Monday, September 27, 1993
                       Parent-Daughter Orbit Divergence
                       8:30 - 10:00 a.m.     Berkner Room


Dermott S. F.*
   Orbit Evolution: Asteroid Families

   No abstract available.


Jackson A. A.*   Zook H. A.
   Some Considerations on Velocity Vector Accuracy in Dust Trajectory Analysis

INTRODUCTION

The relative contributions of comets and asteroids to the reservoir of dust in
the interplanetary medium is not known. There are direct observations of dust
released from comets and there is evidence to associate the IRAS dust bands
with possible collisions of asteroids in the main belt (Dermott et al., 1984).
A means toward sorting out the parent sources has been proposed in the
establishment of a dust collector in orbit about the Earth (Horz, 1990). The
purpose of such a facility would be to collect not only cosmic dust particles
intact but also the state vectors (Zook, 1986), as they arrive at the
detector, the idea bein that one may combine analytical laboratory analysis of
the physics and chemistry of the captured particles with orbital data in order
to help distinguish between bodies and identify parent bodies. The theoretical
study of dust particle orbits in the solar system takes on greatly more
importance if we use collected trajectory data. The orbital motion of dust
when radiation and forces alone are acting is well understood (Wyatt and
Whipple, 1950; Burns et al., 1979). When gravitational forces due to the
planets are included, the motion can become quite complex (Gonczi et al.,
1982; Gustafson and Misconi, 19861; Jackson and Zook, 1992).

In order to characterize the orbits of particles as they crossed the Earth's
orbits, a study of the long-time dust orbital evolution was undertaken. We
have considered various parameters associated with these dust orbits to see if
one may in a general way discriminate between particles evolved from comets
and asteroids.

PRELIMINARY ACCURACY CONSIDERATIONS

We proceed in this study as we have done previously (Jackson and Zook, 1992).
That is, we considered the dust particles as ideal black bodies, of density of
1 gm/cc, spherical, with radii 10-100 micrometers. Particles of this size are
affected by radiation forces, photon pressure, and Poynting-Robertson drag.
Account was also taken of solar wind drag, which amounts to about 30% of the
Poynting-Robertson drag negligible. The gravitational forces due to the
planets are included, unlike in our previous study; the planetary orbits are
those of true n-body interaction so that the possibility of secular resonance
is included. Our method was to calculate explicitly by a numerical procedure
the orbits of dust particles after they left their parent bodies. The motion
is determined numerically with the implicit Runge-Kutta integrator using
Gauss-Radau spacings (Everhart, 1985).

Particles are ejected from comets or asteroids at perihelion. This will happen
due to some outgassing process on the comet or meteoric impact on an asteroid.
Particles then feel the immediate effect of radiation pressure and move on
radiation-modified orbits that have new semimajor axis and eccentricity. Then
the particle orbit was followed until it was well inside Mercury's orbit.

When the ascending or descending node of a test particle fell within the range
of .983 to 1.017 AU from the Sun a collision with an spacecraft in the same
orbit as the Earth's becomes possible. The Earth is not a target for right now
since we did not include the effects of its gravitational acceleration. Two
orbital parameters proved useful for the characterization of parent body
origin, orbital eccentricity e, and magnitude of relative velocity u, of
intersection at the node

For illustration we plot u vs. e in Fig. 1 for 30-micrometer particles. The
circles are particles from asteroids and the squares are particles from
comets. As might be expected, dust from the comets retain their high
eccentricities, while asteroidal grains are of low eccentricity. However,
there is some overlap in eccentricity and velocity. Figures 2 and 3 show
histograms of the magnitude of the velocity and eccentricity of the particle
orbits from comets and asteroids at nodal crossing.

We considered as a measure of accuracy the following quantity:

Acc = (delta v(sub)r^2 + delta v(sub)t^2 + delta v(sub)n^2)^1/2/|v|

where delta v(sub)r is the magnitude of the difference between the relative
radial velocities of the asteroid and comet, delta v(sub)t is the magnitude of
the difference between velocities tangential to the Earth's orbit, and delta
v(sub)n is the magnitude of the difference between the velocities normal to
the Earth's orbit. Relative here means the relative velocity between the dust
particle and a hypothetical spacecraft in a circular orbit at the Earth's
orbit.

Figure 4 shows the results of taking pairwise for each 30-micrometer dust
asteroid particle with each comet particle and keeping only the minimum value
of the accuracy that results.

ERROR ANALYSIS

We take the following as a simple model of the effect of measurement error on
the determination of dust orbits by a detector. In order to make the problem
tractable, take the dust detector to be in a circular heliocentric orbit with
dust particles striking it. Such acceleration processes as the gravitational
field of the Earth (important for a spacecraft orbiting the Earth) are not
considered here.

We also take the collisions to occur with the detector spacecraft at the dust
particle orbit ascending node. Let there be a coordinate system x,y,z erected
at the instantaneous position of the spacecraft and the intersection point of
the dust particle, where x is in the radial direction away from the Sun, y
points tangent to the velocity vector of the circular spacecraft orbit, and z
is along the angular momentum of the spacecraft orbit. Opik (1963) then gives
the components of particles velocity vector u by the following:

u(sub)x^2 = (2 - a(1 - e^2) - l/a)

u(sub)y = [(a(1 - e^2)] ^1/2 cos i -1

u(sub)z^2 = a(1 - e^2)sin^2i

where a (in units of the spacecraft orbit semimajor axis) is the semimajor
axis of the particle orbit, e is its orbital eccentricity and i is its
inclination to plane of the spacecraft's orbit.

In the above, orbital elements are computed relative to a "radiation"-modified
gravitational potential. That is if beta = force of radiation/force of
gravity, then the potential is of the form (1 - beta)mu/r, if mu and r are the
usual gravitational parameter distance from the Sun.

Conversely one may write

a = (1 - u^2 - 2u)^-1

1 - e^2 = (2 - a^-1 - u(sub)x^2)a^-1

tan i = u(sub)/(1 + u(sub)y)

which give orbital elements as a function of the velocity components.

Let the detector measure the magnitude of the partic1e velocity and two
angles, theta the angular distance from the positive y axis and phi the
azimuth angle (see Fig. 5). Then the velocity components can be expressed as

u(sub)x = u sin theta sin phi

u(sub)y = u cos theta

u(sub)z = u sin theta cos phi

so that

cos theta = u(sub)y/u

tan phi = u(sub)x/u(sub)z

Now consider another simplification, take the angular measurements, theta and
phi to be perfect. Let an error occur in the measurement of magnitude of the
relative velocity, u. Holding theta and phi constant we have

da/du = (2u + 2du(sub)y/du)/(1 - u^2 - 2u(sub)y)^2             (1)

de/du = [(2 - a^-1 - u(sub)x^2)a^-2da/du + (a^-2da/du +
2u(sub)du(sub)x/du)a^-1]/2e                                    (2)

sec^2i di/du = du(sub)z/du(1 + u(sub)y)^-1 -
u(sub)zdu(sub)y/du(1 + u(sub)y)^-2                             (3)

Taking the data given at the ascending node from the computed orbital
evolution of dust particles the dependence of da, de, and di on du are given
in Figs. 6-8 for the asteroidal particles and Figs. 9-11 for the cometary
particles.

CONCLUSIONS

The largest error incurred in semimajor axis determination arises in the case
of 10-micrometer particles from comets, da/du ~ 6 (see Fig. 9). This was a
particle from comet Honda-Mrkos-Pajduskova that had at the time of intercept a
semimajor axis of approximately 8 AU and a eccentricity of about .87. One sees
from (1) and noting that the magnitude of u is

u = 3 - 2[a(1 - e^2)]^1/2 - 1/a

that for large a and e - 1 that

da/du ~ a

Thus for particles captured from large a and high eccentricity orbits the
error in u can be substantial.

A value of de/du of nearly 1.5 occurs in a 30-micrometer asteroidal particle
from asteroid Flora (Fig. 7). In this there is nothing unusual about the
orbital elements at arrival; however, the particle has a low azimuth angle and
high elevation of nearly 90 degrees. In this case de/du - u^2.

The errors incurred in measured orbital inclination were always smaller than
those for the other orbital elements.

The current error budget for CDCF has candidate value of 1% (Zook, 1986).

In the worst case above this would result in an error of .07 AU in semimajor
axis determination (can be higher for bigger cometary orbits) and .015 in
eccentricity.

We will continue study of this problem, including effects of the Earth's
gravitation and errors in the measuremnet angles. We will present further
results in a later report.

References: Dermott S. F. et al. (1984) Nature, 312, 505-509. Gonzci R. et al.
(1982) Icarus, 51, 633-654. Gustafson B. A. S. and Misconi N. Y. (1986)
Icarus, 66, 280-287. Horz F. et al. (1990) Cosmic Dust Collection Facility:
Scientific Objectives and Programmatic Relations, NASA TM 102160. Jackson A.
A. and Zook H. A. (1992) Icarus, 97, 70-840. Opik E.(1976) Interplanetary
Encounters, Elsevier, New York. Wyatt S. P. and Whipple F. L. (1950)
Astrophys. J., 111, 134-141. Zook H. A. (1986) In Trajectory Determinatin and
Collection of Micrometeroids on the Space Station, LPI Tech. Rept. 86-05.

Figures 1-11 appear in the hard copy.


Liou J. C.*
   Velocity Distribution of Earth-crossing Asteroid Grains

   No abstract available.





                           Monday, September 27, 1993
                              Trajectory Sensing
                      10:00 - 12:00 a.m.     Berkner Room


Auer S.*
   Charged-Particle Velocity Sensor

   No abstract available.


Tuzzolino A. J.*
   PVDF Flux/Mass/Velocity/Trajectory Systems and Their Applications in Space

Abstract

The current status of the University of Chicago Polyvinylidene
Fluoride (PVDF) flux/mass/velocity/trajectory instrumentation is
summarized. The particle response and thermal stability
characteristics of pure PVDF and PVDF copolymer sensors are
described, as well as the characteristics of specially
constructed two-dimensional position-sensing PVDF sensors. The
performance of high-flux systems and of velocity/trajectory
systems using these sensors is discussed, and the objectives and
designs of a PVDF velocity/trajectory dust instrument for launch
on the Advanced Research and Global Observation Satellite (ARGOS)
in 1995, and of a high-flux dust instrument for launch on the
Cassini spacecraft to Saturn in 1997 are summarized.

1. Introduction

For cosmic dust, the scientific importance of in situ particle
velocity/trajectory measurements and, in particular, of particle
trajectory measurement combined with compositional and isotopic
information on the captured particle residue, has been discussed
in the literature and in various workshop reports [1-9]. In near-
Earth space, where both cosmic dust and orbital debris contribute
to the particulate environment, velocity/trajectory measurements
permit discrimination between these two particle classes
[1,3,10,11].

For regions of space where dust flux intensities may be extremely
high and may coexist with backgrounds of high fluxes of charged
nuclei as well as intense magnetic fields (i.e., cometary comae,
planetary dust rings within magnetospheres, intense cosmic dust,
or orbital debris streams), a dust instrument capable of
providing particle mass spectra as well as accurate high-flux
measurements in the presence of these possible backgrounds would
provide important information regarding the sources, sinks, and
dynamics of the dust [12].

Development of the polyvinylidene fluoride (PVDF) dust sensor at
the University of Chicago began in 1983 and since then the main
design objectives of our PVDF technology program have addressed
the two types of measurements described above, namely:

1. The capability to measure individual particle trajectories
with sufficient accuracy to permit identification of their parent
bodies. This, combined with chemical and isotopic analyses of the
captured material in a capture cell device (and/or returned
sensors), permits a direct study of the physical, chemical, and
isotopic composition of matter from a known parent body.

2. The capability for accurate measurement of particle mass
spectra and high dust fluxes, with immunity to possible intense
backgrounds of charged nuclei (radiation belts) and/or intense
magnetic fields.

In the following sections, we summarize the history of this PVDF
technology program, and discuss the objectives and designs of two
dust instruments scheduled for launch in 1995 and 1997 that have
resulted from this program.

2. PVDF and PVDF Copolymer Sensors

2.1. Dust Particle Response

The theory, construction, and dust particle response of pure PVDF
and PVDF copolymer sensors have been described in detail in
earlier reports [4-7,13]. A PVDF (or PVDF copolymer) sensor,
shown in Fig. la, consists of a thin film of permanently
polarized material. A hypervelocity dust particle impacting the
sensor produces rapid local destruction of dipoles (penetration
hole), which results in a large and fast (ns range) current pulse
at the input to the electronics. The output pulse amplitude, in
general, depends on impacting particle mass and velocity [4-7]
and is sharp in time, as illustrated in Figs. 1b,c. This fast
output pulse permits a high counting rate capability for the
sensor (up to 10^4 impacts s^-1 with no, or small, corrections,
and up to 10^5 impacts s^-1 with significant but known
corrections, as illustrated in Fig. 2.

The fast response of the PVDF sensor, combined with its immunity
to very high background fluxes of charged nuclei and magnetic
fields, made it well suited to carry out dust measurements in
cometary comae, and further studies of the PVDF sensor and
associated electronics led to the development of the University
of Chicago Dust Counter and Mass Analyzer (DUCMA) instruments
that were carried aboard the Vega 1 and Vega 2 spacecraft
missions to Comet Halley [14]. The highly successful performance
of the DUCMA instruments throughout the Vega 1/2 missions proved
the high space reliability of PVDF sensors and their value for
space dust studies [15,16].

2.2. Electrical/Thermal Characteristics

For PVDF sensors made from pure PVDF material, the dielectric
constant eta of the material is eta = 12. For thin sensors, this
high eta results in a large detector capacitance, which degrades
the rise time and amplitude at the output of a charge-sensitive
preamplifier used with the sensor and also results in a large
electronic noise level at the output of the linear electronics
[14]. Further, the volume polarization P of pure PVDF material
will degrade with time when the material is maintained at
temperatures >=~80 degrees C [17].

Recent developments in polymeric materials have led to new
copolymer materials that promised to have lower eta and improved
stability of P under long-term high-temperature (~100 degrees C)
exposure, and a study was undertaken to determine the possible
advantages of copolymer materials over pure PVDF material. The
results of these studies are included in Table 1, which
summarizes the characteristics of PVDF (pure and copolymer)
sensors. For the Trifluoroethylene copolymer material we have
studied [18], we have found this material to have distinct
advantages over pure PVDF material, and have made use of this
copolymer material for nearly all sensor construction over the
last few years.

Although our studies have shown that copolymer sensors may be
operated at temperatures up to ~100 degrees C for long periods of
time (months) with very small (<5%) degradation in dust particle
response, the effective sensor emissivity epsilon and
absorptivity alpha are such that exposure of a sensor in space to
solar illumination will result in sensor temperatures high enough
to destroy the sensor. Since, in general, mission and instrument
mounting constraints may require exposure of sensors to solar
illumination for considerable periods of time, we studied
different sensor coating techniques that would restrict sensor
temperatures to <100 degrees C during extended solar exposure.

The basis for our studies is illustrated in Fig. 4a, where for
our sensors (600-800-Angstrom-thick evaporated Al contacts on the
two surfaces of a thin film of PVDF material), epsilon = 0.025
and alpha = 0.1. Assuming the deposition of a coating material
having emissivity epsilon prime on that surface of the sensor
that views an enclosure maintained at temperature T(sub)0, and
that the sensor is thermally insulated from its surroundings,
Figure 4b shows the calculated sensor temperature T vs. the ratio
epsilon prime/epsilon under solar exposure of the space-facing
surface at 1 AU from the Sun. For an uncoated sensor (epsilon
prime = epsilon), the sensor temperature would exceed (pure PVDF)
or approach (PVDF copolymer) the melting temperature of the
polymer material, i.e., the sensor would be destroyed. However,
for epsilon prime >= 10 epsilon (epsilon prime >= 0.25), the
sensor temperature would remain <100 degrees C for enclosure
temperatures <50 degrees C.

Two techniques have been developed for coating of our sensors
with material having epsilon prime >= 0.25. The first technique
(developed at our laboratory) consists of applying (spray brush)
a Chemglaze Z-306 coating [19] to one of the sensor surfaces, and
measured emissivities epsilon prime vs. Z-306 thickness are
plotted in Fig. 5 [20]. The emissivity values obtained with the
Z-306 coatings are seen to be sufficiently large so as to provide
a solution to the solar temperature problem for PVDF sensors. A
number of sensors coated with Z-306 have been thermal/vacuum
cycled over the range -73 degrees C to 115 degrees C and the
coatings have shown excellent adherence to the PVDF surface and
complete mechanical stability.

A disadvantage to our Z-306 coating technique is that the minimum
thickness coating we have achieved to date is ~17 micrometers.
For sensors used in a system requiring particle penetration of a
coated sensor with minimum particle degradation (ablation,
velocity loss, fragmentation), the Z-306 coating method is not
suitable at present since it would introduce considerable
particle degradation [4-7].

An alternative technique that holds promise for epsilon prime ~
0.4 for a film coating on a PVDF sensor with coating thickness in
the range 1 to 2 micrometers was investigated and developed by
the Aerospace Corp. [21]. Several PVDF test samples were coated
(sputtering) with a SiO2 film with thickness 1.5 micrometers, and
measured values for epsilon prime were ~0.39. Although this
epsilon prime value yields an acceptable solution to the sensor
solar/thermal problem, and the SiO2 coating thickness (1.5
micrometers) represents a dramatic improvement over the minimum
Z-306 coating thickness (~17 micrometers), the SiO2 technique
will require further study in terms of SiO2 coating adherence and
stability.

2.3. Position-Sensing PVDF Sensors

Specially constructed PVDF sensors provide the capability for
determination of the x,y coordinates of particle entry into the
PVDF sensor. The two-dimensional position-sensing PVDF dust
sensors (x,y sensors) we have developed employ resistive charge
division, as shown in Fig. 6. Dust particle calibrations have
shown that typical measured position errors for dust particle
impacts are in the range ~1 to 3 mm for x and y [6]. In near-
Earth orbit applications, the unprotected portions of the x,y
sensor surfaces would suffer from oxygen erosion effects.
However, an overcoat of a thin-film oxide may provide a solution
to this problem, and this possibility will be studied in the near
future.

3. Particle Velocity/Trajectory Measurement Using Thin PVDF
Sensors

Two thin (<~6 micrometers thick) sensors in a time-of-flight
(TOF) arrangement may be used to determine impactor velocity. The
concept is illustrated in Fig. 7a, and several dust accelerator
calibrations were carried out at the Heidelberg and Munich
(Germany) dust accelerator facilities to verify the concept [4-
7,13], and examples of the Munich results are given in Figs. 7b-
d.

For particle trajectory measurement, moderate trajectory accuracy
(~5 degrees trajectory error) may be obtained by using arrays of
non-position-sensing sensors arranged in two planes of given
separation. Approximate particle trajectory is determined by
electronic identification of the upper and lower plane sensors
penetrated by the particle [4,5]. For high trajectory accuracy
(~1 degree trajectory error), two planar arrays of PVDF x,y
sensors would provide this capability, although at higher
electronic complexity [6].

4. Combined PVDF Trajectory-Capture Cell Systems

Following our early calibrations of thin PVDF sensors in a TOF
arrangement, it became clear that a PVDF TOF telescope combined
with a capture cell device would provide the additional
capability of an Earth-based chemical and isotopic analysis of
captured dust. The PVDF trajectory system would provide
measurements of impacting particle flux, mass, velocity (by time-
of-flight), and trajectory. The capture cell system would provide
for capture of particle residues following penetration of the
trajectory system by the impactor. Subsequent Earth-based
analyses would yield chemical and isotopic composition of the
residues. Thus, the combined trajectory-capture cell
instrumentation would provide the capability to measure the
orbital elements of individual particles prior to capture.

Dust accelerator calibrations on combined PVDF trajectory-capture
cell systems established that, for 75% of all impactors with
velocities <~8 km/s, thin PVDF trajectory systems satisfied the
requirements of (1) velocity trajectory determination, (2)
identification of the location of particle fragments in capture
cells, and (3) sufficient mass following penetration of the
trajectory sensors for successful capture and subsequent chemical
and isotopic analysis [7,22].

5. Current University of Chicago PVDF Dust Instrument Programs

Continuing development of PVDF sensors and associated electronics
has led to two current programs described below.

5.1. The Space Dust (SPADUS) Instrument for Flight on the Advance
Research and Global Observation Satellite (ARGOS)

A SPAce DUSt (SPADUS) instrument designed to measure the flux,
mass distribution, velocity/trajectory, and impact-time
characteristics of near-Earth dust (both cosmic dust and orbital
debris) is currently under development for flight on the Advanced
Research and Global Observation Satellite (ARGOS), with a launch
in September, 1995. SPADUS is being jointly developed by groups
at the University of Chicago (dust sensors and linear
electronics), the Lockheed Space Sciences Laboratory (digital
electronics), and the Space Sciences Division of the Naval
Research Laboratory (mechanical design and construction). SPADUS
will be integrated and flown by the DOD Space Test Program, with
funding for the University of Chicago portion of SPADUS
development provided by the Office of Naval Research and NASA.

The ARGOS objective is to demonstrate high-temperature
superconductivity technology, stellar navigation and timing
concepts, electric propulsion, and conduct upper atmosphere
imaging and environment studies, with SPADUS providing continuous
measurements of the particulate environment. The ARGOS mission
and the SPADUS instrument have been described in [10,ll], and the
characteristics of SPADUS are summarized in Fig. 8.

5.2. High Rate Detector (HRD) Instrument for the Cassini Mission
to Saturn

The High Rate Detector (HRD) instrument is currently under
development as a subsystem of the Cosmic Dust Analyzer (CDA)
experiment (E. Grun, MPI, CDA Principal Investigator). The CDA
has been described in [12] and the goals of the CDA are to study
the dust environment at Jupiter encounter, in the asteroid belt,
in the saturnian system, and in interplanetary space throughout
the cruise portion of the mission.

The HRD exploits the high counting rate capabilities of PVDF
sensors, and is designed to measure intense dust fluxes and
particle mass spectra during ring plane crossings, where fluxes
are expected that may saturate the rate capability of the Dust
Analyzer portion of the CDA [12]. The characteristics of the HRD
are summarized in Fig. 9. Launch of the Cassini spacecraft is
scheduled for November, 1997.

6. Future Space Applications of PVDF-based Instrumentation

PVDF-based instruments utilizing new gate-array microcircuitry in
combination with programmable microprocessors would provide low
power, light weight, and flexible instrumentation that would be
ideally suited to a number of possible future space missions. An
instrument similar to SPADUS combined with arrays of capture cell
devices behind the trajectory instrumentation would provide
important near-Earth dust measurements on an LDEF II mission or
MIR. We have begun the design of a SPADUS type instrument that
would provide data comparable to that provided by SPADUS (Fig. 8)
but with significantly reduced weight and power requirements (~2
kg and ~2.5 W, with no capture cells). Such an instrument would
be suited for deep-space missions.

For a simpler dust instrument that would provide measurement of
particle mass distribution and flux, with no velocity/trajectory
information, an instrument similar to the HRD, but with a total
PVDF sensing area of 1000 cm^2, would provide useful dust data in
a variety of space environments. Such an instrument would provide
the dust data listed in Fig. 9, would weigh ~ 1 kg, and require
~0.4 W.

Acknowledgments:  The author thanks J. A. Simpson, R. B.
McKibben, M. Perkins, E. LaRue, and staff at the University of
Chicago for their important contributions to the PVDF development
program, and their contributions to SPADUS and HRD development.
He thanks H. Voss, R. Baraze, R. Fisher, J. Kilner, J. Mobilia,
D. Chenette, and R. Vondrak at the Lockheed Space Sciences
Laboratory and H. Gursky, M. Lovellette, D. Woods, and G. Fritz
at the Naval Research Laboratory for their participation in
SPADUS design and development, and E. Grun and the entire CDA
consortium for their contributions to the interfacing of the HRD
to the CDA. The University of Chicago group is grateful to R. G.
Joiner for his continuing sponsorship of SPADUS, and for his
efforts in obtaining partial support for SPADUS development.
Finally, we thank the DOD Space Test Program for providing the
SPADUS flight on ARGOS, and the Aerospace Corp. and Rockwell
International staff who have made integration of SPADUS into
ARGOS possible. The University of Chicago effort for SPADUS is
supported in part by ONR Grant N00014-91-J-1716 and NASA Grant
NAGW-3078. The Lockheed SPADUS effort is supported in part by the
Lockheed Independent Research Program, and the Naval Research
Laboratory SPADUS effort is supported by in-house funding. The
University of Chicago funding for HRD development is provided by
NASA Subcontract JPL 959531.

References:  [1] Horz F. ed. (1986) LPI Tech. Rep. 86-05, LPI,
Houston. [2] Mackinnon I.D.R. and Carey W. C., eds. (1988) LPI
Tech. Rep. 88-01, LPI, Houston. [3] McDonnell J.A.M., ed. (1992)
Hypervelocity Impacts in Space (University of Kent at
Canterbury). [4] Simpson J. A. et al. (1989), Nucl. Instr. Meth.,
A279, 611-624. [5] Simpson J. A. and Tuzzolino A. J. (1989) Nucl.
Instr. Meth., A279, 625-639. [6] Tuzzolino A. J. (1991) Nucl.
Instr. Meth., A301, 558-567. [7] Tuzzolino A. J. (1992) Nucl.
Instr. Meth., A316, 223-237. [8] Jackson A. A. and Zook H. A.
(1992) Icarus, 97, 70-84. [9] Workshop on the Analysis of
Interplanetary Dust (1993) LPI, Houston. [10] Tuzzolino A. J. et
al. (1993) Second LDEF Postretrieval Symposium, NASA CP-3194,
Part 4, 1535-1549. [11] Tuzzolino A. J. et al. (1993) Adv. Space
Res., 13, No. 8, 133-136. [12] Ratcliff P. R. (1992) J. Brit.
Interplan. Soc., 45, 375-380. [13] Simpson J. A. and Tuzzolino A.
J. (1985) Nucl. Instr. Meth., A236, 187-202. [14] Perkins M. A.
et al. (1985) Nucl. Instr. Meth., A239, 310-323. [15] Simpson J.
A. et al. (1986) Nature, 321, 278-280. [16] Simpson J. A. et al.
(1987) Astron. Astrophys., 187, 742-752. [17] Furukawa T. and
Wang T. T. (1988). The Applications of Ferroelectric Polymers
(Chapman and Hall, New York). [18] Ferren R. A. (1988) The
Applications of Ferroelectric Polymers (Chapman and Hall, New
York). [19] Obtained from the Lord Corp., Erie, PA. [20]
Measurements carried out at the Aerospace Corp., Los Angeles, CA.
[21] Ken Aitchison, Aerospace Corp., El Segundo, CA, private
communication. [22] Simon C. G. (1993) Int. J. Impact. Eng., in
press.

TABLE 1.  Characteristics of PVDF Dust Sensors

- Require no operating bias voltage. Long-term stability during
storage.

- Long-Term High Temperature Stability:

P = polarization of sample at 20 degrees C before thermal test.

P' = polarization of sample at 20 degrees C after thermal test.

a) Copolymer Sensor: 21 days at 115 degrees C results in (P-P')/P
= 5.5%.

b) Pure PVDF Sensor: 14 days at 115 degrees C results in (P-P')/P
= 23%.

- Capacitance:

For sensor of given area and thickness, copolymer sensors have
~1/2 the capacitance of a pure sensor (eta (pure) = 12; eta
(copolymer) = ~6). Also, over the temperature range -73 degrees C
to 115 degrees C, copolymer sensors have smaller temperature
dependence of capacitance than pure sensors (Fig. 3).

- Response to Dust Impacts:

For impactors that penetrate the sensor, copolymer sensors give
~30% larger signal.

- Highly radiation resistant. No measurable change in response up
to ~10^7 rad.

- Response to dust impacts unaffected by high background fluxes
of charged particles.

- Fast detector response (few ns) enables accurate counting for
high dust fluxes.

- Proven space performance on Vega 1/2 missions to comet Halley.

Figure 1, which appears in the hard copy, shows (a) schematic
drawing of a PVDF (or PVDF copolymer) dust sensor. The sensor
film, with thickness L, has a built-in volume polarization P, as
shown. (b) Example of output pulse for a 2.0-micrometer-thick
PVDF copolymer sensor impacted by a glass particle having
velocity V and diameter D(sub)p as indicated. (c) Example of
output pulse for a 6.8-micrometer-thick PVDF sensor impacted by a
glass particle having velocity V and diameter D(sub)p as
indicated.

Figure 2, which appears in the hard copy, shows an illustration
of counting rate capability of PVDF sensor/electronics system.
Data taken using a random electronic pulser to simulate a random
particle impact rate. Up to impact rates of 10^4 s^-1, counting
losses are <5%.

Figure 3, which appears in the hard copy, shows measured
temperature dependence of sensor capacitance for a sensor made
from pure

Figure 4, which appears in the hard copy, shows (a) model assumed
for a PVDF sensor coated on one surface with material having
emissivity epsilon prime. The coated surface views an enclosure
maintained t temperature T(sub)0. The space-facing sensor surface
views the Sun at 1 AU distance, and the sensor (and coatig are at
temperature T. The sensor is assumed to be thermally insulated
from its surroundings. (b) Calculated sensor temperature vs. the
ratio epsilon prime/epsilon for the model in (a).

Figure 5, which appears in the hard copy, shows measured
emissivity for a sensor surface coated with Chemglaze Z-306 vs.
Chemglaze Z-306 thickness.

Figure 6, which appears in the hard copy, shows a schematic
illustration of a two-dimensional position-sensing PVDF dust
sensor using external resistive charge division. Three signals
(P,Q,R) yield the x,y coordinates of particle impact [6].

Figure 7, which appears in the hard copy, shows (a) illustration
of particle impact velocity deterination by time-of-flight. An
impactor with velocity V(sub)0 and diameter D(sub)p impacts a
thin PVDF D1 sensor at time t(sub)0, emerges from D1 with reduced
velocity V(sub)1 and impacts D2 at a later time T(sub)1, with
delta t = t(sub)1 - t(sub)0. With S known and delta t measured,
V(sub)1 = s/delta t. From the amplitude of the D1 output pulse
and V(sub)1, both V(sub)0 and D(sub)p are determined from
calibration data. (b-d) Examples of two-sensor time-of-flight
data obtained during dust claibrations carried out at the Munich
dust accelerator during May 1989. Indicated are the D1,D2 sensor
thicknesses, the glass impactor (2.5 g/cm^3) diameter D(sub)p,
and the D1,D2 separation S.

Figure 8, which appears in the hard copy, shows a summary of the
SPADUS instrument.

Figure 9, which appears in the hard copy, shows a summary of the
HRD instrument.


Peterson R.*
   Plasma Sensor Experiments Using Two Planes

   No abstract available.


Alexander W. M.*   Tanner W. G.   McDonald R. A.   Schaub G. E.
Stephenson S. L.   McDonnell J. A. M.   Maag C. R.
   The Status of Measurement Technologies Concerning Micron and Submicron Space
   Particulate Matter Capture, Recovery, Velocity, and Trajectory

OVERVIEW

The return of a pristine sample from a comet would lead to greater
understanding of cometary structures, as well as offering insights into
exobiology. The paper presented at the Discovery Program Workshop [3],
outlined a set of measurements for what was identified as a SOCCER-like
interplanetary mission. Several experiments comprised the total
instrumentation. This paper presents a summary of CCSR with an overview of
three of the four major instruments [4,9,11]. Details of the major dust
dynamics experiment including trajectory are given in this paper. The
instrument proposed here offers the oppurtunity for the return of cometary
dust particles gathered in situ. The capture process has been employed aboard
the Space Shuttle with successful results in returning samples to Earth for
laboratory analysis. In addition the sensors will measure the charge, mass,
velocity, and size of cometary dust grains during the encounter. This data
will help our understanding of dusty plasmas.

I. Introduction: Remote observation from Earth up to this time has provided
everything we know regarding objects beyond our solar system, and in fact,
most of what we know about the solar system itself. However, experience has
shown that when we move from remote observations of cosmic objects to detailed
laboratory studies on Earth of cosmic materials, we achieve far more than
merely extending the perception of systems. Indeed, our complete outlook may
change and place us on a higher plateau of understanding from which hitherto
undiscovered pathways may lead. These statements are paraphrased from the 1987
report--Rosetta: The Comet Nucleus Sample Return Mission [1]. The issuance of
the letter of invitation from Dr. Wesley T. Huntress to participate in a
workshop to evaluate concepts for consideration of Discovery missions
presented the opportunity to consider a comet coma sample return mission,
whose basic background comes out of these prior studies and workshops. The
invitation letter also contained the statement--"Participation in larger
international missions in which the SSED costs do not exceed Discovery
guidelines and are also within the scope of the program."--therefore, elements
from the SOCCER [2] workshops that provide spacecraft data in somewhat detail,
coupled with very specific analysis capabilities, and appropriate experience
with specific curator needs in both the USA and Europe, provided the basis for
proposing a Comet Coma Sample Return [3] mission to be considered for the
Discovery missions with SOCCER mission capabilities.

II. SCENTIFIC GOALS AND OBJECTIVES.
Statement of scientific problem and specific goals and objectives: The primary
goal of understanding the origin of the solar system and more especially the
origin of life, will not be adequately addressed until primitive bodies, e.g.,
comets, have been sampled. In establishing the core program, the SSEC of NASA
Advisory Council has argued cogently for the inclusion of a comet sample
return mission which will "... provide a detailed elemental and isotopic
composition analysis of gases and dust from the coma of a comet, data
complementary to that acquired by a Comet Rendezvous mission. Ideally, the
material will be returned to terrestrial laboratories from the same comet
observed by the rendezvous spacecraft," (Planetary Exploration Through Year
2000, A Core Program, Washington, D. C., 1983).

Unfortunately, the nonhomogeneity of comets may not allow one to assess from
the sample of two or even three comets at perihelion, the primordial nature of
the solar system. The most often repeated scientific question arising from the
investigations of primitive bodies remains; what is the nature of that
material that composed the early solar system and can that material be sampled
in an unaltered state, and if so where? The primary advantage in utilizing
comets as the source for samples occurs for those short period comets that
pass near the orbit of the Earth in six or less years.

Comets are made up of a wide variety of substances believed to have been
abundant during the origination of our solar system. Composed of frozen gases?
refractory grains, silicates, and carbonaceous compounds, comets are generally
believed to be unaltered bodies condensed in the early stages of solar system
formation. Near perihelion, the frozen gases in the outer layers of a comet
sublime to form the distinctive coma. Unlocked from the nucleus, neutral gases
stream from the illuminated surfaces entraining cometary dust grains embedded
in the frozen composite matrix.

SOCCER-like missions will be scheduled to coincide with perihelion, in
principle, the time when the nucleus will be most active and when gas and dust
released from the nucleus will be maximum. Capture of the largest newly
released grains will require a very close approach to the nucleus. In
addition, the spacecraft should fly by the cometary nucleus on the sunward
side (subsolar point) to increase the probability of sampling "jets," which
emanate material that had not been extensively exposed to the inner solar
system environment.

Cometary samples should provide insights into the early history of the solar
system and are assumed to have been altered little since the origin of the
solar system. Returning even a small sample from a selected comet would
provide a means to identify the properties of cometary grains. In principle,
the percent of cometary materials present in our current collection of
meteorites and dust is uncertain since the processes that led to the recovery
of these samples has homogenized them. Cometary samples examination in
terrestrial laboratories along with a subsequent identification of cometary
constituents would allow differentiation of cometary from the asteroidal
particles.

The return of samples of captured intact cometary dust and volatiles drives
the design of the device used to capture and maintain the intact grains. The
major concern remains the deceleration of the clusters of ices and minerals.
The key design features for the system must be 1) conduction of heat away from
the "burn" track where the grain has come to rest, and 2) conduction of heat
must be maintained once the grain has embedded in the micropore foam.

II.A.1. Collect Cometary Dust Samples Comet Coma Sample Return (CCSR)
experimental package: Capture intact dust and organic grains in a micropore
polyfoarn maintained at low-temperature.

Locate the entry point for each captured grain by means of active trajectory
system data.

Upon return to terrestrial laboratories investigators will:

Perform an examination of the returned samples recovered from micropore
polyfoams.

Maintain recovery unit at a low-temperature to ensure the curation of the
unexamined samples.

When samples are returned to Earth from the cometary environment it will be
necessary to have laboratories for their analysis. Planning for the
establishment of curatorial facilities for returned cometary dust particles
will be an integral part of the mission. This facility needs to be capable of
simulating certain conditions of the comet coma from which the samples were
collected. Existing analytical tools will be adapted to operate in these
conditions and new techniques for the curation and analysis will be developed
to ensure that the most efficient and conservatory methods are used. Expanded
discussion of the analyses associated with the specific instrumentations under
discussion are presented in this workshop by C. Maag et al [4].

However, from the comet P Halley probe, we have learned that cometary grains
also contain semi-volatile organic compounds and, possibly, icy volatile
inclusions. Of special and unique interest are the organic grains that were
observed in the coma of Comet P/Halley, the CHON particles. If we assume that
these grains are common among comets as observational evidence suggest [5],
the groundbased laboratory must have instrumentation that allows for the
identification, separation, and sophisticated analyses of the individual CHON
grains (micron to sub-micron sizes) for an overall chemical and physical
characterization. The survival of organic molecules in the grains returned
after a hypervelocity impact should strengthen theoretical assertions
concerning the possible role of comets in the genesis of life. Therefore,
biological and chemical contamination issues are of extreme importance so that
the purity of the comet samples can be ensured. Based on the P/Halley
experiences, the characterization of organic grains is an area that is
expected to have considerable impact on laboratory activity.

II.A.2. Investigate Dust Environment in the Comet Coma. Establishing the
charge, mass, and velocity vector of cometary dust, the flux of cometary
grains on the capture cells will be determined so that one may:

Delineate the major components of the total dust grain size and mass
distribution for the particulates in the comet's coma.

Utilize the knowledge of the dust grain size distribution associated with the
region of space near the nucleus of a comet to compare with the "jet"
structures of the active comet.

A scientific objective of the proposed experimental package is the
investigation of a comet's dust environment to establish the electrostatic
charge, mass, and speed of cometary dust. With a measure of these parameters
an upper and lower bound on the density of cometary dust grains can be
established. Analysis of the dust grain data will reveal the flux profile for
cometary dust grains emanating from the comet nucleus and will provide an
assessment of the dust size distribution associated with the variations in
cometary activity.

The flyby of Halley's comet by the GIOTTO probe has provided a unique
opportunity to increase our knowledge on cometary structure. Several cometary
models [6] developed prior to the in situ measurements had predicted that the
dust flux would have a simple inverse square of the distance to the comet
nucleus dependence. For most of the Halley encounter this was an accurate
model.

However, five deviations or anomalies were found by DIDSY during the flyby.
Two of these occurred before closest approach and the remaining three were
noted after closest approach. This indicated an in situ derived example of
dust/plasma interaction [7].

Utilizing cometary dust detection data and data provided by a plasma
experiment, an investigation of the interactions between dust and plasma in
the development of cometary structures of comets is strongly desired. Since a
knowledge of the plasma environment is necessary for an understanding of any
type of dust behavior, observations will both complement and be complemented
by experiments that characterize the plasma and energetic particle regimes
from in situ data. CCSR can very well provide important information on the
collective effects of interactions in dusty plasmas.

II.A.3. Investigations of Dusty-Plasma Environment. Utilizing cometary dust
detection, capture, and plasma data, the interactions between dust and plasma
in the development of cometary structures of comets will be analyzed:

To study the dynamical affects on the motion of charged dust grains embedded
in the plasma sheet of the cometary coma.

To investigate the question of whether the number of charges on dust grains
embedded in the plasma of a cometary coma may be a function of the separation
distance between neighboring dust grains.

The physics of dusty plasmas is an emerging field of research with broad
implications for the understanding of comets and the evolution of the early
solar system protoplanetary disk. Many subtle and often surprising effects
have been discovered theoretically [8], but very little experimental work has
been done. Previous measurements of dust in the vicinity of planets and comets
have been limited by their low sensitivity and lack of ability to measure
grain velocities, charges, and masses. An in situ determination of all these
quantities is crucial before further progress in the understanding of dusty
plasmas can be made. All these quantities will be measured by the Dust
Particle Dynamics and Environment Experiment (DPDEE) (section III.B.1).

II.B. Comet Targets: The primary comet target can be Kopff in 2002; additional
targets to be considered are Finlay in 2002, Wirtnaen in 2002, and Churyumov-
Gerasimenko in 2002. The selection of comet targets will be made based on the
activity of the comet, the quality of Earth viewing for a particular
perihelion passage, as well as the best estimate of the dust to gas ratio.

III. INSTRUMENT DESCRIPTIONS AND REQUIREMENTS.

III.A.1. Gas Capture Cell (GCC): Upon impact, a particle (carbonaceous
chondrite or icy grain) traveling with hypervelocity speed is vaporized. In
order to assess, collect, and retain the evolved gases we propose to include a
unique gas capture cell. The cell will consist of numerous devices or sensors
that have been tested in the space environment singly, but heretofore, not as
a collective group. the gas capture cell will consist of a cryoplate, which
will be maintained at approximately 150K for the duration of the mission,
i.e., from just prior to the flyby of the proposed target comet through
rerieval and return of the instrument to the laboratory and subsequent data
reduction. Approximately ten (10) cm above this plate an ultra-thin film (tf <
500 A) will be placed to act as both a primary sensing element and a thermal
radiation barrier.

Attached to the cryo plate will be an array of Gerrnanium ATR (Attenuated
Total Reflectance) crystals. These crystals will be maintained at 150K by
thermal conduction. Attached to the cell will be a quadrupole mass
spectrometer capable of measuring from 1-50 AMU. The mass spectrometer will be
a slightly modified version of the instrument flown four times on the Space
Shuttle Induced Environment Contamination Monitor (IECM) [9].

III.A.2 Organic Foam Capture Cell (OFCC): Intact capture of a cosmic dust
particle will be accomplished by the use of micropore foams. The principal
objectives of the original program were to develop techniques that would
provide the size distribution of A1203 particles (AOS) expelled from a Solid
Rocket Motor (SRM). Polymetric foams, with and without deceleration films,
were extensively used to capture AOS intact. Aerogel materials were also used
as a capture material. Commercial organic polymer foams were ininally used.
Based on testing in light gas gun facilities, it was determined that the
ability of these foams to retain particles impacting at hypervelocities was
marginal at best. These tests found that the more complex polymers had better
stopping ability. Accordingly, it was also determined that the polymers that
had extremely small cell sizes, higher latent heats of fusion and very low
densities (e.g., 0.02-0.7 g/cm^3 had the highest probability of providing
intact capture. In ground tests, the foams have been successfully tested
between 1 and 11 km/s. One of the more interesting highlights of this program
was the intact capture and retention of materials with a much lower density
and material strength than AOS. Since the initial program other materials have
been tested successfully, most notably foams of silicones, polymides, and
fluorocarbons. For this activity, foams of the aforementioned polymers will be
developed and used that have both gradients in axial density, typically from
0.006-0.09 g/cm^3, and imbedded sensors to sense impact parameters as the
particle decelerates in the foam (Figure 1). [4] Figures 2, 3, and 4 show
three conditions of particles captured intact using organic foam capture cells
on Shuttle STS 61-B and STS 41-D. Figure 2 depicts an interplanetary particle
from STS 61-B. Figures 3 shows an AOS particle captured intact in the organic
foam cell and then in Figure 4 the same AOS cell after the pyrolized foam has
been removed using a low-molecular concentration of HCL. Figure 5 shows a
particle captured using Aerogel.

III.A. Multiple Thin Film Array Capture Devise (MTFA): Cometary dust grains
impacting on a very thin film will experience a pressure transient, however,
when the thickness of the film is very much less than the diameter of the
grain, the duration of the 8 transient is extremely short with the destructive
effect being greatly reduced. A succession of these films, spaced to allow
nondestructive energy dissipation between impacts, significantly reduces the
kinetic energy of the grain without allowing its internal energy to rise to
the point of destruction of the projectile mass. (Figure 6).

III.B.1. Dust Particle Dynamics and Environment Experiment (DPDEE) [10]: In
previous sections, statements were presented for inclusion of in situ
determination of the charge and dynamics of each of the cometary grains that
impacted a collection device. The trajectory information is significant in
locating the grain in the OFCC, ACC, and the MTFA capture devices. The dynamic
parameters are the scientific information derived in the investigation of each
captured grain and/or residue collected.

The DPDEE is a group of sensors integrated into a detection array that
measures:
the charge possessed by the grains traversing the system, the trajectory of
the grain within the system, the time of flight (TOF) of the grain within the
system, and the kinetic energy of the grain.

All of the sensors have been previously developed and used in some form in
space. The DPDEE is depicted in Figure 7. The system is shown coupled to an
OFCC. The MTFA capture device will also couple to the DPDEE. The signals to be
derived from the passage of a grain through the unit are depicted in Figure 8.
A laboratory system with these sensors (PZT impact plate instead of a capture
cell) was used in a hypervelocity accelerator to demonstrate all aspects of
the DPDEE with the schematic and results seen in Figures 9 and 10. (Figure 9
shows the schematic of the system). The bottom trace contains the charge
measurement, which also yields a TOF. The top trace shows charge/plasma
associated with the particles passage through the thin film and then the sharp
negative pulse from the plasma of the impact on the PZT plate. In the proposed
array this last signal will come from the plasma generated as the particle
impacts the capture cells. TOF and position data are obtained from the two
plasma pulses while kinetic energy is derived from the second plasma pulse.

Electrostatic charge and trajectory information about an interplanetary
micron-sized dust particle is retrieved from 2 XY grid systems (Figure 11).
Laboratory work has been conducted to optimize the sensor array designed with
respect to grid spacing and separation using both signals from charged beads
traversing two grid systems with induction from the electronically charged
signals. Results around a single wire are seen in Figure 12. Calibration tests
of the 10 wire grid system resulted in data exhibiting considerable variation
in pulse amplitudes between adjacent wires (Figure 13). However, when the
ratios of the outputs from the electronic system are summed and plotted, the
information obtained makes possible the determination of quite precise
particle trajectory. Figure 14 depicts the graph of position obtained from gnd
wires at 1cm centers connected by a capacitor divider as shown in Figure 11.
Resulting "fits" to the data are shown in Figure 14 . These were obtained
using linear regression techniques, having a correlation coefficient of
0.99962 and indicating a standard deviation of less than +-0.8mm. This equates
to an angular resolution of better than +-2 degrees as for particles entenng
the sensor array with 2 grid system detection units.

III.B.2. Detection of Ion and Electron Components of Plasma Within Cometary
Coma (IECCC): The charge on the cometary grain is an important measurement.
The charge primarily results from photoelectric effect and plasma interaction.
Therefore. a measurement of the magnitude of the plasma prior to comet
intercept and throughout passage into the coma will be provided by standard
and ion electron spectrometers.

IV. BENEFITS TO THE DISCOVERY PROGRAM AND SOLAR SYSTEM EXPLORATION.

The proposed mission is designed to meet the requirements for a Discovery-
class mission. The proven success of the capture materials in Low Earth Orbit
combined with the support of the space science community in Japan offers a
clear chance to launch an inexpensive mission to achieve the capture of
cometary material in a pristine state, which has long been one of the
objectives of workers in the field of solar system exploration.

V. UTILIZATION AND BENEFITS TO UNIVERSITY, INDUSTRY, AND NASA COMMUNITY.

In the workshop proposal [3], the CCSR mission is an example of an
international consortium whose domestic partners includes NASA (JSC), industry
(SAIC, Perkin-Elmer), nonprofit (SwRI) and university (Baylor, Johns Hopkins,
APL). International cooperation includes the University of Kent at Canterbury
(UKC) in England and the Institute of Space and Astronauncal Science (ISAS) in
Japan. All of these groups have extensive experience in cometary research.

VI. RELATIONSHIP TO PAST AND FUTURE PLANETARY MISSIONS.

Giotto Halley, Giotto Grigg-Skjellerup, Vegas, Sagisake, and Sushiam cometary
missions produced a wealth of data concerning the make up of comets. However
the compositions of the diverse materials that were intercepted by these
extremely high encounter velocity spacecraft could better be unraveled if
material similar to that intercepted could be analyzed in terrestial
laboratories. The sample return nature of CCSR will allow the first
investigation of newly released material from an active comet, which can be
analyzed in terrestial laboratories. The CCSR mission could examine material
released from a comet that would be or had been the target of a proposed
rendezvous mission to the same comet. The knowledge gained from the samples
returned coupled with data gleaned from the flyby missions could set the stage
for a large-scale international rendezvous and sample return mission like the
ESA/NASA proposed Rosetta mission.

The many questions raised concerning the nature of primordial material by
previous flyby missions, analysis of stratospheric collected IDP's, and remote
sensing of primitive bodies could be answered by analysis of intact captured
cometary grains in situ and in terrestrial laboratories. Benefits derived from
comparative analysis between material detected during the outer planet
missions, i.e., Galileo, Ulysses, Cassini, and that collected by CCSR could
help define the nature of metamorphizing mechanisms acting on the material in
the solar system.

VII. MAJOR ACCOMPLISHMENTS AND PUBLIC PERCEPTIONS OF THE MISSION.

A CCSR-type mission will return data on the composition of the comet nucleus
and the grains ejected into the coma during the high activity of the
perihelion passage. It will allow the determination of the basic properties of
the nucleus: the size distribution of material ejected from the active areas,
"jets"; and the structures arising from the interaction between the plasma and
the fine dust grains that later may be analyzed in terrestrial laboratories.

Cometary apparitions, once a portent of an impending cataclysm, now stimulate
the imaginations of the public to wonder about the make up of these ancient
objects. As Earth-launched spacecraft glide through the icy sheets of a
comet's coma, the scientific experiments link us with the encounter. Now that
material from these prodigious objects can be harvested and returned to Earth,
an final demystification of the ominous nature of comets can reveal the
reality of the solar system's early formation and construction. For these
reasons the CCSR mission would engender the highest measure of public support
and interest.

References: [1] Wood J. A. (1987) ESA SP-278. [2] Uesugi K. et al. (1991)
Japan-US Joint Workshop on Missions to Near-Earth Objects. [3] Alexander W. M.
et al. (1992) Discovery Program Workshop. [4] Maag C. (1993) This workshop.
[5] Huebner W. F. (1988) Advances in Space Science Research. [6] Gambosi T. I.
et al. Reviews in Geophysics, 24. [7] Alexander W. M. et al. (1987) GCHP-BUSSL
Research Report, 007. [8] Huebner W. F. et al. (1988) Icarus, 76. [9] M.
Heppner (1993) this workshop. [10] Alexander W. M. et al. (1992) BUSSL
Research Report, 82992. [11] Tanner W. G. et al (1993) This workshop.





                           Monday, September 27, 1993
              Capture Medium Development: Laboratory Experiments
                       1:30 - 5:30 p.m.     Berkner Room


Anderson W.*
   Penetration of Highly Porous Media

   No abstract available.


Tsou P.*   Griffiths D. J.   Albee A. L.
   The Physics of Intact Capture

The ability to capture projectiles intact at hypervelocities in underdense
media opens a new area of study in physics. Underdense material behaves
markedly different than solid, liquid, or gas upon hypervelocity impact. This
new phenomenon enables applications in science that would either not be
possible or would be very costly by other means. This phenomenon has been
fully demonstrated in the laboratory [1] and validated in space [2]. Even more
interesting is the fact that this hypervelocity intact capture was
accomplished passively. A better understanding of the physics of intact
capture will lead to improvements in intact capture. A collection of physical
observations of this phenomenon is presented here.

INTRODUCTION
Since it was accepted at one time that a projectile going faster than a
speeding bullet could not be captured intact, it was believed that the only
way to stop such a particle, especially at higher hypervelocities, would be to
convert the particle into gas and capture the vaporized condensates, i.e., the
atomization process. Capturing and studying a cosmic dust particle or any
hypervelocity object by atomization is not unlike first reducing the object to
atoms, and then reconstructing the object from the gas condensates. This
approach proved to be very difficult, ununique, and fraught with
uncertainties. By contrast, capturing a portion of a hypervelocity particle
intact would preserve the object's "structure" (phase, morphology, and
mineralogy) and its full chemical and isotopic compositions. Reconstruction of
the object would be easier and carried out with more confidence.

But how can intact capture of hypervelocity particles be possible? A gedanken
experiment provided the inspiration for investigating just such a solution.
Conceptually, there exists an extremely thin diaphragm that, when penetrated
by a hypervelocity projectile, will not affect the integrity of the
projectile. The diaphragm, being extremely thin, will absorb only a very small
amount of the projectile's kinetic energy. However, after penetrating a very
large number of such diaphragms at noninteracting separations, all of the
energy will be absorbed, leaving the projectile intact and at rest. Since the
projectile has a finite initial kinetic energy, the number of diaphragms and
the total separation between them may be very large, but is nevertheless
finite. In actuality, the projectile need not be 100% recovered; by
sacrificing a portion of the projectile, the capture can be achieved with even
thicker diaphragms and fewer of them. This approach is a considerable
improvement on the process of atmospheric re-entry of meteorites, since we can
control the diaphragms' material, thickness, and separation distance. The
challenge then is one of reducing this arrangement to a practical instrument
by using a good energy-absorbing material and reducing the number of
diaphragms or size of the instrument required. This gedanken experiment has
served as the guiding concept for designing experiments and developing
practical materials to achieve intact capture.

PHYSICAL PHENOMENON
To generate physical data on intact capture, systematic and exploratory
capture simulation experiments were performed with two-stage light-gas guns up
to 7 km/s and electrostatic accelerator up to 20 km/s. These simulation
experiments consisted of launching projectiles with known mass and integrity
at known speeds into capture media in a vacuum and at room temperature. The
captured projectiles were then characterized by observing morphology of the
captured projectile and amounts of mass recovery. The capture media were
examined for track dimensions, mass loss, and surface composition. Analysis
tools used included optical microscope and scanning electron microscope.

Recovery Ratio
One of the first obvious measures of hypervelocity intact capture is the
projectile mass recovery ratio, (i.e., the ratio of the recovered projectile
mass to the original mass). Extensive data on metal projectiles have been
presented [3]. Figure 1, which appears in the hard copy, shows the recovery
ratios of three different diameters of glass projectiles captured in
polyethylene foam.Using an ablating re-entry vehicle type of model
formulation, a good descriptive fit of the intact mass recovery ratio data has
been achieved [4].

Conventional hypervelocity impact theory, working in the hydrodynamic limit,
takes the target's bulk density to be the only target parameter influencing
solid impact into solids. Applying this reasoning, the lowest bulk density
target produces lower shock levels and lower temperature and thus should have
the highest recovery; however, our data show that projectile recovery is more
sensitive to media structure (cell size and wall thickness) and material
thermal properties (decomposition point) than media bulk density [5].

Penetration Track
Tracks or cavities produced deep in the underdense media and caused by
hypervelocity projectile penetration are very different from craters formed in
solid-to-solid impacts. The tracks left in underdense media tend to be thin
and long and typically carrot-shaped. A track can be characterized in terms of
three regions: initially, the track quickly widens to a maximum, then
gradually narrows, and finally bores through the medium with approximately the
diameter of the projectile. The surface of the initial region tends to be
rough and broken along the wall of the track cavity. Following the broken
region is the transition zone, typified by smoother walls. The final region of
the track shows shearing of the medium.

The penetration track lengths for the same set of experiments shown in Fig. 1
are presented in Fig. 2, which appears in the hard copy. Track lengths tend to
be longer for lower-density media if the media structures are similar. The
total track length to projectile diameter ratio ranges about 50 to 250
permitting the determination of the projectile direction error to less than 1
degree. Assuming a simple F = ma type of equation of motion and characterizing
the medium erosion in three regions, a descriptive model of the total track
lengths was formulated for various projectile speeds that fitted experimental
data well [4].

The combined effects of mass recovery, penetration track length, and track
profile as functions of the projectile initial speed are shown in Fig. 3,
which appears in the hard copy, as one display. It can be seen that the total
penetration track tends to peak in the region of 2 to 3 km/s. The general
shape of the penetration track curve seems to be invariant: a steep rise
(initially linear) to a peak, then a drop to a near plateau, then a steep
fall, and, finally, termination. A second derivative of this generic
penetration track curve indicates three inflections or four regions: an all-
shear region with no projectile mass loss, a media-melt region with minor
projectile mass loss, a media-pyrolysis region where significant projectile
mass loss begins, and, finally, a projectile disintegration region where the
track is greatly enlarged and branched.

The physical mechanisms of projectile mass loss in underdense media at
hypervelocities are very complex and not yet well understood. Based upon fluid
dynamic formulations, drag force, and thus the peak temperature, favors higher
mass recovery for larger projectiles and is indeed indicated here. The
projectile surface-to-mass ratio increases for smaller projectiles.

Penetration Track Volume
The capture medium absorbs the projectile's kinetic energy as it penetrates
the medium; the manner of this energy absorption is reflected in the shape and
volume of the penetration track. Since the time required to produce pyrolysis
features in the media must be much greater than the projectile penetration
time (about 200 us over a 100-cm distance for a 3.2-mm projectile), the actual
shape of the track cavity must have been developed long after the projectile
had come to rest. Typical track profiles for the polyethylene foam (for a 3.2-
mm pyrex projectile) are shown in Fig. 4, which appears in the hard copy.

The volume of the track cavities correlates well with initial projectile
speeds. Figure 5, which appears in the hard copy, using the same medium as
shown in Fig. 4, plots the track volume with respect to initial projectile
speed. Cavity volume is also proportional to projectile size. Since projectile
velocity has two components, speed and direction, this volume-to-speed
relationship provides the speed component; track orientation gives the
direction. This is another valuable feature of intact capture by underdense
medium: not only is the projectile captured passively, but the projectile
velocity is also recorded passively. With the knowledge of projectile velocity
and given the orientation of the capture instrument, the original projectile
trajectory can be calculated.

Projectile Deceleration Profile
To ascertain the manner in which the projectile loses its mass as it
penetrates the underdense medium and the timing during which the loss occurs,
the projectile's speed profile during capture was sought. A variety of methods
to accomplish this were explored. Early success was achieved with the standard
and simple breakwire approach using 5-micrometer-thick copper labyrinth
imprints on 25-micrometer Kapton film. The breakwire was considerably stronger
than the capture medium; consequently, a nondestructive motion sensor would be
preferred. We adapted the magnetic pickup approach used in light-gas guns to
determine the passage of the projectiles. Deceleration profiles were obtained
for 2.4-mm stainless steel projectiles only (the current induced on Al
projectiles was not sufficient for magnetic pickup); even then, signals beyond
the second station were noisy. Finally, a set of X-ray shadow graphs captured
the deceleration of the projectile in four positions. As it turned out, the
results from all three methods are comparable, as shown in Fig. 6.

These profiles indicate surprisingly near-linear initial deceleration. Our
original hypothesis had been a much steeper exponential drop followed by slow
deceleration to rest. However, this near-linear deceleration was predicted by
Sandia National Laboratories' CSQII Eulerian finite-difference program for
two-dimensional material response, with the underdense medium approximated by
the snowplow model. This deceleration profile provides important insight into
the manner of energy dissipation, useful for theoretical developments and for
developing an improved intact capture medium.

Real-time Dynamics
The real-time action and reaction between the projectile and the capture
medium is immensely important to understanding the physics of the intact
capture phenomenon. Since a hypervelocity intact capture event is completed in
less than a fraction of a millisecond, means to provide a temporal expansion
of the event are desired. Efforts to ascertain the real-time dynamic impact
chemistry of the projectile and the capture media through spectroscopy [6]
have proved useful in understanding dynamic impact chemistry and making
temperature estimates. A wide range of wavelengths and means of spectra
acquisition were examined. To determine the effectiveness of different
approaches from ultraviolet to visible have been acquired by using prisms,
reflection and transmission gratings are then recorded by photographic and
electronic media. The first spectrum of an intact capture event is shown in
Fig. 7, which appears in the hard copy, and was obtained from a 3.2-mm-
diameter Al sphere impacting multilayers of 10-micrometer mylar films with the
NASA Ames reflection grating spectrograph. The black-body temperatures
determined for the different polymer underdense media are in the range of 2500
to 3100 K [6].

Momentum Transfer
A characteristic of intact capture is energy dissipation through momentum
transfer by dragging the capture medium with the moving projectile. For the
polymer capture medium, densely packed and melted foam would form a shell
around the forward portion of the projectile. At times, the mass detached from
the capture medium can be as much as the captured projectile. Fused glass is
often found around the captured projectile in aerogel. This accumulation must
take place after the transitional region and further protect the projectile by
both momentum transfer and heat removal.

Medium Energy Absorption
One criterion that can be used to measure the overall performance of a capture
medium is the percentage of the projectile's kinetic energy that medium
absorbs; the unabsorbed energy produces the projectile damage. This energy
absorption was crudely estimated by recording the mass of segmented foam
pieces before and after a simulation experiment. Estimates of the absorbed
energy were made from the loss of foam mass, the track-cavity volume, and
estimated thermal constants of the capture medium and Al.

For a 3.2-mm Al projectile captured in 32-mg/ml expanded polystyrene at 6 km/s
(875 J total kinetic energy), the foam's net mass loss was about 1.2 g. Using
listed values of specific heat and heat of depolymerization of polystyrene of
400 J/g and 660 J/g respectively, all of the total kinetic energy of the
projectile can only depolymerize about 0.68 g of polystyrene. Photographs of
the back ejecta substantiated this by showing substantial amounts of solid
particulates. Based on measurements of the track volume, the equivalent mass
loss in the track cavity was compared to the actual mass loss. This
difference, about 0.6 g, should be the portion of the foam that was melted and
resolidified along the track wall. Using listed polystyrene foam heat of
fusion of 87 J/g, about 34% of the incoming kinetic energy would be used to
melt the polystyrene. Other components of medium energy absorption, such as
mechanical work, are more difficult to estimate and assumed to be relatively
small.

As a check, the amount of energy absorbed by the projectile can be estimated
from the measured mass loss of the projectile. Projectile mass loss can be due
to vaporization, melting, or mechanical ablation. Since the shock pressure
generated at 6 km/s is much too low to cause vaporization of Al, the energy
absorbed by the projectile was assumed to have melted, an upper bound
estimate. Using a value for Als heat of fusion of 400 J/g and a specific heat
of 9 J/g, a little more than 1% of the incoming kinetic energy contributed to
the projectile mass loss. Compared to about 11% energy absorbed by the Apollo
capsule in atmospheric re-entry, underdense media make good intact captures.

SUMMARY
The intact capture of a hypervelocity projectile would first punch a straight
and narrow track through an underdense medium not much larger than its
diameter in less than a millisecond covering about 100 times its diameter
before coming to rest. The projectile would be in one piece, wedged at the end
of the track. Some time later, as the high-energy ejecta from the medium and
projectile deposited along the track wall have time to dissipate thermally,
the narrow track would be enlarged into a carrot shape, as shown in Fig. 3. At
the medium entry, about the first dozen cells, the projectile and the medium
would not be damaged except for a clean hole left in the medium. Beyond the
entry, energetic ejecta would build up rapidly to a peak in the medium. The
projectile would decelerate in a near linear fashion before gradually coming
to rest. Most of the projectile mass loss seems to occur before the point of
peak diameter in the track. The actual peak shock may precede the point of
track peak due to the forward motion of the ejecta.

Small cellular members in underdense media generate very low shock levels to
both the projectile and medium. Underdense media readily absorb projectile
energy at low thresholds, providing an efficient and low-threshold energy
sink. As the aerogel capture medium is very porous, partial compaction allows
another means of energy dissipation by momentum transfer. Effective capture
media must have sufficient material strengths to hold medium members in place,
preventing prolonged accumulation of damaging plasma while not contributing
added damage to the projectile. In all, underdense media emerge as a phase of
material that decelerates and preserves a hypervelocity projectile gently and
efficiently.

The signature left by an intact capture is a long, thin track in the capture
medium. Low bulk density would be necessary for intact recovery; however,
within a certain range, the medium's bulk density tends to affect the shape of
the track more than the mass recovery. The structure of the medium,
configuration, and size of cells affects mass recovery directly, both in terms
of the quantity and the deformation of the recovered projectile. Other
sensitive parameters are the structural and thermal properties of the capture
medium.

As the density of a medium is lowered, the penetration track tends to become
longer. For the same density medium, but at higher projectile speeds, the
track tends to be both wider and shorter. The projectile entry hole varies
with projectile speed: at low speeds, the entry hole is nearly the same
diameter as the projectile; for higher speeds, the entry-hole diameter
increases--reaching as much as three to four times the projectile diameter.

REFERENCES: [1] Tsou P. et al. (1988) Proc. LPSC 19th. [2] Tsou P. et al.
(1993) Proc. LPSC 24th. [3] Tsou P. (1990) Int. J. Impact Engng., Vol 10. [4]
Peng S. T. J. et al. (1987) LPI Tech. Rept. 88-01. [5] Tsou P. et al. (1991)
APS. [6] Tsou P. et al. (1993) Int. J. Impact Engng., Vol 13,.


Fujiwara A.*   Nakamura A.   Kadono T.
   Laboratory Simulation of Intact Capture of Cometary and Asteroidal Dust
   Particles in ISAS

In order to develop a collector for intact capturing of cometary dust
particles in the SOCCER mission and regolith dust particles released from
asteroid surfaces by the impact of projectiles launched from a flying-by
spacecraft (Fig. 1) [1], various kinds of materials as the collector
candidates have been exposed to hypervelocity projectiles in our laboratory.
Data based on the penetration characteristics of various materials
(penetration depth, hole profile, effectiveness for intact capturing) are
greatly increased. The materials tested for these simulation experiments
includes various kinds of low-density media and multisheet stacks; these are
foamed plastics (polystyrene 0.01 g/cc) silica aerogels (0.04 g/cc), air
(0.001 g/cc), liquid, and multisheet stack consisting of thin Al sheets
(thickness 0.002 to 0.1 mm) or polyethylene sheets. Projectiles used are
spheres or cylinders of nylon, polycarbonate, basalt, copper, iron, and
volatile organics (e.g., paradichrorobenzene) of size ranging from 30
micrometers to 1 cm launched by a two-stage light-gas gun and a rail gun in
ISAS at velocity up to about 7 km/sec. Some results obtained by using nylon
projectiles and of velocity less than about 5 km/s have already been published
elsewhere by the present authors [2-5], and two figures from these works are
reproduced here; the penetration depth vs. bulk density of the collector
naterial for several kinds of materials (Fig. 2) and the velocity at which the
projectiles begin to fragment vs. material density for foamed polystyrene
(Fig.3 ).

Figure 3 shows that if we want to capture particles of velocity 8-10 km/sec,
the expected relative velocity in the SOCCER mission, the density of the
collector media must be as low as 0.001 g/cc, which corresponds to almost the
density of air of 1 atm. Hence deceleration and trapping by gas cells followed
by low-density soft material like aerogel seems to be a possible way of intact
capturing very fragile and volatile dust particles like cometary dusts. At
present, efforts are being made to extend the data for wider experimental
parameters (e.g., higher projectile velocity, variety of lower-density
materials, etc.), and aerogels of lower density are also being developed.

References:  [1] Fujiwara A. et al. (1990) Proc. 12th Solar System Sci.(in
Japanese) ISAS. 167-172. [2] Fujiwara, A. and T. Kadono (1990) Jpn. J. Appl.
Phys., 29, 1620-1624. [3] Ishibashi, T. et al. (1990) Jpn. J. Appl. Phys., 29,
2543-2549. [4] Nakamura A. et al. (1991) Jpn. J. Appl Phys., 30, 2129-2133.

Figure 1, which appears in the hard copy, shows regolith dust particles
released from asteroid surfaces [1].

Figure 2, which appears in the hard copy, shows the hole depth vs. the target
bulk density for PEF: polyethylene film (thickness = 50 micrometers), PES:
polyethylene sheet (thickness = 300 micrometers), ALF: aluminum foil
(thickness = 15 micrometers), ALS: aluminum sheet (thickness= 100
micrometers), FPB: foamed polystyrene,  ALB: aluminum block, and PEB:
polyethylene block.

Figure 3, which appears in the hard copy, shows fragmentation velocities and
depths vs. target densities. Solid line represents the critical fragmentation
velocity V(sub)c, where the fragmentation of projectile occurs between two
impact velocities for each target density as shown by open and filled circles.
Dashed line represents the fragmentation depth, where the depths corresponding
to two impact velocities are indicated by open and filled rectangles
respectively.


Mendez D.*
   High-purity Aerogel: Manufacture and Verification of Quality and Performance

   No abstract available.


Zolensky M. E.*   Barrett R. A.   Horz F.
   The Use of Silica Aerogel to Collect Interplanetary Dust in Space

ABSTRACT

We report on impact experiments to evaluate the suitability of silica aerogel
as a capture medium for interplanetary dust particles in space. The structure
of silica aerogel is such that even micrometer-sized hypervelocity particles
may sense it as a target of very low density; consequently shock stresses and
temperatures due to impact should be minimized. We performed impact
experiments in a light-gas gun that employed projectiles manufactured from
forsterite, pyrrhotite, and calcite. From these experiments we have developed
techniques that will permit the successful extrication of interplanetary dust
particles from aerogel. We note a rough correlation between increasing
particle penetration lengths and decreasing aerogel density, and there is a
poor correlation of track lengths with impact velocity at laboratory-
attainable velocities of 5 to 7 km/s. We conclude that aerogel track length
should not be used even as (crude) velocity indicators. Transmission electron
microscope studies show the recovered particulate residues are encased in
melted silica aerogel, which possibly protects the residue from some impact
damage. Individual minerals within interplanetary dust are expected to exhibit
differential survival, and may be vaporized, melted, dehydrated, or
structurally reordered. However, some mineral grains should successfully be
captured in an unmelted state at encounter velocities <8 km/s.

INTRODUCTION

Several proposed missions will seek to collect individual interplanetary dust
or cometary particles in space. Associated capture media must be deliberately
developed to minimize potential particle degradation and destruction. Upon
retrieval to Earth-based laboratories the captured particles may then be
characterized in detail. An outstanding technical challenge has been to
develop suitable capture media permitting successful deceleration and capture
of particles in a manner permitting laboratory analysis of particle residues.
This capture of hypervelocity particles in the least destructive fashion
depends upon the dissipation of the particle's kinetic energy. A successful
capture medium must dissipate the maximum amount of energy into the target, in
such a manner that the fraction that is unavoidably partitioned into the
impactor is kept below the specific heats of fusion and vaporization of the
projectile. Deceleration by molecular collisions in a gas or by viscous drag
in a liquid seems impractical for most purposes. For this reason, low-density
foams or stacked-foil capture cells are being evaluated at this time (CDCF
Report, 1990). The former material is the focus of this paper, which expands
on the work reported by Tsou (1990) and Tsou et al. (1989, 1990).

We report here on impact experiments using silica aerogel as a deceleration
and capture medium. This material can be manufactured with a bulk density << 1
g/c^3 (Hrubesh and Poco, 1990), and therefore will induce low shock stresses
and associated temperatures even at predicted encounter velocities of 15-20
km/s (e.g., Horz et al., 1986; Anderson and Ahrens, 1988). It has a suitable
microstructure, with irregular chains and clusters of SiO4 tetrahedra,
generally <100 Angstroms in dimension and fairly uniform distribution of
associated void space (Hrubesh and Poco, 1990). The solids composing any
suitable low-density, porous medium must have substantial, nonporous
dimensions significantly less than the projectile diameter, otherwise the
impacting particle will sense them as massive (and successive) plates,
possibly even as infinite half-space targets (in the extreme case), voiding
the low acoustic impedance and thus shock stress properties of the bulk
material. Aerogel meets the requirement of low bulk density at dimensional
scales smaller than the expected micrometer-sized cosmic dust impactors better
than most other low-density materials, and is therefore a top candidate
capture medium for CDCF flight instruments from the viewpoint of shock
processes.

The first projectile-recovery experiments using silica aerogels were reported
by Tsou et al. (1989), who successfully retrieved soda lime glass spheres and
olivine grains that impacted at ~6 km/s. They also analyzed the detailed
geometry and length of projectile penetration tracks, which are generally
visible to the naked eye in transparent aerogels. We have expanded on the
experiments performed by Tsou et al. by employing different, more realistic
projectiles, such as powdered calcite, olivine, enstatite, and pyrrhotite, as
well as artificial interplanetary dust analog spheres. We further tested a
suite of different silica aerogel materials varying in density from 120 to 20
mg/c^3.

STRUCTURE AND COMPOSITION OF SILICA AEROGEL

STRUCTURE.  Silica aerogel is a transparent medium that transmits most
wavelengths of light, but scatters the shorter wavelengths to produce its blue
color. We employed several varieties of aerogel in our experiments. The 250-
mg/c^3 silica aerogel was provided to us by L. Koch-Miramond, Service
dAstrophysique, Gif-Sur-Yvette, France. The 120-mg/c^3 silica aerogel was
produced by Henning Airglass in Staffanstorp, Sweden. Still lower density
varieties of 60, 40, and 20 mg/c^3 were developed and fabricated by Lawrence
Hrubesh at Lawrence Livermore National Laboratory (Hrubesh and Poco, 1990).

We imaged 120-mg/cc silica aerogel samples in a JEOL 2000FX TEM, at 200 kV.
These samples were prepared by vacuum impregnation using the epoxy MBED-812,
followed by ultramicrotomy. Imaging shows the silica aerogel consists of
loosely bonded, nanometer-sized "balls" of silica. We saw no evidence of long-
range structural order or large pores in this particular material.

We also investigated some of the opaque, white silica "droplets" commonly
produced during impact experiments as projectiles penetrate and compress the
silica aerogel; this material typically coats projectile residue. High-
resolution transmission electron microscope (TEM) imaging reveals that these
"droplets" actually consist of welded, 50-200-nm-sized, rounded silica grains
with considerable long-range order. This probably indicates that the particle
capture process (at least at laboratory impact velocities of 5-7 km/s) induces
melting and subsequent crystallization (increasing long-range ordering) of the
originally amorphous aerogel.

CONTAMINANTS.  Trace-element analyses of the silica aerogel were performed by
Instrumental Neutron Activation Analysis (INAA) on two samples of silica
aerogel having densities of 120 mg/c^3 and 60 mg/c^3, and reported by Barrett
et al. (1992). The 120 mg/c^3 aerogel contains Fe (60 ppm), Sn (4 ppm), Br (2
ppm), Zn (0.3 ppm), Cr (0.2 ppm), Sb (10 ppm), Co (8 ppb), and Sc (60 ppt).
The 60 mg/cc sample contains Rb (12 ppm), Na (3.4 ppm), Zn (0.4 ppm), Br (0.2
ppm), and Au (10 ppb). The Sn, Au, and Zn are most likely traceable to
autoclave materials, and Br to the solvents used in aerogel extraction, but
the source of alkalis is unknown. These data show the silica aerogel can be
highly pure. Even if impacting particles were mixed with molten silica aerogel
in amounts many times the particle mass, bulk trace-element analyses would not
be greatly affected.

It is an important goal to develop organic-free capture media, since there is
interest in the biogenic component of interplanetary dust (CDCF Report, 1990).
Hrubesh and Poco (1990) reported the levels of H and C contaminants within the
hydrophobic silica aerogel monoliths manufactured at Lawrence Livermore
National Laboratory to be approximately 1.2 and 4.5 wt% respectively. This
organic contamination is due to incomplete removal of alcohol and other
solvents used in its manufacture; complete removal of water and organic
contaminants is difficult due to unusually large specific free surface area.
Accordingly, we have evaluated the facility with which water and organic
species may be removed from aerogels by heating.

Barrett et al. (1992) reported results of thermogravimetric analysis (TGA) of
three aerogel samples (60, 120, and 250 mg/c^3 samples) heated in a flowing He
atmosphere. The three samples lost 0.9-8.0% weight by simple heating to
approximately 900 degrees C in the dried He carrier gas for 1.5 hr. The
largest weight loss, 8.0 wt%, was noted for the 120 mg/c^3 aerogel sample. The
60 mg/cc sample lost only 2.3 wt%; the smallest weight loss observed was for
the 250 mg/c^3 sample, which lost only 0.9 wt%. The TGA data indicate that to
remove most volatile contaminants, aerogel must be uniformly heated to at
least 600 degrees C and remain in an environment in which significant
volatiles cannot be re-adsorbed by the aerogel.

IMPACT EXPERIMENTS

LIGHT-GAS GUN.  A 5-mm light-gas gun was used to accelerate projectiles of
approximately 100-micrometer diameter via the so-called "shot-gunning" method.
This technique involves an ensemble of particles (typically about 100) being
loaded into a cylindrical cavity of a four-piece sabot. These four pieces will
separate radially once they exit the gun barrel, permitting the particle cloud
to expand in such a manner that at least a few particles will make it to the
target (typically 2-6 grains). The sabot pieces and the majority of the
particulates are intercepted by a massive steel aperture located 2 m in front
of the target. We know from independent experiments that sabot and actual
particle velocity are identical to within 2%, and that the velocity spread
among these particles that reach the target is also <2%.

AEROGEL TARGETS.  We assembled the aerogel targets in two ways, depending on
the size and shape of the aerogel monoliths at hand. The 120 mg/c^3 monoliths
measured 2.5 x 15 x 15 cm each, permitting entire targets to be assembled by
merely stacking pieces. In all other cases, we placed monolith cylinders (with
heights and diameters of approximately 2.5 cm) of the lower-density aerogels
within tight cavities excavated within monoliths of the 120 mg/c^3 aerogel.
Based on a few initial experiments with 120 mg/cc samples that were 2.5 cm
thick, pronounced spallation phenomena occurred at the rear surface, including
total failure of the target if free-standing. As monolith thickness was
limited to 2.5 cm, it was necessary to encase the low-density samples as
described above and to back the first monolith with additional 120 mg/c^3
material in the rear. This arrangement typically resulted in a composite
aerogel block at least 5 cm thick that inhibited spallation at the free
surface of the sample cylinder. The dimensions of the cylinders were
sufficient to terminate the particle's travel, capturing the entire particle
penetration track.

PROJECTILES.  The projectiles consisted of irregular grains, 105-125
micrometers in diameter, obtained by grinding and sieving of large, high-
quality single crystals of calcite (density 2.7 g/c^3), olivine (forsterite,
density 3.2 g/c^3), enstatite (density 3.2 g/c^3), and pyrrhotite (density 4.6
g/c^3). These minerals were chosen for their varying physical properties and
relevance to interplanetary dust studies. The projectiles were initially
accelerated separately and, later, as a 1:1:1 mixture of calcite, forsterite,
and pyrrhotite. A few experiments were also made employing artificial cosmic
dust analog spheres, consisting of the above minerals in an epoxy matrix. The
aggregates were then carefully ground into 1-mm-diameter spheres using a small
ball mill familiar to crystallographers (featuring agitation within an
abrasive-lined, cylindrical container).

RESULTS.  Twenty-one cosmic dust impact simulation experiments were
successfully performed using the two-stage light-gas gun. The shots were made
into the silica aerogel targets at velocities ranging from 5.1 to 7.2 km/s.
Each of the impact experiments employing the large, simulated interplanetary
dust spheres resulted in a crater of significant size (>1 cm) in the aerogel
monolith, made largely by spallation processes. So much mass was displaced
that it was very difficult to locate and extract projectile residue grains
from among the ejected aerogel debris. There was no clean-cut, single
penetration track caused by the main projectile mass, nor any number of
distinct tracks caused by major projectile fragments. This was an unexpected
result compared to that obtained by Tsou et al. (1988, 1989), and our own
experiments with monomineralic powders at smaller projectile scales (see
below). We have not followed up on possible reasons for this difference, which
either relates to dimensional scaling or to possible strength differences of
the projectiles, with the epoxy binder controlling the bulk behavior of the
millimeter-sized impactors.

When the 100-micrometer-diameter mineral grains were shotgunned, they bore
into the silica aerogel, producing gently curving penetration "tracks," as
first described by Tsou et al. (1988). Along the trunk and at the terminus of
each track, small projectile residue grains were evident. We carefully
measured the track lengths for each experiment. We then excavated the
projectile residues at the bottom of the tracks for TEM characterization. The
recovered residues from these experiments were typically highly brecciated,
locally devolatilized and melted, and welded together with fractured and
melted aerogel. However, crystalline domains remained in all residues. A few
projectiles were found to have survived unmelted; however, all recovered
mineral grains showed evidence of abrasion. The minerals successfully
recovered from the shotgun experiments were forsterite and pyrrhotite. No
calcite was recovered, even in experiments that employed only calcite powders.
Indeed, no penetrations or other evidence of impacts were observed in the
latter experiments, indicating that the calcite projectiles disintegrated in
the gun barrel due to rapid acceleration and associated gravitational forces
that seem to exceed the calcite's compressive and/or tensile strength.

Figure 1 shows a target of the 40 mg/c^3 aerogel following impact of
shotgunned mineral mixture at 6.4 km/s. Approximately one dozen tracks are
visible, with the longest measuring 18 mm. This track morphology was typical;
the shorted tracks are due to fragmented projectiles. At higher magnifications
residue particulates are apparent along the entire main track, with the
largest recoverable mass lodged at the track terminus.

RESIDUE EXTRACTION AND CHARACTERIZATION.  An important purpose of this work
was to develop sample preparation techniques for extracting impactor residue
from silica aerogels, preparatory to performing detailed TEM characterization.
To extract a residue particle, we first trimmed the aerogel from around the
particle using an X-ACTO blade, observing the operation under a binocular
microscope, until there remained only a small amount of aerogel enclosing the
particle. This aerogel block was then vacuum-impregnated with EMBED 812 low-
viscosity epoxy. The impregnation with uncured epoxy served to render the
aerogel completely transparent, permitting final removal of the residue
particle. We then set the extracted particle into fresh EMBED 812, for curing
and ultramicrotomy. Samples of aerogel examined with the TEM were prepared in
the same manner.

The TEM analyses show that the recovered projectile residue grains were
typically rounded, abraded, fractured, and encased within melted silica
aerogel. Shots at the highest gun velocities attained and into low-density
aerogels, i.e., best approximation to the expected conditions for the actual
Cosmic Dust Collection Facility, were studied in greatest detail. These shots
were at 6.29 and 7.18 km/s respectively and employed target densities of 40
and 20 mg/c^3 respectively. The projectile residues recovered from all of
these shots were basically similar. A few grains of pyrrhotite were recovered,
but in other cases only iron blebs were observed, resulting probably from
volatilization of sulfur during impact.

In all cases a residue grain was present at the terminus of the projectile
tracks in the aerogels. Much smaller residue grains were also observed at
irregular intervals in off-shoot tracks located about the main trunks of the
tracks. We are not able to confidently determine the masses of any of these
residue grains due to their small size, disrupted nature, and intimate mixing
with aerogel (see below).

Crystalline olivine is the principal and most resistant phase observed in the
current experiments. Figure 2 shows that the forsterite residue is fractured
and mixed with melted and unmelted aerogel.

The projectile forsterite grains contain a flaky, layerlike phase similar to
phyllosilicates. This phase had the composition of serpentine or forsterite,
and prominent basal spacings varying from 6.3 to 9.3 Angstroms. This
description is very similar to the so-called "intermediate" phase pseudomorphs
after serpentine described by Akai (1990) and Zolensky et al. (1993) in the
course of heating experiments with serpentine from the Murchison CM chondrite
(at temperatures between 400 and 600 degrees C). We conjecture that the
forsterite from our impact experiments originally contained small regions of
serpentine, as is ubiquitous within most olivines. During the impact
experiments, this included serpentine was dehydrated and structurally
transformed into the intermediate phase. Zolensky et al. (1993) noted that at
temperatures in excess of 600 degrees C this intermediate phase transforms to
olivine and/or pyroxene. This observation is significant since serpentine and
other phyllosilicates are principal components of interplanetary dust
(Zolensky and Lindstrom, 1992).

PENETRATION TRACK ANALYSES.  We have plotted the longest track lengths made by
impacting forsterite as a function of target density for shots in four
different aerogels, separately showing the shots at 6 and 7 km/s (Fig. 3). We
note that the track lengths are much shorter for the denser aerogels. These
indicate a rough negative correlation between track lengths and silica aerogel
target density, which we can conclude only for these projectile velocities and
target densities. We presume that the larger cells of the low density aerogel
will allow a slower deceleration of the particles. This factor is important in
determining the minimum feasible density of aerogel that can be used to assure
complete deceleration of the projectile within a capture cell.

The aerogel track lengths (T) from eight impact experiments from the current
study are normalized to projectile diameter (Dp) in Fig. 4 and compared to
other penetration studies in porous media of very low densities (<< 1 g/cc),
such as space shuttle tiles (foamed silica, Christiansen and Ortega, 1990),
diverse organic foams (Tsou, 1990) and "Saffile," a porous alumina (Werle et
al., 1981). All of these studies, however, employed metal projectiles, such as
aluminum alloys and stainless steel. Figure 4a plots density-scaled
penetration tracks lengths against target bulk density (d(sub)T), and Fig. 4b
normalizes the widely variable density parameters for all studies, with dP
being projectile density. Where possible, only those experiments between 6 and
7 km/s are compared in Fig. 4; however, the Werle et al. (1981) data are
typical for <5 km/s. Clearly, absolute penetration depth is inversely
proportional to target density (Fig. 4a) and increases with increasing density
contrast between projectile and collection medium (Fig. 4b). A least squares
fit of the data-points in Figure 4b results in

dP/d(sub)T = 1.997 (T/DP)^0.975

with a correlation factor of r^2 = 0.761. Clearly, factors other than density
alone must be important, considering that the materials summarized in Fig. 4b
range from Al2O3 to SiO2 to diverse organic molecules. For these reasons we
deemed velocity scaling of this dataset to be inappropriate (see also below).
However, we note that removal of the non-silica materials from Fig. 4b results
in a greatly improved correlation.

We have compared forsterite track lengths vs. projectile velocity for shots in
the 120-mg/cc aerogel, to explore the effects of impact velocity at otherwise
identical conditions (see Fig. 11 in Barrett et al., 1992). We conclude from
this comparison that track length is not a sensitive indicator of projectile
speed, for the velocities of our experiments.

Werle et al. (1981) were the first to report that penetration tracks in porous
alumina of 180 and 340 mg/cc are substantially deeper at 2-3 km/sec than at
higher velocities. Tsou (1990) reports identical penetration maxima at
relatively modest velocity (2-3 km/sec) in porous organic media.

The fact that density scaling (Fig. 4b) does not substantially reduce the
scatter observed in the raw data (Fig. 4a) is indicative that parameters other
than density alone must exert a substantial role. As was suggested by Anderson
and Ahrens (1988), deceleration by viscous drag may dominate projectile
deceleration in very low density targets, rather than shock-wave interactions
or impact processes in general. The ubiquitous production of a molten aerogel
cap at the projectile's front face may also be evidence of non-shock-related
processes and forces that affect projectile deceleration in aerogel.

The above summaries and comparisons with other low-density target penetration
data are merely offered here to underscore the conclusion that absolute
penetration depth will have a complex relationship with projectile speed and
other initial impact conditions.

CONCLUSIONS

We believe that low-density silica aerogel is chemically and structurally
appropriate as a capture cell medium for the collection and recovery of
interplanetary dust particles in space. Organic contaminants in the aerogels,
unless reduced, may limit its usefulness for study of biogenic components in
interplanetary dust. We have developed techniques that will permit the
successful extrication of interplanetary dust particles from aerogel capture
cells.

We note a rough negative correlation between the particle track lengths and
the silica aerogel target density, a trend consistent with other experiments
into foamed targets. There is only a poor correlation of track lengths with
impact velocity at 5 to 7 km/s. Accordingly, we conclude that (at the
velocities and aerogel densities germane to this study) aerogel track lengths
should not be used as velocity indicators; this conclusion seems to be
corroborated by previous studies as well.

TEM studies show the recovered particulate residues from silica aerogel
capture cells are encased in melted silica aerogel. The aerogel appears to
melt on impact, and forms a plug possibly protecting the impactor from
excessive damage. From our experiments simulating cosmic dust impacts, we
conclude that individual minerals of cosmic dust are expected to exhibit
differential survival. The minerals may be vaporized or melted, as in the case
of sulfur volatilizing from pyrrhotite. They may be dehydrated and
structurally reordered as for the pseudomorphous "intermediate" phase observed
after serpentine. Finally, relatively pristine mineral grains should actually
be captured, as for our forsterite, although this is demonstrated only at
modest encounter velocities (<8 km/s). The "intact" capture of entire complex
projectiles, however, seems impractical at laboratory impact speeds.

Acknowledgments:  We acknowledge L. Hrubesh for his development of the very
low density silica aerogels used in this study. F. Cardenas, W. Davidson, and
G. Haynes ably performed the light-gas gun shots.

References:  Akai J. (1990)  14th Symp. Antarctic Meteor., pp. 22-23.
Anderson W. and Ahrens T.J. (1988)  In Progress Toward a Cosmic Dust
Collection Facility on Space Station, LPI Tech. Report 88-01, LPI, Houston,
pp. 21-22. CDCF Report (1990) Cosmic Dust Collection Facility: Scientific
Objectives and Programatic Relations. NASA TM 102160, 28 pp. Christiansen E.
and Ortega J. (1990) AIAA Space Prog. Tech. Conf., Abstracts, AIAA 90-3666, p.
13. Horz F. et al. (1986) In Trajectory Determinations and Collection of
Micrometeoroids on the Space Station, LPI Technical Report 86-05, LPI,
Houston, pp. 58-60. Hrubesh L. W. and Poco J. F. (1990) Development of low-
density silica aerogel as a capture medium for hyper-velocity particles.
Contract Rept.#T-1143RAM2, Lawrence Livermore National Laboratory, 12 pp. Tsou
P. (1990) Int. J. Impact Engin., 10, 615-627. Tsou P.et al. (1989) LPS XX,
1132-1133. Tsou P. et al. (1990) LPS XXI, 1264-1265. Werle V. et al. (1981)
Proc LPSC 12th, 1641-1647. Zolensky M. E. et al. (1993) GCA, 57, 3123-3148.
Zolensky M. E. and Lindstrom D. J. (1992) Proc. LPSC 22nd.

Figure 1, which appears in the hard copy, shows tracks in 40-mg/cc silica
aerogel made by the impacts of olivine and pyrrhotite particles traveling at
6.4 km/s. The variation in track lengths observed was typical for this study
and our light-gas gun performance in 1989-1990, being due to projectile
fragmentation and differences in projectile masses (see the text); the longest
track shown measures 18 mm.

Figure 2, which appears in the hard copy, shows a TEM image of a 1000-
Angstrom-thick ultramicrotomed section of a forsterite grain. The forsterite
is porous fractured (with minor chattering attributable to the ultramicrotomed
process).

Figure 3, which appears in the hard copy, shows plots of maximum measured
track lengths made by forsterite projectiles into silica aerogels as a
function of aerogel bulk density, all at 6 and 7 km/s. Measurement errors are
+/- 0.3 mm.

Figure 4, which appears in the hard copy, shows a comparison of penetration
track lengths in silica aerogel (this study) with those from previous
experiments in porous materials of extremely low densities. Results from the
current study are shown for velocities ranging from 4.1 to 7.2 km/s for
aerogels with densities of 20, 40, 60, and 120 mg/cc. (a) Actual observations
plotted against bulk density of the targets. (b) The same data as (a)
following scaling impactor and target densities. Some of the scatter inherent
in (a) is eliminated by this normalization of density, yet subsantial scatter
remains. This result serves to illustrate that properties other than density
and velocity must play a substantial role in determining  track lengths in
low-density media. The salient experimental conditions were as follows: Werle
et al. (1981): steel projectiles at 0.4-7.9 km/s, Dp = 1.5-2 mm, targets were
Al2O3 (Saffile) of 180 and 340 mg/cc; Tsou (1990): aluminum projectiles at
0.3-6.5 km/s, diverse organic materials ranging in density from 13 to 68
mg/cc; Christiansen and Ortega (1990): diverse aluminum and nylon projectiles
at 4.4-7.1 km/s, foamed silica shuttle protective tiles of 140-350 mg/cc.





Maag C.*
   Intact Capture Experiments Using Foams

   No abstract available.


Simon C.*
   Penetration of Multiple Thin Films

   No abstract available.


Horz F.*
   Cratering and Penetration Experiments in Teflon and Aluminum

   No abstract available.





                           Monday, September 27, 1993
                         Poster Session and Reception
                        5:30 - 7:00 p.m.     Great Room


No abstract available.





                          Tuesday, September 28, 1993
       Capture Medium Development:  In Situ Collections and Requirements
                       8:30 - 12:00 a.m.     Berkner Room


Walker R. M.*
   Ion Microprobe Studies of LDEF Impacts on Capture Cells and Flat Plates

   No abstract available.


Zook H.*
   LDEF Flux Summary

   No abstract available.


Brownlee D.*
   LDEF Mineralogy and Composition Summary

   No abstract available.


Tanner W. G.*
   EURECA: Preliminary Observations

   No abstract available.


Tsou P.*
   Non-Destructive Time of Capture, Location, and Velocity Sensing in Intact
   Capture

Since cometary dust, as is cosmic dust, is essentially randomly distributed,
knowing the time of the dust capture and the location of the capture on the
collector permits the identification of the specific dust particle captured.
Velocity information helps to determine the trajectory of the dust particle.
Our discovery of a very suitable acoustic sensor and fruitful experimental
results has led to the realization of a flight velocity sensor for the intact
capturing underdense media [1].

Velocity sensing of hypervelocity particles was a significant field of
interest from the 1960s to 1970s. Generically, velocity sensing has been
approached by either of two methods: first, detecting direct plasma or light
emissions [2,3] on to a solid surface, or second, performing time of flight by
film penetration [4] or by detecting charged particles passing a static
electric field [5]. Impact plasma techniques required the destruction of the
particles. Multiple-film penetrations were also destructive, especially for
smaller particles. There is a need for an integrated, reliable, and simple
location and velocity sensor compatible to our intact capture underdense
medium without contributing additional damage to the captured particle.

Film Piezoelectric Sensor
Acoustic sensing was used for micrometeoroid detection in the first free-world
satellite, Explorer. However, traditional acoustic sensors (quartz or ceramic)
have a significant mechanical mismatch with underdense media and can produce
considerable microphonic noise. Kynar piezo film (polarized polyvinylidene
fluoride) is free from both mechanical mismatch and significant noise
problems. Kynar has a very wide frequency response from 0.005 to 10^9 Hz; a
low acoustic impedance, which better suits underdense media; a high voltage
output, 10 times higher than ceramics; good stability against oxidants and UV;
and good noise immunity. This sensor film can be readily made as large surface
areas permitting large-scale integration with standard photo printing
techniques for intricate patterns and electrical contacts.

Capture Experiments with Kynar
We have performed experiments with Kynar on the two promising underdense
capture media, polymer foam and silica aerogel [6]. Tests have ranged from 200
m/s to 6 km/s with solid 50-75-micrometer glass and 1.6-3.2-mm Al projectiles.
A characteristic acoustic signal from a 75-micrometer glass projectile
captured in aerogel at 5.5 km/s is shown in Fig. 1. The distinct delays among
sensors at different distances from the impact point can be seen clearly. The
arrival times of the signal at sensors are linear up to 10-cm separation from
sensor to impact point. From the delay times at different sensors, the
location of the specific particle in the capture medium can be determined.

The peak amplitude of the acoustic signal in the time domain has been related
to a function of the speed of the projectile. Acoustic experts suggest proper
frequency filtering of the acoustic signal will clarify the relationship
between the energy/momentum of the projectile and acoustic signal
characteristics such as rise time and amplitude. Figure 2 shows a normalized
response of two Kynar sensors on the same polyethylene foam respect to initial
projectile speed.

A most troublesome aspect of acoustic sensing is microphonic noise. Spectrum
analyses of signals generated by hypervelocity impact into underdense foam
revealed a small characteristic bandwidth. This allows the Kynar sensor to be
mounted to the foam directly without coupling to surrounding structures that
could produce microphonics. Kynar is not prone to microphonic noise as shown
by one experiment where a sensor was not attached to the underdense medium and
left in suspension. No signal was picked up by the sensor from either the
launch of the two-state light-gas gun or the mechanical reverberation of the
target tank.

Both the time of capture and capture location can be derived from an array of
Kynar sensors. A schematic of this 10 x 8 grid system is shown in Fig. 3. The
second component of the velocity vector, direction, is obtained by measuring
the particle penetration track. This track direction can be measured to better
than 1%. This accomplishment makes available a very simple and integrated
nondestructive velocity sensor with intact capture underdense media.

Applications
For the SOCCER mission, this nondestructive sensor provides a real-time signal
of dust captures as well as temporal and spatial distributions of captured
dust. The dust velocity is expected to be all normal to the collection
surface. For Earth orbital cosmic dust collection, this nondestructive sensor
can provide a mark indicating the orientation of the collector at the time of
capture for a specific particle. With postflight track morphology analysis,
the particles' trajectory can be ascertained, which allows the association of
the dust's parent sources.

References: [1] Tsou P. et al. (1989) LPSC 20th, 1134. [2] Frichtenicht J. F.
(1965) NASA Contr. No. NA SW-936. [3] Eichorn G. (1975) Planet. Space Science,
23, 1514. [4] Berg O. E. et al. (1969) Rev. Sci. Inst., 40, 333. [5] Auer S.
et al. (1968) EPSL, 4, 178.

Figures 1-3 appear in the hard copy.


Maag C. R.*   Borg J.   Tanner W.   Stevenson T.   Bibring J.-P.
   The Intact Capture of Hypervelocity Dust Particles Using Underdense Foams

1. OVERVIEW

Since the first NASA U-2 flight to collect extraterrestrial particles [1], the
interest in the continued collection and analysis of these particles has
increased. The main scientific interest in the analysis of extraterrestrial
microparticles, more commonly called interplanetary dust particles (IDPs), is
due to the fact that part of these particles could be of cometary origin and
thus contain information on the origin of the solar system. Cometary material
is likely to be the most primitive material accessible for analysis. It is
thought that grains once present in the cometary nuclei, and now present as
individual grains in interplanetary space, are still the best candidates for
having properties acquired before or during condensing in the protosolar
nebula. Also present in the low Earth orbit (LEO) environment are orbital
debris (paint flakes, aluminum oxide spheres, etc.) with velocities of the
same order as IDPs, which have resulted from the activities of man in space.

Many materials and techniques have been developed by the authors to sample the
flux of particles in LEO. Through regular in-situ sampling of the flux in LEO
the materials and techniques have produced data that complement the data now
being amassed by the Long Duration Exposure Facility (LDEF) research
activities. Thirteen flight experiments have been conducted on the space
shuttle as part of an ongoing program to develop an understanding of the
spatial density as a function of size for particles 1 x 10^-6 cm and larger.
In addition to the enumeration of particle impacts, it was also the intent
that hypervelocity particles be captured and returned intact.

In addition to the shuttle payloads, an experiment was developed and flown as
part of the Timeband Capture Cell Experiment (TICCE) on the EuReCa 1 payload.
This experiment has provided the opportunity to assess a wide range of
dynamics of ejecta created by hypervelocity impacts on various substrates.

The 12 years of flight experimentation has provided the understanding and
experience for the intact collection, removal, and analysis of particles in
underdense materials. This experience will be used in the development of the
capture cells proposed for the SOCCER-like Comet Coma Sample Return (CCSR)
mission.

2. BACKGROUND

A large body of experimental data exists concerning hypervelocity impacts.
There are several empirical expressions relating the crater volume to the
impacting particle size and mass and many previously flown experiments have
examined these relationships. It is also well established that a part of the
projectile mass is deposited and detectable on the inner surface of the impact
crater. Despite being totally disassociated, elements detected from these
sites allow coarse categorization of the impacting particle type, particularly
with regard to all the important discrimination of space debris.

High-purity metallic surfaces have been used for the collection of all grains
down to submicrometer sizes [2]. During the impact, a characteristic crater is
formed, with rounded habits and a depth-to-diameter ratio equivalent to the
velocity and size of the impacting particle and the encountered metal. During
the impact, the particle is destroyed and the remnants are mixed with the
target material, concentrating in the bottom of the crater and on the
surrounding rims. A major strength of the metallic collectors lies in the fact
that analytical techniques can be applied without modification to the craters.
Also, identification of carbon and organic material is quite possible; this is
essential for the study of extraterrestrial material (C, H, O, N).

The impact of a hypervelocity projectile (> 3km/s) is a process that subjects
both the impactor and the impacted material to a large transient pressure
distribution. The resultant stresses cause a large degree of fragmentation,
melting, vaporization, and ionization (for normal densities). The pressure
regime magnitude, however, is directly related to the density relationship
between the projectile and target materials. As a consequence, a high-density
impactor on a low-density target will experience the lowest level of damage.

Historically, there have been three different approaches toward achieving the
lowest possible target density. The first employs a projectile impinging on a
foil or film of moderate density but whose thickness is much less than the
particle diameter. This results in the particle experiencing a pressure
transient with both a short duration and a greatly reduced destructive effect.
A succession of these films, spaced to allow nondestructive energy dissipation
between impacts, will reduce the impactor's kinetic energy without allowing
its internal energy to rise to the point where destruction of the projectile
mass will occur. An added advantage to this method is that it yields the
possibility of regions within the captured particle where a minimum of thermal
modification has taken place [3].

Polymer foams have been employed as the primary method of capturing particles
with minimum degradation [4]. The manufacture of extremely low bulk density
materials is usually achieved by the introduction of voids into the material
base. It must be noted, however, that a foam structure only has a true bulk
density of the mixture at sizes much larger than the cell size, since for
impact processes this is of paramount importance. The scale at which the bulk
density must still be close to that of the mixture is approximately equal to
the impactor. When this density criterion is met, shock pressures during
impact are minimized, which in turn maximizes the probability of survival for
the impacting particle.

The intact capture of cosmic dust particles has been accomplished by the use
of micropore foams [5]. The principal objectives of the original program were
to develop techniques that would provide the size distribution of Al2O3
particles (AOS) expelled from a solid rocket motor (SRM). Polymeric foams,
with and without deceleration films, were extensively used to capture AOS
intact. Aerogel materials were also used as a capture material. Commercial
organic polymer foams were initially used. Based on testing in light-gas gun
facilities, it was determined that the ability of these foams to retain
particles impacting at hypervelocities was marginal, at best. These tests
found that the more complex polymers had better stopping ability. Accordingly,
it was also determined that the polymers that had extremely small cell sizes,
higher latent heats of fusion, and very low densities (e.g., 0.02-0.7 g/cm^3)
had the highest probability of providing intact capture. In ground tests, the
foams have been successfully tested between 1 and 11 km/s. One of the more
interesting highlights of this program was the intact capture and retention of
materials with a much lower density and material strength than AOS. Since the
initial program other materials have been tested successfully, most notably
foams of silicones, polyimides, and fluorocarbons. For this activity, foams of
the aforementioned polymers will be developed and used that have both
gradients in axial density, typically from 0.006-0.009 g/cm^3, and imbedded
sensors to sense impact parameters as the particle decelerates in the foam.
Figures 1-3 show three conditions of particles captured intact using organic
foam capture cells on shuttles STS 61-B and STS 41-D. Figure 1 depicts an
interplanetary dust particle recovered from the STS 61-B experiment. Figure 2
shows an AOS particle captured intact in the organic foam cell, and in Fig. 3
the same AOS particle after the pyrolyzed foam has been removed using a low-
molar concentration of HCl. Figure 2 also shows a portion of the "burn track"
after the foam had been microtomed to locate the particle. Figure 4 shows a
perforation through a typical 3 ľm deceleration film.

Aerogel has also been used as the capture medium in capture cells. Aerogel is
particularly useful where extremely small cell sizes (150 Angstroms) are
necessary. This material in its silicon form is commonly produced for the
nuclear industry Cherenkov radiator and has, in fact, been used in both space
(HEOS, Ulysses) and balloon instrumentation. In comparison with polymer foams,
however, the extremely low densities (0.035 g/cm^3) cannot be achieved in any
aerogel without producing great fragility. An aerogel capture cell was first
used on shuttle mission STS 61-B. Figure 5 shows the placement on the Remote
Manipulator System arm. While a particle was captured essentially intact, the
fragility of this capture medium was demonstrated by the inability to remove
the particle intact. In addition to the inherent fragility, aerogel has
suffered from the effects of the space environment. Low temperatures seem to
alter the shape of the "burn track," atomic oxygen exposure erodes the
surface, and it has been reported by the Centre National E'tudes Spatiale
(CNES) that the material embrittles after exposure to the radiation
environment. Irrespective of these reported problems, experimentation with
aerogel will continue.

3. SHUTTLE EXPERIMENTATION

One of the means to test the aforementioned capture cells has been on the
Interim Operational Contamination Monitor (IOCM) developed under the auspices
of the U.S. Air Force/Space and Missile Systems Center. The IOCM contains an
array of passive and active sensors that continuously sample three orthogonal
directions in the shuttle cargo bay (Fig. 6). The IOCM has successfully flown
on four shuttle missions, the two most recent being STS-32 and STS-44.

Although the primary objective of the IOCM on STS-32 and STS-44 was to verify
the effects of the space environment to the cargo element, a secondary
objective was to sample the LEO space debris and micrometeoroid complex using
an array of passive sensor experiments. An additional design goal for these
experiments was to test the survivability of thin-film sensors with a
thickness of less than 750 Angstroms.

The Limited Duration Space Environment Candidate Materials Exposure (LDCE)
experiment was launched on STS-46. LDCE-1 and -2 were mounted in GAS canisters
with door assemblies. LDCE-3 was mounted on the top of the Space Complex
Autonomous Payload (CONCAP). Figure 7 depicts the layout of the LDCE-1
exposure plate after integration into the GAS canister. The GAS canisters were
located in Bay 13 of OV-104. The samples mounted on LDCE-3 were exposed for
the entire duration of the mission. After the door assemblies of GAS canisters
open, the samples mounted in the LDCE-1 and -2 were exposed for a period of 40
hours. The exposure occurred toward the end of the mission, near 200 km, with
a continuous payload bay attitude into the velocity vector. The LDCE
experiment flew principally to understand the influence of atomic axygen on
materials. Our array of sensors were uniquely designed for the detection of
hypervelocity impacts (HVI).

The LDCE-1 HVI package contained a gold foil (nominal T(sub)f ~ 4.0
micrometers) that covered a low-density micropore foam. Similar foams had been
used on past missions to collect hypervelocity particles, intact. A similar
piece of gold foil covered a highly polished aluminum strip coated with vacuum
deposited gold. This aided in the understanding of the distribution of ejecta
material. Also included was a thin aluminum film (nominal T(sub)f < 500
Angstroms) stacked above a coated substrate. It was hoped that an estimate of
the trajectory of grains within the experiment could be derived from the
analysis of penetrations made in the thin film and impact sights (these data
have not yet been reduced). The last group of passive sensors were high-purity
metallic surfaces used for the collection of grains down to submicrometer
size.

The LDCE-1 and -2 aluminum films, with a total surface area of 3.24 X 10^-4
cm^2, should have experienced 5.2 particle impacts in 10 days while in LEO.
This estimate was based on Pegasus data published in 1970. Using a foil
thickness of 7.24 X 10^-5 cm, a density of 2.8 g/cm^3, and a velocity of 7
km/s, the minimum mass that could penetrate the thin film was calculated. The
thin films could be penetrated by a grain possessing a mass greater than a
picogram.

4. EURECA EXPERIMENTATION

As a consequence of the experimental data developed during both recent and
earlier STS missions and the data expected from this mission, the authors have
produced and delivered an experiment for the European Space Agency, European
Retrievable Carrier (EuReCa). The HVI experiment was flown as part of the
TICCE experiment. The EuReCa payload was launched on OV-104 (Atlantis) on 31
July 1992, providing a total mission exposure of nearly 11 months.

4.1. Objectives of the TICCE/HVI Experiment

The primary objectives of the experiment were to: (1) Examine the morphology
of primary and secondary hypervelocity impact craters. Primary attention will
be paid to craters caused by ejecta during hypervelocity impacts on different
substrates; (2) Determine the size distribution of ejecta by means of witness
plates and collect fragments of ejecta from craters by means of momentum
sensitive micropore foam; (3) Assess the directionality of the flux by means
of penetration hole alignment of thin films placed above the cells. (4)
Capture, intact, the particles that perforated the thin film and entered the
cell. Capture medium consisted of both previously flight-tested micropore
foams and Aerogel. The foams had different latent heats of fusion and
accordingly will capture particles over a range of momenta. Aerogel was
incorporated into the cells to determine the minimum diameter that can be
captured intact.

4.2. Data Analysis and Expected Results

Primary analyses will be performed using a Scanning Electron Microscope (SEM)
outfitted with a Princeton Gamma Tech (PGT) elemental analysis system
(beryllium window). Since each unit cell will be ~10 mm square, samples will
be easily prepared for viewing in the SEM. The SEM is sufficiently large to
support the viewing of 5-cm substrate material.

Count of hypervelocity impact craters on the witness plates whose diameters
are larger than 4 micrometers will be accomplished by the use of SEM
photographs. Once digitized by means of a high-resolution optical scanner,
these data will be analyzed using a hypervelocity impact morphology system.

Analysis of the substrate will be of particular importance. The same procedure
outlined above to analyze the witness plates will be applied to the substrate.
Of primary interest will be the recovery of data concerning the effects on the
substrate's optical properties, which have been subjected to primary and
secondary hypervelocity impacts. Also recoverable from the substrate (and
perhaps the witness plates) will be data pertaining to the fragmentation of
grains by the thin films.

Principal theoretical analyses will be conducted using hydrocodes to establish
the limiting mass that will penetrate all, two, or only one of the thin films.
Comparisons of the computational results with experimentally derived
parameters will be carried out.

Based on the present knowledge of the space debris and micrometeoroid fluxes,
all cells should be penetrated by grains with the properties: m(sub)p = 3.4 X
10^-13 g; r(sub)p = 3.8 g/cm^3;
v(sub)p = 7.00 km/s; thus, r(sub)p = 0.3 micrometers.

Since EuReCa was in orbit for approximately one year, there should be ~50,000
impacts/m^2; with 100 cm^2 (1.0 X 10^-2 m^2), the HVI experiment should expect
3.75 impacts/cell from grains that are this size and larger. In addition to
the 200 unit cells, the HVI contains high-purity metallic surfaces plus 200
cm^2 of micropore foam.

5. SUMMARY

The experimentation on the shuttle (1983-1993), although snapshots in time,
has yielded interesting data. One of the more salient results suggest that the
smaller particles, 10^-4 to 10^-2 cm diameter, have a much higher flux rate
(~2 orders of magnitude) than previously considered. Another interesting
result of the long-term study, irrespective of the high uncertainty, suggests
that the growth in this size range is approximately 6% per year; whereas, the
growth of the 10^-2- to 10^-1-cm-diameter size is approximately 2% per year.
LDEF, unfortunately, due to its extended stay on orbit, could not provide
population growth data.

The research program also developed/modified a family of materials useful in
intact capture particle studies. A reasonable number of particles (250) have
been captured intact using micropore polymeric foams. Four bonafide IDPs have
been captured and returned intact.

The passive space debris/micrometeoroid sensors suggest a flux of 0.2 X 10^2
impacts/m^2 occurred during the STS-32 mission (seven-day duration). The
average diameter of the perforations was ~12.5 micrometers. The largest
perforation measured was 65 micrometers.

STS-44 has proved to be one of the most interesting HVI experiments placed on
the shuttle. During the course of the STS-44 mission, the space shuttle
corrected its altitude by 26 km to evade a spent upper stage. The object,
which was slightly outside the given collision ellipsoid, was determined to be
the Kosmos 851 rocket body. Kosmos 851 was launched on 27 August 1976 into an
81 degree inclination orbit. The results of our data suggest that a cloud of
irregularly shaped particles, most of which were aluminum oxide, impacted the
shuttle during the mission (Figs. 8-10). Data also suggests that the
associated debris cloud caused an increase in flux of nearly 2 orders of
magnitude over the background flux typically experienced.

Other major impacts on STS-44 were primarily due to low-velocity, low-density
substances. Investigation of these sites revealed no hypervelocity impact
features and only small evidence of residual materials left by the impactor.
Shallow and wide impressions in the films were seen immediately adjacent to
the perforation sites. The morphology of these sites suggests that low-density
material formed the impressions. It is thought that the impacts come from
water ice dumped by the shuttle. Water and waste dumping from the orbiter have
been observed contributing significant amounts of gaseous and particle
contaminants. These materials and the propulsion contaminant mass remain near
the orbiter for some time before they disperse.

Very preliminary data from STS-46 suggests that nine impacts and perforations
occurred during the course of the mission. One of the particles captured
intact has tentatively been identified as an IDP.

5.2. "Quick Look" Results from EuReCa

Data from the two-dimensional (2D) computer simulations of hypervelocity
impact events for the TICCE/HVI thin films conform to a high degree with the
Carey, McDonnell, and Dixon (CMD) equation for all densities tested.

Early examination of the aerogel samples flown on the EuReCa TICCE exhibit
signs of shrinkage (~6% in both length and width). Recovery as a function of
time will be monitored.

Visual impacts were observed in the deceleration films covering the polymeric
foam capture cell experiments. Work is proceeding to analyze the perforations
and remove the grain.

6. CONCLUSION

The proposed underdense foam capture cell experiment has the capability to
capture intact cometary grains. More importantly, the techniques to remove the
particles have been developed over 10 years of research. This experiment when
flown in concert with the other instruments in the CCSR package will permit an
in-depth understanding of cometary composition.

References: [1] Brownlee D. E. et al. (1976) NASA TMX 73, 152-168. [2] Bibring
J-P. et al. (1985) LPS XVI, 55-57. [3] Tanner W. G. (1993) Int. J. of Impact
Engng, in press. [4] Maag C. R. and Linder W. K. (1992) Hypervelocity Impacts
in Space (J. A. M. McDonnell, ed.), University of Kent at Canterbury, 186-190.
[5] Maag C. R. (1987) Final Report to USAF/AFRPL, JPL Task 80-1547A.

Figure 1, which appears in the hard copy, shows IDP (C1) recovered from HVI
experiment on STS 61-B.

Figure 2, which appears in the hard copy, shows an aluminum oxide sphere, 8
micrometers in diameter, recovered from foam. The encrusted pyrolyzed foam is
easily removed, leaving an undegraded specimen.

Figure 3, which appears in the hard copy, shows AOS (see Fig. 2) after cleanup
with HCl.

Figure 4, which appears in the hard copy, shows film perforation occurring
during STS-32 flight.

Figure 5, which appears in the hard copy, shows the capture cell experiment
flown on STS 61-B.

Figure 6, which appears in the hard copy, shows the IOCM experiment as flown
on STS-44.

Figure 7, which appears in the hard copy, shows the LDCE-1/HVI experiment as
mounted on STS-46

Figure 8, which appears in the hard copy, shows an irregularly shaped Al2O3
captured particle (STS-44).

Figure 9, which appears in the hard copy, shows an intact captured Al2O3
particle with EDS spectra (STS-44).

Figure 10, which appears in the hard copy, shows a second irregularly shaped
Al2O3 particle (STS-44).





Flynn G. J.*   Sutton S. R.
   Collection Requirements for Trace Element Analyses of Extraterrestrial
   Samples

Introduction
Trace element abundances and abundance patterns in meteorites have proven to
be useful indicators of (1) nebular [1] and parent body [2] fractionations,
(2) formation temperatures [3], (3) thermal metamorphism [4], and (4)
cogenesis [5]. Trace-element measurements on individual interplanetary dust
particles have been useful in separating an "igneous" subgroup from the
chondritic particles [6], identifying particles that were severely heated on
atmospheric entry [7], and suggesting a possibly volatile enrichment of the
interplanetary dust over the most volatile-rich type of meteorite [8].

Synchrotron X-Ray Fluorescence (SXRF) is a nondestructive trace-element
analysis technique particularly useful for the analysis for small samples. In
the present X-ray microprobe instrument installed on beamline X26 of the
National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory
samples are excited with filtered, continuum synchrotron radiation from a
bending magnet on a 2.5-GeV electron storage ring [9]. Flynn and Sutton [6]
have demonstrated that the present SXRF microprobe at the NSLS can detect
elements from Cr to Mo in concentrations down to a few parts per million in
10- to 30-micrometer-diameter interplanetary dust particles (see Fig. 1).

A particular advantage of SXRF analysis is its low energy deposition in the
sample, making possible the analysis of volatile elements and even ices. The
NSLS X-ray microprobe deposits approximately 10-5 cal/sec into a 100-
micrometer cube of ice/silicate aggregate [10]. The corresponding power
density is 10^-6 times that of a typical electron microprobe [10].

SXRF also has the potential of performing in situ analyses of particles
imbedded in the collection medium, allowing preliminary characterization of
particles prior to extraction.

Instrument Improvements
The commissioning of the Advanced Photon Source (APS) at the Argonne National
Laboratory in 1995 will significantly improve the element analysis capability
by providing a more highly focused X-ray beam of higher X-ray energy [11]. The
X-ray microprobe to be installed at the Advanced Photon Source is expected to
be capable of analyzing individual micrometer-sized grains with an element
sensitivity better than 100 parts per billion [11].

The element detection limits of the present X-ray microprobe at the NSLS is
compared to the expected sensitivities of X-ray microprobes installed on the
APS bending magnets and wiggler magnets in Fig.2.

Sample Contamination Requirements
All sample collection techniques require some contact, and possible
contamination, between the sample and the collector. Even in the case of the
relatively low velocity collection of interplanetary dust from the Earth's
stratosphere onto impact collectors coated with silicone oil, traces of
residual silicone oil (and any contaminants it contains) remain on the
particle surface [12].

High-velocity collection, as proposed for in-orbit capture of interplanetary
dust or flyby capture of cometary dust, has the potential for more severe
particle contamination and alteration of the collected samples. For example,
particles impacting into aerogel have been observed to acquire a "glassy"
coating of melted aerogel material [13].

We have previously measured the content of trace elements from Cr to Rb in two
aerogel samples (provided by M. Zolensky), and found those samples to be
sufficiently clean to be suitable for trace-element measurements at the NSLS
detection limits of particles collected at high velocities [14]. However, the
abundance of Fe was distinctly different in the two samples, indicating that
aerogel from different sources is likely to contain different amounts of
contamination [14]. Zolensky et al. [13] measured Fe, Sn, Br, Zn, Cr, Sb, Co,
and Sc in an aerogel sample produced by Henning AG, while a sample from
Lawrence Livermore Laboratories contained detectable Rb, Na, Zn, Br, and Au.
The importance of testing proposed flight materials for contamination must,
therefore, be emphasized. In addition, the wider range of elements and
increased element sensitivity that will become available using the X-ray
microprobe at the Advanced Photon Source will necessitate retesting flight
materials with the new instrument.

It is essential that sample collection hardware be constructed from high-
purity materials, but it is equally important that the selection of these
materials be given considerable thought. For example, one class of
stratospheric particles has a high Al content. These particles are generally
believed to be terrestrial, but Flynn et al. [15] have reported roughly
chondritic Fe/Ni and Mg/Si ratios in these particles, possibly suggestive of
an extraterrestrial origin. The use of Al collection material would seriously
compromise the identification of material of this type.

In-Situ Characterization of Particles Imbedded in Collector Material
Dust collection in low Earth orbit poses a particular problem of
descriminating between orbital debris and interplanetary particles. Although
particles of both types are inherently interesting, the experimental protocols
for the subsequent study of these different types of particles are likely to
differ. The X-ray penetration depths and fluorescence X-ray escape depths
indicate that in situ chemical analysis of particles imbedded to a depth of
about 100 micrometers in low-density material such as aerogel is possible.
Small fluid inclusions in quartz have been analyzed to a depth of tens of
micrometers by SXRF [16]. Thus SXRF should provide a technique for in situ
classification of collected particles.

Individual Grain Analyses
The availablity of high X-ray brightness at X-ray energies up to 40 keV using
the APS undulator will allow efficient excitation of heavier elements than are
accessible with the present NSLS microprobe. It is anticipated that rare earth
and platinum group elements will be detectable in microparticles using the APS
facility. Trace-element determinations on individual particles as small as 1
micrometer in size should be possible with a detection limit of about 100 ppb.

Computed Microtomography
When intact samples of cometary material (ice and dust mixtures) are
recovered, measurement of the spatial relationships between mineral phases,
ices, and voids will be important. X-ray-computed microtomography could be
used to obtain this information in a nondestructive manner with a spatial
resolution better than 5 micrometers (see discussion in [10]). If samples are
to be cored from a cometary or asteroidal parent body, a coring technique that
preserves the inherent structure of the parent body is critical.

Conclusions
Trace-element abundances have proven important in understanding the evolution
of and interrelationships between different meteorites. Preliminary
investigations of the trace-element contents of interplanetary dust particles
indicate that trace-element abundances will prove equally important in
distinguishing between micrometeorites of different types, comparing the
interplanetary dust to the meteorites, and assessing the degree of thermal
alteration experienced either on the parent body or during the collection
process.

Sample collection, delivery, and curation must be accomplished in a manner to
avoid contamination with even trace amounts of the elements to be analyzed.
The present SXRF sensitivity for micrometeorite analysis is of order 1
femtogram, but anticipated improvements in sensitivity will require sample
contamination substantially below this level. Sample collection and handling
equipment should be constructed from materials selected for ultra-high purity,
and serious consideration should be given in selecting the particular set of
elements from which the collection apparatus is composed so as not to
compromise useful information.

References:  [1] Larimer J. W. and Anders E. (1967) GCA, 31, 1239-1270.  [2]
Schnetzer C. C. and Philpotts J. A. (1969) in "Meteorite Research" (P.
Millman, ed.), Reidel, Dordrecht, 206-216. [3] Keays R. R. et al. (1971) GCA,
35, 337-363.  [4] Ikramuddin M. et al. (1977) GCA, 42, 1247-1256.  [5] Schaudy
R. (1972) Icarus, 17, 174-192.  [6] Flynn G. J. and Sutton S. R. (1990) Proc.
LPSC 20th, 335-342.  [7] Flynn G. J. and Sutton S. R. (1992) Proc. LPSC 22nd,
171-184.  [8] Flynn, G. J. and S. R. Sutton (1992) LPSC XXIII, 373-374.  [9]
Gordon B. M. (1987), X-Ray Microscopy II, 56, 276-279.  [10] Flynn G. J.et al.
(1993) Proceedings of Workshop on  Returned Comet Nucleus Samples, in press.
[11] Sutton S. R. et al. (1988) in Synchrotron X-Ray Sources and New
Opportunities in the Earth Sciences, Argonne National  Laboratory Technical
Report ANL/APS-TM-3, 93-112. [12] Sandford S. A. and Walker R. M. (1985)
Astrophys. J., 291, 838. [13] Zolensky M. E. (1990) LPSC XXI, 1381-1382. [14]
Flynn, G. J. and Sutton S. R. (1990) LPSC XXI, 371-372. [15] Flynn G. J. et
al. (1982) LPSC XXII. [16] Vanko D. A.et al. (1993) Chemical Geology, in
press.

Fig. 1, which appears in the hard copy, shows a synchrotron X-ray fluorescence
spectrum for a 30 micrometer chondritic interplanetary dust particle (W7027C5)
obtained during a 30-minute data acquisition at National Synchrotron Light
Source beamline X26A at the Brookhaven National Laboratory. Energies less than
9 keV are plotted logarithmatically while those above 9 keV are plotted
linearly. The Kr fluorescence results from the air along the path from the
sample to the detector. The Fe escape peak is labeled "esc."

Fig 2, which appears in the hard copy, shows estimated minimum detection
limits for elements from Z = 12 to 38 for a 10 mg/cm^2 thick ice/dust sample
analyzed using three different synchrotron radiation sources: the NSLS bending
magnet (currently in use), the APS bending magnet, and the APS wiggler A.


Bunch T.*
   Shock Stability of Amino Acids

   No abstract available.


Heppner R. A.*   Niu W.   Maag C. R.
   Techniques for In Situ Collection and Measurement of Volatiles Released
   During Hypervelocity Impact

INTRODUCTION
The proposed Comet Coma Sample Return, CCSR, payload contains a variety of
instrumentation for characterizing and collecting cometary dust (Alexander et
al., 1993). In this suite of instruments the Gas Capture Cell (GCC) is unique
in that it not only collects the vaporization products resulting from the dust
particle impacts, but also provides chemical characterization information
prior to return of the dust particles for analysis on Earth. The GCC provides
near real-time characterization of the volatile species, such as low- and
medium-molecular-weight organic compounds that evolve from dust particles on
impact with metal targets. Instrument sensitivity is sufficient for analyzing
the volatile impact products resulting from single, individual dust particles.
This capability will enable chracterization of near-pristine dust particles,
including the CHON particles, to be performed at a level not previously
possible.

Collection of interplanetary dust particles has been performed since the mid-
70s when U2 and ER2 flights at high altitudes were utilized for this purpose.
Numerous particles were collected; however, the degree to which particle
nature was changed by the relatively long residence time in the Earth's
reactive upper atmosphere was unknown. Recent shuttle experiments have
collected particles in space, enabling gross particle composition to be
determined. However, the capability to detect the potential presence of more
volatile, organic species was lost because of the nature of the hypervelocity
impact. The question as to whether primordial seed material was contained on
interplanetary and/or cometary dust grains could not be answered.

In the GCC experiment dust particles impact on metal targets causing all
organic and significant amounts of the inorganic material to vaporize. Low
volatility inorganic elements/compounds will immediately condense on the walls
of the gas collection chamber. Less volatile organic/inorganic species will
condense more slowly while undergoing multiple collisions with gas collection
chamber walls until finally collecting on the actively cooled surfaces within
the GCC. Volatile organic/inorganic molecules resulting from the impact will
not condense and are difficult, if not impossible, to transport back to Earth
for study. Both the volatiles and the semivolatile species will be
characterized according to their m/e ratios with a small mass spectrometer.

The combination of the actively cooled surfaces, the cell walls, and the
targets themselves serve as carriers to return dust particle components for
Earth study. These components are integrating-type collectors because material
is collected over the full duration of the experiment, whereas the mass
spectrometer performs near real-time analysis. The two coldplates are
continuously maintained at an operating temperature of nominally 150 K by
small Peltier coolers. One of the plates has Ge Internal Reflection
Spectroscopy (IRS) crystals attached to its surface enabling IR analysis of
the surface film of accumulated semivolatiles once the payload returns to
earth. Both coldplates maintain sample material in the condensed phase until
the examination phase under curation laboratory conditions.

SCIENTIFIC OBJECTIVE
The GCC supports the CCSR objective of characterizing the distribution and
composition of both interplanetary and cometary dust particles. Mass
spectrometer (MS) analysis of volatile material and cryoplate collection of
semivolatiles will complement analytical information from other CCSR
experiments that collect intact particles after hypervelocity impact. By
comparing QCM profiles with mass spectral data not only can the amount of
material being deposited be determined, but the chemical composition can be
determined as well. Observation of vaporization products in situ will aid in
determining the degree to which hypervelocity impact has altered amounts of
volatile and semivolatile material on the dust particles that are collected by
other CCSR experiments and returned to Earth for analysis. These analyses and
comparisons will provide new insights into the detailed composition of gases
and dust within the coma of a comet and on the makeup of interplanetary dust
particles.

GAS COLLECTION CELL CONCEPT
GCC Configuration. Figure 1 shows the proposed Gas Collection Cell concept.
The cell has nominal dimensions of 10 x 10 x 10 cm^3 to provide an expansion
volume of approximately 1 liter for the vaporization products. Dust particles
enter the cell by penetrating a thin (500-1000 Angstrom) vapor-deposited Al
film. The film is thin compared to particle diameter and previous work (Tanner
et al., 1993) has shown that the brief particle/film encounter has very little
effect on particle composition or integrity. A 90% transmission screen backs
the film to provide structural support during launch and return phases of the
mission. Once inside the cell, particles travel unimpeded to the target plate
on the floor of the cell. The target is a polished nickel plate with a vapor
deposited gold coating to provide a clean, specular, well-characterized
surface. Upon impact with the plate, volatile and semivolatile dust components
are released into the cell where they are sampled by the MS and are collected
on the coldplates and the Temperature-controlled Quartz Crystal Microbalance
(TQCM). Two coldplates are used in order to provide redundancy and prevent
loss of organic semivolatiles collected on the surfaces should one cooler
fail.

The cell is protected by a door that is closed during ground, launch, and
retrieval operations through a double-seal arrangement that prevents air
leakage when the cell door is closed. The volume between the door and the thin
film and the internal cell volume both connect to a common solenoid vent valve
that opens to space. Connecting both volumes to a common vent valve prevents
pressure differentials from developing that could potentially damage the thin
film.

GCC Block Diagram. The block diagram for the GCC is illustrated in Fig. 2.
The GCC interfaces with the spacecraft computer for instrument control and
data storage. A limited set of discrete digital commands control basic system
functions, such as system power, MS power, Peltier Cooler/TQCM power, and door
actuation.

Data acquisition and associated instrument control functions, such as scanning
the MS and measurement of mass spectral peak intensities and TQCM frequency
shifts, are performed through the Serial Bus Communication Interface.
Monitoring of various housekeeping parameters, such as internal temperatures
and power supply voltages, is also performed through this interface.

Various analog electronic modules support operation of the TQCM, the Peltier-
cooled coldplates, and the MS. These modules are either controlled directly by
the discrete digital lines or communicate with the data bus through an Analog
to Digital Converter and/or a Digital to Analog Converter.

GCC Operational Scenario. During ground testing and prior to launch the cell
door will be closed and both the internal cell volume and the space between
the door and the vapor-deposited film will be evacuated and baked out through
the solenoid vent valve. Once evacuated, the Peltier coolers, the TQCM, and
the MS can all be exercised and their performance verified. After launch and
orbital injection are complete, the vent valve and cover can be opened to
enable the GCC to acquire dust samples.

The GCC can be activated at any point in the mission cycle. Although its
principal purpose is the acquisition of dust samples while in the comet coma,
operation at larger distances could help determine whether the features
observed in the Halley GIOTTO dust flux data (Alexander et al., 1988) have
corresponding compositional variations. If necessary, power can be conserved
by operating the MS intermittently, otherwise keeping it in a standby
configuration. A few seconds will be required to scan the mass spectrum,
depending upon the desired sensitivity. Thus scans may require a longer time
period when particle fluxes and evolved gas levels are correspondingly low. At
peak particle flux levels (estimated at approximately 50/s and perhaps
higher), the scan time will be reduced. At mission completion and prior to
CCSR retrieval, the door and vent valve will be closed to prevent accumulation
of shuttle-associated contaminants. The GCC will then remain sealed, with
Peltier coolers operating, until opening occurs in the controlled environment
of the curation facilities. Here the semivolatile organic material trapped on
the coldplates will be analyzed using a variety of spectroscopic and
chromatographic techniques. Material absorbed on the IRS crystals can be
directly analyzed using FT-IR spectroscopy without extensive sample
preparation. Some of the dust particles trapped in the metal target plate will
be extracted and examined for compositional comparison with those particles
trapped in foam- and aerogel-based collection devices.

Analysis of GCC Performance. A number of factors will affect GCC performance,
such as particle size and flux, GCC cell volume, and impact magnitude. These
factors will be fully considered during the conceptual design phase; however,
preliminary performance estimates can be made.

A 1-micrometer-diameter particle having density of 1 g/cm^3 has a mass of
approximately 0.5 pg. This particle impacting within the GCC will cause
vaporization of a number of molecules given by

                m x vf x (1/M) x (6 x 10^23 molecules/mole)

where m = the particle mass, vf = the vaporization factor, and M = the molar
mass

For a 0.5-pg particle with molar mass of 30, and 0.1 vaporization factor,
approximately 1 x 10^9 atoms/molecules are vaporized. Assuming the cell
temperature is nominally 300 K, and scaling from the molar volume relation
(22.4 liter at STP), the 1 x 10^9 molecules will produce a pressure rise of

(1 x 10^9 mol) x 22400 cm^3 x 760 torr/(1000 cm^3) x (6 x 10^23 mol)

= 2.8 x 10^-11 torr

A portion of the material contributing to this pressure rise will be
nonvolatile and will immediately deposit on chamber walls; also semivolatile
material will slowly be removed by the coldplates. If 10% of the material
remains as volatile species, then the resultant pressure rise from a single 1-
micrometer particle will be 2.8 x 10^-12 torr.

The MS sensitivity under normal operating conditions is 5 x 10^-6 Amp/torr, so
the above pressure rise produces a current of 1.4 x 10^-17 Amp, which with ion
counting, corresponds to approximately 100 ions/s at the detector. The MS
background count rate is typically 1 or 2 cts/second and thus 100 ions/sec is
detectable. If this count rate is spread over several ion masses it may be
necessary to increase the sensitivity to provide sufficient signal when the
mass range is scanned. Increasing the MS filament emission level by a factor
of 10 provides a simple means to increase sensitivity by an order of
magnitude. Since the MS only has to operate over a total time period of a
month or so, high emission (which reduces filament life) could be employed, if
required.

The primary conductance out of the gas cell will be the 40 cm^3/s through the
MS system. This conductance provides a time response of approximately 25 s for
the GCC and provides sufficient time for evolved gas to condense on cryoplate
surfaces and be sampled by the MS. At high particle impact rates, near coma
center, impact rates may be as high as 60/s of particles with mass of 4 x 10^-
10 g or greater. Using the assumptions made above for particle vaporization,
this corresponds to a gas load into the cell of approximately 1.3 x 10^-6
torr-liter/s. The MS pumping speed will limit the pressure rise in the cell to
approximately 3 x 10^-5 torr. In the event extremely high gas evolution rates
are encountered, then the vent valve can be opened to limit the internal cell
pressure rise.

The coldplates and the TQCM will remove semivolatiles from the cell by
cryogenic pumping. For each 0.5-pg particle. we calculated earlier that
approximately 0.045 pg would be semivolatile. If the TQCM sensor area of 1
cm^2 is approximately 10% of the total cryoplate area, then it will receive
approximately 4.5 fg per 0.5-pg particle. A 1-Hz frequency change from the
TQCM corresponds to approximately 1 x 10^-10 g/cm^2 mass change. Thus the TQCM
will be capable of sensing events resulting from individual large particles
(10^-9 g). At the high particle flux rates near coma center the mass increase
rate of the TQCM will be

60 part./s x (4 x 10^-10 g/part.) x 0.1 (vap. fac.) x 0.1 (area frac)
= 2.4 x 10^-10 g/s

Using the sensitivity factor given above, this mass deposition rate will
result in a several Hz change every second.

INSTRUMENT DESCRIPTION
The GCC system contains a number of important elements. These are described in
some detail below.

Cell and Targets. High-purity metallic surfaces will be used for collecting
gaseous species and dust particles/grains on the cryoplates and targets.
During impact of a high-density particle, a characteristic crater is formed
with rounded edges and a depth-to-diameter ratio determined by the encountered
metal and the velocity and size of the impacting particle. Typically the
particle is destroyed and the remnants are mixed with target material,
concentrating in the bottom of the crater or on the surrounding rim. Gold and
Ni are metals suitable for such targets. A top layer composed of 50 nm of
evaporated Au facilitates identification of the impacting position, as the
torn film indicates impact position.

The chemical and isotopic properties of the impacting particle can be
identified by analyzing the rim material. The major strength of the metallic
collectors lies in the fact that analytical techniques can be applied without
modification to craters ranging from tens of nanometers up to millimeters in
size, limited only by the thickness of the plate. Also, identification of C
and organic material is made possible, which is essential for the study of
extraterrestrial material.

A low-weight latching solenoid valve will be used for the cell vent. A
stepping motor will drive the cell cover between open and closed positions.

Vapor Deposited Thin Film. The basic approach to implementing an ultrathin
film is shown in Fig. 3. The thin Al film of 500 Angstroms or so is initially
deposited onto a C foil over a Buckbee Mears 30-line-per-inch 90% transmissive
grid (Tanner et al., 1991). After deposition, the C is removed, leaving a
free-standing 500-Angstrom Al film for particles to transit. This approach to
film fabrication has already been expanded to the required 10 x 10 cm^2
entrance area of the GCC. A film of the required size was recently returned
from the European Retrievable Carrier/Timeband Capture Cell Experiment,
EuReCa/TICCE.

Mass Spectrometer (MS). The MS will be adapted from the Induced Environmental
Contaminant Monitor (IECM) instrument design, which has flown on numerous
shuttle and satellite payloads. On several of these missions it monitored ram
and wake gas fluxes with equivalent pressures in the 10^-11 to 10^-13 torr
range (Naumann et al., 1985). Most recently one of these instruments was
refurbished and upgraded for use on the MSX Contamination Monitoring
Experiment. The MS analyzer and electronics packaging is illustrated in Fig.
4. For the GCC application we will investigate the potential for reducing the
rod length consistent with the resolution and mass range requirements of the
GCC. The IECM electronics were also upgraded for the MSX mission and these
designs will serve as the basis for the GCC MS. This upgrade replaced analog
current measurement with pulse counting, which will provide high sensitivity
by detecting single ion events.

Temperature-controlled Quartz Crystal Microbalance (TQCM). The TQCM measures
material deposited on the sensing head by detecting the frequency shift of a
quartz crystal oscillator. A cross-sectional view of the TQCM is shown in Fig.
5. Two matched quartz crystal, one for sensing contamination and the other
operated as a reference crystal, provide a beat signal that is totally
independent of temperature and power supply fluctuations, but shifts by 1 Hz
for 1 x 10^-10 g/cm^2 accretion of mass on the sensing crystal. A two-stage
thermoelectric cooler cools the crystals to the 150 K operating temperature. A
Pt temperature probe senses operating temperature.

Cryoplate/IRS. Witness samples will be attached to at least one of the
cryoplates to facilitate identification and quantitation of species that
condense on the cryoplates. One potential witness sample uses a dielectric
mirror consisting of a highly polished substrate (such as Ni) with an
evaporated Al coating and overcoat of aluminum oxide with 8 lambda/4
thickness. With this plate conventional reflectance spectroscopy could be used
to observe any changes in spectral or total reflectance by observing shifts
and amplitude changes in reflectance maxima or minima due to the accumulation
of vaporized material from particle impact. If a change occurs, it indicates
an absorbing layer of another substance has been deposited on the surface.

A second witness material uses a Ge prism to aid in analysis by Internal
Reflection Spectroscopy (IRS). With IRS the optical spectrum of a deposited
film in contact with an optically dense, but transparent medium (the Ge
prism), can be obtained by launching light into the denser medium. The
measured reflectivity depends upon the interaction of the evanescent wave with
the sample material. One application of IRS is the measurement of material
optical constants by measuring perpendicular and parallel polarization at a
properly selected angle of incidence, or alternatively, using incident-
polarized light and measuring reflected intensity at two angles of incidence.
Once material optical constants are known, the thickness of a contaminant film
can be determined from the first term of the series expansion of Fresnel's
equation.

CONCLUSION

The GCC experiment has the capability to both collect particles and
semivolatile species released as a result of particle impacts, and, in
addition, characterize the noncollectable volatile species produced during
impact. Every element of the proposed GCC experiment has gone through proof-
of-concept and flight testing. Most of the equipment has considerable flight
experience and provides mature, proven hardware. The GCC experiment and the
other instruments in the CCSR package will permit better understanding of
comet composition and dynamics, as well as help in resolving whether comets
contain and distribute the primordial organic compounds necessary for life.

References: Alexander W. M. et al. (1988) Proc. LPSC 19th.  Alexander W. M. et
al. (1993) Abstracts of Workshop on Particle Capture, Recover, and
Velocity/Trajectory Measurement Technologies, Lunar and Planetary Institute,
Houston TX. Naumann R. J. et al. (1985)  NASA TM-86509. Tanner W. G. et al.
(1991) Proceedings of the Workshop on Hypervelocity Impacts In Space,
University of Kent (J. A. M. McDonnell, ed.). Tanner W. G. et al. (1993) Int.
J. of Impact Engng, in press.

Figure 1, which appears in the hard copy, shows the Gas Collection Cell (GCC)
concept showing the cell with the Mass Spectrometer for analyzing volatile
impact products and the Peltier-cooled coldplates and TQCM for collection of
less volatile products.  All GCC components have proven flight heritage.

Figure 2, which appears in the hard copy, shows a block diagram of the GCC
electronic system. The spacecraft microprocessor is used for instrument
control and data acquisition.

Figure 3, which appears in the hard copy, shows an exploded view of the thin-
film assembly. The Baylor University Space Science Laboratory has flown this
assembly on three shuttle missions. Recently this approach has been used for a
10 x 10 cm^2 foil on EuReCa/TICCE.

Figure 4, which appears in the hard copy, shows an exploded view of the Mass
Spectrometer (MS) used on the GCC experiment to monitor in real-time volatile
and semivolatile species resulting from dust particle collisions with the GCC
target. This instrument is derived from the Induced Environmental Contaminant
Monitor (IECM).

Figure 5, which appears in the hard copy, shows a cross section of the
Temperature-controlled Quartz Crystal Microbalance (TQCM) used to monitor
deposition rate of impact products from dust particle and GCC target
collisions.
1




Shimizu M.*
   Scientific Objectives of the Primitive Body Sample Return Missions-An
   Approach from the LED Effort On Water

Water is doubtlessly one of the most crucial components of the solar nebula to
determine the planetary composition: Planets were formed from the accretion of
the dust particles in the nebula, and the redox state of Fe in the particle
could be determined by the reaction of Fe with water vapor diffused into the
interior of the particle in the early stage of the solar system formation.

It has been discussed from various observations that the cores of Mercury,
Venus, and the Earth might be metallic Fe, although the core of the Earth may
be somewhat oxidized by the high pressure and temperature reaction of liquid
Fe with perovskite at the boundary of the mantle and the core [1]. Whereas,
the core of Mars may be much oxidized, being suggested by its low density.

Isotopic anomalies of various elements had been frequently observed in the
solar system (in the planetary atmospheres and in the meteorites) and some of
them could be attributed to the injection of exotic particles formed in other
stars into the solar nebula. Hydrogen and D anomalies in the planetary
atmospheres were frequently discussed to correlate with the differential
escape of H and D from the exospheres of Venus and Mars, although no one knows
the primordial D/H ratios before thermal escape. However, an explanation of
the decrease of the observed D/H ratios with the distance from the Sun was
attempted by considering the light-induced drift effect [2] to displace H2^l6O
alone to the outside in the solar nebula .

Oxygen isotopic anomaly was observed in the carbonaceous meteorites: the ratio
of ^17O and/or ^18O/^16O in the meteorites deviated from those of tap water on
the Earth to increase ^16O with the distance from the Sun [3]. The phenomenon
could be explained by the contamination of the solar nebula from the injection
of exotic materials containing pure ^16O alone of the supernova, but the
tendency of the increase could also be interpreted in terms of the light-
induced effect, since H2^16O alone would be moved outside, as well as the
variation of the redox state of Fe in the cores of the terrestrial planets.

The light-induced effect was first advocated by Gel'mukhanov and Shalagin [4]
and its photomechanical parameters were measured by Bloemink et al. [2] to
suggest that transport distance of H2O during the solar nebula phase would be
3.5 x 10^7 m, 3 times the diameter of Venus, by assuming 2500 K black-body
radiation from the protosolar envelope. The effect is isotopically selective,
additive and uses particle momentum instead of light momentum. Therefore the
effect could cumulatively be rather strong, although it is based upon the very
weak gravitational redshift of the infrared radiation. However, the estimated
distance is too small, since the vacancy of water on Venus would soon be
filled by water in the vicinity of Venus at the solar side. The transported
distance should be at least of the order of 0.1 AU.

We speculate that the proto-Sun would be the infrared laser star, whose
radiation was strong enough to cause the above three phenomena (in the same
direction); iron oxidization, H and O anomalies. Millimeter emission from
water vapor due to light pumping was observed by irradiation of large
ultraviolet flux [5] Laser effect was also detected at the wavelengths of 28
and 33 micrometers [6]. Although direct observation of water laser in near-
infrared region in the laboratory is not yet available, we may expect that the
proto-Sun was a strong infrared laser star, pumped by violent molecular
collisions in its envelope caused by turbulence from interior (ultraviolet
radiation would also contribute to pumping). We hope to test it by the
observation of T Tauri stars by using the ISO spacecraft in near future. The
analysis of oxygen anomolies of the returned sample from the primitive bodies
in the solar system is interesting for this purpose, too.

References: [1] Knittle E. and Jeanloz R. (1991) Science, 251, 1438. [2]
Bloemink H. I. et al. (1993) Phy. Rev. Lett., 70, 742. [3] Clayton R. N.
(1976) EPSL, 30, 10. [4] Gel'mukhanov F. Kh. and Shalagin A. M.(1979) JETP
Lett., 29, 711. [5] Gebbie H. A. and Apsley N.(1988) Infrared Phys., 28, 337.
[6] Gebbie H. A.(1964) Nature 201, 250, and 202, 169.





                          Tuesday, September 28, 1993
                          Future Flight Opportunities
                       1:00 - 4:00 p.m.     Berkner Room


Albee A. L.*   Uesugi K. T.   Tsou P.
   SOCCER-Comet Coma Sample Return Mission

Comets, being considered the most primitive bodies in the solar system,
command the highest priority among solar system objects for studying solar
nebula evolution and the evolution of life through biogenic elements and
compounds. SOCCER, Sample Of Comet Coma Earth Return, a joint effort between
NASA and the Institute of Space and Astronautical Science (ISAS) in Japan.
SOCCER has two primary science objectives: (1) the imaging of the comet
nucleus and (2) the return to Earth of samples of volatile species and intact
dust. This effort makes use of the unique strengths and capabilities of both
countries in realizing this important quest for the return of samples from a
comet. Science from this mission will far exceed the Atomized Comet Sample
Return Mission, a NASA Core Program of Planetary Exploration Through Year 2000
[1]. Without the Comet Rendezvous and Asteroid Flyby (CRAF) mission, it is all
the more critical to realize SOCCER.

1.  INTRODUCTION

The intact capture and return of cometary coma material, both dust and
volatiles, has an advantage over orbiter and rendezvous missions in that the
captured material can be made available to all complex, sophisticated
laboratories here on Earth. Samples, if properly stored and preserved, can
also be examined by analytical techniques, presently unknown but almost surely
to be developed in the future. Lander- or rendezvous-type of sample return
missions require rather complex spacecraft, intricate operations, and costly
propulsion systems. By contrast, it is possible to take a highly simplified
approach for sample capture and return in the case of a comet. This can be
accomplished by an Earth free-return trajectory to the comet, in which passive
collectors intercept dust and volatiles from the cometary coma. Bringing
samples back from within the zone of parent molecules of a known comet will
provide valuable information on comets, will serve as a rosetta stone for the
analytical studies of interplanetary dust particles for the last two decades,
and will provide much needed samples for the analysis community.

The Halley Sample Return (HSR) Mission was the first comet coma sample return
mission studied by the Jet Propulsion Laboratory (JPL) in 1981. The Sample Of
Comet Coma Earth Return (SOCCER) was initiated with this proposal team
associated with National Aeronautics and Space Administration (NASA) and the
Institute of Space and Astronautical Science (ISAS) in Japan. This effort
makes use of the unique strengths and capabilities of both countries in
realizing this important quest for the return of samples from a comet, and in
sharing the cost and the science findings.

SOCCER has two primary science objectives: nucleus imaging and dust and
volatiles sample return. SOCCER can achieve first-class science by being the
first mission to bring back samples from a known comet, and by obtaining high-
quality close-up images of a comet nucleus. Science from this mission will far
exceed the Atomized Comet Sample Return Mission, a NASA Core Program of
Planetary Exploration Through Year 2000 [1].

2.  COMET COMA SAMPLE RETURN BACKGROUND

Interest in cometary missions began in the early 1970s. The first JPL mission
proposed was the Halley Flyby with a Probe and Tempel Rendezvous (HFPTR)
mission [2]. The development of cometary flyby sample return technology was
triggered by the JPL's HSR Mission [3]. Due to the 70-km/s encounter speed
with Comet Halley, atomized sample return rather than intact sample return was
considered for HSR. The first comet coma sample return mission with intact
capture was jointly proposed with Goddard Space Flight Center (GSFC) as a NASA
mission for the Planetary Observer Program and proposed direct re-entry via a
Discoverer Capsule [4]. Mission costs from GSFC and JPL were compared [5].
Using the spare Giotto spacecraft, JPL proposed Giotto II jointly with the
European Space Agency (ESA) [6]. Based upon the Giotto II concept, an ESA
Comet Atmosphere Encounter and Sample Return (CAESAR) was studied in 1986 [7].
Since 1987, low-cost flyby sample return missions to comets, SOCCER, have been
jointly studied in U.S. and Japan [8].

2.1 SOCCER Mission

NASA's interest in comets has been reflected in four major JPL efforts
beginning with the HFPTR, Halley Intercept Mission, HSR, and finally the
recently canceled Mariner Mark II Comet Rendezvous and Asteroid Flyby (CRAF)
mission [9]. The high percentage of comets with Japanese surnames clearly
indicates the interest of the Japanese in comets. For some time now, ISAS has
demonstrated its unique ability to develop low-cost spacecraft for a variety
of missions (SUISEI & SAKIGAKE to Halley, HITEN to the Moon, GEOTAIL to the
distant geomagnetic tail). The dominant video and camera industries in Japan
give ISAS a strong basis for developing an imaging camera system. Since 1981,
JPL has developed the enabling technology for flyby sample return-intact
capture of hypervelocity particles [10], and validated techniques for the
capture of volatiles [11]. NASA has an operational shuttle capable of
retrieving objects in low Earth orbits, and its Deep Space Network (DSN) can
track deep-space signals around the clock. JPL has mounted sophisticated
missions throughout the solar system. Consequently, both NASA and ISAS have
common interests in exploring comets and unique capabilities, and SOCCER has
been conceived as a joint mission taking advantage of each agencies'
strengths. In a tight funding environment, the total cost of a comet flyby
sample return mission can be jointly shared, making the acceptance of such a
mission easier for both countries.

2.2 SOCCER Joint Working Group

The first meeting of this current NASA/ISAS proposal team was held in Japan
[12]. The team was formally installed as a joint study group [13],
subsequently promoted to a joint working group, and met again at ISAS [14].
The second working group meeting was held jointly with the Japan-U.S. Workshop
on Missions to Near-Earth Objects [15], while the third working group's
meeting was held in May of 1992 [16].

The working division of responsibility between NASA and ISAS follows
designated capabilities. The imaging system and radio science will be provided
by ISAS, while NASA will provide the sample collection system. The spacecraft
will be designed and built by ISAS, and launched by an ISAS M-V, three-stage
solid propellant rocket with a NASA medium-expendable launch vehicle (MELV) as
backup. The returned samples will be retrieved by the shuttle. Operations will
be led by ISAS, supported by NASA, for launch, cruise, tracking, navigation,
encounter, Earth insertion, and aerobraking.

3. SOCCER SCIENCE PAYLOADS

Each part of the SOCCER science payload serves a dual function, science and
engineering: the imaging camera will perform navigation for comet locating and
encounter targeting; the sample collector will serve as a dust shield for the
spacecraft during encounter, and, of course, the radio transponders will serve
for spacecraft communication as well as for radio science.

3.1 SOCCER Imaging System

The closest image of a nucleus was obtained by Giotto, a spin-stabilized
spacecraft. There is a need for images of much higher resolution. The world's
first high-resolution imaging of the comet nucleus can be achieved by SOCCER
using a three-axis stabilized platform. A charge-coupled device (CCD) camera
can achieve fine resolution of the dynamics of the comet nucleus surface in
both the pre- and postperihelion stages. The goals of the SOCCER imaging
system are the following: (1) Obtain nucleus surface images at the closest
distance with the greatest resolution; (2) Obtain observatory, wide coverage
of large-scale dynamic coma phenomena; and (3) Obtain pre- and postperihelion
images of the comet coma development at regular and frequent intervals
beginning several months before encounter and extending several months after
encounter.

The SOCCER camera will be an improved version of a solid-state camera designed
for several ongoing ISAS flight programs. These include the Planet B mission
to be launched to Mars in 1996, and the LUNAR-A mission to the Moon in 1997.
The camera's resolution will be specified to be 10 microradians or better,
with images being focused on a CCD, having a field of at least 1024 x 1024
pixels. Keeping the CCD cooled to approximately 200 K will ensure a large
signal-to-noise ratio. With filter wheels, this CCD camera can capture both
color imaging and surface spectral data. With the high-gain antenna oriented
toward Earth, selected images can be transmitted in real time. To facilitate
both pre- and postencounter imaging, wide-angle, as well as narrow-angle
optics will be used.

3.2 SOCCER Sample Return Collection System

The current collection concept for SOCCER [17], has evolved through a sequence
of previous studies: HSR [18], Planetary Explorer, Comet Intercept Sample
Return [18], ESA/NASA Giotto II [20], space station attached payloads
experiments: Intact Particle Capture Experiment [21], Cosmic Dust Intact
Capture Experiment [22], Interstellar Dust Intact Capture Experiment [23],
Exobiology Intact Capture Experiment [24], Second Generation Space Station
Cosmic Dust Collection Concept Development [25], and finally, the Shuttle Get-
Away Special Sample Return Experiment [26]. The sample collection system
consists of designated regions for dust collectors and volatiles collectors.
Many of the engineering details have yet to be defined but the sketch provides
an overall perspective of the collection system. All collector regions will be
modular in sign. Dust collectors must allow for depth of penetration and so
will be cubic in shape, whereas, volatile collectors, and fine dust impactors
will be flat surfaces.

4.  ENGINEERING FLIGHT SYSTEM

The engineering system includes the spacecraft, dust shield, and an aerobrake
cone to enable matching of the shuttle retrieval circular orbit from a highly
elliptical Earth insertion orbit. The spacecraft and launch vehicle will be
designed and fabricated by ISAS. ISAS has demonstrated the technique of
aerobraking, having utilized it for HITEN's cislunar aerobraking in 1991.
Sample return requires close targeting, which dictates the need for optical
navigation in addition to ephemeris tracking. SOCCER's navigation capacity is
examined in section 4.3.

4.1 SOCCER Spacecraft Description

The SOCCER spacecraft is built around an octagonal bus as illustrated in Fig.
1. The sample collection system is mounted on one end of the bus with the
aerobrake drag cone mounted on the other. Since the dust collector serves as
the dust shield during the encounter, no part of the spacecraft will be within
5 degrees from the edge of the collector. 250 W of power come from two
articulated solar paddles that are wrapped about six flat faces of the bus
during launch and encounter phases. The solar cells face outward to permit
power generation during encounter and aerobraking phases. The articulated 1.8-
m high-gain antenna, as well as the fixed medium and low-gain antennae are
mounted under the aerobrake cone shown in Fig. 1. The aerobrake cone will be
composed of SiO2, which is transparent to radio frequency transmission. On one
of the uncovered bus faces, a camera port provides side field of view during
encounter. A mirror to facilitate encounter imaging of the nucleus is under
study. S- and X-band transponders, having 10 W and 25 W power, respectively,
will provide uplink and downlink communications. At 2 AU, the data rate at X-
band is about 32 kbps and 3 kbps at S-band with the 64-m antenna at Usuda. All
midcourse trajectory correction maneuvers (TCMs) and Earth insertion burns
will be accomplished by a bipropellant propulsion system with the main
thruster mounted in the bus. Attitude control and spin control thrusters are
mounted on the edge of the bus, between the bus and the aerobraking cone. The
sample collection system will be separated for shuttle retrieval and placed in
a cooled sample collector container aboard the shuttle. A standard shuttle
grapple is mounted below and at the center of the aerobrake cone to facilitate
spacecraft stabilization for sample collector removal.

4.2 SOCCER Launch System

SOCCER will be launched by ISAS's newly designed M-V vehicle. In late 1994,
the MUSES B mission will be the first launch to use this system. M-V is a
three-stage solid propulsion rocket, 2.51 m in diameter and 31.4 m long. The
vehicle can carry a two-ton payload to low Earth orbit. With a fourth stage
kick motor, interplanetary missions can be accomplished.

4.3 SOCCER Navigation

Reliance upon comet ephemeris alone will not suffice to achieve a targeting of
the cometary nucleus better than 100 km. The uncertainties of cometary
trajectories at successive apparitions are due to gravitational perturbations,
as well as nongravitational effects arising from jetting, which generates the
coma in the first place. With favorable orientation conditions of Earth,
comet, and Sun, groundtracking of the comet can reduce the ephemeris error. It
is more likely, however, that sample return targeting will require optical
navigation. A prime determinator of the accuracy achievable by optical
navigation is the camera's pixel size. Using a 9-micrometer-sized pixel in a
frame CCD camera, with an assumed ephemeris error of 750 km, and with TCM 1
and TCM 2 executed 10 days and 1 day before encounter, respectively, a
delivery accuracy of 20 km can be achieved for a 10 km/s flyby speed. Figure 2
shows the delivery accuracy with respect to the time for the last TCM. Given
this accuracy, the uncertainty in locating the nucleus within the coma will
also need to be considered in specifying a flyby distance. For a very close
flyby distance, less than 10 km, a pathfinder spacecraft would enhance the
confidence; otherwise, a pathfinder would not be necessary.

5. SOCCER MISSION DESCRIPTION

Important considerations in designing a free-return trajectory for SOCCER are
a low encounter speed to enhance intact capture of dust samples, the
restricted launch vehicle capability of ISAS's M-V rocket, and the need for a
dependable trajectory of the targeted comet. Based upon these three
considerations, the 2002 perihelion encounter with Comet Finlay has been
selected as the baseline comet. A plot of this baseline trajectory to Finlay
is shown in Fig. 3. The encounter speed for this trajectory is 7.98 km/s,
which is within the range of speeds for which successful intact capture has
been demonstrated (i.e., up to speeds of 13 km/s).

5.1 Cometary Candidate

Possible comet candidates for SOCCER in the 2000 time frame include Churyumov-
Gerasimenko (2002), Wirtanen (2002), DuToit-Hartley (2003), Kopff (2003), and
Finlay (2002). For this proposal Finlay, has been chosen as a baseline comet.
Comet Finlay, known since 1886 and observed at 11 apparitions, has a well-
determined and relatively stable orbit with perihelion just outside the
Earth's orbit (near 1.1 AU) and a revolution period of slightly less than
seven years. Since 1953, the comet has been observed at every return to the
Sun with its orbital motion having been subjected to modest and somewhat
variable nongravitational perturbations. However, the comet has a history of
at least one major orbital anomaly, this occurring either late in the 19th
century or early in this century [27].

Comparison of P/Finlay's lightcurve in 1960 (the only apparition since 1953
from which adequate information is available on the comet's brightness) with
the lightcurves in and before 1926 indicates that the object's intrinsic
fading has been dramatic, the peak normalized magnitude being 10 in 1960,
compared with 6.5 in 1906, and 8 in 1886 and 1893. Brightness fluctuations
with an amplitude of about one magnitude were reported during one week of
intense observing shortly after perihelion in 1960, which suggests that the
production was quite variable on a timescale of days. Still, this comet is
considered to be a good candidate for particle-collection.

5.2 Mission Scenario

Ground and space telescopes would begin the tracking phase to acquire Comet
Finlay before launch and continue after launch to refine Finlay's orbit. After
payload integration and tests, SOCCER will be launched by M-V on August 24,
2000, at Kagoshima Space Center. The kick motor will be ignited after the
ejection of the third-stage motor of the M-V and will insert SOCCER into the
heliocentric trajectory needed to encounter comet Finlay. During the pre-
encounter cruise phase the solar cell paddles will be deployed and the
spacecraft placed into spin stabilized mode. During cruise, the covers of the
sample collectors will be kept closed. For the pre-encounter observation
phase, the spacecraft will be despun for three-axis stabilization by the use
of a momentum wheel and to initiate pre-encounter observation imaging science
and optical navigation imaging to aid in precise orbit determination. A
deterministic maneuver of 139 m/s will be performed on December 31, 2001. A
week before encounter, the covers of the sample collection system will be
opened. Final targeting from optical navigation update will be performed one
day before the encounter. Before the encounter phase, which lasts for
approximately one hour, volatiles capture getters will be deposited, solar
paddles will be retracted, and the spacecraft will be poised for the encounter
phase. A summary is shown in Fig. 4.

SOCCER's two primary science systems require a very close encounter with the
comet nucleus to achieve good close-up imaging and to capture cometary samples
with minimum processing. A target encounter distance of 10 km from the comet
nucleus is highly desirable. Detailed study, involving orbit determination
simulations (including the effect of center-finding error) and assessment of
the coma environment, is necessary to determine the minimum feasible encounter
distance. During the final encounter, nucleus images will be acquired at the
maximum rate. The acoustic counter data along with some selected, compressed
images will be processed and transmitted in real time. After the sealer
getters are deposited and the mission begins the post-encounter observation
phase, the sample collector covers will be closed. The solar paddles will be
redeployed and observatory imaging will resume. The spacecraft will then
commence the trip back to Earth. After having completed three orbits around
the Sun in four years, the spacecraft returns to Earth on August 24, 2004.

After a final trajectory correction maneuver, a 660-m/s Earth insertion burn
is performed, bringing the SOCCER spacecraft into a highly elliptical orbit.
Aerobraking is subsequently used to circularize the orbit at an altitude low
enough to allow rendezvous with the shuttle. Before aerobraking is initiated
(with a 20 m/s maneuver reducing perigee to 120 km), the spacecraft loiters in
order to allow its orbit to precess until the spacecraft's spin axis is normal
to the Sun line. This assures the availability of full power during
aerobraking. About 20 apogee maneuvers are needed to navigate the spacecraft
during aerobraking, totaling less than 10 m/s of delta V. After sighting
SOCCER, the shuttle will make final adjustment for an extravehicular activity
to attach the Remote Manipulator System to the grapple. After grapple
attachment, the sample collection system will be separated from the spacecraft
and placed into a refrigerated container for Earth return. On reaching the
ground, the entire container will be moved into the laboratory for
deintegration, sample examination, and sample distribution.

All flight hardware handled by the shuttle is subject to stringent safety
requirements. Since SOCCER is not launched by the shuttle, and is not designed
to meet shuttle launch requirements, the shuttle cannot retrieve it.
Furthermore, the retrieval of a spacecraft into the bay of the shuttle after
the craft has been in deep space for four years would have severe safety, and
thus, cost implications. Consequently, it is cost-effective for the shuttle to
retrieve the sample collection system only.

6. MISSION OPERATIONS

The spacecraft and instruments will be integrated at ISAS, Sagamihara,
Kanagawa Prefecture. The mission operations will be performed at ISAS by its
operational staff with the support of JPL's engineering teams from sample
collection systems, navigation, and tracking. At the time of shuttle
retrieval, the ISAS spacecraft team will support the NASA shuttle retrieval
team. Thus, operations, as much as possible, will be handled by ISAS. The
typical ISAS operations team is small and very efficient, making low costs
possible; uplink sequences will be designed by ISAS with participation from
the sample collection team. Uplink data are transmitted in S-band and
downlinked in both S- and X-bands. Imaging data and dust capture data are
downlinked with minimal processing.

7. SUMMARY

As was stated in the Solar System Exploration Committee's report of 1983, the
next step in cometary exploration following the flyby missions to Comet Halley
should be a rendezvous and sample return with a short-period comet. Without
CRAF, it is all the more critical to the achievement of the goals of the Solar
System Exploration Committee that SOCCER be accomplished. Through a joint
effort with ISAS, it is cost-effective to acquire thousands of samples from a
known comet and thousands of images of a comet nucleus and dynamics of that
comet coma at half the cost for each agency. During development, SOCCER
extends participation to NASA and ISAS facilities, and more than one
university. With returned samples, many analysis laboratories throughout the
world will be involved. SOCCER will make available for analysis not only the
only extraterrestrial samples from a known source since the Apollo missions,
but specifically cometary material captured in the zone of parent molecules.
Such material rates highest among all solar system objects for study of the
planetary evolution and that of compounds and phases containing the biogenic
elements.

References: [1] SSEC (1983) NASA. [2] Atkins K. L. (1979) AIAA Paper 79-2066,
AIAA/DGLR 14th International Electric Propulsion Conference, Princeton, New
Jersey, Oct 30-Nov 1. [3] JPL (1991) JPL, 715-140. [4] Tsou P. et al. (1983)
International Conference on Cometary Exploration, Vol. II, , pp. 215-224. [5]
Tsou P. et al. (1985) AIAA-85-0465. [6] Albee A. L. (1985) Giotto II Workshop,
Caltech. 7] ESA (1986) ESA SCI (86)3. 8] Uesugi K. T. et al. (1987) ISAS. [9]
Neugebauer M. et al. EOS (1989) 70, 633. [10] Tsou P. et al. (1984) LPS XV,
866-867. [11] Tsou P. et al. (1987) LPS XVIII, 1024-1025. [12] ISAS (1987)
ISAS. [13] ISAS (1988) Proceedings of ISAS/NASA Joint Study Group Meeting.
[14] ISAS (1988) Proceedings of ISAS/NASA Joint Working Group Meeting. [15]
ISAS (1991) Proceedings. [16] ISAS (1992) Proceedings of 3rd ISAS/NASA Joint
Working Group Meeting. [17] Tsou P. (1990) JPL D-7777. [18] Tsou P. (1983) JPL
D-797. [19] Tsou P. et al. (1983) JPL D-1153. [20] Tsou P. et al. (1985) JBIS,
38, 232-239. [21] Brownlee D. E. (1988) Proposal to The Small-Class Explorer
Program. [22] Brownlee D. E. (1989) Proposal to Space Station Attached Payload
Program. [23] Brownlee D. E. (1989) Proposal to Space Station Attached Payload
Program. [24] Carle G. (1989) Proposal to Space Station Attached Payload
Program. [25] Tsou P. (1989) Proposal to Space Station Attached Payload
Program. [26] Tsou P. et al. (1993) LPS XXIV. [27] Sekanina, Z. (1993) Astron.
Astrophys., 271, 630-644.


Nishioka K.*
   An Innovative, Flexible Conceptual Instrument Design for SPICE

   No abstract available.


Kinard W.*
   Future Investigations of the Near-Earth Environment

   No abstract available.


Nishioka K.*   Maag C.*   Horz F.*
   MIR Plans

   No abstract available.





                          Tuesday, September 28, 1993
              Discussion:  The Future of Cosmic Dust Instruments
                           4:00 p.m.     Berkner Room


No abstract available.






                             Additional Abstracts



Borg J.   Bibring J. P.   Maag C.
   Main Characteristics of the Comet/Comrade Experiments

INTRODUCTION

Both the COMET (Collection en Orbite de Matiere ExtraTerrestre) and the
COMRADE (Collection of Micrometeorites, Residue and Debris Ejecta) programs
are devoted to the collection and analysis of the particles of various
origins orbiting around the Earth at low altitudes (between ~300 and ~500
km).

These particles can be roughly divided into categories of orbital debris
resulting from manmade effects and extraterrestrial particles. The goal is
to collect the extraterrestrial particles before they are processed by the
Earths atmosphere, which can cause severe alteration. The main interest in
their study is due to the fact that part of these particles contains
information on the origin of the solar system. In particular, cometary
material is likely to be the most primitive material accessible for
analysis. It is thought that grains once present in the cometary nuclei and
now present as individual grains in interplanetary space are the best
candidates for still having properties they acquired before or during the
condensation of planetary objects. A second minor component is also
present, originating from the asteroidal belt. The smaller size fraction
(grains less than 10 micrometers in diameter) is supposed to be enriched in
grains of cometary origin [1]. The grains we analyze are thus of various
origins from inside the solar cavity and should reflect in their elemental,
isotopic, molecular, and mineralogic composition the variety in the
components of the primary solar nebula. In addition, they have been
subject, inside the past and present solar cavity, to various kinds of
irradiations. It is now well known that these different irradiations of
grains can result in different physical, chemical or isotopic properties
[2]. Also present is orbital debris with velocities of the same order and
resulting from manmade activities (paint flakes, aluminium oxide spheres,
etc.). The small-sized grains are the most frequent ones orbiting around
the Earth, manmade debris having, for all sizes, much larger fluences than
extraterrestrial grains.

The COMET experiment is more specifically designed to be flown for a short
period of time (a few days), in concordance with a meteor stream crossing
the Earth. Thus, it results in a considerable enrichment in the collection
of grains related to a given comet.

The COMRADE experiment has been selected as a proposal for long-duration
flights (a few months), in order to gain information on all sizes of
particles present on low Earth orbits, including submicrometer grains. It
has been accepted by ESA authorities for use on the EURECA 2 platform.

We are concerned simultaneously, for the future exposure of both
experiments, with a destructive capture of orbiting grains, using metallic
collectors, improved since the COMET-1 experiment [3] and a nondestructive
capture, using new low-density targets in which the impacting grains are
slowed down and stopped, practically intact. The advantage of the first
type of capture is twofold: it allows us to gain information on the
smallest size fraction, and to detect the presence of light elements such
as C. The interest of the second type of capture is to allow the extensive
study of intact IDPs. Up to now, this technique has only been applied to
short flights of the NASA space shuttle for studies of orbital debris [4].
Grains a few micrometerss in size can be stopped in those low-density
materials and recovered for further studies.

The objectives of these studies are multiple. The use of passive detectors
gives access to the chemical and isotopical properties of the grains in the
micrometer size range, by analyzing either the particle remnant mixed with
the target material, or the intact particle captured in a specific low-
density material. The particle remnants of the micrometer-sized
extraterrestrial grains, having impacted on purposely designed metallic
collectors, are identified for complete and detailed chemical, isotopic,
and organic analysis, thereby determining grain composition as well as the
existence of organic and inorganic molecules, to be related with the
possible cometary origin of the grains. Micrometer/submicrometer dust
grains are also captured in a manner that insures minimal particle
degradation. The captured intact particles are returned to Earth for
complete and detailed chemical, isotopic, spectral, mineralogical, and
organic analysis.

These investigations, which will collect micrometer/submicrometer
particles or their remnants, can at the same time measure the dynamic
particle parameters (determination of its mass, velocity, trajectory, and,
for some, charge) with a high degree of confidence, if active detectors
are exposed in parallel with the collecting passive detectors. This part
of the investigation is more adequately meant to be part of the COMRADE
program, as long-term in situ dynamic measurements of these particles, in
this spatial region, do not exist reliably and are of great importance by
themselves, since detailed trajectory information is currently needed to
calibrate experimental laboratory measurements. Such dynamic measurements,
coupled with collection analysis, will help in advancing current theories
on the evolution of the universe and the solar system.

THE COMET EXPERIMENT

The specificity of the COMET experiment is to expose collectors only during
the encounter of the Earth with a given meteor stream, so that the
collection is enriched in grains of a known origin, related to a specific
comet. For this purpose, the collectors are installed in boxes that can be
automatically closed and opened by the astronauts from inside the station.

Such a grain collection on low Earth orbit has started with the COMET 1
experiment that was designed to allow the collection of grain remnants by
impacts on targets installed outside Salyut 7 station, orbiting at 350 km
altitude [5]. The collectors were exposed to space in October 1985, while
the Salyut 7 station was crossing the Draconides meteor stream, in order to
study the chemical properties of grains originating from the Giacobini-
Zinner comet. The collectors consisted of high-purity gold and nickel
modules, of 2 cm^2 area, covered by an ultrathin (100 nm) gold film, having
two main functions: (1) protection against contamination and (2)
identification of impact positions. Modules with a total collecting area of
1152 cm^2 were allocated in four boxes. The boxes were opened and closed
through an electronic unit from inside the station by the cosmonauts.

The collectors, after their exposure in space, are brought back in the
sealed boxes to the clean rooms of the lab. There, they are opened and a
first optical scanning is performed in order to identify the impact
positions of the larger grains. The impact positions of the micrometer-
sized grains are identified directly on the surface of the metallic
collectors: after contaminant removal, the various metallic targets are
thoroughly scanned, using a JEOL 840 scanning electron microscope (SEM) at
750x magnification, in order to select events showing typical hypervelocity
crater features (round circular habits, ridge). Such a magnification allows
the identification of crater features down to diameters of ~0.5
micrometers. We can thus analyze the size distribution of the impacting
particles, down to these sizes, allowing the evaluation of the incident
microparticle flux in the near Earth environnment.

In a second step, it is possible to determine the chemical composition of
the impacting particles. The JEOL SEM is equiped with an EDS analysis
TRACOR system, allowing semiquantitative analysis down to Na, and
qualitative detection down to C. Such a chemical identification can be
followed by a high-resolution analytical protocol including instruments
such as FESEM (Field Emission Scanning Electron Microscope, for high-
resolution imagery of the impacting events), LIMS (Laser Induced Mass
Spectroscopy, for molecular identification of the carbon compounds), or
SIMS (Secondary Ions Mass Spectroscopy, for isotopic identification), for a
complete characterization of the impacting particles.

The main results obtained up to now from the COMET 1 experiment are very
encouraging. By counting the number of holes in the protective films and of
impact craters on the bars of the grids holding the films and directly
exposed to space, it is possible to estimate the flux distribution of
particles at the moment of exposure. The correlations between hole and
crater diameter and particle size were determined from simulation
experiments that took place at the Dust Accelerator of the Max Planck
Institut in Heidelberg.

We found for the number of impacting particles smaller than 10 micrometers
in diameter a cumulative flux of ~8 x 10^-2 m^-2 s^-1, consisting of ~90%
orbital debris, as confirmed by chemical analysis. This value induces a
large enhancement as compared with the known estimations of the
micrometeroid particle mass distribution [6]. We attribute this enhancement
to the fact that the collection occurred during the encounter of the
Giacobini-Zinner meteor stream. The extraterrestrial particles, supposed to
be mainly of cometary origin, show various proportions of the following
elements: Na, Al, Mg, Si, S, Ca, and Fe, associated in most cases with
various proportions of C and O. For some extraterrestrial particles, C and
O are found alone. The systematic presence of low Z elements, either
exclusively or associated with other elements whose abundances reflect a
chondritic type composition, can be compared to results obtained by the
PUMA and PIA experiments that analyzed the grains in the close environment
of the Halley nucleus [7] or with chemical identifications on
extraterrestrial particles from the stratospheric collections [8].

After the COMET 1 collection, we proceeded to improve the collectors: the
protective 100-nm film has been eliminated and the collectors are now
clean, flat metallic surfaces, either of Au or Ni, on which 50 nm of Au is
evaporated in order to "decorate" and enlarge the impacting positions.

Also, collectors have been conceived to recover intact grains, whatever
their density and size, as the return of extraterrestrial material to the
laboratory is a primary goal of the future investigations. Such collectors
are made of very-low-density material, either foam or silica aerogel, that
would provide a unique means to slow down all the grains with minimal
destruction and thus allow their further analysis, not only for elemental
and isotopic composition, but also their molecular composition, and
mineralogical and physical properties.

In order both to visualize the impact hole of intact particles at the
surface of the low-density material, and thermally protect the material,
the surface of the collectors will be covered by 50 nm of a highly
reflective metal (either Al or Au) and 200 nm of evaporated silicon oxide.
An optical survey of the surface allows the identification of the impact
positions. By following the track inside the foam, the particle can be
reached, picked up with a clean needle and recovered on a golden microscope
grid for future analysis [9]. A quick EDS identification can indicate
whether the particle is an IDP (chondritic elements: Mg, Fe, Si,...) or an
orbital debris (Al, Ti, Zn,...). The same high-resolution analysis protocol
(FESEM, LIMS, SIMS,...) as for grain remnants can be applied to the intact
particles for their further identification.

An improved version of the COMET instrument has been designed,
manufactured, and tested. It will hopefully be installed in the near future
on board one of the two last automatic spacecraft to be docked to the
Russian MIR station. The results already obtained with the COMET 1
experiment show that such an instrument can constitute an important step
toward the analysis of cometary material and the understanding of the early
evolution of the solar system.

THE COMRADE EXPERIMENT

As an extension of the COMET program, and taking into account the future
possibilities of long-duration flights, we proposed in 1991 to ESA the
COMRADE program, which was to be integrated as a program to be flown on the
EURECA 2 carrier, devoted to the determination of the main properties of
the particles on LEO. Therefore, it includes a more complete set of
detectors, both passive and active, in order to gain the maximum
information on the impacting particles [10].

The concept, underlying the proposed investigation, is to conduct a long-
term investigation into micrometer/submicrometer-charged dust grains in
near Earth orbit. During this investigation, the proposed experiment will
capture intact dust grains/particles and ejecta as well as remnant
particles for chemical analysis and at the same time conduct state-of-the-
art in situ measurements of the fundamental grain parameters (trajectory,
velocity, mass, and charge).

The specific concepts/methods underlying the various parts of the
integrated instrument are quite similar to what has been described for the
COMET experiment, concerning the passive detectors, except that it is
proposed that the various collectors are exposed for the totality of the
mission, and, thus, do not have to be mounted in boxes to be opened and
closed. Independant cells can be devoted to specific tasks, as the
detectors are conceived as independant units that need not sit together if
needed. The use of the multi-user facility ESTEF (European Science and
Technology Exposure Facility) is provided on EURECA, the external faces of
which could receive our experiment tray of passive detectors. The exposure
geometry of EURECA is supposed to be essentially Sun pointing; this
situation is such that the Earth orbital particle population component will
be randomized by opposition to the heliocentric interplanetary component.
All crossing meteor streams will thus have grains impacting our collectors,
enriching our collection in grains of cometary origin.

What is specific to the COMRADE experiment, in opposition to the COMET
experiment, is the possibility of having access at the same time to the
measurement of the fundamental parameters of the particles.

The reliable determination of the trajectory of each individual dust
particle is a high priority of the proposed investigation. Historically,
particle trajectories (as well as particle time of flight) have been
determined using the thin film/plasma technique. This technique is based on
the fact that a cosmic dust particle that impacts an extremely thin film
will create a minute plasma cloud. The collection of this plasma cloud then
allows the analytic determination of dynamic particle parameters. The use
of multiple thin films thereby yields a method whereby particle
trajectories and time of flight can be determined.

In addition to the particle trajectory, it is vital that dynamic particle
parameters also be measured with a high degree of reliability. The basic
parameters that the proposed experiment could measure and/or determine are
the particles velocity, mass, and mass flux. The particle charge and time
of flight can be measured by examining the thin film/plasma technique
discussed previously. Also, by examining the amplitude of the plasma pulse
produced, the kinetic energy of the particle can be obtained, which in turn
enables a determination of the particles mass. Since one of the major
goals of the proposed instrument is to capture the particle while causing
minimum particle degradation, it is necessary that extremely thin films be
used in this sensor. The thinner the foil, the smaller the plasma produced
and the more difficult it is to capture the signal produced. However, the
experiments previously listed, along with the production and collection of
ions and electrons from current laboratory hypervelocity impact studies,
have yielded the data necessary to optimize the collection ability of the
proposed instrument [11]. Collection of plasma from rear plate
hypervelocity impact will imply penetration of thin films by an incident
dust grain. Thus a lower bound on particle mass is set which depends on
inferred density, and the penetration mass limits established during
calibration studies conducted using materials with various densities.

In order to determine the montentum of individual particles, piezoelectric
transducers have been used as impact impulse sensors since Explorer I. The
distinctive characteristic of any linear elastic sysem, like a
piezoelectric crystal (PZT), is that the maximum displacement of the system
is directly proportional to the impulse imparted, and the displacement of
the crystal produces a proportional potential. Through calibration, a known
impulse may be equated with a specific charge produced on the electrodes of
the PZT crystal. With precise knowledge of the plasma collected at the PZT
impact plate, and with accurate measurement of velocity vector, one can
with calibration data from hypervelocity impacts on PZT plates establish an
upper bound on the particle mass. The accuracy of this method depends on
the composition of dust grains utilized in laboratory simulation studies.

Therefore, by coupling plasma collection after thin film penetration and
hypervelocity impact PZT measurements, one may establish a lower bound and
an upper bound on the mass of the dust grain encountered. If one assumes
that the incident dust grains are spherical, one can then determine the
volume from the radius derived by the electostatic charge measurement. The
density of the dust grain can thus be deduced using the lower and upper
bound values provided by the other sensors of the detector package.

One aspect of measurement not yet exploited is the density difference of
impactors. If the density of a dust grain is less than the thin film and/or
the impact plate material then the hypervelocity impact will convert more
of the dust grain into plasma than thin film or PZT impact plate material.
If calibration studies are performed to characterize the plasma signature,
e.g., pulse width, received from an Al thin film and a gold coated PZT
plate, then impact plasma signals collected after hypervelocity impacts of
less dense material could show anomalies. With these data a greatest upper
bound on particle density may be established, i.e., anomalous plasma
signature with too narrow a pulse width implies low atomic number material.
With an estimate of density, rho, and with mass of the grain values,
rho(sub)g (density of the grain) may be calculated and finally bounds set
on the ambient potential V(sub)c.

Independant unit cells have been designed to accommodate the myriad of
subexperiments, needed for the determination of the various parameters
described. Each unit will possess one or two thin Al films (nominal t(sub)f
< 50 nm) stacked above a coated substrate. Beneath the thin films and above
the substrate will be a network of collimating plates. These divisions will
assure that grains whose velocity vectors make a large angle with respect
to the surface normal of any film will not impinge on the another cell, but
will impact the witness plates of a specific cell or be stopped by a thin
film.

CONCLUSIONS

It is in the context of the search for organic material in extraterrestrial
particles down to submicrometer sizes that our proposal must be perceived,
exposing our types of passive detectors (metallic collectors and low-
density material) on board the EURECA 2 flights, as well as on the MIR
flight, in boxes opened for exposure only during the encounter with a given
meteor stream. Any collection facility designed to identify particles of
cometary origin should contain some high-purity metallic targets for
chemical and isotopical identification of particles. The coupling of
metallic collectors and low-density material is an unique opportunity to
complete information on all sizes of grains from submicrometer sizes to a
few micrometers.

All grains down to submicrometer sizes can be collected on our metallic
targets. Because of their high relative velocity (>=5 km/s), the impacting
grains are physically destroyed, leaving a melted remnant that is mixed
with the crater material. This process is more favorable for the smaller
grains, with sizes in the micrometer size range; the larger grains can
vaporize, leaving no analyzable remnant. Our previous results have shown
that Au and Ni collectors are favorable for the collection and analysis of
the small sized grains orbiting around the Earth. For the less frequent
larger grains, their collection is possible in the large surfaces of low-
density material we will expose.

The analysis of the grains, either remnants or entire, will be performed
with the high-resolution instruments we will have access to (optical
microscopy, SEM, EDS, ionprobe...). By the time the collectors will be back
from space, new techniques will have been developed and accessible for our
analysis; for instance IR spectroscopy of individual grains, or double
laser probe, promising techniques for eventually identifying organic
molecular species present inside the grains. The possibility of recovering
particles of cometary origin, in which the organic phase can be analyzed,
is a very exciting one, as the comet grains remain privileged witnesses of
the beginning of the solar system.

The present COMET and COMRADE proposals expand upon a program initiated a
few years ago with the COMET 1 experiment. It concerns the collection of
cometary dust and space debris by exposing various detectors on board
spacecraft orbiting the Earth. It takes into account not only the
possibility of gaining information during the given flights, but also, and
more importantly, the fact that these experiments could fit into the
general perspective of proposing a permanent collection facility.

REFERENCES:  [1] Bell J.F.(1991) Met., 26, 4, 316. [2] Bénit J. and Bibring
J-P. (1990) LPS. XXI, 65-66. [3] Bibring J-P. et al. (1985) LPS.XVI, 55-57.
[4] Maag C. et al. (1993) Proc. ESA Conference on Orbital Debris 1st
(Darmstadt, 1993). [5] Borg J. et al. (1993) Meteoritics, in press. [6]
Borg J. et al. (1993) LDEF - 69 months in space, NASA Publication 3194,
347-356. [7] Langevin Y. et al. (1987) Astron. Astrophys., 187, 761-766.
[8] Rietmeijer F. J. M. (1992). Asteroids, Comets, Meteors 1991, 513-516.
[9] Maag C.R. and Linder W.K. (1992) in Hypervelocity Impacts in Space
(McDonnell, ed.) 187-195. [10] Borg J. et al. (1993). Proc. ESA Conference
on Orbital Debris 1st (Darmstadt, 1993). [11] Tanner W.G. et al. (1992) in
Hypervelocity Impacts in Space (McDonnell, ed.) 239-243.


Albee A.   Brownlee D. E.   Burnett D. S.   Tsou P.   Uesugi K. T.
   Comet Coma Sample Return Instrument

1. INTRODUCTION

Comets have preserved volatiles and refractory materials that were in the
outer regions of the solar nebula. The study of comets and, more especially,
of material from them provides an understanding of the physical, chemical, and
mineralogical processes operative in the formation and earliest development of
the solar system. Bringing samples back from within the zone of parent
molecules of a known comet will provide valuable information on comets, will
serve as a rosetta stone for the analytical studies of interplanetary dust
particles for the last two decades, and will provide much needed samples for
the analysis community. The intact capture and return of cometary coma
material, both dust and volatiles, has an advantage over orbiter and
rendezvous missions in that the captured material can be made available to all
complex, sophisticated laboratories here on Earth. Samples, if properly stored
and preserved, can also be examined by analytical techniques, presently
unknown but almost surely to be developed in the future.

2.  SAMPLE RETURN SCIENCE

The scientific goals of this Flyby Sample Return are to return coma dust and
volatile samples from a known comet source, permitting: accurate elemental and
isotopic measurements for thousands of individual solid particles and
volatiles; detailed analysis of the dust structure, morphology, and mineralogy
of the intact samples; and identification of the biogenic elements or
compounds in the solid and volatile samples. Having these intact samples,
morphologic, petrographic, and phase structural features can be determined.
Information on dust particle size, shape, and density can be ascertained by
analyzing penetration holes and tracks in the capture medium. Time and spatial
data of dust capture will provide understanding of the flux dynamics of the
coma and the jets. Additional information will include the identification of
cosmic ray tracks in the cometary grains, which can provide a particle's
process history and perhaps even the age of the comet.

The measurements will be made with the same equipment used for studying
micrometeorites for decades past; hence, the results can be directly compared
without extrapolation or modification. The data will provide a powerful and
direct technique for comparing the cometary samples with all known types of
meteorites and interplanetary dust. This sample collection system will provide
the first sample return from a specifically identified primitive body and will
allow, for the first time, a direct method of matching meteoritic materials
captured on Earth with known parent bodies.

It is important to target SOCCER to within 100 km of the comet nucleus in
order to ensure that sample collection takes place within the zone of parent
molecules. While water molecules will resist photodissociation out to great
distances, more fragile molecules like CS will be dissociated at distances of
the order of a few hundred kilometers from the comet nucleus, for an encounter
distance of 1 AU from the Sun.

3. SAMPLE COLLECTION TECHNOLOGY

Cometary coma sample collection makes use of distinct and different techniques
for solid particles and volatiles. The fundamental technology for capturing
hypervelocity solid dust particles intact has been under development at JPL
since 1984. Today, submicrometer particles can be captured intact at speeds up
to 10 km/s. Experiments, conducted in 1987, have also demonstrated the
efficacy of physisorption of noble gases on passive foil under cometary
encounter conditions.

3.1 Dust Collection Technology

The development of cometary flyby sample return technology was triggered by
the Halley Sample Return Mission (HSR) [1]. Due to the 70 km/s encounter speed
with Comet Halley, only an atomized sample return was considered for HSR. The
need for and the value of capturing dust intact was made apparent by
subsequent validation of atomized sample collection [2]. The capture of solid
dust particles is rooted in intact capture technology--the ability to capture
solid particles, intact, at hypervelocities. At first, hypervelocity "intact"
capture was thought to be impossible. Since the demonstration of intact
capture [3], a wide range of capture media has been systematically explored
[4], finding that passive underdense media in the form of certain foams
perform the best [5].

Microcellular underdense polymer foams can capture solid 1.6-mm Al projectiles
with 95% of the original mass recovered intact and unmelted at 6 km/s [4].
Glass spheres 5-50 micrometers in diameter have been similarly recovered
intact in the 6 km/s speed range [6]. Iron grains have been captured intact in
the submicrometer range to beyond 10 km/s [7]. Transparent and
ultramicrocellular aerogel was introduced for optical visibility of
micrometer-sized captured particles [6]. The effect of the capture medium's
mesostructure on intact capture is being investigated and analyzed [8,9].
Clearly, the technology of intact capture of hypervelocity projectiles by
passive underdense media has been proven.

The capability to fabricate clear aerogel in 10 cm x 10 cm x 1 cm cell size
has been developed at JPL. Since no number of laboratory simulations can
substitute for a realistic space environment, concerted efforts have been made
to gain space test opportunities. An intact capture experiment with aerogel
capture cells has been flown on STS-47 & STS-57 in September 1992 and June
1993 respectively, with 20-30 mg/ml density aerogel cells made at JPL.

3.2 Volatiles Collection Technology

Two approaches have been conceptualized for volatile capture: physisorption
and chemisorption. Initial experiments of physisorption of noble gas under a
simulated cometary flyby encounter environment proved surprisingly successful
[10]. A simple device for capturing volatile species consists of a getter
freshly deposited by electroevaporation just before the coma flythrough, with
a second, sealant surface, being deposited over the collected volatiles at the
end of the collection period. A similar technique is used in the energetic
particle composition and interstellar gas experiment (GAS) on board Ulysses.
The surface area for each type of volatile will be about 100 cm, allowing for
some damage by dust impact.

Other techniques commonly used to collect volatiles in the Earth's environment
can be adapted for cometary sample collection. For example, there are
adsorbents already known to serve the physisorption of organic compounds.
Examples of such adsorbents are silica gel, alumina, and activated carbon. On
the other hand, chemisorption makes use of absorbent compounds to react with
selected volatiles. An example is sodium hydroxide used to detect carbon
monoxide and carbon dioxide.

4.0 INSTRUMENT CONCEPT

The comet coma sample return instrument serves a dual function, science and
engineering, in that the sample instrument will serve as a dust shield for the
entire spacecraft system during encounter.

4.1 Sample Collection System

The current collection concept for SOCCER [11] has evolved through a sequence
of previous studies: HSR [12], Planetary Explorer, Comet Intercept Sample
Return [13], ESA/NASA Giotto II [14], Comet Dust Intact Capture Explorer [15];
space station attached payloads experiments: Intact Particle Capture
Experiment [15], Cosmic Dust Intact Capture Experiment [16], Interstellar Dust
Intact Capture Experiment [17], Exobiology Intact Capture Experiment [18],
Second Generation Space Station Cosmic Dust Collection Concept Development,
and finally, the Shuttle Get-Away Special Sample Return Experiment [19]. The
sample collection system consists of designated regions for dust collectors
and volatiles collectors. An artist's rendition of the sample collection
system, not to scale, during encounter is shown in Fig. 1. Many of the
engineering details have yet to be defined but the sketch provides an overall
perspective of the collection system. All collector regions will be modular in
design. Dust collectors must allow for depth of penetration and so will be
cubic in shape, whereas volatile collectors and fine dust impactors will be
flat surfaces.

4.1.1 Sample Collection System Goals

The greatest number of dust particles will be very small grains, but the large
particles will contribute the greatest mass. Thus, for sample collectors, the
large particles will be of the greatest scientific value. The goals of the
sample collection system are stated quantitatively as follows: (1) To collect
intact at least 10 large dust particles (0.5-1 mm in diameter); (2) To collect
intact at least 100 medium dust particles (10-100 micrometers in diameter);
(3) To collect intact at least 10,000 small dust particles (0.1-5 micrometers
in diameter); (4) To collect at least 10 fluence molecules/cm of volatiles.

These goals are to be achieved under a set of desired constraints: (1) To
minimize contamination among collected samples; (2) To permit temporal and
spatial identification of the samples collected; (3) To maintain an
environment for preserving the condition of the samples at the time of
capture; (4) To allow significant depth in all intact collection media so as
not to preclude large dust particle capture; (5) To protect against impact
damage to the spacecraft and collection system mechanisms.
4.1.2 Dust Sample Collection

The size and location of the dust collectors are allocated according to the
expected flux of samples, the collection goals stated above, and the validated
capture efficiency of the collection media. A portion of the surface area is
reserved for novel collector concepts, as yet not fully validated. Silica
aerogel at 20-30 mg/ml density is the primary candidate underdense medium for
dust collection. Modular collection cells of 10 cm x 10 cm x 10 cm cube
facilitate fabrication, handling, and replacements. The dust collection cubes
are placed on top of a gridded polyvinylidenefluoride film to provide a
nondestructive acoustic capture signal recording the time and location of the
capture. Information of the temporal and spatial distribution of captured
particles will enable the reconstruction of jetting dynamics relative to dust
flux profiles. In order to maintain cleanliness, the collectors are kept
covered and sealed, before and after the encounter, with purge gas to maintain
a fixed partial pressure within the collector compartment.

The flux of large particles is expected to be low; however, since the science
value of large particles is immensely greater, the depth of the dust
collection cells will be dictated by the desire to capture large particles.
The very small particles will have very high flux; consequently, ultrafine-
mesostructure capture media dedicated to fine particles need only have a small
surface area. Although sample fragmentation is expected for solid soft metal
collection surfaces, for very fine dust, less than a micrometer, embedded
fragments will be concentrated and offer easier analysis.

For a very close encounter with a comet nucleus of about 10 km, there is a
possibility of capturing trapped nonvolatilized organics or interstitial
frozen volatiles embedded within conglomerated particles; thus, some portion
of the collection region will be refrigerated to a temperature of 140 K. In
this cool environment, volatiles generated during capture or captured
volatiles can be best preserved. This temperature can be maintained by placing
these collectors in double-walled compartments. The outer compartments are
maintained at 200 K and 300 K, respectively, by passive radiators. Due to the
excellent insulation properties of underdense media, active refrigeration by
means of a cooler is necessary only after Earth insertion, during aerobraking,
and Earth descent; fortunately power will be plentiful at 1 AU.

4.1.3 Volatiles Sample Collection

The area allocated for physisorption of volatiles will be determined by the
capture efficiencies of the capture media and the expected gas flux to be
captured. Due to the need to maintain ultrapure getters for physisorption,
each capture surface will be freshly deposited, just before encounter, by
electrovaporization of an ultrapure filament on ultrapure semiconductor grade
Si or Ge (see Fig 2). After the encounter, a sealer will be vapor deposited to
prevent escape of trapped volatiles and to prevent external contamination.
Since the flux of volatiles will be quite high, the total surface area
allocated to the collection of volatiles will be small (hundreds of
centimeters). For active volatile species, chemisorption will be used as the
capture mode. These active capture cells will be uncovered during encounter
and sealed afterwards to prevent further reactions or contaminations.
Capturing molecular volatiles may not be possible due to the capture energy
being greater than the molecular binding energy, but this is under study. The
collection surface requires a field of view along the ram direction during
collection. A second field of view for a radiator for cooling the getter
surface will also be required.

5.0 SUMMARY

As was stated in the Solar System Exploration Committee's report of 1983, the
next step in cometary exploration following the flyby missions to Comet Halley
should be a rendezvous and sample return with a short-period comet. Without
Mariner Mark II Comet Rendezvous and Asteroid Flyby Mission, it is all the
more critical to the achievement of the goals of the Solar System Exploration
Committee that a comet coma sample return be accomplished. Through a joint
effort with ISAS, it is cost effective for both NASA and ISAS to acquire
thousands of samples from a known comet and thousands of images of a comet
nucleus and dynamics of that comet coma at half the cost for each agency. This
instrument will make available for analysis not only the only extraterrestrial
samples from a known source since the Apollo missions, but specifically
cometary material captured in the zone of parent molecules. Such material
rates highest among all solar system objects for study of the planetary
evolution and that of compounds and phases containing the biogenic elements.

References: [1] JPL (1991) JPL 715-140. [2] Tsou P. et. al. (1983)
International Conference on Cometary Exploration, Vol. II, pp. 215-224. [3]
Tsou P. et al. (1984) LPS XV, 866-867. [4] Tsou P. et al.(1989) LPS XX, 1132-
1133. [5] Tsou P. et al. (1987) LPS XVIII, 1026-1027. [6] Tsou P. et al.
(1988) LPS XIX, 1205-1206. [7] Tsou P. et al. (1992) LPS XXIII. [8] Tsou P. et
al. (1991) LPSC XXII. [9] Griffiths D. J. et. al.(1991) J. Appl. Phys., 70,
4790-4796. [10] Tsou P. et al. (1987) LPS XVIII, 1024-1025. [11] Tsou P.
(1990) JPL D-7777. [12] Tsou P. et al. (1983) LPS XIV, 794-795. [13] Tsou P.
et al. (1983) JPL D-1153. [14] Tsou P. et al. (1985) AIAA-85-0465. [15]
Brownlee D. E. (1988) Proposal to The Small-Class Explorer Program. [16]
Brownlee D. E. (1969) Proposal to Space Station Attached Payload Program. [17]
Brownlee D. E. (1989) Proposal to Space Station Attached Payload Program. [18]
Carle G. (1989) Proposal to Space Station Attached Payload Program. [19] Tsou
P. et al. (1993) LPS XXIV.

Figures 1 and 2 appear in the hard copy.


Burchell M.   McDonnell J. A. M.   Cole M. J.   Ratcliff P. R.
   The Hypervelocity Impact Facilities at the University of Kent at Canterbury
   (UK)

The University of Kent at Canterbury (UK) facilities for production of
hypervelocity impacts are described. Micrometeroids are simulated by
electrostatic acceleration of small (10^-12 to 10^-17 kg) particles using a 2
MV Van de Graaff accelerator. This machine has been operational for many
years; both the machine and experimental area are currently being upgraded.
Larger particles (10^-10 to 10^-4 kg) are accelerated using a more recently
installed light gas gun. The status of all hardware (including experimental
areas) is given, along with brief details of recent, current and future
projects making use of them.

Introduction: The Unit for Space Sciences at the Univ. of Kent (UK) has an
extended history of studies of micrometeroids. The design, manufacture, and
calibration of micrometeroid detectors has been one of the group's major
activities. An important tool in this process has been the laboratory
simulation of hypervelocity impacts on selected target materials.
Traditionally the Unit has achieved this using electrostatic acceleration of
(charged) microparticles in a 2 MV Van de Graaff accelerator (operational at
Kent since 1974). More recently a light gas gun has been installed,
explosively accelerating larger particles. This extends the projectile mass
regime from micro (order 10^-15 kg) to macro (10^-4 kg) particle size. Details
of these devices are given below, along with descriptions of their current
use.

2. Van de Graaff Accelerator (2MV): This has been operational at Kent since
1974. It is one of only two such installations that have been continuously
available for microparticle acceleration over the years. The other is also a 2
MV machine, at the Max Planck Institut fur Kernphysik, Heidelberg, Germany
[1]. (Note also that a 6 MV machine was used at Los Alamos for similar work at
the end of the 1980's [2]).

The machine has been described in detail previously [3], so only brief details
or new information are given here. A dust reservoir is at the high voltage end
of the machine (top terminal). This is kept at 15 kV above the 2 MV of the
rest of the top terminal, and is pulsed at 1 Hz to just 1 KV above 2 MV.
Charged dust is then directed onto a needle tip positioned in front of a small
hole in the base plate of the top terminal. The most efficiently charged
particles arrive at the needle tip and are exposed to the potential difference
between the top terminal and the earthed far end of the machine. They thus
accelerate through the hole in the top terminal base plate and along a flight
tube that runs the length of the machine. The 2 MV potential difference is
linear along the length of the flight tube, and is maintained by 52 equally
spaced ring electrodes. Between the top terminal and the first of these is a
cylindrical tube maintained at a voltage set by the machine operator. The
field at the entrance and exit of this cylinder serves as a focusing element
for the beam. The main axis of the accelerator is horizontal, and the flight
tube is approximately 2 metres in length. While the flight tube is maintained
under a vacuum of some 10^-6 mbar, the rest of the accelerator is filled with
a dry gas (20% CO2 and 80% N, mixed with a trace of SF(sub)6) at 100 to 150
psi. This is to prevent sparking and breakdown of the potential.

Although the potential is 2 MV, due to the mass of the microparticles the
acceleration is totally non-relativistic. Thus we can write that the kinetic
energy is simply the energy gained by acceleration:
0.5 mv^2 = q V
where m is particle mass, v its final velocity, q its charge, and V the
accelerating potential. It can thus be seen that v is a function of q/m. For
fully efficient charging, the charge is a surface effect, and thus (for
perfectly spherical particles of a given type) q/m is fixed at a given mass.
So (with a small spread) there is only one velocity for a given mass particle,
and the highest velocities are associated with the smallest masses. This is
shown in figure 1, where data for iron particles accelerated in the machine
are given. Note that for a normal dust sample (one that has not been finely
graded for particle size) the bulk of the population is at high mass, and
hence low velocity. Figure 1 shows that the typical mass range for iron
particles is 10^-13 to 10^-16 kg, with corresponding velocities 1 to 25 km/s.
Iron is typically used in the accelerator, but other conducting materials can
be used as required. Indeed, nonconducting materials can also be used,
provided they can be coated with a conducting surface.

The accelerated particles (the flux above 1 km/s is typically 5 or so per
minute) leave the Van de Graaff as a beam with diameter of up to 2 cm. Some
dispersion of the beam is present, and typically represents a maximum opening
angle to the beam axis of tan(theta) = 1/250. In part this can be controlled
by the focusing element mentioned above.

In order to improve the understanding of the machine and ensure better
operator control, a new top terminal monitoring system is currently being
installed (Summer 1993). This consists of a programmable circuit board mounted
on the top terminal. Via ADC's it can sample up to 8 inputs at a frequency of
15 kHz. Since the top terminal cannot be directly linked to earth via an
electric cable, this monitor is read out via a fibre optic link connected to
the base (Earth) of the machine. Circuits to convert the electrical signals to
light pulses (and vice versa) are included at both ends of the cable. This
permits a two way data flow. Voltages of components inside the top terminal
are to be monitored, as is the voltage of the focus element and the
temperature. Control of the system is via a microcomputer used by the machine
operator.

The user area consists of three main elements linked by 6 cm diameter
stainless steel tubes. Any or all of these main components may be present at
one time. The first component is a stainless steel 'pot' (a cylinder of height
50 cm and diameter 30 cm). This is equipped with high vacuum flanges with
electrical feedthroughs and view ports. It is positioned in the beam line with
the cylinder's main axis vertical. It contains a stage, which can be moved
vertically over a distance of 10 cm by an external crank. This is in the
process of being changed, so that the new stage will be motorized, being
capable of 10 cm movement vertically and 10 cm horizontally along an axis
perpendicular to the beam. A motorized rotatable small stage will be mountable
on the main stage if required. This system will be driven by the operator from
a touch panel, and the position will be continually monitored, with the
information available to the user. This should be installed and operational by
end September 1993.

The 'pot' is used as close to the accelerator as possible, and is connected to
it via a T shaped beam pipe. The extra outlet from the T junction leads to an
oil diffusion pump, which provides a vacuum of 10^-6 mbar in both the
accelerator flight tube and the 'pot'.

The second main component is a velocity selection unit. This has four parts.
The first is inserted in the beam pipe. It consists of three tubes positioned
sequentially along the beam axis. They serve as pickups when a charged
particle passes through them. The induced charge is amplified by an Ortec
charge sensitive amplifier (type 142A). The first and third tubes are 10 cm
apart, and give signals, the leading edges of which serve to provide the
timing information necessary for determining the velocity. The central tube is
used to provide an accurate charge measurement (necessary when combined with
the velocity to obtain the particle mass). The second element of this system
is the hardwired electronics, which takes the signals, looks for a leading
edge above a threshold on the first and third signals and finds their time
separation in units of 20 ns. This is then compared to a preset velocity
window and a yes/no decision and the particle velocity and charge are then
available on outputs. The third element is simply a 50 cm long tube, which
serves to introduce a particle flight time sufficient to permit the
electronics to reach a decision. The final element is a coffin shaped box, 165
cm long, 13 cm high and a maximum of 56 cm wide. An oil diffusion pump is
mounted on the coffin to provide an interior vacuum of 10^-6 mbar. The coffin
contains two pairs of kicker plates that can be charged and discharged to
deflect the beam particle. This system operates in two modes. The first is to
keep the plates charged and to discharge them if the particle's velocity is
acceptable (i.e, the chosen particles suffer no deflection and travel straight
through the system). The second method is to charge the plates only when a
particle is selected and to deflect it a fixed amount into a new, off axis,
beam line.

The final user component normally present is a large stainless steel 'churn'.
This is a cylinder 2 m long, with a diameter of 1 m. It is positioned so that
the main (2 m) axis is horizontal and perpendicular to the beam direction. It
is equipped with view ports and high vacuum flanges with electrical
feedthroughs. The vacuum is suppled by an oil diffusion pump mounted directly
on the churn, giving a vacuum of 10^-6 mbar. It is planned to replace this
pump with a new cryopump. This will provide a vacuum of at least 10^-7 mbar,
and which, locally, should be free of any contaminating oil vapor. To reduce
oil contamination from the rest of the system, a turbomolecular pump will be
mounted on the beam line just before the churn to provide an isolating vacuum
region of at least 10^-8 mbar. The specification for the cryopump has an over-
capacity on the gas compressor, so that extra cold heads can eventually be
installed to replace the other oil diffusion pumps in the experimental area.

Other improvements to the user area are also under way. A new more compact
velocity selection system is being designed and built. The coffin will be
replaced with a tube of length 50 cm, and diameter of 10 cm (slightly larger
than the normal beam tubes). This will contain one pair of kicker plates of 35
cm length. This is under construction. The three element charge pickup will be
replaced by a new two element device. The central element of the previous
device is removed, and the last element used to provide not only a timing
signal but also the measure of the charge. Again this is under construction. A
new electronic decision making circuit is being designed and should make the
system more flexible.

Beam position monitors are also being installed. These consist of parallel
plates followed by a conducting tube, all inserted into the beam line. In each
pair of plates, one is earthed and the other read out via an Ortec charge
sensitive amplifier. The passage of a charged particle between the parallel
plates produces an output signal whose magnitude is dependent on the
particle's charge (measured by the tube) and its relative distance between the
plates. The calibration of this is achieved by collimating the beam so that it
passes along a known path between the plates, the resulting signals then being
studied. This is checked by a calculation of the expected induced charge on
the plates. One device, consisting of two pairs of parallel plates (the second
pair rotated by 90 degrees around the beam relative to the first) has already
been installed and calibrated. It provides position information on the
location of a particle in two axes perpendicular to the beam direction good to
1 mm. Refinements in the calibration method are hoped to improve this. Two
more such devices are being constructed.

One feature of these beam monitors is that the signals are read out not just
on an oscillosope, but also into an IBM 486 compatible personal computer. The
data acquisition system used is a software package from National Instruments
called LABVIEW [4]. This is interfaced either directly to the data (via a
standard DAQ board) or to the oscillosope (via an IEEE 488.2 standard GPIB
board). The user writes his own application program using the LABVIEW software
and can not only acquire the data (and store on 3.5 inch disk) but also
display and analyze online. Thus in a time of order 1 sec after acquisition
the x,y coordinates are available (as is a measure of the velocity). The
LABVIEW language is not a text based system, rather it is symbolic, using
icons and circuit drawing tools to permit operations upon the data similar to
that which occurs in a hardwired electronic circuit.

3. Experimental Programs Using the Van de Graaff Accelerator: The Van de
Graaff has been used for many years to support the activities of the Unit for
Space Sciences at Kent. During the 1980's, data from the machine was used in
the calibration or interpretation of data from the Space Shuttle STS-3
microabrasion foil experiment, the Long Duration Exposure Facility (LDEF)
microabrasion foil experiment, the Giotto Dust Impact Detector System and
Particulate Impact Analyser and for prototype work for several other projects.
The recently retrieved ESA satellite EuReCa, carried an experiment (TICCE)
built at Kent, the analysis of data from which will owe much to studies made
using the accelerator. Indeed, the studies of thin foil penetration and crater
size and depth in semi-infinite (thick) targets, which has been carried out at
Kent has been widely published and used (e.g. [5]).

Similar programs of work are continuing. As is work studying the impact
processes themselves, and comparing the results with the predictions of
calculations. In particular, plasma production, the role of ejecta, the
influence of oblique angle impacts and the energy partitioning occuring during
impacts are all being studied.

Current detector work includes the design and testing of the Cosmic Dust
Analyser [6] for the Cassini/Huygens mission to Saturn and its moons,
scheduled for launch in October 1997. This is an elaboration of the dust
detectors on the Ulysses and Galileo missions now operational [7]. The
detector is an ionization sensing device. A schematic is shown in figure 2.
The dust enters through the grids at the top of the device. Any charge is
detected as a pulse in the electronics connected to the grids. The dust
particle then impacts an ionization target (a curved surface made of gold) or
a chemical analyzer target (a curved surface made of rhodium). The ions
liberated in the impact are attracted to an ion detector (a discrete dynode
electron multiplier) mounted at the focus of the curved surfaces. Over the 3
mm just above the chemical analyzer target surface an intense field of 1 kV is
applied. There is a further potential difference of 350 V between the entire
target surface and the ion detector. For the chemical analyzer this
combination is sufficient to make any initial ion momentum negligible, thus
permitting a chemical decompostion of the ions by the time of flight method.
After subtraction of the target contribution, the remaining chemical species
present are a measure of the dust particle's composition. Due to the high
field and small drift length, a fast digitization of the signal from the ion
detector is required. A 100 MHz system is under separate development at the
Rutherford Appleton Laboratory (UK). The incident particle velocity is found
by one of several methods (listed in decreasing accuracy). If there is
sufficient charge the shape of the pulse on the entrance grids yields the
velocity. Next, if the particle impacts the ionization target (figure 2) the
rise time of the pulses on the target and ion collector give the velocity. If
the particle hits the chemical analyzer, the rise time of the signal is too
fast to give the velocity, however there is a broadening of the signals in the
mass spectrum that can give velocity to a factor of two. The particle mass is
found from the total ion yield, since this has been shown to be proportional
to mass for a given velocity [6].

That the detector design was feasible was demonstrated with a prototype used
in the Van de Graaff accelerator in 1989-1990. A typical mass spectrum is
shown in figure 3. This was for an iron particle (25 km/s) impacting on an
aluminium target doped with silver. Since the velocity and mass are
independently measured for accelerated particles, the calibration of the
detector for measuring these quantities can also be obtained. Testing and
calibration of the project laboratory model of the detector is scheduled to
take place during late 1993.

A new project, which will shortly start to make use of the accelerator, is a
proposal to construct a dust flux analyzer for the Pakistani satellite Badr-B.
This is scheduled for launch in December 1994. It is proposed to include on
the satellite a plate for detecting dust impact. This will be multiply
instrumented, including PVDF coatings, which when penetrated are locally
depolarized, producing a signal proportional to the incident kinetic energy.
Piezoelectric Zirconium Titanate sensors and Lead Zirconite transducers will
also be present, allowing both a confirmation of the impact and, by arranging
several sensors on the plate, the relative timing of the signals will permit a
position determination for the impact. This concept will be tested in the
accelerator, making particular use of the new motor driven stage and beam
position monitors. Although, the piezoceramic sensors require a relatively
large mass impact to be fully sensitive, sufficient masses can be accelerated
to make calibration and testing practical.

A separate program of work is also planned to observe the degree of
degradation of optical surfaces after exposure to the micrometeroid population
in Earth orbit. This will involve use of both the accelerator and the light
gas gun below.

4. Light Gas Gun: The Light Gas Gun was installed at Kent in 1989, and has
since undergone development. It is approximately 6 m in length. At one end is
a gun, firing a 32 g cartridge (diameter 2 cm). The volume of powder is
typically 10 to 13 g. The expanding combustion gases causes a piston to
advance down a shaft, compressing a chamber of hydrogen gas (initially at 40
bar). At the far end of this chamber is a pressure sensitive disc, which
ruptures when a sufficient pressure has been reached. This then permits the
acceleration of a nylon sabot (a cylinder 4 mm long, 4 mm diameter, and mass
of 80 milligrams). This can serve as the projectile and reaches velocites of
up to 5.45 km/s. Alternatively it can be pre-cut and loaded with a chosen
projectile. In this case, due to rifling in the barrel down, which it
initially travels, plus the cuts, the sabot is discarded and flies away from
the projectile leaving it free to fly to the target by itself.

During the sabot's (or projectile's) flight its velocity is measured by the
time interval between its passage through two light curtains. These are from
white light sources mounted perpendicular to the direction of flight, and are
50.5 cm apart. The flight through a curtain is detected from light scattered
into a photomultiplier tube positioned perpendicular to the flight direction
and at 90 degrees to the light source.

The target is mounted in a target chamber at the far end of the system. A
target diameter of 10 cm can be accommodated. This chamber (as is all the
system after the rupture disk) is pumped by a rotary pump to a minimum vacuum
of 10^-3 mbar.

The gun may be fired approximately once per day.

5. Light Gas Gun Experimental Program: The gun is being used for a variety of
programs. Chief among these are studies of crater shape (in particular
ellipticity and depth variations across the crater) for oblique impacts. The
distribution (in both direction and mass) of ejecta is also being studied.
Both these projects use the 80 milligram nylon sabot as the projectile. Recent
firing of the gun for this work has consistently attained velocities of around
5 km/s, with the highest reliably recorded velocity being 5.45 km/s.

A separate project is underway where the sabot is loaded with small glass
beads. The sabot has been successfully discarded in flight and a sample target
bombarded with a cloud of the beads. These beads are typically around 50 to
200 microns in diameter (and thus mass ranges from 10^-9 to a few 10^-7 kg).
It is clear that masses greater than those obtainable in the Van de Graaff
machine are being accelerated. Fine grading of the beads by size will be
carried out during the full program of work to produce impacts of just a
limited mass range on each target. The targets to be used correspond to
materials flown on previous space flights, where interpreting the data has
proved difficult due to suspect or questionable calibrations. Thus it is hoped
to recalibrate the results of micrometeroid fluxes from past experiments, to
check the results and to increase the size of usable data sets in such
studies.

6. Conclusion: The hypervelocity impact (simulation) facilities of the Unit
for Space Sciences at the Univ. of Kent (UK) have been described. Their main
features have been given, along with indications of their typical performance
and the use to which they are put. Although the internal program of work is a
vigouros one, care is taken to ensure that, where possible, time is made
available to external collaborators or to groups who wish to visit. This is
essential given the limited number of such facilities.

References: [1] Fechtig H. et al. (1972) Naturwiss, 59, 151. [2] Keaton P. W.
et al. (1990) Int. J. Impact Engng, 10, 295-308. [3] Green S. F. et al. (1988)
Journ. British Interplanetary Soc., 41, 393. [4] LABVIEW, a product of
National Instruments. Contact the local sales office for details. [5]
McDonnell J. A. M. and Sullivan K. (1992) Proc. of Hypervelocity Impacts in
Space, (J. A. M. McDonnell, ed.), Univ. of Kent, pp. 39-47. [6] Ratcliff P. R.
et al. (1992) J. Brit. Interplanetary. Soc., 45, 375. [7] Goller J. R. and
Grun E. (1989) Planet. Space Sci., 37, 1197-1206.

Fig. 1, which appears below in the hard copy, shows measured mass vs. velocity
for iron particles accelerated in the 2 MV Van de Graaff Facility.

Fig. 2, which appears below in the hard copy, shows a schematic of the Cassini
Cosmic Dust Analyzer and representation of signals obtained [6].

Fig. 3, which appears below in the hard copy, shows a mass spectrum obtained
by the Cosmic Dust Analyzer for an iron particle impacting at 25 km/s a silver
doped aluminium target [6].


Tanner W. G.   Maag C. R.   Alexander W. M.   Stephenson S.
   Assessment of Velocity/Trajectory Measurement Technologies During a Particle
   Capture Event

1. OVERVIEW

The creation of the system composed of specific components described in
several presentations of this workshop (Alexander et al., 1993; Maag et al.,
1993; Heppner et al., 1993), has been conducted in a manner that will produce
an integrated package for flight on EuReCa 2 as COMRADE (Borg et al., 1993),
on SOCCER, and on LDEF II. The function of each component of the integrated
package has been described in general (Alexander et al., 1993) and in detail
(Maag et al., 1993; Heppner et al., 1993), but one other component to be
described in detail, i.e., ultra-thin-film time-of-flight determination, may
be mated with any capture package envisioned.

The authors have been involved for many years in the development and
application of thin-film sensors to detect a hypervelocity penetration by a
dust grain. The major electronic circuit design has been conducted over the
span of several decades, but the utilization of ultrathin metallic films as
charge generation media has only recently reached an acceptable level. Since
the penetration event of a hypervelocity dust grain will be dominated by the
material strength and thickness of the target surface, the most efficient
sensor would be that for which both parameters have been minimized. The
minimization of density and material strength is most appropriate for intact
capture; therefore, results of the variation of that parameter will be
discussed in other papers (cf. Maag et al., 1993).

The primary parameter of a thin metallic film that has been systematically
altered in this investigation was the film thickness while holding constant
film density, i.e., rho = 2.70 g/cm^3 (2024 Al). Recently the thickness of the
films has been made sufficiently small to assure perforation, and only partial
fragmentation of the dust grain while providing a significant charge
liberation media. Numerous experiments have shown the repeatability of
penetrations and perforations with minimal fragmentation of the projectile.
That the projectiles have both been simulated in the laboratory and through
computer simulations allows for the repeated variation of parameters of the
hypervelocity interaction.

Through this research an optimized thin-film sensor has been produced that
presents a sufficient amount of matter to detect the charge liberated during
perforation while providing a significant support structure for the
survivability of the sensor. Combinations of thin-film substances,
thicknesses, and support structures have been tested in the laboratory, on
many STS flights and during an 11-month orbital flight on board the EuReCa
spacecraft. The experience gained will be utilized to develop the
velocity/trajectory system for the SOCCER-like Comet Coma Sample Return (CCSR)
mission.

2. BACKGROUND

Since the early 1960s, the means to measure the time of flight (TOF) of a dust
grain within a mechanical detection array has existed, first, in the
laboratory and then in space experiments. Laboratory hypervelocity dust
particle accelerators have used electrostatic detection of charge on
accelerated particles for TOF and particle mass determinations. These
laboratory studies have led to the development of ultra-thin-film sensors that
have been used for TOF measurements in dust particle space experiments. The
prototypes for such devices were ultra-thin-film capacitors that were used in
the OGO series of satellites (Alexander et al., 1971).

The main goal of the experimental work to be described is the development of
the capability to determine the velocity vector or trajectory of a dust grain
traversing an integrated dust detection array. The results of these studies
have shown that the capability of detecting the charge liberated by
hypervelocity dust grains with diameters in the micrometer range can be
detected. Based on these results, detection systems have been designed to
provide a precise analysis of the physical and dynamic properties of
micrometer and submicrometer dust grains, viz., Fig. 1. design verification
unit (DVU). Through unique combinations of in situ detection systems, direct
measurements of particle surface charge, velocity, momentum, kinetic energy,
and trajectory have been achieved. From these measurements, the remaining
physical parameters of mass, size, and density can be determined.

The heritage for the measurements providing trajectory determination is based
on a long line of successful dust particle experiments. However, laboratory
research efforts have been accomplished during the past decade using: (1) data
provided from sensing the specific charge on a dust grain traversing an
experiment array designed to accurately determine the path of the grain within
the array, and (2) data provided from plasma generated by a dust grain passing
through very thin films within an experiment array designed to accurately
determine the path of the grain within the array. The first method is used
when the particle velocities within the array are low; i.e., between 10 and
100 m/s. The second method is used for particle velocities greater than 2
km/s. Recent laboratory studies have also been conducted to measure the charge
present during a hypervelocity impact event.

The presence of an intense light flash upon impact has suggested that
ionization of the impact site material as well as the impactor has occurred.
The impact flash phenomenon has been studied extensively by several groups
over the last three decades and has been attributed to the "jetting" of
material near the impact site, e.g., shaped charge explosion. Luminous matter
created near the impact site has also been seen to produce a spontaneous
magnetic field along the axis of the impactor. Even though a complete
understanding of the event still eludes researchers, the collection of charge
liberated during the event has been well documented. It is that collected
charge that can be utilized to establish time of flight and, consequently, the
velocity vector of the penetrating particle. The assessment of the efficiency
of a dust grain to penetrate a specific thickness of thin film has been
conducted by both theoretical and experimental means.

The principal theoretical approach has been to perform calculations using a
hydrodynamic computer code CTH (McGlaun et al., 1990). This tool provides a
computational laboratory where hypervelocity impacts may be controlled and
observed. The complete perforation of a thin film will create a significant
amount of data depicting the event. The most useful utilization of the modeled
event has been the determination of the hole size generated by the penetrating
dust grain. Provision must be made for the most crucial parameter controlling
the simulation, i.e., time to maximum expansion of the penetration hole. That
time must be commensurate with the expansion of the shock front and the
rarefaction wave. Pressure, density, and temperature in the film must also be
tracked to assess the progression of the energy in the material. Of equal
interest is the strain rate, which signals the relaxation of the material
following the passage of the shock front through the material. Once the strain
rate has reduced in value near the walls of an impact site, a commensurate
decrease is seen in the other extensive variables of the interaction, and thus
confirms that the progression of the crater's diameter has been halted.
Penetration mechanics suggests that the more the diameter of the impactor
exceeds the thin-film thickness, the less will be the erosion of the
projectile during passage through the film. Thus, Dp/Tf must be maximized in
order that the fragmentation of the dust grain may be minimized.

3. PENETRATION MECHANICS

Hydrodynamic computer programs have benefited greatly from the data provided
by many experiments designed to assess the effects of high-shock conditions
present in materials. Principally the "hydrocode" calculations possess
equations of state which describe both the elastic-plastic and the phase
transition with melt of the high-shock regime. For several years research has
been underway to assess the association between specific thermodynamic
properties of materials and the process of fragmentation, i.e., the
catastrophic failure of materials.

The primary description of the event has evolved from the study of the very
early stage creation of ejecta spray patterns liberated from the semi-infinite
target via hypervelocity impact and the subsequent shock wave disruption of
the target and projectile material. The formation of ejecta patterns seen in
experimental work was utilized to test the validity of a hydrodynamic computer
code designed to track the progress of a hypervelocity cratering event.
Hypervelocity impact experiments have suggested a set of preferred angles,
viz., 15 degrees, 45 degrees, 60 degrees, 80 degrees, at which the ejecta
spray size distribution is spread by some as-yet unexplained process. In
practice most hydrocodes will reproduce results commensurate with those
derived from normal and oblique semi-infinite target impacts.

Perhaps the most important results of these analyses will be tested, i.e.,
angles of incidence with surface controls size distribution of ejecta, by data
collected via EuReCa TiCCE while on orbit. Upon return last June 1993
materials exposed to 11 months of LEO have yielded data of oblique impacts
with a specific capture material located near impact sites to capture ejecta
particles for size and velocity distribution analysis.

Using CTH, many properties of hypervelocity particle thin-film capture
techniques have been theoretically analyzed. Hypervelocity perforation of thin
films will fragment glass spheres that have been used to simulate
interplanetary dust particles (IDPs). Upon impacting a thin film with
hypervelocity a small IDP analog will fragment if and only if the film
thickness and the IDP analogs velocity are sufficient. Hence the coupled
parameters of velocity and Dp/Tf ratio will determine the degree of
fragmentation the thin film will cause in the IDP analog. With a hydrocode
calculation one can investigate many different values for velocity and film
thickness. The fragmentation process arises from the activation of various
fragmentation sites within the nonperfect crystal. The sites that are crystal
defects can be stimulated into action by a shock waves passage through the
sites, which will cleave the crystal along a defect boundary. Defects have
been found to activate at a specific shock velocity and thus can be by size
sensitive to high or low velocity (2-10 km/s).

CTH has been used to investigate the penetration mechanics of small particles
impacting and perforating a thin film. The fragmentation of IDP analogs has
been investigated to determine penetration parameters of thin films. The same
analysis can be applied to the fragmentation of targets or projectiles or even
secondary impacts due to ejecta sprays. Molecular dynamics (MD) computer
programs can provide a means to establish the velocity distribution of the
small particles created during a fragmentation event. Utilization of MD can
provide a means to describe particle fragment motions. Fragmentation events
can therefore be fully characterized using MD since particles will move under
Newtonian kinematics and thus articles can be employed in the interaction to
develop macroscopic scaling rules.

3.1. PASSAGE OF PARTICLE THROUGH A THIN FILM

The pressure an impacting dust grain experiences during a hypervelocity impact
can be sufficient to alter the state of matter of the particle. However, very
short duration high-pressure pulses can be sustained in large dust grains
without fragmentation or complete phase change occurring. In this class of
events the cross-sectional area of the impinging dust grain and the thickness
of the target are important components of the interaction. The surface area
over which a force is administered and the length of time in which the impulse
is delivered define the magnitude and the duration of the pressure pulse that
gives rise to a sustained shock front in the material. The duration of the
shock front wave is also determined by the depth of penetration and therefore
the thickness of the target, Tf. If one considers the dynamics of an impact
event from the perspective of a penetrating particle the ratio that defines
the aspect ratio of the dust grain, i.e., L/Dp, may be investigated to
determine the residual length of the particle upon encounter with a thin
target. In the case of a thin-film penetration event, the ratio of interest is
that between the diameter of the dust grain, Dp, and the thickness of the
film, Tf. It has been well documented (Zukas, 1990) that a projectile with a
high aspect ratio will penetrate to a depth defined by the following
relationship:

p = L(rho(sub)p/rho(sub)T)^0.5

The penetration depth, p, of a rod into a thin film can be equated with the
film thickness, Tf, and the residual length L(sub)R of the penetrating rod can
be equated with the residual diameter of the dust grain. The change in the
diameter of the dust grain can be roughly estimated to be

L(sub)R/D(sub)p ~ 1 - Tf/Dp (rho(sub)p/rho(sub)T)^0.5

In the case of a ratio of Dp / Tf = 30 the residual diameter of the dust grain
would be greater than 90% by this estimation. Even though the not eroded
nature of the material composing the incident dust grain can only be assessed
by other measurement means, the foregoing analogy may serve as a metric for
further analysis (Fig. 2).

Of particular interest in these investigations is a specific empirical form
that relates penetration hole size with the diameter of the penetration hole.
This experimentally derived equation for the description of the penetration
relationship for iron projectiles impacting Al films of various thicknesses
was developed by Carey, McDonnell, and Dixon equation (CMD) (Carey et al.,
1985). The CMD empirical equation has been compared with the results of
computer simulation of hypervelocity impacts and has been plotted for various
velocities of interest for surfaces flown in LEO (Fig. 3.).

4. EXPERIMENTAL RESULTS

Results of both two-dimensional and three-dimensional computer simulations of
the hypervelocity impact events that penetrate the STS and the EuReCa 1 thin
films will be reported. A relationship between the particle diameter, Dp, and
the diameter, Dh, of the hole created in a 500-Angstrom Al thin film (Tf) and
micropore foam (Tm) for relevant particle and target parameters will be
derived and will be compared with empirical equations. That relationship will
be used to analyze in situ data of the thin film experiments flown in LEO, and
to determine the size distribution of grains that penetrate the thin films and
are captured intact in the micropore foam (Tanner et al., 1992).

Thin-film perforation events liberate a substantial quantity of charge that
can be collected near the impact site. The collection sites can be so
subdivided that a location in a plane can be established with high accuracy.
However, in order to provide the highest confidence for the survival of a dust
grain while penetrating a thin film, the thickness and the density of the
material composing the film should be minimized. The magnitude of charge
liberated by a thin-film perforation has been assumed to be a strong function
of the film thickness. The use of thinner films would thus imply a degradation
in the position accuracy, especially for the very smallest dust grains (dp <=
0.1 micrometers). The preliminary results of experiments conducted at the
University of Kent at Canterbury, Unit for Space Science (UKC-USS) indicate
that the charge yield by perforation of a 250-Angstrom Al film (Fig. 4) is
commensurate with a 500-Anstrom film (Fig. 5).

Further testing has provided data for particle velocities approaching 40 km/s.
Each of the thin-film perforation events shown above were verified by a charge
collection immediately in front of an underdense foam intact capture cell
situated 5 cm behind the thin film. The coincidence of all sensors recorded
the time of flight, electrostatic charge, thin film perforation, and impact
charge liberation.

Charge liberation events due to hypervelocity impacts that have penetrated an
ultrathin film (Al 250 Angstrom and 500 Angstrom) and are captured in an
underdense foam. Figure 6. illustrates the configuration for the experiment.
Figure 7, a representative trace downloaded from the LeCroy scope during the
experimental activity can be interpreted as follows: Each point (10,000 points
per division) represents 5.00 x 10^-9 s of time. Given that the separation
distance between the two electrostatic charge sensors is 5 cm, the velocity of
the dust grain is 3.3 km/s. The amplitude of each pulse in the electrostatic
region measures the inherent charge on the dust grain. The velocity of the
dust grain allows for the determination of the mass of an assumed sphere.
Efficient charge collection immediately in front of the thin film is the
result of a focusing potential of -45 V on the grid wires. The charge
collected reflects a jet initiation event and thus will be most pronounced on
the front side of the thin film. Charge collected behind the thin film is much
more diffuse and thus is broader in time. It should be apparent that the
impact into foam liberates charge in excess of the charge measured at the rear
of the film. A time of flight between the thin film perforation and the foam
impact can be analyzed to determine a change in the velocity of the
perforating dust grain. Charge liberated from the carbon-containing material
is evidenced by the excess of charge on the charge collection grid immediately
in front of the foam.

5. CONCLUSION

The proposed ultrathin-film experiment has been tested in the laboratory to
establish the quantity of charge liberation during an intact capture event.
The entire ultrathin-film system has proved to be mechanincally sound after
experiencing the launch loads of the shuttle. Also, after 11 months exposure
to the LEO space environment the HVI/BUSSL 500 Angstrom Al films exhibited no
penetrations due to mechnical effects. Theoretical calculations have
facilitated the determination of the penetration efficiency of the thin-film
materials selected. Subsequent laboratory studies as well as analysis of the
thin-film experiment flown on EuReCa have demonstrated the optimization
process has produced a highly successful combination of materials. These tests
constitute an initial calibration of the thin film charge detection system
that will provide data depicting the dynamics of grains that COMRADE, SOCCER,
or LDEF II will collect.

References:  [1] Alexander W. M. et al. (1971) Space Research XI, 279. [2]
Alexander W. M. et al. (1993) In the Proceedings of LPI Workshop on Particle
Capture, Recovery, and Velocity/Trajectory Measurement Technologies. [3] Borg
J.et al. (1993) 1st European Conference on Space Debris, in press. [4] Carey
W. C. et al. (1985) 85th Proceedings of IAU. [5] Heppner W. (1993) In the
Proceedings of LPI Workshop on Particle Capture, Recovery, and
Velocity/Trajectory Measurement Technologies. [6] Maag C. R. (1993) In the
Proceedings of LPI Workshop on Particle Capture, Recovery, and
Velocity/Trajectory Measurement Technologies. [7] McGlaun J. M. et al. (1989)
Intl. J. Impact Engineering, 10. [7] Tanner W. G (1992) Intl. J. Impact
Engineering, in press. [9] Zukas J. A., ed. (1990) High Velocity Impact
Dynamics, Wiley.