2. Asteroid Compositional Types and their Distributions

Abstract

A sample of 359 minor planets with available colorimetric, spectropnotometric, thermal-radiometric, and/or polarimetrie data are classified into broadly defined compositional types C (carbonaceous), S (silicaceous), M (metal-rich), E (metal-free; enstatite?), O (ordinary-chondritic?), T (Trojan; unidentified composition), and U (unclassifiable; none of the above).The small asteroids in Mars- or earth-crossing orbits are almost invariably of type S or O. For the main belt between 2.2 and 3.5 AU., distributions of the various types over diameter and orbital parameters are derived with corrections for observational selection bias. For 560 main-belt asteroids with diameters >50 km, 76% are of type C, 16% of type S, 5% M, and 3% of other types. The S objects become progressively less common with distance. The shapes of the diammeter-frequency relations for primitive (C) and evolved (S + M) types are statistically indistinguishable, both showing a change of slope at 160-km diameter.While large asteroids avoid the Kirkwood gaps more strongly than small ones, we find no significant gap-related anomalies in the relative frequencies of compositional types. Thus most of the observational basis for models in which primitive and evolved asteroids respond differently to collisional evolution has been removed.