- images of a C-type object. Eros, with dimensions of 14 × 14 × 40 km, is an S-type asteroid with heterogeneous surface composition. Data to be obtained for Eros include measurement of bulk properties (size, shape, volume, gravity field, spin state), surface properties (elemental and mineralogical composition, texture, topography, geology), and internal properties (mass distribution, magnetic field).
The NEAR spacecraft, launched in February 1996, carries an instrument payload that includes a multispectral imaging system, an x-ray/gamma-ray spectrometer, a near-infrared spectrometer, a magnetometer, a laser rangefinder, and a radio science experiment. After insertion of the spacecraft into polar orbit around Eros, the science payload should provide sufficient data for nearly complete topographic and geologic maps showing features as small as a few meters across. Spectral types will be mapped at a resolution of a few meters, mineral abundances at a resolution of several hundred meters, and major and minor radioactive elements at a similar or coarser scale. Thus, the NEAR mission should provide a major advance in our knowledge of the composition and geologic evolution of this asteroid.
- Deep Space 1, NASA's first New Millennium program deep-space technology demonstration mission, is planned for launch in July 1998. Although this mission will be driven by the requirements of technology validation (especially solar electric propulsion), it will encounter an asteroid, a comet, and the planet Mars. The first encounter will be with the Amor asteroid (3352) McAuliffe in January 1999, and the second with the periodic comet West-Kohoutek-Ikemura in June 2000. No physical studies have yet been carried out on McAuliffe. The asteroid may be about 2 km in diameter if it is an S type, or about 5 km in diameter if it is a C type. An integrated camera-spectrometer will be used to determine the sizes and shapes of both bodies, as well as the spectral reflectance of their surface materials.
- Stardust, the fourth of NASA's Discovery missions, will be launched in February 1999. It will capture a sample of dust particles from comet P/Wild 2 and return it to Earth for laboratory analysis. A deflection of this comet during an encounter with Jupiter in 1974 reduced its perihelion distance from 5 to 1.5 AU, and so this object is now Earth approaching. The spacecraft will approach the comet nucleus within 50 km, using on-board optical navigation, and a particle mass spectrometer provided by Germany will obtain in-flight data on very fine particles. During interplanetary cruise, a second set of collectors will collect dust grains currently entering the solar system from interstellar space. The samples will be returned to Earth in January 2006.
- Muses-C is an asteroid sample-return mission to be carried out by Japan's Institute of Space and Astronautical Science, with participation by NASA. Its scheduled launch is January 2002. It will rendezvous with (4660) Nereus (approximately 1 km in diameter, probably either a C-type or an M-type asteroid) in September 2003. Nereus is one of the most accessible NEOs so far discovered. In addition to a ballistic sampling device, the nominal instrument payload of Muses-C includes a CCD camera, an x-ray spectrometer, a secondary ion mass spectrometer, a dust collector, and a laser rangefinder. Prior to sampling, the asteroid will be mapped with the on-board instruments. After two months on station at Nereus, the spacecraft will return a sample to Earth, arriving in January 2006. Muses-C is also intended to demonstrate a solar electric propulsion system, autonomous guidance and navigation, and direct hyperbolic reentry of the sample capsule.
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5. Space Science Board, National Research Council, Strategy for the Exploration of Primitive Solar-System Bodies—Asteroids, Comets, and Meteoroids: 1980–1990, National Academy Press, Washington, D.C., 1980, p. 15.