightness indexes increase with discovery date for long-period comets.


Feldman P. D.   McPhate J. B.   A'Hearn M. F.   McFadden L. A.   Haken M. E.
   The Evolution of the Coma of Comet P/Swift-Tuttle During November 1992

The International Ultraviolet Explorer (IUE) satellite observatory was used to
determine the abundances of OH, CS, O, C and S in the coma of periodic comet
Swift-Tuttle (1992t) during the period 3-23 November 1992 on 4 separate dates.
The heliocentric distance decreased from 1.17 AU to 1.01 during this period.
On 3-4 November, during a 16 hour observing shift, visual photometry using the
IUE Fine Error Sensor showed a variation of 40% with a clear maximum near
02:00 UT on November 4. The ultraviolet emissions, sampled three times during
this period, appear to follow this variation, although with characteristic
phase lags. The water production rate, deduced from the observed OH emission,
increased rapidly with decreasing heliocentric distance, following an
approximate r^-n law with n between 6 and 9 (the uncertainty is due to the
short-term variability) and reaching a value of ~5 X 10^29 molecules s^-1 on
23.9 November. This makes P/Swift-Tuttle the most active comet at 1 AU to
appear since P/Halley. While our data are insufficient to determine a rotation
period, they are consistent with periods in the range of 2.6-2.9 days and can
exclude a period of less than one day.


Fomenkova M.*   Jones B.   Pina R.   McFadden L.-A.
   Thermal-Infrared Imaging of Comets

Comets are known to be strong infrared sources because of thermal emission of
dust grains ejected by their nuclei. Observations at thermal wavelengths are
very useful in assessing the physical properties of the dust and of the
nucleus itself.

Fifteen images of a dynamically "new" comet, Austin (1989c), were obtained on
May 5-12 (UT), 1990, on the Mount Lemmon Observing Facility's 1.5 meter
telescope (Tucson, Arizona) with the UCSD mid-infrared array camera. The
camera has a format of 64x16 pixels with a plate scale of 0.83 arcsec/pixel
(which twice samples the core of the telescope diffraction pattern at 10
micrometer). The somewhat marginal quality of the data reflects the early
engineering status of the camera at the time the observations were made and
mandates caution in interpretation of the data. Flux calibrations were
obtained using the stars ,Beta Peg, alpha Lyr, and alpha Sco.

The finite spatial resolution of cometary measurements yuts a limit on the
time scale of variability which can be detected. For an aperture of angular
size phi centered on the nucleus, the "aperture escape time" for dust ejected fr
the nucleus is [1]:
tau(phi) [hrs] = 0.35 x phi [arcsec] Delta [AU] r^1/2 [AU], where Delta is the
geocentric and r is the heliocentric distance. With an array format of 53x13
arcsec, the aperture escape times for the Comet Austin images are 6.6 and 1.6
hours respectively, and the 2-pixel (minimum resolution) crossing time is 0.4
hours. Thus, images obtained on different nights are not expected to appear
the same and they do not. Images obtained on the same night may show
variability but appear similar within the accuracy of the data.

Radial brightness profiles have been constructed for each date of observation
by averaging all images obtained on that date. All the profiles indicate a
slope of -1, consistent with a steady state model, up to an angular distance
of 4-5 arcsec (10^3 km) from the nucleus. Beyond that, the profiles steepen to
about -1.25. One hypothesis is that the steepening may be due to the shaping
of the coma by solar radiation pressure. However, the radial distance at which
the dynamical effect of solar radiation pressure becomes important is
typically 3-5x10^2 km (>>10^3 km). A more plausible hypothesis is that the
number of grains begins decreasing faster than p^-2 Hanner [3] has estimated
that icy grains should have a maximum radial extent of only 10^2-10^3 km. This
is somewhat more consistent with our data.

The majority of our images were obtained with a broad 9- 12 micrometers filter
bracketing the peak in black-body emission. Thus, the integrated flux is
proportional to the total thermal emission and is predicted to vary as r^-4.
The measured dependence r^-5 5 can be accounted for if the dust production
rate falls as r^-3 (r^-2 due to the temperature decrease and r^-0 5 due to the
decrease of the grain terminal velocity).

Comet Austin had an unusually wide silicate emission feature [4l. On May 12,
we acquired one image with the 9-12 micrometer filter and two images with a
narrower 8-9 micrometer fi1ter. A ratio of these images was examined to
determine the spatial distribution of the silicate emission. The emission was
found to be strongest within a 90 degree angle in the sunward direction with
the peak at a distance of 2000 km. Observations of Comet Halley [5] also
indicated silicate emission in the sunward direction. However, the peak
emission was located at the nucleus.

REFERENCES. [1] D. Jewitt, in Comets in the Post-Halley Era, R.L. Newburn et
al. (eds.), V. 1, 19-65, Kluwer Academic Publishers (1991). [2] D. Jewitt, K.
Meech, Astrophys. J. 317, 992 (1987). [3] M. Hanner, Icarus 47, 342 (1981).
[4] M. Hanner et al., submitted to lcarus (1992). [5] Hammel, H. B. et al
(1987), Astron. Astrophys. 187, 665-668.


Gajdos S.
   On the Brightness of the Comet P/Halley in the Past

The article deals with behaviour of a brightness of comet the P/Halley in the
past. 30 confirmed apparitions of the comet, since 239 B.C., are analysed in
this paper. From Sun-Earth-comet configurations in the moment of perihelion
passages it can be determined several groups of individuaL apparition with
very similar geometry. These groups were analysed with emphasis to examining
of development of brightness of the comet on different long time scale.


Geyer E. H.   Kiselev N. N.   Chernova G. P.   Jockers K.
   Surface Polarimetry of Comet Tanaka-Machholz 1992d Using a Novel Double
   Wollaston Prism

Surface polarimetry, when done with a Wollaston prism, usually
requires at least two separate exposures, and it is necessary to
rotate the instrument between these exposures by 45 degrees. With
comets, this technique has the disadvantage that, because of the
proper motion of the comet, the background star field will change
between the exposures. Also, there may be temporal changes either
in the sky or within the comet between exposures. In this paper
we describe a double Wollaston prism that splits the exit pupil
of a focal reducer and thus simultaneously produces four images
with polarization directions 0 degrees, 90 degrees, 45 degrees,
and 135 degrees, from which the linear polarization vector can be
derived. The combination of Wollaston prisms is shown in the
panels A and B of Fig. 1. Panel C illustrates the arrangement of
the four images generated on the detector. Naturally, the
exposure time must be increased by a factor of two, i.e., there
is no gain in efficiency, except for the saved time necessary to
rotate the instrument by 45 degrees. To avoid image overlap a
mask can be put into the Cassegrain focal plane.

The first object we observed with this instrument was comet
Tanaka-Machholz 1992d. The paper will be concerned with the
results of these observations and will critically discuss the new
technique.

Figure 1 appears here in the hard copy.


Goidet B.   Levasseur-Regourd A. Ch.   Clairemidi J.   Moreels G.
   Dust Distribution in Comets; Comparison Between Models and In Situ Data

A three-dimensional mathematical model with an r^-2 (where r is the nucleus
distance) dust spatial distribution has been used to estimate the distance of
the closest approach for the Giotto/Grigg-Skjellerup flyby. The dust
brightness profiles obtained by the OPE experiment on the dark side of the
coma were indeed found to nearly obey a r^-1 law (see Le Duin et al., this
volume). It is concluded that the nucleus distance was of the order of 200 km
and that the nucleus was in the vicinity of the plane scanned by the line of
sight.

Another physical model including fountain effect and fragmentation has been
found to be reasonably well fitted with the results obtained by the three-
channel spectrometer TKS on board the Vega/Halley mission, and to explain the
color effect. It is now tentatively compared with the OPE dust brightness
profiles obtained on the sunny side.


Grudzinska S.
   Some Remarks on the Radial Velocities in the Head of Comet West 1976 VI

A spectre of Comet West 1976 VI was obtained by Herbig on 15.03.1976 with the
3-m telescope of Lick Observatory. The dispersion was 11 A/mm and the radial
velocity of the Comet +21 km/sec. In the region from 623.7 to 636.2 m the
emission lines--mostly from NH2--were measured in 4 different positions on the
comets head: in the cener, at the distance of ~500 km from the center in the
direction of the Sun and in the tail, and at the distance ~900 km in the tail.
For those distances the difference in the radial velocity between that of the
comet was determined.The results were compared with the fountain model of
cometary head calculated for the physical conditions of Comet West.


Hadamcik E.   Levasseur-Regourd A. Ch.   Renard J. B.
   Synthetic Curves for Polarization of Comets

Synthetic curves of the evolution of the polarization degree with phase angle
have been derived from all available cometary polarimetric data. These curves,
presented for different wavelengths in the visible domain, are relevant of the
nature of the cometary comae.

Preliminary, the curves could be represented by a third order polynomial fit.
A trigonometric fit is a better approach to derive the main parameters of the
curves (inversion angle alpha(sub)0, slope at inversion h, minimum of
polarization P(sub)min at phase angle alpha(sub)min, maximum of polarization
P(sub)max at phase angle alpha(sub)max). In the red domain (670+/-50 nm),
these characteristic parameters are found to be:

alpha(sub)0 (degrees)           23 +- 1

h (10^-2 deg^-1)              0.25 +- 0.05

alpha(sub)min (degrees)         11 +/- 3

P(sub)min (percent)           -1.5 +/- 0.5

alpha(sub)max (degrees)         90 +/- 15

P(sub)max (percent)             16 to 26

While there is a good agreement between all polarimetric values at small phase
angles, the dispersion at large phase angles can be an indicator of
differences in the activity of comets.


Ivanova V.   Komitov B.   Shkodrov V.
   Photometry of Comet Levy (1990c) Around 380nm. Physical Parameters of the CN
   Coma

Main results of the processing of Comet Levy's photographical observations
were analyzed. The plates were taken in August 1990 with 50/70cm. Schmidt
telescope through a narrowband filter. The spatial parameters of the coma of
CN parent molecules are determined using two plates. According to our
preliminary evaluations on one of the two plates a slight fountain effect is
visible. Estimations for radial expansion velocity of CN coma are given.


Jackson W. M.*   Mohammad F.   Bao Y.   Morris V.   Jones A.
   Cometary Implications of the Transient IR Emission Spectra of the
   Photofragments Formed in the Laser Photolysis and Laser-Induced Reactions of
   Small Molecules

The experimental results from the transient infrared emission spectroscopy
(TRIES) studies recently performed in our laboratory will be described.
Vibrationally excited radicals are produced when an ArF laser is used to
photolyze molecules because the photon energy of this laser is usually much
greater than the bond energy of the molecules. Infrared emission can also be
observed when radicals formed by laser photolysis undergo exothermic reactions
with stable molecules or with each other.

To do these TRIES laboratory studies an ArF laser was combined with a
Perkin-Elmer infrared monochromator, a fast infrared detector, a 2 micros
transient digitizer, and an IBM compatible computer. The vibrationally excited
species that have been observed using this experimental system are C2H, C2,
CH3, HCN, HNC, CO, C, and C3H3. Some of the species may also be important in
cometary atmospheres. These laboratory studies suggest that the infrared
emission from the inner coma may be a diagnostic for some of the chemical
processes that are occurring in the coma. We will discuss how the existence of
certain chemical reactions may be reflected in infrared observations of the
coma that probe region near the nucleus.

Acknowledgment: The authors gratefully acknowledge the financial support of
the planetary astronomy program under grant NAGW-1144.


Jockers K.   Chernova G. P.   Bonev T.   Ivanova V.
   Search for a CO+ Tail in Comet Schwassmann-Wachmann 1

CO^+ was discovered in spectra of comet Schwassmann-Wachmann 1 by Cochran et al.
[1]. Images of the CO^+ tail of comet Schwassmann-Wachmann 1 have been presented
by Jockers et al. [2,3], but the result did not seem convincing. In a new
attempt the comet was observed again with the 2-m telescope of the Bulgarian
National Observatory and the focal reducer of the MPI for Aeronomy in December
1992 during opposition. A tunable Fabry-Perot interferometer with resolution of
about 10 angstroms was employed to obtain narrow-band images of this comet in
the (3-0) ^2Pi(sub)3/2 - X^2 Sigma^+ subband of CO^+ at 4020 angstroms and in a
nearby continuum window at 4060 angstroms. Both windows are contaminated by C3
emission, which, however, has never been observed in comet Schwassmann-Wachmann
1. Red continuum images were obtained in a Gunn R filter. For comparison with
another comet, the same technique was applied to comet Shoemaker 1992y, which at
that time was at a heliocentric distance of 2.54 AU and a geocentric distance of
1.80 AU. We will present the results and discuss the evidence of a CO^+ tail in
both comets.

References:  [1] Cochran A. L. et al. (1980) Astron. J., 85, 474. [2] Jockers K.
et al. (1992) in Asteroids, Comets, Meteors 1991 (A. W. Harris and E. Bowell,
eds.), 261-264, LPI, Houston. [3]Jockers K. et al. (1992) Astron. Astrophys.,
260, 455.


Jorda L.   Hainaut O.   Smette A.
   Photometric Study of Comets P/Faye 1991 XXI and Zanotta-Brewington 1991g1

We observed comet P/Faye at ESO, La Silla from July 1991 to February 1992
using IHW filters mounted on the 1m and the ESO 50cm telescopes. CCD images
were also obtained at Danish 1.54m telescope on August 26, 1991 with IHW image
quality filters. These data are used to study the dust production rate along
the orbit, through the Af rho parameter. No gas was detected during these
observations. Comet Zanotta-Brewington was observed on March 31, 1992 at La
Silla. Using the spectro-imager EMMI at the NTT, we got a low resolution
spectrum, showing C2(Delta v=0,1) and CN(Delta v=0) bands. IHW image quality
filters were used simultaneously at the Danish 1.54m telescope to get narrow-
band images. We deduced C2 and CN production rates, as well as the Af rho
parameter.


Kim S. J.   A'Hearn M. F.
   The Swings Effects of the A-X System and v' = 1-0 Band of CO

We have constructed a model of the A-X system of C0 in order to analyze the C0
bands appearing in the ultraviolet spectra of comet P/Halley and other comets
obtained with the IUE. The major bands of the A-X system occur in the 1200 -
1800 A range where the temporal variation of solar irradiance is significant.
The model includes electronic, rotational, and vibrational transitions,
excitations by solar ultraviolet radiation, and effects of neutral (Private
communications with S. Green, 1992) and electron collisions. The solar
spectrum in this spectral range shows many solar emission lines, which causes
a pronounced Swings effect. Comet P/Halley was observed during a period when
the solar activity was quiet. We derived fluorescence efficiencies of the
bands (Private communications with H. Weaver, 1993) and estimated that the
ultraviolet Swings effects are less than 20 % of the fluorescence efficiencies
for most bands of the A-X system. The study of the A-X system also requires
knowledge of vibrational and rotational fluorescent processes in the infrared
and radio regions because the majority of C0 molecules in the coma is in the
ground vibrational and rotational states. The solar infrared spectrum around 5
microns, where the fundamental band of C0 occurs, contains strong absorption
lines of the fundamental band and hot bands of C0 and its isotopes. These
solar absorption lines cause a 20 % reduction of the g-factor of the
fundamental band at heliocentric velocities close to 0 km/sec.


Kolzer G.   Grun E.   Heidrich R.   Hesselbarth P.   Hoppner B.   Kochan H.
Kohl H.   Lammerzahl P.   Lorenz D.   Michel H.-J.   Thiel K.
   Particle Emission in the Comet Simulation Project KOSI: A Review of Five
   Different Experiments

In a series of comet simulation experiments mineral-ice mixtures are studied
under space conditions [1]. The emission of ice, mineral-ice, and dust particles
during the illumination phases is investigated by different devices: The
particle emission was first established in the form of a spatial distribution of
the deposited dust particles in front of the sample after the experiment. From
this distribution the total mass loss of the sample with respect to minerals can
be estimated. The time dependence of the emission activity is recorded by
piezoceramic impact detectors [2], ice/mineral and dust collectors [3,4,5] and a
laser particle counter [2].  The particle emission can be directly observed by
video cameras [6]. The time dependence of the particle emission rate is related
to the thermal evolution of the sample [7] that in turn depends on the sample
composition and the duration and intensity of the illumination. The relevant
near-surface parameters for the particle emission are the flux density of the
outflowing gas [8] and the surface temperature of the sample [9]. These
parameters also give information about the development of a dust mantle on the
sample surface.

The present paper gives a review on the dust results of five comet simulation
experiments. In all experiments the maximum gas flux was observed immediately
after the start of illumination. This high flux is accompanied by the maximum
emission rate of ice containing particles. The gas and dust emission activity in
all experiments decreases more or less steeply with increasing irradiation time,
whereas the surface temperature increases to an asymptotic value. Near-surface
parameters that are relevant for the particle emission activity are discussed
for the five experiments investigated.

References:  [1] Gruen E. et al. (1991) Geophys. Res. Lett., 18, 245-248.
[2] Kohl H. et al. (1989) Planet. Space Sci., 38, 567-573. [3] Mauersberger K.
et al. (1991) Geophys. Res. Lett., 18, 277-280. [4] Thiel K. et al. (1991) Proc.
LPSC, Vol. 21, 579-589. [5] Thiel K. et al. (1991) Geophys. Res. Lett., 18, 281-
284. [6] Kochan H. et al. (1991) Geophys. Res. Lett., 18, 273-276. [7] Benkhoff
J. and Spohn T. (1991) Geophys. Res. Lett., 18, 261-264. [8] Hesselbarth P. et
al. (1991) Geophys. Res. Lett., 18, 269-272. [9] Gruen E. et al. (1991) Geophys.
Res. Lett., 18, 257-260.


Korsun P. P.
   Analysis of P/Brorsen-Metcalf H2O+ Brightness Profiles by Means of the Monte
   Carlo Simulation

The long-slit spectra of Comet P/Brorsen-Metcalf were obtained on the high-
altitude station "Terscol" of the Main Astronomical Observatory of Ukraine
using image intensifier "Topaz." To investigate the H2O+ ionosphere we
extracted the brightness profiles of the 8-0 and 6-0 H20 bands.

On the basis of the Monte Carlo formalism we developed a computer program to
simulate the distribution of observed species in cometary atmospheres. Using
this model we calculated the spatial distribution of the H2O+ ions in
P/Brorsen-Metcalf atmosphere and fitted the computed H20 profiles to the
observed ones. We obtained the H2O acceleration in the tailward direction to
be 10^-13 cm x s^-2, According to our simulation the ionization H2O lifetime
is 2.2 x 10^5 s. The lifetime of H20+ against its disappearance turned out to
be (1.9-2.0) x 10^5 s. Our simulations have shown that the comet-solar wind
interaction is crucial in the formation of the H2O cometary ionosphere.


Korsun P. P.   Lipatov S. V.
   Spectroscopic Observations of Comet P/Brorsen-Metcalf at 3500-7550
   Wavelength Region

The spectroscopic observations of Comet P/Brorsen-Metcalf were made on the
high-altitude station "Terskol" of the Main Astronomical Observatory of
Ukraine on August, 1989. The spectra were taken using image intensifier
"Topaz" with the UMK-91VM image tube as the basic unit. The spectra cover the
wavelength region from 3500 to 7550 Angstroms at resolution ranging from 1.8
to 7.7 Angstroms.

We carried out the identification of 329 spectral features in P/Brorsen-
Metcalf spectra. We found emissions of CN, C2+, 12C/13C, CH, C3, NH2, Na, O[I]
neutrals, and N2+, OH+, CO+, CH+, CO2+, H2O+ ions in our spectra. We detected
the transitions from the rotational levels up to J" = 99 for the DELTA upsilon
= O C2 sequence for the first time. We found five features that coincide with
the laboratory band heads wavelength for MCO, and three that coincide with the
heads of the Swan "tail" bands as well.


Kotsarenko N. Ya.   Verkhoglyadova O. P.   Churyumov K. I.
   Three-Dimensional Model of Ray Structure Formation in Cometary Plasma Tails

The physical model of large-scale ray structure formation, which has been
proposed in a previous paper [1], resides in the consequential development of
ion-acoustic instability and filamentational one due to the solar wind flow
penetrating into the cometary plasma. The latter instability consists in the
solar wind flow division and that of cometary plasma into separate current
layers.

In order to study a real form of the filaments and instability development in
space a study of the proposed mechanism in three dimensions in terms of
hydrodynamic equations for ions and electrons was made. Our main interest here
was with the large spatial scales, which are not affected by thermal effects
[1]. Consequently, all the components were considered to be cold. Cometary
plasma ions and those of the solar wind seemed to be collisionless, otherwise
collisions of electrons dealing with scattering on ion-acoustic oscillations
were characterized by effective collision frequency [1]. The cometary plasma
filaments, which are observed as bright rays, were formed due to instability
development. They took the form of cylindrical plasma fibers with thickness
determined by cometary plasma parameters and those of the solar wind. Plasma
density in the filaments varies as J(sub)0(kr), where J(sub)0 is the zero-
power Bessel function, k is the parameter playing the role of transverse wave
number. Under some assumptions we obtained characteristic length, which may be
thought of as the maximum ray size.

In the hard copy, equation (1) appears here.

The obtained spatial scales are in a good agreement both with one-dimensional
theory [1] and the observational data. We can evaluate the number of rays
within one lobe of cometary plasma tail, which is approximately equal to L x
k(sub)0/11.2, where L is the system length, and varies at range of 2/30 rays.

References:  [1] Verkhoglyadova O. P. et al. (1993) Lett. Astron. J. (in
Russian), in print. [2] Verkhoglyadova O. P. et al. (1992) Bull. AAS, 1024,
24.


Kresak L.   Kresakova M.
   Updating of the Catalogue of Absolute Magnitudes of Periodic Comets

Our catalog of absolute total magnitudes of periodic comets has been updated by
including the apparitions of cornets that passed their perihelia in 1988-1992.
The total number of objects has increased from 144 to 172, that of the objects
of more than one apparition from 89 to 106, and the sum of apparitions from 575
to 661. In addition, seven entries from 1959-1987 were changed by observations
published after 1987.

Comparison of the new data with the older does not reveal any signatures of
progressive fading. On the contrary, 60% of the objects were brighter than at
the previous apparition and 67% were brighter than the long-term average. The
corresponding weighted means of Delta H are -0.2 and -0.5 magnitude
respectively. This brightening is spurious and can be attributed to the failure
of rediscoveries of a few objects that became fainter, to a better time coverage
by photometric observations, and to the reduction of instrumental corrections
for faint comets by the application of modern techniques, such as the CCD
Spacewatch.

Quite exceptional is P/Metcalf-Brewington, which remained unobserved for 10
returns and this time was 3.4 magnitudes brighter than at the discovery
apparition in 1906. Three other long-lost comets--P/Peters-Hartley,
P/Swift-Gehrels, and P/Tempel 1--were also substantially brighter than ever
before, indicating a renewal of activity, and the increased brightness persisted
for P/Tempel 2 and P/Honda-Mrkos-Pajdusakova. Unusual activity (more than 2
magnitudes above the average) was recorded far from the perihelion for P/Gunn,
P/GiacobiniZinner, P/Churyumov-Gerasimenko, and P/Sanguin. However, with the
exception of P/Gunn, the instrumental effects tied with a very low apparent
brightness do not allow us to draw any definite conclusion about their possible
outbursts. The comets discovered during their immediately preceding perihelion
passage tend to become fainter; for P/Machholz, P/Takamizawa, and P/Hartley 2
this suggests a decline following a sudden renewal of their activity.

Our main data sources were IAU Circulars, Minor Planet Circulars, and
International Comet Quarterly. If there are any unpublished photometric
measurements/estimates from around the brightness peaks we would greatly
appreciate receiving them.


Kubacek D.   Pittich E. M.   Zvolankova J.   Ticha J.   Tichy M.
   Plasma Tail of Comet Bradfield 1987 XXIX

A set of thirteen large-scale exposures of comet Bradfield 1987 XXIX taken
between 1987 November 8 and December 25 is evaluated. The observations were
obtained with the 63/85/187 cm Maksutov telescope of the Klet' Observatory.
The studied plasma tail with visible plasma rays, kinks, and disconnection
events extends out of the field of the plates. Therefore, the tail must have
been longer than 1.7 degrees. The dust antitail on the best exposures is long
up to 0.5 degrees.

For this period the solar wind velocity is determined from the aberration
angle of the plasma tail in the comet's environment. The determined value of
the solar wind velocity are compared with satellite data recorded near the
Earth. For the visible plasma kinks a radial and a tangential velocity is
determined.


Lamy P.*   Malburet P.
   Absolute Photometry of the Dust Tail of Comet P/Halley

Postperihelion observations of the tail of comet Halley were performed with a
wide-field CCD camera set up at the European southern observatory. One hundred
images were collected over a period of several months, through B, V, R, and,
occasionally, I filters. We implemented a quasiautomatic procedure for the
reduction of the images, using a large number of stars for absolute
calibrations. The procedure, as well as the results for the B and V filters
processed so far, will be presented.


Lamy P.   Toth I.
   Observation of Comet Faye (1991n) with the Hubble Space Telescope

Periodic comet Faye was observed with the Hubble Space Telescope when it came
to within 0.6 AU of the Earth in October-September 1991. At this distance, the
projected pixel of the planetary camera corresponds to 20 km. The observations
through a broadband red filter were performed seven times to follow the
possible activity of the comet. The outer coma is highly asymmetrical clearly
deviating from a simple 1/r model. The analysis of the innermost coma is
complicated by the spherical aberration of the HST. Various deconvolution
schemes were tested and indicate also a nonisotropic emission of dust. The
contribution from the nucleus was delineated and may give access to a direct
characterization.


Lamy P.*   Cosmovici C.   Schwarz G.
   Changing Properties of the Dust Coma of P/Halley: Implications for an
   Inhomogeneous Nucleus

High-resolution maps of the color and polarization of the coma of comet Halley
derived from CCD images obtained during the first half of March 1986 reveal
considerable temporal as well as spatial variations. They remain however
restricted to the inner coma and are well correlated with the activity of the
comet. The varying reddening and polarization are best explained by a
population of organic grains that do not survive beyond a few tens of
thousands of kilometers. Color and polarization do not exhibit a unique
correspondance. Taking into account the rotation of the nucleus, this implies
an inhomogeneous nucleus. The maps of the color, the polarization and the jet
structures as revealed by unsharp masking are correlated to delineate the
physical properties of the dust grains in the jets and in the background coma.


Larson S. M.   Spahr T.   Shirley Y.   Hergenrother C.   Scotti J.
   Activity Levels of Comets at Large Heliocentric Distances

The activity level of 60 comets observed in red-band CCD imagery since 1985
have been measured in terms of coma size, tail length and total brightness
when at r > 1.5 AU. About 30 of these could be followed over substantial
segments of their orbit, or multiple orbits. For this range of r, it is
assumed that only dust is being measured, but the intrinsic dust/gas ratios of
many of the comets are not known. The substantial homogenous data sample is
sufficient to consider studying the range of activity (and appearance) among
comets of varying evolutionary histories (aging) and possibly different
formation environments. Consistent with other studies, we find that new comets
are more active at large r inbound, and that several comets display
substantial tail development even at r > 8 AU. At this preliminary stage of
analysis, there is no clear correlation between the measured and the orbital
characteristics. The brightness behavior of many of the periodic comets
exhibit the common postperihelion dust production increase.


Le Duin T. L.   Levasseur-Regourd A. Ch.   Renard J. B.
   Dust and Gas Brightness Profiles in the Grigg-Skjellerup Coma from
   OPE/Giotto

The OPE experiment on board the Giotto spacecraft has provided the first in
situ optical probing of the very inner coma of a short period comet. Along the
trajectory, brightness profiles (both of solar light scattered by cometary
dust grains and of light emitted by cometary gases) have been obtained in
various channels for distances to the nucleus below 10,000 km.

These profiles allow us to estimate the size of the outer coma (dark and sunny
side). On the dark side of the inner coma, the dust brightness profiles nearly
obey a r^-1 law (where r is the nucleus distance), to be expected from a r^-2
dust spatial distribution. On the sunny side of the inner coma, the power law
is found to be of the order of r^-1.7+/-0.1 and the profiles exhibit a
negative curvature. The gas emission profiles are also estimated of both sides
of the coma. The relative contribution of dust and gas are discussed for the
local increases of brightness detected in the inner coma.


Long Y.*   Chen D.-H.   Liu L.-Z.
   On the Stability of the Cometary Ionopause

This paper discusses the stability of the sunlit cometary ionopause. Utilizing
the ionosphere model of Comet Halley, which was discovered by Giotto Mission, we
release our discussion to compressible condition. Meanwhile the distribution of
plasma density is taken to vary as r^-1 throughout the entire region, and then
the magnetism magnitude varies as shown by the equation that appears in the hard
copy.


Matese J. J.   Whitman P. G.
   Is the Perihelion Asymmetry in the Nongravitational Force Acting on Comet
   1960II (Burnham) Due to Its Passage Through the Taurid Stream?

Comet Burnham has been cited (Marsden, 1969) as unique among Oort cloud comets
in having high-precision observations that exhibit a large, systematic trend
in the gravitational orbit residuals. Over a six-month interval, 37 individual
measurements by Roemer et al. (1966), illustrated in the accompanying figure,
showed a minimized mean residual of 3''.06 when fit to a gravitational orbit.
The gravitational solution for the original value of the reciprocal semimajor
axis is 1/a = -135 +/- 23 in units of 10^-6 AU^-1, where the uncertainty
quoted is the formal measurement error. Comet 1960II is also distinguished as
one of only seven dynamically new comets that have original gravitational
orbits that are nominally hyperbolic at a level >=5x the formal measurement
error (Matese et al., 1991, 1992). In these references it was demonstrated
that this hyperbolic family uniquely correlates with orbits that preferably
have small perihelion distances and move in a retrograde sense to the
planetary motion. It was argued there that the physical explanation for this
correlation is the catalytic enhancement of nongravitational forces due to
outgassing of volatiles.

Whipple (1950) recognized the importance of the rocketlike forces on a comet
nucleus when he formulated his conglomerate model. A standard model of these
nongravitational forces has emerged that embodies reactive forces that are
symmetric about perihelion (Marsden, 1968, 1969; Marsden et al., 1973).
However, there is a growing body of evidence that these forces may be
asymmetric about perihelion (Yeomans and Chodas, 1989; Rickman et al., 1991).
Since the nongravitational force parameters deduced by the two approaches can
be quite different, it becomes crucial to determine which approach is a better
representation of reality if one hopes to use the parameters to characterize
the physical properties of comets.

To date, no asymmetric model of Comet 1960II has been constructed. We present
arguments here that indicate that an asymmetric model of nongravitational
forces will yield a better fit to the orbit and that the onset of peak
outgassing will be delayed well beyond perihelion. These results are visually
suggested in the figure, which appears in the hard copy, in which the
gravitational residuals are dated relative to perihelion passage and connected
sequentially by straight lines. One observes that the residuals random walk
during the initial phases. The large systematic trend mentioned by Marsden
only begins well beyond perihelion.

We have found that Burnham passed through the Taurid stream in the interval
16-19 days subsequent to perihelion, during which the relative velocity of the
comet and meteoroidal stream material was ~80 km s^-1. We will argue that
there is a causal link between the stream passage and the onset of substantial
nongravitational forces. Meteoroidal impacts can be expected to create craters
up to 1 m in size, exposing the underlying volatiles and releasing any
subsurface pressures that may have built up as the mantle formed. Related
observations of the intrinsic light curve of Comet 1960II will also be
presented.


Medvedev Yu. D.
   On the Dust Cloud in the Comet's Head and Its Role in Forming the
   Photocentre Shift Phenomenon

Within the head of a comet there exists the point of equilibrium around which
the dust is accumulated. The point is located on the comet-Sun line and is
stable with respect to the motion of a dust particle along this line. The
model computations support this theoretical conclusion. The estimation of
brightness of the dust cloud and the comparison with that one of the comet
nucleus have been done.

The simple formulae have been deduced for the distance of this point from the
center of the comet nucleus in terms of the gas productivity and the
heliocentric radius of the comet. The computed values of this distance for the
P/Halley in the 19851986 apparition are equal to 180 km and 990 km for the
preperihelion and postperihelion motion of the comet correspondingly. The last
of these values is in agreement with the observed value of the post-perihelion
photocentre shift of the P/Halley. As for the first one, it is small and is
difficult to observe.


Meisser W.   Schmidt-Kaler Th.
   Production Rates of Comet P/Halley

We report the results of ground-based photometric observations of comet P/Halley
between 1986 February 24 and April 16. The measurements were obtained in the
IHW-filter system (H2O+, red-continuum, C2, blue-continuum, CO+ and CN) each
with 9 diaphragmas (11".32, 18".23, 28".9, 45".4, and 1'.19, 1'.917, 3'.014,
4'.764, 7'.525). This great number of diaphragmas is useful for improving the
parameters of Haser models.


Moreels G.*   Clairemidi J.   Rousselot P.   Goidet B.
   Detection of Polycyclic Aromatic Molecules in Comet P/Halley

Spectra of the inner coma of P/Halley were obtained during the approach session
of Vega 2 on March 9, 1986. The instrument, called three-channel-spectrometer or
TKS, covered the 275-710 nm range. It produced a spectroscopically resolved
scanning of the inner coma at distances less than 40,000 km. The scientific data
are presented under two forms: (1) spectra at various projected distances p,
i.e., distances between nucleus and optical axis; and (2) radial profiles of
given emission intensities at selected wavelengths in the 40,000-400 km distance
range.

The spectra show the molecular emissions of OH, NH, CN, C3, CH, C2, and NH2 and
the continuum intensity scattered by dust grains. This emission becomes rapidly
more intense than the molecular bands with decreasing projected distance
parameter p. For instance, when p = 1000 km, the scattered intensity is ten
times brighter than the CN emission. At distances shorter than 1000 km, the
residual spectrum after subtraction of the intense solar continuum shows the
presence of two new spectral features: an isolated band at 323 nm and a broad
band with four peaks at 347, 356, 364, and 373 nm. The intensity of these bands
rapidly increases with decreasing distance to the nucleus. It follows
approximately a 1/p law, which implies that the molecules responsible for the
emission are either parent-type molecules or are released by grains of the dust
flow.

A search for candidates that could possibly be responsible for the new emission
at p < 1000 km shows that the spectra of CN, OH, CO2+, and H2CO cannot explain
the spectral features and the radial variation of the detected bands. Since the
existence of polycyclic aromatic compounds was suggested following the
identification of an X-CH vibration band at 3.2-3.5 micrometers in Halley's IR
spectrum, we compared the UV laboratory spectra of PAH having 2 and 3 benzenic
rings with our cometary spectra. A good fit was obtained at 323 nm with the
fluorescence spectrum of jet-cooled naphtalene [1]. In the case of the broad
band at 342-375 nm, a very good agreement was found between the cometary spectra
and the fluorescence spectrum of phenanthrene [2]. In conclusion, we can
confidently suggest the identification of naphtalene and phenanthrene in
Halley's comet. It may also be suggested that the unidentified features at 321.9
and 323.2 nm detected in comet Austin by Valk et al. [3] might be due to
naphtalene. The detection of two specific polycyclic aromatic molecules in
Halley's comet supports the idea of an analogy of composition between cometary
material and interstellar matter. It has important implications in the scenario
scheme of the earlier solar system formation period.

References:  [1] Beck et al. (1981) 74, 1, 43-52. [2] Karcher et al. (1985)
Spectral Atlas of Polycyclic Aromatic Compounds, Reidel. [3] Valk et al. (1992)
Ap. J., 388, 621-632.


Mueller B. E. A.*
   An Interpretation of P/Halley's Photometry at 5 AU Pre-Perihelion

Belton et al.'s original spin model for P/Halley [1] was successfully tied in
with photometric observations at 8 AU pre-perihelion when the comet was rather
inactive [2,3]. Most of the observed light-curve near 8 AU is adequately
explained by light scattered off a homogeneous nucleus, spinning as described
by Belton et al. [1] and with the shape determined by Sagdeev et al. [4].
Superimposed on this variation are transitory and periodic bursts of coma
activity that probably originate in an active region near the small end of the
nucleus (area 1, Belton et al. [1]). This region is probably identical to one
of the most active areas found in the March-April, 1986, time frame.

We present the results of the comparison of the spin model with photometric
observations at 5 AU pre-perihelion. Special consideration will be given to
the active areas, in particular to active area 1.

References. [1] Belton, M.J.S., W.H. Julian, A.J. Anderson, and B.E.A.
Mueller, 1991: Icarus 93 183. [2] Mueller, B.E.A., 1992: Proc. 30th Liege
Internat. Astrophys. Coll. pp. 129. [3] Mueller, B.E.A. and M.J.S. Belton,
1992: Proc. Lenggries Workshop on Activity of Distant Comets, in press. [4]
Sagdeev, R.Z., K. Szego, B.A. Smith, S. Larson, E. Merenyi, A. Kondor, and I.
Toth, 1989: A.J. 97, 546.


Nakamura T.   Yoshikawa M.
   Exponential Growth of Round-Trip Error and Instability of Orbits of Small
   Bodies in the Outer Solar System

Exponentially growing behavior of the round-trip (RT) error in a closure test
of numerical integration for orbits of comets and come-like bodies was
examined in relation to the time scales of their orbits becoming chaotic and
unstable. For the majority of the orbits of about 160 SP comets that we
integrated for 4400 years, the relative RT error for each of Kepler elements
increased approximately exponentially from a very small value (say, 10-22) to
the order of unity within 1000-2000 years; this period of time was found to be
nearly the same as the capture/ejection time scale for the SP comets.
Similarly, as a result of 0.1-0.2 My integration, we found that the error
growth time scale for the orbits of giant comet-like objects such as Hidalgo,
Chiron, an Pholus (5145,1992AD) is consistent with the time scale in which
those orbits change substantially from the present ones.

If this parallelism between error growth and orbital change is valid for the
objects with much slower orbital evolution, it is expected that long-term
orbital stability/instability of an object can be discussed to some extent,
without performing direct numerical integrations for a very long period of
time. We here applied this proposed conjecture to the orbital evolution of
1992 QB1, which travels outside the orbit of Pluto with an eccentricity as
small as those of outer planets: a 2.4 My numerical integration of this orbit
(IAUC No. 5684) toward the past did not show any appreciable changes in the
orbital elements. An extrapolation of the RT error growth curve of 1992 QB1
obtained from 0.1-0.4 My integrations seems to indicate that this orbit will
be unstable in 5-10 My. Although this value needs further confirmation in
various respects, this result, if taken at its face value, may suggest that
the time period that 1992 QB1 had stayed in the present orbit is likely to be
much shorter than the age of the solar system.



Nazarchuk G. K.   Chorny G. F.
   Luminescence of Organic Molecules in the Halley Comet

It has been previously observed (December 1985, G. K. Nazarchuk) that there
are two broad features with the centers at 3950 Angstroms (-10,+40 Angstroms)
and 5100 Angstroms (+50 Angstroms) in the continuum of the Halley comet. They
are parts of the scattered solar continuum. The feature at 5100 Angstroms has
been observed as far as 2000 km from the nucleus. The feature at 3950
Angstroms was more long lived and has been observed at the nucleocentric
distances up to 20000 km.

Assumption is making that there are the luminiscence features of unknown
origin. The luminiscent spectra of some alternative sources in the optical
region are qualitatively analyzing: (a) the sputtering of water ice crystals
at temperature below 100 K by electrons with the energy between 20 and 300 eV;
(b) continuous spectrum of C2H radical that arises under photolysis of the
C2H2 molecules; (c) the photoluminiscence of lonsdaleite crystals, which are
the white grains of diamond and may be included in composition of graphite
particles; (d) the photoluminiscence of the polycyclic aromatic hydrocarbons
(PAHs)

The analysis shows that the possible contribution of (b) and (c) sources is
sufficiently slight. Further, process (a) may contribute to the detail at 3950
Angstroms if there are the regions in a cometary atmosphere 1n which the
temperature of the ice crystals is in the range from about 77 K to 100 K. The
photoluminiscence of the small PAHs, in environment where these PAHs are
slightly pushed by H2O and OH molecules, is the most probable.

The candidates amongst PAHs are anthracene (Cl4H10), pyrene (Cl6H10), chrysene
(Cl8H10), anthrol (Cl4H9OH), and other small PAHs with adjacent O, OH, or H2O,
and, probably, ionized anthracene.


Nazarchuk H. K.
   Anomalous Distribution of the Density of Molecules Near the Nucleus of Comet
   Halley

The distribution of the surface brightness in the inner coma of Comet Halley
has been obtained. The series of the spectra that had been observed by TV-
scanner of the 6-m telescope was used for this purpose. On November 15, 1985,
Comet Halley occulted a star. One of two input diaphragms of the spectrograph
was pointed onto the occulted star while another traced the inner coma. There
were 7 spectra in different points of coma within 135 arcsec. The nearest
point of the tracing was at 12 arcsec (6450 km) from the nucleus. The Sun-
comet-Earth angle was about 5 degrees.

The intensities of the continuum (at 4132A, 4387A, and 4594A) and molecular
bands of CN (4214.7A), C3 (4019.4 and 4051.6A), and C2 (4713.2 and 4734.9A)
were measured in the same relative units. The result are given in the table on
the hard copy.

Points #1 and #2 are on the Sunward side of the coma; the other points are
situated on the opposite side. One can see an interesting anomaly in the space
distribution of the monochrome brightnesses. In the narrow range of
nucleocentric distances the intensities of C2 and C3 have inverse dependence
on the nucleocentric distance. This inversion (the minima at 90 arcsec for C3
and at 60 arcsec for C2) is real and can not be interpreted as an instrumental
distortion or error. The continuum as well as the emission of CN does not show
this anomaly clearly. There may be only suspicion that it is present too.

The phenomenon looks like the action of an unknown weak additional source of
C2 and C3 molecules at 30000 and 70000 km correspondingly. Can the CHON-
particles be this source? Why not?


Okamoto H.   Mukai T.   Kozasa T.
   The 10-micrometer Feature of Aggregates in Comets

We have examined the thermal emission from the cometary dust aggregates with
fractal structure, consisting of multi-components. The aggregates are modeled
by (i) 2 components-fractal particles with randomly mixing of olivine and
magnetite, (ii) 2 components-fractal particles with core-mantle structure (the
core is magnetite and the mantle is olivine), and (iii) 2 components-compact
sphere. By using DDA (Discrete Dipole Approximation) and Mie theory with
Maxwell-Garnett theory, numerical simulations of thermal flux from the
cometary grains with the mass distribution detected in comet P/Halley (Mazets
et al. 1986) have been performed for various values of fractal dimension D and
a mixing mass ratio R of olivine to magnetite. It is found that the 10 micron
meter feature mainly depends on D, whereas the temperature of aggregate
strongly depends on R. Types (i) and (ii) generally have the similar emission
features, although type (ii) tends to show slightly higher temperature than
type (i). Our results have also revealed that, when D ~ 2 and R ~ 1, this
aggregate shows the remarkable twin-peaked structure in the 10 micrometer
silicate band, which fits well to that observed, e.g. in comet P/Halley, and
furthermore gives a reasonable temperature close to that observed in P/Halley,
e.g. 385K at a solar distance of 0.79AU.

We have also investigated the far-infrared spectrum of these aggregates and
compared it with observations. Figure 1, which appears here in the hard copy,
shows thermal flux normalized at a wavelength of 8 micrometer from the
aggregates with a fractal dimension D consisting of olivine and magnetite. A
mass ratio of olivine to magnetite is unity. The mass distribution of the
aggregates is applied from that deduced from the dust measurements in comet
P/Halley by Mazets et al. (1986).


Osip D.   Schleicher D.   Campins H.
   Near Opposition Phase Effects in the Dust Coma of Comet Austin (1989c1) and
   Other Comets

Asteroids, the Moon, and other atmosphereless solar system bodies are known to
exhibit phase effects including a brightness surge when observed near
opposition, due presumably to preferential backscattering and/or reduced
shadowing. An opposition surge or other phase effects in the continuum of an
active comet can serve as a diagnostic of the physical properties of the
scattering particles in the coma. We are currently investigating phase effects
as the explanation for the peculiar post-perihelion behaviour of the continuum
of Comet Austin (1989c1).

The figure, which appears here in the hard copy, shows plots log Af-rho, a
measure of the dust production vs. log R, heliocentric distance, with pre-
perihelion observations as open symbols and post-perihelion observations as
solid symbols.

While the post-perihelion dust production originally diminished in a similar
fashion to the volatile production rates, a pronounced surge in the Af value
is quite evident beginning at 1.15 AU (0.06 in log R). This sharp increase in
activity coincides with a two week span of observations in which the phase
angle changed rapidly from 60 degrees to 3 degrees. This increased production
was not observed in any of the emission species, confirming that it is an
observational effect of the changing phase angle on the scattering grains.

The scattering function for the cometary grains is currently being
quantitatively determined for Comet Austin. Furthermore, we are investigating
similar phase effects in the dust coma of Comet P/Halley during the near
opposition observations of Nov. 1985 and in other comets from our photometry
database.


Randall C. E.   Schleicher D. G.
   Long-Slit Spectral Imaging of Comets Austin and P/Brorsen-Metcalf

In this study we present long-slit CCD spectrophotometry of comets P/Brorsen-
Metcalf (1989 X) and Austin (1990 V). The analysis presented here is a
continuation of work previously presented in which we determined Haser
scalelengths for CN, C2, C3 and CH (BAAS 24, p.1002, 1992). The motivation for
this project derives from attempts to understand and define the comparative
compositions of comets. In particular, it is necessary that one precisely
determine the scalelength of a species in order to determine its production
rate, and thus relative abundance, from measurements such as aperture
photometry. Here we will present long-slit CCD spectra acquired over
wavelengths ranging from about 3300 to 10,000 Angstroms. Some of the new data
presented here were acquired with the slit offset from the nucleus, which,
when combined with the nuclear-centered spectra, yield a much larger spatial
extent than otherwise possible. We will also discuss the effects of solar
variability on scalelength determinations, as these can constitute a
significant complication, especially relevant in understanding and modelling
spatial profiles.


Renard J. B.*   Levasseur-Regourd A. Ch.   Le Duin T.
   Polarization Profiles of the Grigg-Skjellerup Coma from OPE/Giotto

Optical polarization probing of the coma of comet P/Grigg-Skjellerup has been
performed by the OPE experiment on board the Giotto spacecraft. Because of the
geometry of the encounter, scans of the inner coma were obtained at a phase
angle of about 90 degrees. Polarization profiles, with a resolution up to 56
km, and a distance to the nucleus smaller than 10,000 km (dark and sunny
sides), are presented for two dust continuum channels and for the C2 gaseous
channel.

Significant spatial variations of the scattered light polarization are
detected and compared with the corresponding brigthness evolution. Four
regions are pointed out: two regions of low polarization correlated with
maxima of brigthness, a region of high polarization associated with a jet from
the nucleus, and an outer coma region (distance to the nucleus in the 1500-
10,000 km range) with no significant variation. These structures suggest some
evolution in the physical properties of the grains.


Reuter D. C.*   Mumma M. J.   Hoban S.
   Sources of the 3.4 micrometer Feature in Comets

Infrared spectra of recent comets have shown excess emission in a relatively
narrow feature centered about 3.36 micrometers, the so-called "cometary
organic feature", which has been ascribed to molecular C-H stretching modes.
Recently, methanol (CH30H) has been detected in several comets in both IR
(1-3) and millimeter wave (4,5) observations. Methanol production rates have
been determined from the 3.52 micrometer V(sub)3 band flux in the infrared and
from individual line intensities in the millimeter wave region. For those
comets observed in both spectral regions, the production rates obtained from
the two techniques agree to within their respective uncertainties. If, as
seems probable, the 3.52 micrometer feature is due to v(sub)3 in CH30H, then
part of the 3.4 micrometer feature must be due to the v(sub)2 and v(sub)9
bands of this species. Using a model developed for the cometary emission
spectrum of CH30H (6) the v(sub)2 and v(sub)9 cometary flux may be estimated
by scaling the model by the same factor required to match the observed v(sub)3
flux. This process is illustrated in the below, where the data are from recent
observations of comet P/Swift-Tuttle on 1992 November 8 (obtained with the
CRyogenic SPectrometer at the Kitt Peak National Observatory lambda/Delta-
lambda = 88), and the solid line is the methanol model. The residual spectrum,
obtained by subtracting the model from the data and shown in the bottom frame,
must then be due to something other than the ground state of CH30H. The
appearance of this residual vries somewhat among the six comets for which IR
data are available, and methanol contributes between 25% and 60% to the 3.4 m
flux. The heliocentric variation and possible progenitors of this residual
emission will be discussed.

1. Hoban, S. et al. 1991. Icarus 92:122-134. 2. Davies, J. et al. 1992. IAUC
5659; Mon. Not. Rov. Astr. Soc., submitted 1993. 3. Hoban, S. et al. 1993.
Icarus submitted 4. Bockele'e-Morvan, M. et al. 1990. ESA SP-315 ,143-148 5.
Paubert, G et al. 1991. IAUC 5653; IAUC 5664. 6. Reuter, D. C. 1992. Ap. J.
386:330-335.


Sandhu S.   Solc M.
   Does the Oort Cloud Exist?

Once accepted, the existence of Oort cloud was never questioned. Nobody except
Lyttleton questioned the existence of the Oort cloud. In this paper, the
formation and existence of the Oort cloud is reviewed. It is found that there
are some serious mistakes in the data of the original orbits of comets. How a
comet such as 1941I can have a bound orbit, which has aphelion distance of
about 31.6 Ly. Two more comets 1897I and 1952VI also come from interstellar
space with elliptical orbits. So the data regarding the original orbits are
erroneous and cannot be relied upon. This single point is sufficiently strong
to discard the concept of the Oort cloud. According to the formation scenario
reviewed in this paper, a spherically symmetrical cloud cannot be formed. Some
more problems, like the clustering of perihelia and aphelia of long-period
comets toward the general directions of solar apex and solar antapex
respectively and the inclination distribution of long-period as well as short-
period comets are also addressed. If we discard the concept of the Oort cloud,
we must search for a reliable alternative for it. A concept of satellite
cloud(s) is proposed to replace the Oort cloud. And it can solve many of the
existing problems regarding cometary origin and dynamics, and opens new
windows to look upon the expanding horizons of new ideas.


Schloerb F. P.*   Lis D.   Schilke P.   Sanders D.   Deane J.   Ziurys L.
   Observations of Parent Molecules in Comet Swift-Tuttle

We present observations of millimeter and submillimeter wavelength rotational
transitions of the cometary parent molecules HCN, H2CO and CO in Comet Swift-
Tuttle. Detections of the HCN J=3-2, HCN J=4-3, H2CO 5(sub)1,5-4(sub)1,4, and
H2CO 3(sub)1,2-2(sub)1,1 transitions were obtained during November 1992 with
the Caltech Submillimeter Observatory 10m antenna. A search for the CO J=3-2
transition was also carried out with this instrument and yielded negative
results. Finally, a marginal detection of the J=1-0 transition of HCN was made
with the Five College Radio Astronomy Observatory 14m antenna during this
period. A spectrum of the J=3-2 HCN transition obtained at the CSO on 12
November 1992 is presented below.

Analysis of the data to determine production rates and other physical
properties of the comet is somewhat complicated by uncertainties in the
ephemerides used to track the comet, since they contained errors comparable to
the beam size of the antenna. However, typical values of the HCN production
rate appear to be in the range of 2 X 10^26 s^-1 during the month of November,
which would be consistent with a nominal abundance of HCN in the comet.
Comparison of observations of the H2CO 5(sub)1,5-4(sub)1,4 transition on 16
November with those of the H2CO 3(sub)1,2-2(sub)1,1 transition on 25 November
suggests a possible increase in the H2CO production rate from approximately 2
X 10^27 s^-1 to 4 X 10^27 s^-1, although we caution that different epherides
were used to track the comet on these two dates. The antenna temperature of
the H2CO 3(sub)1,2-2(sub)1,1 line on 25 November is comparable to that of the
HCN 3-2 line on that same day, which is unusual compared to observations of
previous comets. The H2CO line shape at this time is also unusual and has a
noticable asymmetry with a strong blueshifted wing. Figure 1, on the hard
copy, shows HCN J=3-2 line observed in Comet Swift-Tuttle on 12 Nov. 1992


Schulz R.   Schnur G. F. O.   West R. M.
   Comet Bradfield 19791: Inner Coma Structures in High-resolution Photographs

High-resolution photographs of the long-period comet Bradfield (19791) taken
with the ESO 3.6 telescope at La Silla, Chile were used for a structural
analysis of its inner coma. The photographs were taken on Jan. 23 and 24,
1980, using red-sensitive as well as blue-sensitive emulsions and broadband
filters. They were digitized with a PDS microdensitometer using a step-width
of 20 x 20 micrometers, which resulted in a resolution of about 55 km per
pixel.

To search for structures, in particular jets, azimuthal derivatives of the
images have been calculated (shift angle = 5 degrees) with an algorithm
described by Larson & Sekanina (1984). The blue-light images, reflecting the
gas components for the most part, show a prominent jet originating at the
nucleus, while jetlike features are only slightly indicated in the red-light
images, representing the cometary dust. During the observations the comet was
moving fast across the sky, near the celestial South Pole, which caused
dramatic changes in its position angle. Although this led to a more difficult
guiding and allowed only short exposures, it eventually turned out to be an
advantage for checking the applicability of the enhancement technique. While
the geometry of the comet on the photographic plates rapidely changed during
the observations, the jet in the blue light keeps its position with respect to
the star trails. This proves the jet to be a real feature, which was
permanently present without major positional changes during the whole
observational period of about 19 hours.

Before the discovery of gaseous jets in Comet P/Halley (A'Hearn et al., 1986),
jetlike features have commonly been associated with cometary dust. The
existence of gas jets was confirmed by several authors (Larson et al., 1987;
Cosmovici et al., 1988; Schulz, 1992), but all investigations on the gas and
the dust jets found in Comet P/Halley led to the result that there is no clear
correlation between the geometry of the gas jets and dust jets visible in the
coma (Larson et al., 1987; Cosmovici et al., 1988). For Comet Bradfield also
no relation between gas and dust features could be pointed out, and even the
evidence for jets in the red-light images is only vague. The weakness of the
enhanced dust features (if actually present) in comparison to the distinct
gaseous features may be due to the unusual high emission-to-continuum ratio
measured in Comet Bradfield, which was one of the gasiest comets ever observed
(A'Hearn et al. 1981). Since gaseous jets can be observed in short-period
comets like P/Halley as well as in long-period comets like Bradfield (19791),
their existence in cometary comae seems to be a more ordinary phenomenon than
expected. However, they are not necessarily related to the dust particles
visible by the refection of the sunlight, which leads to constraints on their
formation mechanism.

References: A'Hearn M. F. et al., Astron. J., 86, 1559-1566. A'Hearn M. F. et
al. (1986) Nature, 324, 649-651. Cosmovici, C. B. (1988) Nature, 332, 705-709.
Larson S. M. and Sekanina Z. (1984) Astron. J. 89, 571-578. Larson S. M. et
al. (1987) A & A 187, 639-644. Schulz, R. (1992) Icarus 96, 198-203.


Scotti J. V.
   Comet Nuclear Magnitudes

Calibration of the sizes of cometary nuclei is dependent on the determination
of accurate comet nuclear magnitudes. The measurement of nuclear magnitudes
are hindered by the intrinsic activity of comets. When the nucleus would
otherwise be most visible, the comet is most active and the nucleus itself
contributes only a small fraction of the total light from the comet.
Therefore, measurement of comet nuclear magnitudes has generally been limited
to when the comet is at relatively large distances. During the course of a
systematic program of faint comet astrometric observations using CCD's
attached to the Spacewatch Telescope on Kitt Peak, a large and relatively
consistent set of measurements of faint, nearly quiescent comets has been
obtained. Table 1 contains nuclear absolute magnitudes for some comets for
which Roemer (1966) measured photographic nuclear magnitudes. In the table,
the observation interval is given; r(sub)max and r(sub)min is the range in
heliocentric distances (in AU) over which the measurements were made; Sigma
H(sub)n is the average nuclear absolute magnitude; sigmaH(sub)n is the
standard deviation of the absolute magnitude; R(sub)0.02 and R(sub)0.03 are
the derived nuclear radii assuming albedoes of 0.02 and 0.03 respectively. The
albedo 0.02 is the lower limit used by Roemer, and 0.03 represents the present
best estimate of the typical cometary albedo. Once reliable nuclear magnitudes
have been determined, the size-frequency distribution of the comet population
can be studied (cf. Fernandez et al. 1992). Table 1, which appears here in the
hard copy, shows nuclear absolute magnitude measurements made with Spacewatch
of comets whose sizes were estimated by Roemer.

References. Fernandez, J.A., H. Rickman, and L. Kamel 1992. The population
size and distribution of perihelion distances of the Jupiter Family. In
Periodic Comets (J.A. Fernandez and H. Rickman, Eds.) Proceedings of a
Workshop held in Montevideo, Uruguay, August 5-7, 1991, pp. 143-157.
Universidad de la Repubilca - Facultad de Ciencias. Roemer, E. 1966. The
Dimensions of Cometary Nuclei. Mem. Soc. R. Sci. Liege 12, 23-28.


Seidensticker K. J.
   Gas Dynamics of a Sublimating Cometary Analogue

The gas released by sublimation from a cometary nucleus cannot be in
thermodynamic equilibrium near the surface because it can only expand into the
half-space of the vacuum side. Only after many collisions between molecules of
different velocities, many free paths away from the surface, the gas will re-
establish a new (drifting) Maxwell-Boltzmann velocity distribution. Huebner
and Markiewicz (1992, unpublished) described the velocity structure of such a
gas and calculated some essential parameters. But in reality it is difficult
to predict the correct velocity distribution. Many poorly known parameters
like surface temperature or the size and structure of a dust mantle lying
above the sublimating surface modify the primary velocity distribution.

The KOSI collaboration (KOSI = German abbreviation for COmet SImulation)
studies the behaviour of ice-dust mixtures (cometary analogues) under
simulated space conditions. Within this program we try to determine the
dynamics of the sublimated gas which is necessary for an understanding of the
gas-particle-interaction. Apart from testing the model mentioned above, one
can determine the important factors which influence the velocity distribution
and one can support the calculation of the energy balance and of the thermal
evolution of a sample.

In order to measure the gas velocity we constructed a device, called gas-
chopper,  where the gas flow from the sample is periodically interrupted
(chopped) and the resulting density variation measured. As the gas has a
finite velocity and the distance between chopper and gauge is about 0.2 m we
measure time-delayed convolution of the velocity distribution and the
interruption profile. The device and the results achieved so far will be
described with emphasis on the last KOSI experiments where measured for the
first time the velocity distribution at about 1 m distance from the sample
surface.


Sitarski G.   Todorovic-Juchniewicz B.
   Linkage of All the Apparitions of Comet P/Wolf

We collected 596 observations from 14 apparitions of the comet during 1884-1992.
Equations of motion of the comet have been integrated by the recurrent power
series, including all the planetary perturbations. We also took into account
nogravitational effects in the comet's motion using the known Marsden's method
but with the four parameters A, eta, I, Phi connected with the rotating nucleus
of the comet; a precessional motion of the spin-axis of the comet's nucleus was
taken into account by determining the linear daily change of the angle Phi. Thus
it was possible to link all the observations of the comet with the RMS residual
equal to 1."65.


Suzuki B.   Kurihara H.   Watanabe J.
   Gaseous Jets of P/Hartley 2

CCD imaging observations of P/Hartley 2 were carried out with 188 cm reflector
on August 12-15 1991. Four narrow-band filters were used for C2, CN emissions
and their corresponding continuum. After processing their images carefully by
applying azimuthal enhancement technique, we obtained asymmetric coma
structure as shown in figure 1 on the hard copy. Furthermore, we found a
difference in radial profiles between the enhanced region of coma and the
background. These results indicate an existence of gaseous jets of C2 and CN
Components in P/Hartley 2. We discuss about the structure of the gaseous jets
in this paper.


Svoren J.
   Photometry of Comet P/Brorsen-Metcalf at the Skalnate Pleso Observatory

The analysis of the pre-perihelion photoelectric measurements of  P/Brorsen-
Metcalf made at the Skalnate Pleso Observatory is presented. The photoelectric
measurements of P/Brorsen-Metcalf in August 1989 were carried out with IHW/IAU
filters CN, C2, C3, CO+, Cont.365.0 and Cont.484.5. Two focal diaphragms,
49.0" and 220.5" in diameter, were used. The magnitudes, fluxes and total
numbers of molecules are presented.

The results with the values of P/Halley at equal heliocentric distance,
obtained with the same method and instrument, are compared. P/Brorsen-Metcalf
emitted considerably less molecules in the same column of the coma than
P/Halley. The ratios in the CN-emission and C3-emission are 1:91 and 1:23,
respectively.

Correlation of CN with dust was confirmed.

The photoelectric C2-magnitudes in the larger diaphragm agree with the mean
values of the total visual estimates.


Szutowicz S.
   Connection Between Nongravitational and Rotational Motion of Comet P/Kopff

The orbital and rotational motion of periodic comet Kopff was investigated using
observations made during its 12 returns to the Sun. To correct the orbit of the
comet a model of nongravitational effects varying with time was used. The
magnitude of the nongravitational acceleration was computed from the variation
in the sublimation rate with position of the comet in orbit. The sublimation
rate derived from light curves was taken from Sekanina [1]. Direction cosines of
the nongravitational acceleration in orbital coordinates depend on the lag
angle, the equatorial obliquity of the cometary nucleus, and the cometocentric
longitude of the Sun. These angular parameters and orbital elements are
calculated from observational equations in the iterative process of the orbit
improvement.

References:  [1] Sekanina (1984) Astrophys. J., 89.


Tozzi G. P.   Baffa C.   Di Giacomo A.   Lisi F.   Hunt L.   Stanga R.
   Infrared JHK Imaging of Comet P/Schaumasse (1992x) During the Pre and
   Post-Perihelion Phases

Images of comet P/Schaumasse (1992x) in the J, H and K bands were obtained
during the middle of February and March 1993 with the new infrared imaging
camera (ARNICA) at the Italian TIRGO telescope. The field of view was about 4
arcmin with a pixel size of 1 arcsec. Since the emission in these IR bands is
due to solar radiation scattered by the dust, these images are particular
useful for the study of dust component of the inner coma. The data have been
analyzed in order to determine the possible presence of fans and jets. The IR
color index has been also measured in selected regions with different
nucleocentric distances and position angles to check the possible change of
the dust composition in the coma.


Tozzi G. P.   Feldman P. D.   Weaver H. A.
   Search for the Near Infrared Intercombination Lines of Carbon in Comet
   P/Swift-Tuttle (1992t)

We have previously discussed the question of the formation of carbon atoms in
the metastable level ^1D in a cometary coma produced by photodissociation of
CO (Tozzi, Feldman and Festou - 1992, BAAS, 24, 1019), and have proposed that
a measurement of the prompt CI (^1D-^3P) emission at 9823.4 and 9849.5
Angstrom would provide a way to measure Q(CO) from the ground. We have
attempted such a measurement on comet P/Swift-Tuttle (1992t) which was
observed on 12th and 14th of November 1992 in the near-IR, at Loiano Obs. (BO,
Italy). The spatial and spectral resolutions were about 3 arcsec and 5
Angstroms respectively and the slit length projected on the sky was 4 arcmin.
Near simultaneous observations with IUE were made to determine the brightness
of the CI (^1D-^1P) transition at 1931 Angstroms and of the carbon resonance
multiplets at 1657 and 1561 Angstroms. The CN (1-0) and (2-1) bands are
clearly visible in the whole 9000-10200 Angstrom spectra. Of the two carbon
lines, the weaker one is undetectable over the noise level, while the other
one at 9849.5 Angstroms is blended with the OH (3-0) P(sub)2(2) sky line at
9848.48 Angstroms. To identify this CI line it has been then necessary to
accurately subtract both the dust scattered solar continuum and night sky
lines. Since residual OH (3-0) lines remain on the cometary gas spectra we
consider the possibility that they are produced by OH prompt emission.


Valtonen M. J.   Innanen K. A.   Matese J. J.   Whitman P. G.
   Solar Motion in the Galaxy and Its Effect on the Comet Flux via Changing
   Tides

Galactic tides are known to cause a significant and probably dominant
perturbing effect on the orbits of Oort Cloud comets. The magnitude of
Galactic tides varies during the quasiharmonic oscillation of the Sun about
the Galactic plane as well as with epicyclic motion of the Sun in the Galactic
plane. We construct models where these two periods are about 60 m.y. and 200
m.y. The corresponding variations in the flux of new comets show strong
variations in the quasiperiods of 30 m.y. and 200 m.y. The flux of Earth
crossers can vary by an order of magnitude during the shorter cycle and it is
further modulated by a factor of 3 in the longer cycle.


Verkhoglyadova O. P.*   Kotsarenko N. Ya.   Churyumov K. I.
   Dynamics of Cometary Tail Plasma Structures During Comet Motion Within the
   Solar System

On the basis of a previous study [1], we believe that the large-scale ray
structure of cometary plasma tails is the result of the filamentation
instability development due to the solar wind flow penetrating into the
cometary plasma. Visual change in above structure during comet motion around
the Sun is the theme of our consideration. Timescales of the variations of
plasma parameters are assumed to be sufficiently larger than instability
development time. Cometary rays appear at the distances of r = 9 AU from the
Sun as were observed for Humason (1959 X) comet [2]. A minimum distance from
the Sun where the rays are distinguished may be thought of as 0.6 AU for
Halley (1910 II) [3] and even as 0.46 AU for Kohoutek (1973 XII) [4] comets.

Cometary plasma density and that of solar wind may have a profound effect on
the maximum ray size l(sub)max [1]. Inasmuch as cometary plasma ion density
dependence on r may be considered as n(sub)i2 = n(sub)0i2 x r^-3.3 [5] and
that of solar wind protons as n(sub)i1 = n(sub)0i1 x r^-2 ,where n(sub)0i1 and
n(sub)0i2 are the proper densities at r = 1 AU, l(sub)max depends on r
approximately as r^0.65. By virtue of the fact that the condition
omega(sub)pi2 >> omega(sub)pi1 is one of the assumptions of the theory the
maximum ray size is expected to be 2 x l(sub)max (r = 1 AU) at r = 2.5 AU. The
low limit of this value is reached at r = 0.46 and is estimated as 2l(sub)max
(r = 1 AU)/3. The above values are not too different from the mean value [1].
At greater distances where omega(sub)pi1 >> omega(sub)pi2 cometary rays
disappear gradually. Generation of helical forms in cometary plasma tails is
also discussed.

References:  [1] Verkhoglyadova O. P. et al. (1993) Lett. Astron. J. (in
Russian), in press. Verkhoglyadova O. P. et al. (1992) Bull. AAS, 1024, 24.
[2] Guigay G. (1966) in Nature et Origine des Cometes, Institut
d'Astrophysique Cointe-Sclessin, Belgique, 369. [3] Donn B.et al. (1986) Atlas
of Comet Halley (1910 II) NASA SP-488, Washington, DC. [4] Jockers K. (1985)
Astron. Astrophys. Suppl. Ser., 791, 62. [5] Churyumov K. I. et al. (1990) Ap.
J., 687, 354.


Voelzke M. R.   Schlosser W.   Schmidt-Kaler Th.
   Analysis of the Plasma-Coma of Comet P/Halley with Image-Processing of Data
   from Bochum's Archive

Thirty-four glass plates (32 with CO+ filter and 2 with N2+ filter) with a field
of view of 28.6 x 28.6 degrees and a resolution of approximately 30" were
digitized with a PDS 2020 microdensitometer (1 pixel = 25 x 25 micrometers =
47" x 47"), analysed in relative intensities, or when possible in absolute
intensities. The CO+ filter glass plates were taken from 1986 March 29, to April
17 with an exposure time between 20 and 120 minutes. With the help of
Stellingwerf-Theta-Minimum-Method, a period of 2.14 days results from the
analysis of the structure in the plasma-coma obtained by subtracting subsequent
images.


Wagner S.   Kiselev N. N.   Jockers K.   Kuppers M.
   Spectra of Comet Swift-Tuttle 1992t Obtained with the Twin Spectrograph at
   the 3.5m Telescope of Calar Alto Obseratory

The twin spectrograph of Calar Alto Observatory consists of two
spectrographs (arms) that are fed from a single slit via a color
divider. This allows two spectra (one in the red arm, the other
in the blue arm) to be recorded simultaneously, so that emissions
widely separated in wavelength can be compared.

On the evening of November 18, 1992, four twin spectra of comet
Swift-Tuttle 1992t were obtained with this instrument at the
Calar Alto 3.5-m telescope. In the blue arm an RCA CCD with 1024
x 640 pixels of 15-micrometers-squared size and in the red arm a
GEC CCD with 1152 x 770 pixels of 22.5-micrometers-squared size
was used. The dispersion was 1.6 angstroms per pixel in the blue
arm and 1.1 angstroms per pixel in the red arm respectively. The
first two twin spectra cover the range 3000-4720 angstroms in the
blue arm and 6000-7250 angstroms in the red arm. The remaining
two twin spectra cover the range 3300-5160 angstroms in the blue
arm and 7130-8200 angstroms in the red arm. The slit length,
projected in the sky, is 4 arcmin. The slit was oriented along
the solar-antisolar direction, which at the time of the
observations was not far from perpendicular to the horizon, to
minimize refraction effects. The comet was held at the center of
the slit. At the end of the observations the elevation of the
comet was 11 degrees. A spectroscopic standard star and the solar
type stars 16 Cyg A and B were recorded to allow absolute
calibration and subtraction of the dust continuum. Unfortunately,
the observing conditions were mostly nonphotometric. The spectra
show with high signal/noise the neutral emissions of OH, NH, CN,
C3, CH, C2, [OI], and NH2. The ion emissions are much fainter.
CO2^+, OH^+, CO^+, N2^+, and H2O^+ are present.

The spectra are being processed to extract and compare the
emissions of the blue and red CN bands, of OH and [OI], of NH and
NH2, and of all the ions.


Watanabe J.*   Watanabe H.
   Activity in Distant Comets by Dust Tail Analysis: A Case of Comet Levy
   1990XX

Some of new comets show the extended dust tail, which was formed at large
heliocentric distance such as 10 A.U. or more. This "old" tail can be used to
study the cometary activity in the large heliocentric distance. Moreover, the
history of the activity can be explored by the analysis of the intensity
distribution of the tail on the basis of some assumptions. We analyzed the
dust tail of Comet Levy 1990XX, which was observed on August 20, 1990, with
105-cm Schmidt telescope at the Kiso Observatory, and found a gradual increase
of dust production rate from 15 A.U. as shown in figure. This result indicates
that volatile material such as CO2 or CO controlled the cometary activity of
this new comet at the large heliocentric distance.

The figure, which appears in the hard copy, shows relative production-rate
variation with heliocentric distance derived from the dust tail analysis of
Comet Levy 1990XX.


Wootten A.   Latter W.   Despois D.
   Molecular Rotational Emission Lines Observed in Comet P/Swift-Tuttle

The 12-m telescope of the NRAO on Kitt Peak, Arizona, observed comet P/Swift-
Tuttle during two periods in December 1992, before and after its December 12,
1992, perihelion.  The facility NRAO receivers, using SIS mixers, were used in
the two 3-mm bands (65-90 GHz and 90-115 GHz) and in the 1-mm band (200-270
GHz).  Single sideband system temperatures on the sky were 150 K in the 3-mm
band, and 500 to 1000 K in the 1.3-mm band, the latter during poststorm
clearing. The spectrometers consisted of two pairs of banks of filters, two
polarizations times 128 channels of 100 kHz resolution and 128 channels of 250
kHz resolution at 3 mm.  At 1.3 mm, two polarizations times 128 channels of 250
kHz resolution and 128 channels of 500 MHz resolution were employed. In
addition, the NRAO Hybrid Spectrometer was used at its 37.5-MHz bandwidth at
3 mm and 75 MHz at 1.3 mm, resulting in resolutions of 48.8281 kHz at 3 mm and
97.6562 kHz at 1 mm, with 768 channels in each of two polarizations. Orbital
elements calculated from 1992 observations by Marsden, communicated to us by
Crovisier, were used on December 2, 1992.  On December 30-31, 1992, elements
from Minor Planet Circular 21235 were used. Emission was detected on all three
days of observations from the HCN molecule, while no emission was observed from
either HDO or CH3OH lines at the observed frequencies. The observed molecular
transitions are given in Table 1 along with the dates of observation, distance
from Earth, integrated intensity, and radial offset of the beam center from the
comet position as reported in Minor Planet Circulars.

TABLE 1 appears here in the hard copy.


Wu C. Y. R.   Yang B. W.   Judge D. L.
   Study of Photon Sputtering of H2O Ices, D2O Ices, CH3OH Ices, and H2O-CH3OH
   Clathrates

Measurements of the total yields of ejected ions and electrons from H2O ices and
D2O ices have been carried out using the He I 584 angstrom resonance radiation
as the incident photon beam. The ices were prepared at 77 K in an ultrahigh
vacuum system. The total yield of the ejected ion species and electrons in D2O
ices was determined to be 8.6 x 10^-5 and 4.8 x 10^-4 respectively. The results
agree very well with those of H2O ices [1], suggesting that the isotopic effects
do not play any significant role in the ion desorption processes of water ices
at 584 angstroms. We are currently obtaining similar data on CH3OH ices,
H2O-CH3OH clathrates, and D2O-CH3OH clathrates. The detailed results and the
experimental setup will be discussed. The data obtained will be useful in
understanding the atmospheres of comets, asteroids, icy planets, satellites and
rings, and interstellar dust grains.

References:  [1] Wu C. Y. R. and Judge D. L. (1990) in NASA CP-3077, 414.


Xie X.*   Mumma M.
   Evidence for Rotational Cooling of Water Molecules: Monte Carlo Simulation
   of Cometary Atmosphere

The model of an isotropic cometary atmosphere, made up of H2O and its daughter
radicals (H, OH, O and H2) has been successfully run on the sunstation. The
model was checked against the cases of purely adiabatic expansion and
vectorial model of OH and was shown to be consistent with the results from
numerical techniques for each case. The simulated velocity profile is in good
agreement with Giotto measurement of Comet Halley in March, 1986 (Fig. 1).
Since the semi-classical cross-section model gives a more realistic
description of collisional process than the hardsphere model, therefore
rotation cooling of water molecules is needed to fit the measured outflow
velocity.


Yi Y.   Brandt J. C.
   Analysis of 4 Disconnection Events (DEs) in Halley's Comet

Study of DEs for the purpose of determining the physical processes responsible
involves the kinematics and solar-wind conditions for each event. We present
the circumstances for 4 DEs in Halley's Comet listed by the kinematic time of
disconnection:

1) Dec. 13.5, 1985. The sector boundary was detected by PVO, Vega-1, and shown
by geomagnetic indices. Additional solar-wind data is sparse;
2) Dec. 31.2, 1985. The sectory boundary was detected by IMP-8, PVO, ICE, and
Vega-1. The solar wind conditions measured by IMP-8 are as follows. At the
sector boundary, the density was around 10/cm^3, the solar wind speed was
about 400 km/sec, which was decreasing slowly from 700 km/sec three days
earlier.
3) Feb. 21.7, 1986. This is a DE with but a single known image.  The time of
disconnection is determined by assuming average kinematics. The sector
boundary was detected by IMP-8, ICE, and Vega-1. The solar wind conditions
come from IMP-8; the density was 7/cm^3, and the speed was 550 km/sec.
4) Feb. 28.7, 1986. The sector boundary was detected by the geomagnetic
indices and PVO. The plasma data from PVO shows that the density was 35-
70/cm^3 (corresponding to 17-34/cm^3 at 1 AU) and the speed was 340 km/sec.

These DEs show various degrees of correlation with different physical models,
and for some the in situ data is sparse.  However, consideration of all
obvious DEs in Halley's Comet, 1985-1986, greatly reduces the impact of
uncertainties in single events. These DEs are included in the investigation by
Brandt, Caputo, Niedner, and Yi (this meeting) which concludes that DEs are
strongly associated with sector boundaries and not with high-speed streams.


Zheng J. Q.   Valtonen M. J.   Korpi M.   Mikkola S.   Rickman H.
   Orbits of Short Period Comets Captured from the Oort Cloud

Oort Cloud comets occasionally obtain orbits that take them through the
planetary region. Then perturbations by the planets are likely to change the
orbit of the comet to the extent that it is either lost into interstellar
space or it becomes a short-period comet. We model this process by using a
Monte Carlo method and cross sections for orbital changes, i.e., changes in
energy, inclination, and perihelion, in a single planet-comet encounter. The
influence of all planets is considered. We study the distributions of orbital
parameters of observable comets, i.e., those that have their perihelia smaller
than a given value. Both Jupiter family and Halley group comets can be
identified with reasonable distributions of orbital elements.






                                POSTER SESSION
                            METEORS AND METEORITES



Adolfsson L. G.   Gustafson B. A. S.
   Dependence of Meteor Beginning Heights on Meteoroid Rotation

The temperature profile for a meteoroid as it penetrates the Earth's atmosphere
is highly dependant on the dimension of the meteoroid. The first approach was
taken by Whipple (1950) where he neglected the heat capacity in
micrometeorites. The theory was further developed to account for thermal
capacity as well as thermal conduction (Levin (1965)). By neglecting therrnal
radiation from the surface Ceplecha and Padevet (1961) were able to get an
analytical solution for a rapidly and randomly rotating sphere. In the same
manner, i.e. neglecting thermal radiation, Pecina (1991) could account for
rotation of the spherical meteoroid to get analytical expressions for the
temperature.

We integrate the general heat conduction equation, which is shown here in the
hard copy.

In our approach we can quite generally let our heat flux W contain both thermal
radiation and thermal capacity. Since we solve the problem for any point in the
body we obtain an exact expression for the surface boundary conditions.

Using this method we investigated the dependance of meteor beginning heights on
rotation of the meteoroid. The beginning height of a non-rotating sphere can
differ by as much as 10 km from the beginning height of a rapidly and randomly
rotating sphere, when the meteoroids are large enough to have a temperature
gradient built up within the body. In conclusion, the rotational state of a
large meteoroid during atmospheric flight can not be ignored in calculating
beginning heights.

Ceplecha, Z., Padevet, V., Bull. Astron. Chech. 12, 191-195, (1961).
Levin, B.Y., The Physical Theory of Meteors and Meteoric Matter in the Solar
System, Izdatel'stvo Akad. Nauk SSSR, (1965), [in Russian].
Pecina, P., Bull. Astron. Chech., 42, 359-376, (1991).
Whipple, F.L., Proc. Nat. Acad. Sci. Amer., 36, 687-695, (1950).


Adolfsson L. G.   Gustafson B. A. S.
   Meteoroid Material and Beginning Heights Within Meteor Streams

One of the most reliably determined quantities from stereoscopic meteor
observations is the beginning height. The luminosity of a meteor increases
abruptly at the onset of ablation where an observable meteor trail soon
begins. This usually precedes any fragmentation, which simplifies modeling.
The beginning height depends on the meteoroid trajectory, entry geometry and
meteoroid morphology. Ceplecha (1968) may have been first to use beginning
heights to distinguish classes among photographed meteors (McCrosky and Posen
(1961)), which he ascribed to differences in meteoroid densities.

We develop a formalism for modeling beginning heights accounting for heat
diffusion with thermal radiation as well as vaporization and cooling due to
removal of latent heat. Restricting the study to stream meteors simplifies the
problem in that meteoroids within a given stream arrive along parallel
trajectories at near equal velocities and may be assumed to be of similar
material. Stream meteor data cover a range of zenith distances and meteoroid
dimensions which we use to constrain the meteoroid material and morphology.

Ceplecha, Z., Smithsonian Astrophysical observatory Special Report No. 279, 54
pages, (1968).
McCrosky, R.E., Posen, A.,, Smithsonian Contributions to Astrophysics, 4,
15-84, (1961).


Andreev G.
   Determination of Some Meteor Streams Characteristics Based on the Earth's
   Path Length

The length of the Earth's path in meteor streams is one of the most exactly
measured values (3-4 orders better than others). This article shows the
possibilities of using the Earth's path length for the solution of the
following tasks:

* Determination of the size and form of the cross section of the meteor
stream's observed parts;

* Estimation of the upper limit of the real dispersion of orbital elements,
coordinates of radiants and velocities of meteoroids in the meteor streams;

* Calculation of the upper values of particles' ejection velocities from
family bodies by comparison of the measured length of the path depended on
dispersions of orbital elements with the theoretical one, which is a function
of the ejection velocities;

* Determination of the total mass of streams (lower estimation of mass of the
family body) by means of known activity curve of the flux particles and the
length of the Earth's path between the lines of equal density of the meteor
flux;

* Calculation of the upper estimations of the meteor stream's age.

It was shown that in many cases even the upper estimations of the above-
mentioned parameters are less than their observed values obtained with the
help of the most exact photographic method. This is not only telling of the
poor precision of the observed data but demonstrates that it makes sense to
use the estimations obtained with the help of Earth's path length in the
statistical problems of meteor astronomy.


Andreev G. V.   Kashcheyev B. L.   Kolomiets S. V.
   Hyperbolic Meteoroid Flux Near Earth Orbit

This article deals with the hyperbolic meteors observed in Earths atmosphere.
A very highly sensitive radar method of meteor observation has been developed
at the Kharkov Radioelectronic Institute. During a two-year cycle of
observations the orbital elements were determined for 100,000 meteors; the
number of hyperbolic meteors is 1.1%. This paper analyzes the reality of
hyperbolic orbits of observed meteors. Specifically, we have been able to show
that 25% of such orbits can turn into elliptic orbits by using more correct
celestial mechanics expressions. Nevertheless 5% of the hyperbolic orbits are
unexplained by errors of observations and calculations.

The distributions of the orbital elements of observed meteors were compared
with the theoretical one in the search for the hyperbolic meteor sources. This
report analyzes such possible sources as cometary decay, collisions in the
asteroidal belt, close approaches to the planets, and capture of interstellar
dust by the solar system.


Andreev V. V.
   Comparison of Orbital Element Distributions for Different Meteor Catalogues
   and Possible Structure of Sporadic Meteoroid Complex

The division of the sporadic meteoroid complex depending on inclination and
Tisserand's constant of meteoroid orbit into several groups has been carried
out in previous author's papers (Andreev, 1988, 1992). As the result of
peculiar orbits of particles has been found. The orbits are not similar ones
of comets and asteroids.

The conditional orbital element distributions of meteors for fixed values of
inclinations have been analysed for five meteor catalogues to determine
dependence of the distributions on inclination and mass, and to carry out more
accurate division of meteoroid complex into groups. The catalogues are:
catalogue of photographic meteors (McCrosky and Posen, 1961), two catalogues
of radar meteors obtained at Adelaida, the results of radar meteor
observations in Soviet equatorial expedition in Somalia, and in Kharkov.

References: Andreev V. V. (1988) Second GLOBMET Symposium, Abstracts, 17-18.
Andreev V. V. (1992) Asteroids, Comets, Meteors 1991 (A. Harris and E. Bowell,
eds.), 5-8, LPI, Houston. McCrosky R. E. and Posen A. (1961) Smithson.
Contrib. Astrophys., 4, No. 2, 15-84.


Andreev V. V.   Belkovich O. I.   Evdokimov O. I.
   Mass Distribution of Sporadic Meteor Bodies

It is well known that the masses m of sporadic meteor  bodies are distributed
as P(m) = (s-1) m(sub)o^(s-1) m^-s, where m(sub)o is the minimal value of mass
and s is the mass law exponent. The problem is the variations the value of s
over the celestial sphere, with time, and whether it depends on the value of
meteoroid mass and on some of elements of meteoroid orbits.

Radar observations carried out in Kazan were used to find out the variations
of the mass exponent s over the celestial sphere. Two methods of determination
of s were applied: the first one is the calculation of s from the comparison
of the sporadic meteor flux densities (Andreev et al., 1992) at several
amplitude levels and the second one is the determination of s from amplitude
distributions obtained for different areas at the celestial sphere find out
similarly to the flux density. Both methods have given similar results:  The
value of s does not depend on the times of a year but it is the function of
the elongation angle epsilon from the Earth apex only. The approximation is

s(epsilon) = 2.025 - 0.21 cos(epsilon) + 0.076 [sin^3 (1.52 epsilon) -
|sin^3 (1.52 epsilon)|].

The minimal value of s = 1.82 corresponds to epsilon = 0 and maximal one o s =
2.16 is at epsilon = 140 degrees. Analysis of photographic (McCrosky and
Posen, 1961) and television (IAU Meteor Data Center at the Lund Observatory,
Sweden) observations of meteors confirms the radar results but variations of s
are more strongly pronounced: minimal and maximal values of s are 1.44, 1.22
and 2.52, 2.31 for photographic and TV data respectively. Selections of
observation methods have been taken into account.

References:  Andreev V. V. et al. (1992) Asteroids, Comets, Meteors 1991 (A.
Harris and E. Bowell, eds.), 17-22, LPI, Houston. McCrosky R. E. and Posen A.
(1961), Smithson. Contrib. Astrophys., 4, No. 2, 15-84.


Andreev V. V.   Belkovich O. I.   Suleymanova S. L.
   Some Peculiarities of Velocity and Radiant Distributions of Sporadic
   Meteoroids

It was found from radar observations (Andreev et al, 1992) that sporadic
meteor radiant distributions P(epsilon,psi) have a symmetry relative to the
plane crossing the Earth's apex, ecliptic pole and antapex. Here epsilon is
the meteor radiant elongation from the Earth's apex, and psi is the azimuth
angle measured between the ecliptic plane and plane crossing through apex,
radiant and antapex. The radiant distribution averaged over a year has also
the symmetry relative to ecliptic plane. This fact confirms Opik's suggestion
about the uniform distribution of perihelion arguments of meteoroid orbits
with given values of a, e, i. Then averaged sporadic meteor radiant
distribution over entire celestial sphere can be represented by one quarter of
the sphere.

Conditional distributions of meteor velocities u P^x(sub)epsilon,psi (u)
obtained from catalogues of sporadic meteors from photographic and radar
observation have been analyzed. It was found that the distributions do not
depend on the azimuth angle psi.

Conditional distributions of meteoroid velocities P(sub)epsilon (u) have been
obtained taking into account the selectivity of the observation method and the
variation of the mass exponent s over the celestial sphere.

References:  Andreev V. V. et al. (1992) Asteroids, Comets, Meteors 1991 (A.
Harris and E. Bowell, eds.), 17-22, LPI, Houston. Opik E. J. (1951) Proc. R.
I. A., 54, Sec. A, No. 12, 165-199.


Brown P.   Wetherill G. W.   Beech M.   Hawkes R. L.
   Video Observations of the Fall of the October 9, 1992 Peekskill Meteorite

At 2350 UT on October 9, 1992 a "brighter than full moon" fireball was
observed over an approximately 10^6 km2 area of the eastern USA and Canada.
Visual observations were reported from locations as distant as Kentucky.  The
object had a very shallow trajectory and a visible flight time of about 30
seconds.  The approximately SW to NE atmospheric flight path began near the W.
Virginia-Virginia border in the vicinity of Roanoke, VA, and passed over
Maryland and East-central Pennsylvania.  A 12.4 Kg fragment, identified as a
metamorphosed H-class ordinary chondrite, was recovered in Peekskill, NY,
about 75 Km north of New York City.

The event was recorded by a number of professional and amateur video
photographers and is the first meteorite fall for which video recordings are
available.  We have acquired 17 videotapes of observations extending from
North Carolina to Ohio and New York State.  By use of surveying instruments,
we are measuring the positions of foreground objects in the videos, permitting
the meteorite's trajectory to be placed in an absolute reference frame.  The
video data is being analyzed with the intention of extracting the atmospheric
trajectory and preatmospheric orbital elements of the meteorite.  We are also
studying the fireball's fragmentation and tail dynamics, which the video
record shows in detail never previously available.  The video data show that
the meteorite broke into dozens of pieces, of which at least several large
fragments survived its terminal break-up.  These fragmentation events will be
illustrated, together with a map showing the meteorite's flight path.  So far,
only one of the fragments has been recovered, after it struck a parked car.


Ceplecha Z.
   Impacts of Meteoroids Larger Than 1 m into the Earth's Atmosphere

Statistical data on meteoroids based on precise photographic and television
observations enabled rather detailed classification of bodies in the size
range from 0.3 mm to 1 m. This classification is based on nontrivial
differences of the atmospheric penetration ranging over air density ratio of
1:1000. Orbits cannot be used to classify bodies of this size as cometary or
asteroidal.

                        characteristic orbit    assumed
population      rho     sigma   for 0.1-1 m size                composition
        g/cm^3  s^2/km^2        a       e       i
I       3.7     .017    2.4     .68     ~6 deg  stony
II      2.0     .041    2.3     .61     ~5 deg  carbonaceous
IIIA    0.75    .10     2.4     .82     ~4 deg  cometary
IIIAi   0.75    .10     ~*      .99     random  cometary
IIIA[C3]        0.75    .10     2.7     .67     random  cometary
IIIB    0.27    .21     3.0     .70     13 deg  soft cometary

rho     bulk density
sigma   ablation coefficient

Except satellite tracking of the daylight fireball of Aug. 10, 1972, available
data have been published on only 13 bodies with initial sizes over 1 m (all
below 10 m) photographed within the fireball networks. Data on them all will
be given in this paper. Except the Pribram body, all these bodies are only of
type II and IIIB. The terminal heights and the heights of maximum light differ
by several tens of kilometers for these two types of meteoroids.

Taking advantage of the detailed knowledge in the 0.1 to 1-m size range, we
can extrapolate to the 1 to 10-m size range and compare the results with the
small number statistics of 14 largest observed meteoroids. We can also compare
the total number of all meteoroids with the statistics of the smallest
Spacewatch objects. Differences in importance of individual populations
(types) of meteoroids are rather large and very much changing with size.
Numbers of meteoroids coming to the whole Earth's surface per year for two
size intervals follow.

size [m]        I       II      IIIA    IIIB    all     bar rho
0.1-1   20,000  48,000  38,000  21,000  127,000 1.1
~1-10   ~~190   ~1,290  ~~340   ~~980   ~~2,800 0.5

Average bulk density of meteoroids, bar rho, can also be computed by comparing
the mass dependence of meteoroid numbers with the size dependence. The change
of average bulk density corresponds to relative increase of the cometary
populations, namely of the IIIB bodies, with increasing size.

In the following table, the relative percent numbers of the three basic
populations demonstrate the large relative deficiency of the stony material
among the 10-m-sized bodies (I * stony, II * carbonaceous, III * cometary).
The majority of the 10-m-sized meteoroids are cometary bodies with the weakest
known structure.

size    [m]     0.2     0.5     ~1      ~2      ~5      10
I       %       17      15      10      ~6      ~3      ~1
II      %       41      46      48      47      42      30
III     %       42      9       42      47      55      69

Most of the meteoroids in the 1- to 10-m size range are decelerated in the
Earth's atmosphere to surface impacting velocity below 4 km/s and regular
meteorite falls follow. Only stony bodies with low initial velocities (less
than 20 km/s) and larger than 8 m can reach the surface with velocities higher
than 4 km/s giving rise to an explosive crater.


Cevolani G.   Bortolotti G.   Franceschi C.   Grassi G.   Gottardi S.
Trivellone G.   Hajduk A.
   The Bologna-Lecce Forward Scatter Radar Experiment: Activity and Reflection
   Properties of Meteor Showers in 1992

Observational results of meteor showers in 1992--Quadrantids, Lyrids, Perseids
and Geminids--as obtained by using a forward scatter (FS) bistatic radar over
the Bologna-Lecce baseline (about 750 km) in Italy, are presented in terms of
structural aspects and signal amplitude-rate dependence.

Continuous radio monitoring at 42.7 MHz of Perseid activity in August 1992
reveals the presence of two outbursts at 20-21 UT in two successive days (11
and 12) mainly due to large particles with long-lasting echoes (the first
outburst on August 11 with  overdense meteor trails being about 80% of the
total amount is the most relevant as shown in the figure, which appears in the
hard copy). These anomalies appear to be clearly connected to the presence of
P/Swift-Tuttle being in the vicinity of its descending node.

The distribution of energy reflected by meteor trails is shown to be
correlated with the electron line density q, differently for underdense and
overdense echoes. It is pointed out that in the cumulative distribution
function of meteor trails vs. peak signal amplitudes, the number of underdense
echoes decreases with the increasing signal more rapidly than predicted [1,2].
By comparing sets of observations and specified theoretical distributions and
using the Kolmogorov-Smirnov (K-S) procedure, an exponential model is proposed
in order to provide a better fit to trends of underdense meteor trails
expecially at middle-high levels of received power and for values of q >=
10^13 el/m. The present FS observations of meteor showers indicate that the
transition region where contributions from both overdense and underdense are
important, extends over a wide amplitude range.

References: [1] G. Cevolani and A. Hajduk (1993) Proceedings of the Intern.
Astron. Symposium, in press.[2] G. Cevolani, A. Hajduk, and S. Kingsley (1993)
Proceedings of the Intern. Astron. Symposium, in press.


Fulle M.
   Actual Meteor Production of Comet P/Swift-Tuttle 1992t

The numerical inverse model of comet dust tails (Fulle 1989) is applied to
wide field Schmidt images of the dust tail of comet P/Swift-Tuttle 1992t, in
order to establish the actual production of meteors of the well known Perseid
stream. The input images contain information on grains of sizes between 0.01
and 10 mm ejected during the time interval -150 < t < -20 (days related to
perihelion). The tail fits are extremely sensitive to the anisotropy of dust
ejection: it is impossible to fit the observed tail with isotropic or strongly
anisotropic ejections, whereas Sun faced hemispherical dust shells allow to
build up model dust tails in good agreement with the observed ones. This
closely agree with the conclusions of Sekanina (1981), who pointed out that
the well known jets of the inner coma can be explained only admitting a strong
activity of many spots even very far from the Sun culmination. The dust
ejection velocity of particles of 0.15 mm diameter increases from 20 m/s at t
= -150 days, to 50 m/s at t = -20 days. The loss rate is about constant over
the considered time interval at about 5000 kg/s (for a dust albedo for the
phase function of 0.06, Hanner and Newburn 1989). The power index of the time
averaged size distribution is -3.3, thus pointing out that the dust production
of P/Swift-Tuttle is dominated by meteoroids. In fact about the 95% of the
produced mass (reaching at least 10^11 kg for the 1992 passage) is injected in
the Perseid meteor stream.

References

Fulle M., 1989, A&A 217, 283. Hanner M.S., Newburn R.L.Jr., 1989, AJ 97, 254
Sekanina Z., 1981, AJ 86, 1741.


Hajduk A.   Hajdukova M.   Gajdos S.   Kostecky P.   Zigo P.   Cevolani G.
Trivellone G.
   The Bologna-Modra Forward Scatter Radar Experiment: Reflection Properties of
   Meteor Trains

Observational results on the reflection properties of meteor trains obtained
by a forward scatter bistatic radar over the baseline Bologna - Modra (about
700 km) in 1992-1993 are presented.

The observations, carried out during the activity of meteor showers with the
different geometric conditions, allow to deduce the dependence of the received
signal amplitude and the meteor echo rates on the motion of the radiant of a
meteor shower. It is shown that the forward-scatter operation may
significantly contribute to the knowledge of meteor train properties and may
be used for the determination of physical properties of the meteoroid
particles.


Jones J.   Brown P.
   The Radiant Distribution of Sporadic Meteors

The radiant distribution of sporadic meteors has been determined for the ten
orbital surveys catalogued by the IAU Meteor Data Center. In addition to the
previously known apex, helion, antihelion, and northern toroidal sources, we
have discovered a southern toroidal source and have also found that the apex
source appears to be split into northern and southern components. We present
the characteristics for each of the sources and comment on their possible
origins.


Jones J.   Brown P.   Webster A. R.   Ellis K.
   Forward-Scatter Determination of the Radiant Distribution of Sporadic
   Meteors

We show how the observability function for meteors detected on a short-hop
forward-scatter system may be calculated. Using this method we have determined
the response of a specific short-hop forward-scatter system between London
(Ont) and Ottawa to the diffuse sources found by Jones and Brown (1993) in the
orbital surveys catalogued in the IAU Meteor Data Centre. Using echo rate data
collected continuously between 1991 and 1992 on the London-Ottawa system we
have been able to determine the strengths of the sporadic meteor sources.


Jones W.   Jones J.
   The Forward Scattering of Radio Waves from Meteors

We show how the WKB or phase integral method can be applied to the scattering
of radio waves from overdense cylindrical trains at general incidence and
present calculations for initial line densities of 10^16 and 10^17 electrons
per metre; previous calculations over the life of the train, based on the
direct numerical integration of the differential equations for the partial
waves, have been limited to 10^15m^-1. To extend results to densities greater
than 10^17m^-1 we consider the ray tracing method, showing that at 10^17
electrons per metre agreement with full wave calculations is reasonable.

Recent results of forward scattering experiments show an unexpectedly high
proportion of short lived echoes, one interpretation of which is in terms of
head echoes. We apply the ray tracing method to a spherical cloud of
ionisation in an investigation of how a forward scatter head echo might be
expected to vary with time.


Klacka J.   Pittich E. M.
   Meteor Stream of Comet Encke

The 20,000 years long-term orbital evolution of six model dust particles ejected
from the comet Encke at its perihelion is investigated. Particles with ejection
velocity 10 m s^-1 were released in these directions (two orientations for each
direction): tangential to the cometary motion, normal to the comet's orbital
plane, and perpendicular to the preceding ones. Planetary perturbations, Pluto
excluding, and the simultaneous action of the solar radiation--both
electromagnetic and corpuscular--are taken into account. Simulations are done
for dust particles of several sizes. Obtained dispersions in orbital elements
are compared and discussed.


Korlevic K.*   Valdre G.
   New Method for the Study of Airborne Meteoritic Particles Trapped on
   Tree-Resin: Some Results from the Tunguska Region

After powerful air explosion of large meteoroids and small asteroids that have
not the structural strength to survive the aerodynamic pressure during
penetration in the dense part of atmosphere, the only possibility to
reconstruct the bodies compositions after the event are either (1) to search
in the strewnfield for possible meteorites that entered the atmosphere
separately from the main body, or (2) to search micrometer-sized particles
condensed from the plasma globe. The natural traps for the search of deposited
small particles have, until now, been peat bogs, glaciers, and lake, swamp,
and sea bottoms. The difficulty of working with samples collected in those
places is the enormous number of ballast particles that complicate the search
enormously, as well as the problem of the dating of the layers.

During the first International Tunguska Expedition ( ITE ) four kilometers
southeast of the epicentre of the Tunguska explosion, in the test excavation
in search of the 1908 ash layer the first piece of tree resin in the ground
was found. The number of ballast particles was lower that in the peat, swamp,
or lake sediments but still very high. Fortunately, by serendipity, on the
cross section of a red Siberian tree trunk (Picea obovata ) about 160 years
old, cut 5 km northwest of the epicentre of the explosion, one very
interesting feature was found after polishing. The cross section revealed a
horizontal branch emerging from the stem, debarked long before the explosion.
Resin on the scar was renewed year by year. The tree resin was then completely
embedded in the growing wood. In such a clean trap the particles are preserved
from the following airborne contamination in the next years. The dating of the
trapped particles is possible by counting the tree rings, and only small shift
in position was possible due to the pressure of the growing tissue. The tree
slice was cut into pieces corresponding to the year 1908, and earlier and
later years, and resin samples were prepared for conventional scanning
electron microscopy (SEM) and field emission SEM (FE-SEM )observations, and
for X-ray microanalysis in order to detect if and what kind of particles are
trapped in the resin.

During the morphological investigation on the 1908 ring the number, type, and
size of particles embedded in the resin are found : 14 Ca carbonates and/or
oxides 1-5 micrometers, 6 Fe chloride and sulphide 1-5 micrometers, 5 (Na, K,
Ca) chlorides 10-15 micrometers, 5 Bi 1 micrometers, 4 Ba sulphide 1-5, 3 (Na,
Al, K) silicates 2-3 micrometers, 2 Pb-Br 1 micrometer, 1 Co-W 1 micrometer, 1
Ca-Ti-Fe 1 micrometer, 1 Fe-Ni 1 micrometer. In the period from 1908 to 1910
also an enstatite crystal of size 30 micrometers was found, with submicrometer
Ba sulphide particle, and submicrometer and micrometer-sized particles of Ca,
Fe, Cu, Zn, Au. The presence of aggregated particles in the resin 1908-1910
similar to those found in the cosmic dust collection confirmed to us the
validity of the tree-resin traps. Except for Ca and chloride particles the
composition reported above had been found only in the years 1908-1910, none in
the confining rings. Calcium carbonates and/or oxides are common in plants and
are not to be considered related to the Tunguska body composition.

The SEM, FE-SEM, and X-ray microanalysis of the tree resin traps are the
formidable new way to study the unsolved collision of Earth with the small
bodies of the solar system in the last time as Tunguska 1908, Rio Curaca 1930,
Marudi Mountain 1935, Revelstoke 1965, and the last Emilia 1993. Also with
these new methods it will be possible to search for fallen cosmic particles
before the contamination of the atmosphere in the last centuries. If we use
mineralized tree resin (amber) for the analysis it is also possible to study
the trapped cosmic dust particles during the geological ages.

References: [1] S. Cecchini, M. Galli, K. Korlevic, and G. Valdre (1991)
Internal Rapport, University of Bologna, Italy. [2] K. Korlevic and G. Andreev
(1990) WGN, 18, 215-216. [3] J. Rendal (1990) WGN, 18, 95-96. [4] G. Valdre
and K. Korlevic (1991) Internal Report, University of Bologna, Italy. [5] G.
Valdre and K. Korlevic (1993) Ultramicroscopy, in press. [6] N. Vasilyev and
G. Andreev (1989) WGN, 17, 245-247.


Lindblad B. A.
   The Orbit of the Eta Aquarid Meteor Stream

A precise photographic orbit for the Eta Aquarid meteor stream has been
difficult to determine, since the radiant (in Northern latitudes) is low in
the sky and the shower is only visible for a short period of time before
sunrise. Until recently the orbit quoted in the literature was based on one
meteor only. In (Lindblad 1990) the mean orbit and radiant motion of the Eta
Aquarid meteor stream was computed based on eleven two-station photographic
meteors. An orbit based on five radio orbits observed by the Harvard Radio
metor project was also presented. In the present study an improved orbit based
on 21 photographic Eta Aquarid orbits is computed. The revised mean orbit
agrees closely with that of comet Halley.

References: Lindblad B. A. (1990) in Asteroids, Comets, Meteors III (C.-I.
Lagerkvist et al., eds.), pp. 551-553.


McBride N.   Taylor A. D.   McDonnell J. A. M.
   Ecliptic North-South Asymmetries in the Natural Meteoroid Population as
   Sampled by LDEF

It is generally assumed that the time-averaged meteoroid influx to the Earth
from the zodiacal dust cloud and the meteoroid sporadic background are
symmetrical with respect to ecliptic north and south. However, localized
asymmetries at the Earth's orbit due to components from passing comets and well-
known meteor streams may be expected. Work on the Long Duration Exposure
Facility (LDEF) shows that there is a pronounced ecliptic north bias (north to
south ratio of around 1.7) from large interplanetary particles (diameter 30
micrometers). By considering the LDEF orbital history, we find that a +6-degree
shift in the median position of the ecliptic latitude distribution of incoming
meteoroids is sufficient to explain this asymmetry. We identify possible sources
for the observed asymmetry as cometary sources that pass within 0.1 AU of the
Earth's orbit, high-activity meteor streams, or single large atmospheric
fragmentation events that subsequently encountered the spacecraft.


Neslusan L.   Porubcan V.   Svoren J.
   Applicability of Meteor Radiant Determination Methods Depending on the Type
   of Orbit: II. Low-Eccentric Orbits

All known parent bodies of meteor showers belong to the group of bodies moving
in high eccentric orbits (e >= 0.5). Recently, the asteroids in low eccentric
orbits (e < 0.5) approaching to the Earth's orbit were suggested as another
population of possible parent bodies. The present paper deals with the problem
of calculation of theoretical meteor radiants connected with the bodies in low
eccentric orbits from the point of view of optimal results depending on the
method used. The paper is a continuation of our previous analysis of high
eccentric orbits published in Contrib. Astron. Obs. Skalnate Pleso. The
analysis covers Porter's, Steel-Baggaley's and Hasegawa's methods together
with some additional methods resulting from mathematical modelling.

In order to be able to compare suitability of application of the individual
radiant determination methods, it is necessary to determine the accuracy with
which they approximate real meteor orbits. To verify the fitness of the
original orbit of the parent body with that of the meteoroid having one node
at 1 AU, Southworth-Hawkins D-criterion was applied (Southworth and Hawkins,
1963). D <= 0.1 means good fitness of the orbits, 0.1 < D <= 0.2 is considered
for probable fitness of the orbits and D > 0.2 the probability of fitness is
rather low and the change of the orbit unreal.

The optimal methods referring to the smallest values of D for given types of
orbits are shown in two series of six plots. The largest application in the
population of low eccentric orbits has an adjustment of the orbit by variation
of both the perihelion distance and eccentricity considering a minimal change.
A few applications are shown.


Porubcan V.   Gavajdova M.
   A Search for Fireball Streams Among Photographic Meteors

Terentjeva (1990) in her analysis of 554 fireballs has reported to discern 78
fireball streams. The present paper summarizes a similar analysis of a set of
over one thousand photographic orbits of meteors brighter than the magnitude
-3. Fireball streams are searched by a computerized stream-search procedure
based on Southworth-Hawkins D discriminant, considering radiants, their daily
motion, sizes and shapes of the radiant areas. Except of the fireballs
belonging to the known meteor streams, substantially lower number of possible
fireball streams was found. The mean orbits, radiants and periods of activity
of the streams are presented.

References: Terentjeva, A.K.: l990, Fireball Streams, in Asteroids, Comets,
Meteors III, eds. C.I. Lagerkvist, H. Rickman, B.A. Lindblad and M. Lindgren,
Uppsala Univ., 579-584.


Simek M.
   The Giacobinids 1985: Canadian Radar Observations

Fine structure profiles of the Giacobinids 1985 derived from the Ottawa
megawatt meteor radar are presented. Recorded meteor echoes were divided for
the analysis into two basic groups: under-dense represented by meteor echoes
having durations T<0.4s and overdense one with T-0.4s. Maximum activity
occured at 09h 25m UT when ten minute zenithal rate reached about 1157 in
underdense range and about 669 overdense echoes at 10h 25m UT.


Simek M.
   The Perseid Meteor Stream Observed Before Return of Comet P/Swift-Tuttle in
   1992

The nature of Perseid meteor shower activities in 1991 and 1992 is
characterized by high shower rates placed around the solar longitude of 138.8
degrees (1950.0). Such a position of maximum activity is well before that one
observed regularly in this century. Considering the ejection velocities of the
particles from parent comet it was found that the ejection took place during
previous passage of the comet through its perihelion in 1862.


Spurny P.
   Recent Fireballs Photographed in Central Europe

The atmospheric trajectories, velocities, brightnesses and orbits of
exceptional fireballs photographed in Central Europe during last three years
are presented. The operation of Czech part of the European Fireball Network is
briefly described.


Terentjeva A. K.   Bayuk O. A.
   Meteor Streams of the Mars Family

In view of the future investigations of Mars by means of spacecrafts the
studies of the meteor situation in the vicinity of this planet's orbit is of
much importance. For this purpose, first of all, meteor streams approaching
the Mars's orbit in their orbits' nodes at <=0.2 AU have been revealed. It is
found that the meteor situation in the region of Mars's orbit is determined by
about 70 meteor streams (by photographic data). Some of these streams consist
of large meteor bodies which produce fireball showers and, therefore, may be a
more serious danger for spacecrafts, than ordinary meteor streams. Conditions
of Mars's encounter with some of the meteor streams are studied: the shortest
distances between the orbits and the duration of the approaches are defined.

A possible mechanism of the formation of meteor bodies' orbits with aphelion
near Mars orbit is considered. Such orbits are shown to occur as a result of
single close encounters between the inner planets and minor bodies moving
along the orbits with aphelion distances q'<= 4.2 AU. In computing scheme the
Laplace method of the unperturbed problem of two bodies was used.


Tokhtasjev V. S.
   Ablation of Meteoroids

We have carried out the numerical solution of a system of three differential
equations, describing drag, heating and ablation of a single non-fragmenting
meteoroid. All main effects - the thermal radiation, sputtering, evaporation,
deceleration and heat capacity have been taken into account in these
computation. The heat conduction process was also taken into consideration for
meteoroids with radii greater than critical.

The ionization and light curves were calculated with various combinations of
the variable values four fundamental parameters: the preatmospheric velocity
V, cos z, the initial meteoroid mass M and the meteoroid bulk density delta. Fro
the analysis of numerical results we have obtained the relation between the
maximum light height Hm and variables (M, V, cos z, delta). In particular for co
z = 0.667 and delta = 3.5 g/cc we find:
Hm = 37.55 + 26.74 log V - 3.86 log M + DeltaH, (M >- M*), (1)
Hm = 40.62 + 34.46 log V - n log (M/M*) + DeltaH, (M < M*),

where M* = 0.16 /(V^2) , n = 2.0 + 0.01 V, Delta H = 28 exp(-0.2 V), log x =
log10 x, Hm is in km, M is in grams and V in km/s. For small of meteoroids (-7
< log M < log M*) n < 3.86 for efficient cooling by radiating.

The analysis of heights was made for two sets of data: the A group of 98 TV
meteors (Sarma and Jones, 1985) and 132 the A group meteors in the Super-
Schmidt data (Hawkins and Southworth, 1958). In the first play we have reduced
the heights Hm to the standard value cos z = 0.667. Then meteors were divided
into intervals of velocities and for every interval the mean values heights
and velocities were found. Equation (1) has been used to calculate mean values
of mass within each group. The average values <M> are independent of V and
practically the same in both sets of data. From the analysis we can come to
the following conclusions: 1) In the magnitude range from +6 to +1 the main
mechanism ablation of the A group sporadic meteoroids is its fragmentation to
particles of roughly equal masses. 2) The mean masses of fragments (about
0.002 g) independent of initial mass and velocity of the parent body.

Unless the meteoroid mass is small (M < 0.002 g), equations (1) gives
approximately the dependence of Hm on velocity and mass. For values of M
between 0.002 and 0.1 g a better approximation is
Hm = 48.24 + 26.74 log V + Delta H.

We see that Hm is independent of M (Bel'kovich and Tokhtas'ev, 1974).

REFERENCES: Sarma T, Jones J. (1985), Bull. Astron. Inst. Czechosl. 36, 9-24.
Hawkins G.S., Southworth R.B. (1958), Smithsonian Contr.Astrophys. 2, 349-364.
Bel'kovich O.I., Tokhtas'ev V.S. (1974), Bull. Astron. Inst. Czechosl. 25,
370-374.


Tokhtasjev V. S.
   Meteoroids from Comets and Fractal Clusters

The analysis of heights of the C group sporadic TV and
Super-Schmidt meteors was made. The variation of height of
maximum light Hm with initial velocity V and mass M (for cos
z = 0.667) can be described by the equations in the hard
copy.

For values of M smaller than Mc the height of maximum light
is independent of mass (Bel'kovich and Tokhtas'ev, 1974).
This independence Hm from M can be explained if we would
assume that meteoroids of the C group are of cometary origin
and in the range of size 2Rc < 2R < 2Rc have fractal
structure. If meteoroid radii are in the above mentioned
range then the mean bulk density of meteoroids is given by
the equation that appears here in the hard copy.

Note that PM is independent of meteoroid size and its
density. Empirical values Ro and Rc has been determined by
the Eq. (1). We have obtained Ro = 31 micrometers, log(Rc) =
0.64 - 1.09 log V, where Rc is in cm and V in km/s.

According to Eq. (2) in the range of size 2Ro < 2R < 2Rc
densities of meteoroids decreases with increasing of its
radii. As the mechanical strength of meteoroids decrease
also so they should be fragmented completely before the
beginning ablation of grains. For these type of ablation the
Eq.(1) also is valid. The mean size of grains are 30
micrometers. This value is several times lower than one
adopted by Bel'kovich and Tokhtas'ev (1974).

REFERENCES: Bel'kovich O.I.and Tokhtas'ev V.S. 1974, Bull.
Astron. Inst Czech. 25, 370-374.


Zhuang T.   He M.
   A Study on the Historical Records of Leo Shower in China

39 new historical records about Leo Shower have been identified from Defang
Zhi (city chronicles) and other ancient books of China. It is an important
supplement to the relevant records as known before. At present time, the
amount of the published records of Leo Shower increases to 70. Especially
among them, 21 records from different cities gave much information during the
appearance of Leo Shower in 1533. The main conclusions, obtained on the
basis of the study on these old and new data, are as follows:

(a) The conventional positions of apparent luminous points were located near
to the southern part of UMa, about 20-30 Deg. away from the radiant.
(b) It seems to the meteroids escaped from mother body-Temple comet through
collapse, instead of explosion. Then, the collapse process continued in
smaller scale when they moved around the sun.
(c) The distribution of meteorids along its orbit is very nonuniform. Leonids
may be consisted of two large pieces of 'meteoric clouds', which have
different sizes and densities. The distance between these two clouds was about
28000000 km.
(d) The earth has only moved across the leading cloud before 1533 and the
following cloud after 1533, but across both one by one in 1533. It is probably
the reason why Leo Shower has subsisted rather longer time in this year.
(e) The dispersing speed of the meteoric cloud over the orbit was about  0.7-
1m/sec.
(f) The brightness of the sky while Leo Shower appeared in 1533 was around  -
1-1Mag.

References: Zhuang Tianshan, 1991, STUDIES IN THE HISTORY OF NATURAL SCIENCES,
Vol. 10, No. 2, pp. 133-144 (Chinese) Zhuang Tianshan, 1992, Ibid, Vol. 11,
No. 4, pp. 316-324 (Chinese).






                                POSTER SESSION
                                     DUST



Baguhl M.*   Grun E.   Linkert D.   Linkert G.   Siddique N.
   Identification of Small Dust Impacts in the Ulysses and Galileo Dust Data

Two similar dust detectors on the Galileo and Ulysses spaceprobes
registered dust impacts for over 3 and 2 years, respectively, traversing
radial distances from 0.7 to 5.4 AU. The instruments are multi-
coincidence impact ionization detectors. The amplitudes and risetimes of
risetimes of the charge signals obtained from an impact allows us to
derive masses of dust particles ranging from 10^-16 g to 10^-8 g with an
error of aproximately factor 10 and speeds above 2 km/s with an error of
approximately factor 2. All impact and noise signals are divided into
amplitude and quality classes which refer to coincidences between the
different signals. The two lowest signal amplitude and quality classes
are contaminated by noise events triggered e.g. by solar UV flux or
energetic particles. For this reasons all events classified in these
classes had been discarded so far. The number of small particles with
masses less than 10^-12 g (at 20 km/s speed) is affected by this
selection. A close analysis of the signal properties of small events
shows that, nevertheless, noise can be reliably discriminated from
impacts in these lower classes. The distribution of dust particle masses
follows a power law, therefore, the total number of identified dust
impacts increases by about a factor of two. This increased number of
impact events allows us a better definition of the distribution of
particle properties. An example are the 'Jupiter dust streams' [1]
registered by the Ulysses detector at a distance from Jupiter ranging
from about 0.5 AU before to 1 AU after the flyby in February 1992. Since
these streams consist of very small particles throughout many additional
particles could be identified (factor 3 more). The already known streams
show up even more strongly and additional streams can be seen before and
after the the Jupiter flyby. Improved mean values for mass, speed and
impact direction of the stream particles and mean time and duration of
the streams are derived. This may help to better understand the source
and the emission mechanism of this new phenomenon.

References: [1] Gruen E. et al. (1993) Nature, in press.


Bohsung J.   Jessberger E. K.*   Stephan T.
   Concerted Elemental Analyses--PIXE and TOF-SIMS--of Interplanetary Dust
   Particles

Interplanetary dust particles (IDPs) collected in the terrestrial stratosphere
- beside meteorites and lunar material - constitute the third kind of
extraterrestrial matter available for laboratory study. Per se these particles
thus are interesting objects and more so, since they may be linked to comets.
However, because they are small (typical dimension 10 micrometers) they are
difficult to analyse, especially when it comes to minor and trace elements. In
the past we had implemented Proton-Induced X-Ray Emission (PIXE) and
Synchrotron-Fluorescence-X-Ray Analysis (SYXFA) techniques [1-6] and the
latter also was heavily used by others [7-13]. More recently we explored and
used the tremendous capabilities of Time-of-Flight SIMS (TOF-SIMS) for
analysis that are exemplified by high lateral resolution (~0.2 micrometers)
and high sensitivity. Here we report on both, new TOF-SIMS analyses and first
results obtained with the newly constructed Heidelberg-Micro-PIXE (lateral
resolution ~1 micrometer).

A striking feature of many IDPs is the large (up to 100x) bulk enrichment -
relative to cosmic abundance - of some volatile elements, especially Br. It
had been argued that this feature is signifying a new kind of extraterrestrial
matter [13]. We contended, however, that before far reaching conclusions on
the composition of major portions of the solar system are drawn, one would
have to examine if the enrichments indeed are indigenous and would have to
exclude that they are related to the likely contaminating atmospheric
processes [14,15]. The most striking result of our present investigations
using both PIXE and TOF-SIMS is the clear demonstration that elements, that
are present in non-cosmic abundance, are distributed very inhomogeneously
within IDPs. In one IDPs for example Br is enriched in extremely fine grained
(~20 nm) Fe-rich portions that appear to be most susceptible for reactions
with atmospheric aerosols. In another IDP all elements that in the bulk are
present in cosmic abundance are homogeneously dispersed while the
concentration profiles of some of those elements that are either enriched (Cu,
Co, Cr) or depleted (Ca) in bulk relative to CI chondrites deviate from
homogeneity.

We are aware that the present data set on IDPs - and especially the very
limited data on minor and trace element distributions within them - is far
from being complete and certainly do not allow to be firm on the origins and
histories of individual IDPs, but we also are convinced that the results of
such analyses ultimately will provide rigourous boundary conditions for both.

[1] Jessberger E.K., Wallenwein R., Blank H., Traxel K. (1985) Meteoritics
200, 673. [2] Jessberger E.K., Wallenwein R. (1986) Adv. Space Res. 6, 5-8 [3]
Wallenwein R., Blank H., Jessberger E.K., Traxel K. (1987) Analyt. Chim. Acta
195, 317-322 [4] Wallenwein R., Antz Ch., Jessberger E.K., Traxel K. (1987)
Proc. 10th Regional IAU Meeting, Prag, CSSR, (Eds. Z. Ceplecha & P. Pecina)
Vol.2, 245-248 [5] Antz Ch., Bavdaz M., Jessberger E.K., Knvchel A.,
Wallenwein R. (1987) Proc. 10th Regional IAU Meeting, Prag, CSSR, (eds. Z.
Ceplecha & P. Pecina) Vol.2, 249-252 [6] Wallenwein R., Antz Ch., Jessberger
E.K., Buttkewitz A., Knvchel A., Traxel K., Bavdaz M. (1989) Lunar Planet.
Sci. XX, 1171-1172 [7] van de Stap C.C.A.H., Vis R.D.. Verheul H. (1986) Lunar
Planet. Sci. XVII, 1013 [8] Flynn G.J. Icarus 77, 287-310 (1988) [9] Sutton
S.R., Flynn G.J. (1988) Proc. 18th Lunar Planet. Sci. Conf., 607-614 [10]
Flynn G.J. (1989) Proc. Lunar Planet. Sci. Conf. 19, 673-682 [11] Flynn G.J.,
Sutton S.R. (1990) Proc. 20th Lunar Planet. Sci. Conf., 335-342 [12] Flynn
G.J., Sutton S.R. (1991) Meteoritics 26, 1991 [13] Flynn G.J., Sutton S.R.
(1992) Lunar Planet Sci. 22, 373-374 [14] Jessberger E.K., Bohsung J.,
Chakaveh S., Traxel K. (1991) Meteoritics 26, 352 [15] Jessberger E.K.,
Bohsung J., Chakaveh S., Traxel K. (1992) Earth Planet Sci. Lett. 112, 91-99.


Ibadov S.
   Search for Plasma Production Mechanisms in Space Objects: Comets and
   Asteroids

Space plasma being a mixture of neutral and excited atoms, ions, electrons and p
onset for the possibilty of studying celestial bodies by both in situ measuremen
and space-borne/ground-based observations. In the light of this conception produ
impacts of interplanetary/circumsolar dust particles with gaseous and dusty comp
investigated. The efficiencies of the two ionizing mechanisms are found and comp

Calculations show that in regions where the density of interplaentary dusty matt
comet heliocentric distances R<-1 AU the ionization rate of circumnuclear region
atmospheres due to impacts of cometary and zodiacal dust particles may exceed th
cometary molecules by solar short-wave radiation.

Grain-grain collisions are essential plasma and radiation generating process for
Halley 1986 III type of asteroids.





Ishimoto H.   Mukai T.
   Phobos Dust Rings

Taking into account the gravity forces of the sun, Mars and Phobos, and solar
radiation pressure forces on dust particle, we have performed the numerical
calculations of orbital trajectories for dust particles (m >- 10^-14g) ejected
from Phobos based on fourth-order Runge-Kutta method. It is found that the
orbital evolution of such small particles is strongly affected by solar
radiation pressure forces. Their orbits differ from that of Phobos and
consequently a collisional probability of these grains on highly eccentric
orbits with Mars increases. On the other hand, large particles take similar
orbits to that of Phobos after values of these time-scales for arbitrary mass
of particles. As the result, it is found that the particles which could form
dust rings for a significantly long time near the Phobos orbit have a narrow
size (or mass) distribution (10^-10g<m<10^-7g). Furthermore, in the dust
rings, orbital segregation of particles by their sizes or masses occurs. We
have also examined a spatial density of particles in dust rings. Although the
expected dust rings are very thin, a possibility to detect this ring particles
by the planned dust detector on board of the Mars mission (Planet-B) of Japan
in 1996 is briefly discussed.


Klacka J.
   Dust Particle in the Solar Gravitational and Electromagnetic Fields

The action of the solar electromagnetic radiation on the motion of
interplanetary dust particle in the gravitational field of the SUTI is discussed
from the theoretical point of view. Results are presented to the second order in
v/c (c - speed of light, v - orbital velocity of the particle), so also general
theory of relativity is used.


Klacka J.
   Dust Particles: Solar Wind and Perturbation Equations of Celestial Mechanics

The problem of the action of the solar wind on the motion of interplanetary dust
particle is discussed in detail. Differences between the action of
electromagnetic solar radiation and that of the solar wind are explained not
only from the point of view of the physical nature of these phenomena but also
from the point of view of dust particle's orbital evolution. The standard speed
of inspiralling toward the Sun is compared with the result corresponding to the
fact that solar wind particles spread out from the Sun in nonradial direction.


Liou J.-C.*   Dermott S. F.
   Contribution of Encke-type Cometary Dust to the Zodiacal Cloud

Liou and Dermott (B.A.A.S., 1992) showed that cometary particles play an
important role in the structure of the zodiacal cloud. We now estimate the
contribution of Encke-type particles to the Zodiacal cloud. In our model we
continuously release Encke-type dust particles from all points in an Encketype
orbit over the past five thousand years, then numerically integrate their
evolutions including all planetary perturbations, Poynting-Robertson light
drag, and corpuscular solar wind effect. We found that (1) Trapping in mean
motion resonance with Jupiter does occur, however the overall effect is not
significant. (2) Jupiter produces a similar effect on the arguments of
perihelion of the dust particles as it does to the short period comets.
(3) While there is a huge dispersion in the orbital semimajor axes, particles
with a given semimajor axis still define very well the forced elements. We
apply those elements to a three-dimensional model SIMUL (Dermott et al., 1993)
to calculate their overall flux contribution to the zodiacal background and
compare the results with the observation from the Infrared Astronomical
Satellite (IRAS).


Marzari F.   Vanzani V.
   Orbital Evolution of Dust Particles Near J/J+1 Resonances with the Earth

We have performed a numerical study of the orbital evolution of interplanetary
dust particles in the region of the first order outer resonances with the
Earth. With a systematic approach, based on a large number of simultaneous
numerical integrations, we have explored (1) the location of stable resonance
trapping regions in the e-omega bar plane, (2) the long-term dynamical
behaviour while in resonance, and (3) the mechanism forcing the escape from
the resonance. The physical model is basically a seven-body problem, the Sun,
the five major influencing planets (Venus, Earth, Mars, Jupiter, and Saturn),
and the dust particles. As a numerical method for solving the equations of
motion we have adopted the RA15 integrator by Everhart [1].

The choice of the e-omega bar plane as representative of the six-parameter
orbital phase space in studying the resonant capture of dust grains was
motivated by the strong dependence of both the resonant and dissipative (P-R
and wind drag) perturbations on the eccentricity of the particles and by the
relation existing between the angle variable omega bar and the mutual
geometrical configuration of the particle orbit with respect to the Earth's
orbit, in particular for low inclination orbits. Our results show how in each
j/j+1 resonance the maximum probability of capture occurs in correspondence to
eccentricity values for which the dust grains and planets orbits begin to
cross. Moreover, the location of the trapping regions is shifted toward
smaller eccentricities with increasing j. As a consequence low-eccentricity
dust particles are captured more freq