ENCOUNTER WITH EROS:

THE NEAR-EARTH ASTEROID RENDEZVOUS MISSION

-- by Scott L. Murchie, Andrew F. Cheng, and Andrew G. Santo

The Near-Earth Asteroid Rendezvous (NEAR) spacecraft, the first launch of the Discovery program in February 1996, will be the first to achieve orbit around an asteroid. The objective of NEAR will be the unusually large near-Earth asteroid 433 Eros, a member of the S group of asteroids that dominates the inner solar system. NEAR will make the first comprehensive scientific studies of a small body.

During the last decade scientists have gained a new appreciation of how important bombardment by so- called "small bodies"--comets and asteroids--has been in shaping the major planets. One need only look at the pockmarked, cratered faces of the Moon, Mars, and Mercury to recognize that impacts are a major geologic process. However, the impact of comet Shoemaker-Levy 9 with Jupiter in 1994, and recognition of the Chicxulub structure in the Yucatan peninsula as the impact that triggered a mass extinction including the dinosaurs 65 million years ago, have reemphasized that the impact process continues and will inevitably have major effects on Earth again.

SEARCHING FOR ASTEROID- METEORITE CONNECTIONS

The most tangible evidence for bombardment of Earth by small bodies is meteorites, fragments of asteroids or possibly comets that survive passage through the atmosphere to reach the surface. Their chemistry and mineralogy provide direct evidence for processes that occurred in the earliest history of the solar system.

Ordinary chondrites, which make up some 80% of the meteorites in museum collections, are primitive assemblages of iron-nickel metal and silicate minerals that have changed little since they formed from the solar nebula some 4.6 billion years ago. Basaltic and stony-iron meteorites, which make up about 10% of the collections, bespeak instead active geology in which melting and volcanism occurred in the first hundred million years of the lives of their source bodies, presumably some of the larger asteroids.

Despite what meteoriticists have learned about the early solar system, the asteroids themselves remain precious little more than points of light in the sky to telescopic observers. It is generally agreed that the more than 5000 known asteroids are mostly shattered remnants of a much smaller number of larger "parent" bodies. Spectroscopic surveys of the several hundred brightest asteroids show that their compositions differ, and that different asteroid compositions dominate different regions of the solar system. The dominant type in the outer solar system is C asteroids, low albedo bodies thought to be rich in carbon compounds. S asteroids, made of metal and silicate minerals like those of most meteorites, dominate the inner solar system.

However, spectroscopists have not been able to determine whether S asteroids are primitive mineral assemblages, little changed since they first formed like the ordinary chondrites, or material like basaltic and stony-iron meteorites that have undergone extensive melting and evolution. A composition of ordinary chondrite-like material would imply that parent bodies of S asteroids have had quiescent histories except for impacts, but a stony-iron compostion would imply early histories during which they were geologically active. Even less is known about asteroids' large-scale structure; for example, it is not clear whether the small asteroids are coherent chunks, loosely bound rub-ble, or some combination of both.

The Galileo flybys of the S type asteroids 951 Gaspra and 243 Ida (each asteroid is numbered in order of its discovery) opened our eyes to what an asteroid looks like up close: both are heavily cratered, with grooves or fractures on the surface and subtle but definite color variations that hint at different rock types. Ida even has a small moon, Dactyl. But neither flyby determined what rock type S asteroids are made of or where they come from. The composition, bulk properties, and provenance of S asteroids are key links in establishing the connection between meteorites and the history of asteroids, and in better quantifying the nature of the impact hazard that the asteroids pose to Earth.

NEAR trajectory for a launch on February 16, 1996.

EROS LARGEST OF THE NEAs

Most asteroids reside in a broad belt between Mars and Jupiter, but a substantial number lie in orbits that bring them close to Earth. These are the "near-Earth asteroids," prime candidates for the origins of meteorites. Dynamicists who study the evolution of asteroid orbits believe that the near-Earth asteroids or NEAs are mostly pieces cast out of the main asteroid belt by the gravity of Jupiter, with some fraction of extinct comets.

By far the largest and most important of the near-Earth asteroids is 433 Eros, which accounts for over half the volume of all near-Earth asteroids. Eros orbits the sun at an average distance of 1.46 astronomical units (AU), and approaches to 1.13 AU at perihelion. It rotates once each 5.37 hours. As with Uranus a high axial inclination results in the asteroid lying nearly on its side. Eros is also one of the most elongated asteroids, with estimated dimensions of 35 x 15 x 13 kilometers. It is an S type but is known to be compositionally varied, with opposite sides having slightly different mineralogies.

Eros is the prime objective of the NEAR mission, scheduled for launch on February 16, 1996, aboard a 7925 Delta II from Cape Canaveral, Florida. NEAR is being built by The Johns Hopkins University Applied Physics Laboratory and will be the first launch in NASA's Discovery Program. The spacecraft will swing by Earth for a gravity assist in January 1998, approach Eros in January 1999, and be injected into orbit to analyze Eros for nearly one year.

The most important scientific objectives of NEAR are (1) to characterize Eros' physical and geological properties; (2) to infer its elemental and mineralogical composition and variations; (3) to clarify the relationships between asteroids, comets, and meteorites; and (4) to further understanding of the formation and early evolution of the solar system.

THE SPACECRAFT AND ITS INSTRUMENTS

As a Discovery mission, NEAR faces the challenge of delivering first-rate science on a limited budget and tight schedule. The programmatic guidelines of the Discovery Program are that the development cost (to launch plus 30 days) can be no more than $150 million, the development time must be less than 36 months, and the mission must use a launch vehicle of capability no greater than the Delta II. The launch-plus-30-day budget for NEAR will be approximately $120 million, and development time will be 27 months.

These constraints are met by thoughtful design of the overall spacecraft, use of modular subsystems, and use of off-the-shelf, proven components. The instruments are supplied with data processors sharing a common design. The propulsion system, central data handling system, and several of the instruments represent heritage from previous civilian and military space missions. Major systems are redundant functionally, to avoid the cost and mass of duplicate hardware. For example, the prime attitude determination will be made using a star camera with on-board processing; the backup is the main imager with processing done onground.

The spacecraft design also takes advantage of the particular properties of Eros. The asteroid's elongated shape necessitates use of a near-equatorial orbit in most situations. This combined with the high axial inclination results in the lines of sight to the Sun and Eros typically lying near right angles. This situation is used to advantage by equipping the spacecraft with body-fixed, side-looking instruments that share a common aimpoint. Thus, while the solar panels are aimed at the sun, the instruments can be trained on Eros. Aiming at specific points on the asteroid will be accomplished by small rotations of the spacecraft. The guidance and control software is particularly robust for providing data coverage, and makes use of algorithms like those used in military satellites. Downloaded images from the Multispectral Imager (MSI) will be processed onground to derive a shape model for the asteroid that will be maintained onboard the spacecraft. The instruments can then be pointed at a fixed location in the asteroid interior, so that spacecraft orbital motion builds up mosaicked coverage in a "pushbroom" mode, or custom coverage can be provided by pointing at a sequence of locations on the surface. The spacecraft can even track a specific surface feature to build up stereo imagery.

The basic shape of the spacecraft is defined by fore and aft decks and side panels enclosing the bipropellant propulsion system and the electronics boxes. The high gain antenna dish is fixed to the fore deck, as are the four deployable gallium arsenide solar panels. Attitude is controlled by hydrazine thrusters and reaction wheels. Most of the science instrument complement is body-fixed to the aft deck.

View of the fore deck and side panels, shrouding the main thruster, high-gain antenna, and solar panels.

The science instrument complement includes a balance of heritage from previous missions and innovative designs that deliver the data necessary to accomplish the mission objectives. Quality of all the instruments is assured by a combination of careful design, thorough ground testing, and redundant testing of instrument characteristics in flight. MSI is adapted from a military remote sensing system and provides a 2.25 x 2.9 field-of-view, using a CCD with a frame size of 244 x 537 pixels. Brightnesses are encoded to 12 bits instead of the 8 used for Voyager and Galileo, providing 16 times the brightness resolution of imagers on those spacecraft. A filter wheel has 7 color filters covering the wavelength range 0.4-1.1 micrometer, and one clear filter for low-light imaging and optical navigation. The NearInfrared Spectrograph (NIS, also adapted from a military remote sensing instrument) has a 0.38 x 0.76 field-of-view and also reports 12-bit data. Sixty-four spectral channels covering the wavelength range 0.8-2.6 micrometer are measured by germanium and indium-gallium-arsenide detectors. A scan mirror slews the field-of-view over a 140 range; mirror scanning combined with spacecraft motion will be used to build up hyperspectral images.

MSI and NIS will operate synergistically to provide both imaging and determination of mineralogic composition by measuring the spectrum of reflected sunlight. At Eros, MSI will resolve features smaller than 10 meters in size, and NIS will measure spots as small as 300 meters. NIS will measure silicate spectral features which are diagnostic of the composition of iron-containing minerals. It features an on- board, solar-illuminated gold calibration target for determination of instrument responsivity on demand. MSI has 70 times the spatial resolution of NIS, and four of its filters are designed to extrapolate the spectral measurements from NIS down to small spatial scales. Instrument performance throughout the mission will be tracked by repeated imaging of bright astronomical objects.

The X-ray Spectrometer and Gamma-ray Spectrometer (XRS-GRS), in contrast to MSI and NIS, will measure and map elemental abundances. These data will remove ambiguity involved in measuring surface composition using reflected sunlight. XRS contains three gas proportional counters, whose field-of-view is restricted to 5 by a honeycomb of beryllium-copper foil, and solar X-ray monitors. Hard X-rays emitted by the sun stimulate different elements in Eros' surface to emit characteristic spectra of soft X-rays. By building up X-ray measurements of the surface throughout the time in low orbit, XRS will be able to map the abundance and abundance variations in magnesium, aluminum, silicon, calcium, iron, and possibly sulfur and titanium down to a 4 kilometer spatial scale. Variations in X-ray emissions from the surface induced by solar activity such as solar flares will be accounted for by the solar monitors. XRS also features an on-board calibration source. This design is updated from that used during the Apollo missions. GRS contains a sodium iodide detector, enveloped by an active bismuth germanate anti-coincidence shield to provide a 45 field-of-view, and is the first instrument of its design to be flown. GRS measures characteristic emission spectra of elements from radioactivity or excitation by cosmic rays. Gamma-ray measurements over the course of the mission will give abundance estimates of silicon, iron, potassium, thorium, and uranium.

The NEAR Laser Rangefinder (NLR), adapted from a similar instrument on Clementine, contains a neodymium-doped yttrium-aluminum-garnet laser and a detector that measures the delay time between firing of a laser pulse and its return reflection from the surface. The instrument typically will fire once per second. While NEAR orbits Eros, NLR will build up numerous range measurements at a spot size of 4-9 m and an accuracy of ±6 meters limited by ephemeris errors, providing detailed topography of the surface. This will complement measurements of gross asteroid shape from MSI by measuring Eros' night side, which MSI cannot, and will provide detailed topographic profiles of major morphologic features including craters and grooves. NLR is the first such spacecraft instrument to have an internal calibration.

The final two elements of NEAR's instrument complement are a magnetometer (MAG) and the radio science experiment (RS). MAG is a 3-axis fluxgate sensor mounted in a bracket off the high gain antenna. It will measure the strength of Eros' magnetic field to within 45 nanoTeslas, and may be capable of detecting variations in the intrinsic field depending on their magnitude and scale. RS will use Doppler tracking to determine acceleration of the spacecraft by Eros' gravity. Tracking, attitude data from the star camera and MSI, and knowledge of Eros' shape from MSI and NLR will allow highly accurate determination of Eros' density and large-scale density variations.

An experienced international science team supports the NEAR mission throughout the prelaunch development, mission operations, and data analysis phases. This team has representatives from universities, government laboratories, and small businesses. It is headed by Drs. Joseph Veverka of Cornell University (MSI-NIS), Jacob Trombka of NASA-GSFC (XRS-GRS), Maria Zuber of Johns Hopkins University (NLR), Mario Acuńa of NASA-GSFC (MAG), and Donald Yeomans of JPL (RS).

THE MISSION PROFILE

The 14-day launch window for NEAR opens on February 16, 1996. NEAR will follow a "2-minus- DVEGA" trajectory that returns to Earth's vicinity for a gravity assist on January 22, 1998. On June 27, 1997, the spacecraft will pass close to the main belt C-type asteroid 253 Mathilde. NEAR will then perform a deep space DV maneuver shortly after aphelion to direct it to the Earth swingby. The gravity assist at swingby will bend the trajectory into Eros' orbital plane. This sets up the optimal geometry needed for approach to the asteroid.

In January 1999 a series of rendezvous maneuvers will slow NEAR to a velocity relative to Eros of only 5 meters per second. The spacecraft will first fly by the asteroid and then be inserted into a high orbit. Full-color imaging will reveal asteroid shape, morphologic and color features down to 250 meters in size, and NIS will map the surface spectrally at optimal illumination conditions. Imaging and RS will provide a highly accurate mass determination. Then the orbit will be lowered stepwise to 35 kilometers in radius, during which MSI and NIS will observe the surface at progressively higher spatial resolutions. The lowest orbits will be the primary opportunity for NLR and XRS-GRS to conduct detailed mapping of the asteroid's topography and surface elemental abundances, for MAG to measure the magnetic field properties, and for RS to search for variations in interior density.

From the data returned by NEAR, we will have the first comprehensive picture of the physical geology, composition, and geophysics of an asteroid. High resolution imagery can be expected to yield detailed maps of craters, grooves, and other landforms. More detailed analyses will provide insights into the thickness and distribution of regolith and the history of impacts recorded in the crater population. Spectroscopic analysis will provide maps of mineralogy at 300-meter resolution and elemental composition at 4-kilometer resolution. The radio science and magnetometer experiments will yield information on the strength and character of the magnetic field, and on global density and density distribution. All these data will be used to determine which meteorite types may be similar to Eros. This link, if made, will allow the extensive database on meteorites to be associated with a specific, common type of asteroid to help clarify the early solar system history recorded in small bodies.

(The authors are members of the NEAR Mission Science Team and are staff scientists with The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland.)