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The CSM classification serves as the starting point on the geography of the asteroid belt. Raw data on asteroid types are corrected for observational biases (against dark objects, for instance) to derive the distribution of types throughout the belt. Recent work on family members indicates that dynamical families have a true physical relationship, presumably indicating common origin in the breakup of a parent asteroid.
Laboratory experiments and computer modeling are used to predict the development of regoliths on all asteroids more than a few tens of kilometers in diameter, allowing for a wide range in the intrinsic strength of asteroidal surface materials. The high frequency of interasteroid collisions requires nearly all asteroids to be fragments of precursors.
Spacecraft observations of asteroids involve rendezvous or orbit with several main belt asteroids. This concept of a multi-asteroid rendezvous mission provides a baseline for mission planning.
The options for asteroid missions are evaluated within the constraints of existing or planned launch vehicles and low-thrust propulsion systems. A wide variety of missions is possible, including flyby, rendezvous, and even sample return. The multi-asteroid rendezvous concept requires an ion drive low-thrust propulsion system of the type being developed for a comet rendezvous. It is indicated that there are plentiful opportunities for such missions to visit four asteroids with stay times of 60-90 days each and with transit times between rendezvous of the order of a year.
Hard lander missions to asteroids are examined using the Westphal penetrator study as a basis. Imagery and chemical information are considered to be the most significant science to be obtained. The latter, particularly a detailed chemical analysis performed on an uncontaminated sample, may answer questions about the relationships of asteroids to meteorites and the place of asteroids in theories of the formation of the solar system.
A table is presented listing the compositional types chosen for 336 main-belt asteroids, twelve Mars-crossers, two Hildas, and nine Trojans. There are about 100 asteroids with polarimetric data, 126 with reflection spectra, 165 with radiometric diameters, and 300 with UBV colors. Absolute visual magnitudes V(1,0 were computed from the blue magnitudes listed by Gehrels and Gehrels (1977) and B-V colors either known or estimated according to the type. The sample of main-belt asteroids classified in the table is complete to mean opposition apparent magnitude B(a,0) = 12. The frequencies of the compositional types are discussed. Attention is also given to the distributions over diameter and the distributions over heliocentric distances.
The paper reports JHK colors observed for ten asteroids and synthesized JHK colors for seven meteorite groups, samples of iron and nickel metal, pyroxene, olivine, feldspar, a lunar anorthite and some terrestrial mineral samples. Pronounced differences are apparent between the chondritic and achondritic meteorite classes; the chondritic classes show less subdued trends in J-H color which reflect their metamorphic grade. We find small but significant differences between the JHK colors of the predominant C and S classes of asteroids. All JHK colors of asteroids observed here fall within the limited domain defined by the various chondritic and iron-rich meteorites but are strikingly different from those of most achondritic meteorites.
Small asteroids, with sizes of about 1 km, appear to rotate faster than do the larger, about 200 km, asteroids. There is a lack of highly elongated objects, probably due to a collisional erosion process in the main asteroid belt.
Laboratory reflection polarimetry is presented for crushed rocks, lunar soils, and 24 meteoritic samples. The polarization-phase curves found for the meteorites closely resemble telescopic results for minor planets, but only if the samples are crushed finely enough to coat the larger particles with dust. The results generally confirm the identifications previously made from reflection spectroscopy for the meteoritic analogues of various types of asteroids; the more metal-rich ordinary chondrites, for example, match telescopic observations of the S-type minor planets well enough, as do the basaltic achondrites for Vesta. No meteorite measured, however, has a geometric albedo less than 0.05 as required for many of the C asteroids. A calibration of the polarimetric slope-albedo law, now determined strictly for meteoritic samples, confirms the asteroid albedo and diameter scale derived by the independent method of thermal radiometry.
Among the major features of the asteroids as a group is the fact that they are small and numerous rather than being a single planet, and that they have unusually high eccentricities and inclinations. Regarding the first, this paper presents two lines of argument concerning the concept that the asteroids formed with a mass-distribution similar to what we observe today. Considerations of planet accretion are used to clarify the role of Jupiter in preventing coalescence of asteroids into a single planet. Regarding the eccentricities and inclinations, it is proposed that they were excited by secular resonances associated with the presence of Jupiter within a dissipating solar nebula.
Radio observations of the asteroids can provide information on the thermal and dielectric properties of the surface materials and because the radio emission arises somewhat below the surface, the data give some indication of layering. Observational difficulty has limited the investigations to only 6 asteroids: 1 Ceres and 324 Bamberga appear to have a layer of dust covering a more compacted material; the data on 4 Vesta cannot be matched by any current models for the surface; and the results for 18 Melpomene, 31 Euphrosyne and 433 Eros are too incomplete for firm conclusions. Future possibilities include more accurate radiometry of a few selected asteroids of different taxonomic classes and actual resolution of some of the larger objects by aperture synthesis techniques.
Radio observations of the asteroids can provide information on the thermal and dielectric properties of the surface materials and because the radio emission arises somewhat below the surface, the data give some indication of layering. Observational difficulty limited the investigations to only 6 asteroids:1 Ceres and 324 Bamberga appear to have a layer of dust covering a more compacted material; the data on 4 Vesta cannot be matched by any current models for the surface; and the results for 18 Melpomene, 31 Euphrosyne and 433 Eros are too incomplete for firm conclusions. Future possibilities include more accurate radiometry of a few selected asteroids of different tuxonomic classes and actual resolution of some of the larger objects by aperture synthesis techniques.
On the basis of remote-sensing evidence and meteorite studies, a substantial fraction of small asteroids in near-earth space should contain abundant volatile (water, carbon, carbon compounds) and/or free metal (nickel-iron) materials. Although there are only about 40 known earth-approaching asteroids, this number could be increased by at least an order of magnitude in a few years by a dedicated search program with a large telescope (48 in. Schmidt). A discovery rate of 20 objects/yr could be expected from a facility costing about $3 million to establish. Relatively high-resolution, broad-wavelength coverage reflectance spectra could be made for many of these objects with a large telescope and instruments under development. Such spectra are necessary for detailed mineralogic, characterizations of these asteroids. Low-resolution survey survey spectra with similar wavelength coverage (UBVRIJHK filters) would provide useful characterizations of fainter objects.
Photographic coverage of about 80,000 sq deg of sky with the Palomar 46-cm Schmidt camera has yielded 12 new planet-crossing asteroids as well as many objects in the main asteroid belt. The estimated population of planet-crossing asteroids includes approximately 100 Atens, 700 + or- 300 Apollos, 1000-2000 Amors, 10,000 + or- 5000 Mars crossers, and approximately 5000 Mars grazers.
A review of the geochemical data on basaltic achondrite group meteorites, here defined as diogenites, eucrites, howardites, mesosiderites, main-group pallasites and IIIAB irons, suggests that the following processes and properties were important in determining the compositions of igneous materials on asteroidal sized bodies: (1) inhomogeneities in source rock compositions, (2) variable degrees of partial melting from 5-100%, (3) multiple melt genesis from a single source region, (4) fractional crystallization, and (5) remelting of earlier formed igneous rocks. All of this activity started during or shortly after the major asteroidal accretion phase as suggested by the numerous 4.6 Gyr ages for eucrites and may have lasted for at least 200 m.y. The above properties imply an early, intense heat source that was both spatially and temporally variable. A review of asteroid thermal modeling for various heat sources shows that heating by short-lived Al-26 alone appears to be incapable of supplying the observed geochemical variations.
Radio observations of the asteroids can provide information on the thermal and dielectric properties of the surface materials and, because the radio emission arises somewhat below the surface, the data give some indication of layering. Observational difficulty has limited the investigations to only 6 asteroids. 1 Ceres and 324 Bamberga appear to have a layer of dust covering a more compacted material; the data on 4 Vesta cannot be matched by any current models for the surface; and the results for 18 Melpomene, 31 Euphrosyne and 433 Eros are too incomplete for firm conclusions. Future possibilities include more accurate radiometry of a few selected asteroids of different taxonomic classes and actual resolution of some of the larger objects by aperture synthesis techniques.
The collisional evolution of various initial populations of asteroids is simulated numerically and compared with the present asteroid size-frequency distribution to find those populations which collisionally relax to the present belt. Both orbital and size distributions are treated, as well as the simultaneous evolution of two collisionally interacting populations with different physical properties. If the initial belt distribution was a power law, the initial belt population at the time when the present high-collision speed was established was probably only modestly larger than the present population. However, other distributions allow a more massive early belt. The rotational evolution due to collisions of asteroids with power-law distributions is also examined and compared with observations, leading to conclusions generally in agreement with those of size evolution. The high-collision speed in the present belt is likely due to Jupiter. Gravitational stirring by massive Jupiter-scattered planetesimals or secular resonances sweeping through the belt are the most probable mechanisms.
Because meteorites are derived from asteroidal objects, the chronology of meteorites give age information on the histories of the asteroidal parents. With a few possible exceptions, the radiometric chronology of less than 4.4 Gyr determined in meteorites can be attributable to collisions among meteorite parent bodies. Three types of collisional chronologies which have been determined are: (1) cosmic ray exposure ages, which generally date the time of the last fragmentation of the parent object when the meteorite is reduced to meter size, and which generally fall over the wide range of 1-1000 Myr; (2) the collisional shock ages shown by many meteorites, which are believed to date the times of major collisions among parent bodies in the asteroid belt in the time interval of 30-700 Myr; and (3) the brecciation ages of several meteorites which are believed to date the times of their collisional formation from regoliths of larger parent bodies over the period of 1.4-4.4 Gyr.