Characteristics of Asteroids
Photometric studies of the asteroids have for some time been interpreted as showing that they differ in size, shape, and rotation. All but the largest of the asteroids are too small to show a measurable disk. From the variation in their brightness, it has been assumed that most have somewhat irregular shapes with periods of rotation measured in hours. Recent evidence suggests, however, that the brightness variations for some are the resu It of two or more asteroids in mutual orbit about each other; that is, some asteroids are binary systems.
Their colors put nearly all asteroids into two categories: Some are bright reddish, a sign of silicates and iron-nickel or other metallic grains, and they populate mostly the inner part of the asteroid belt; but most have the darker neutral color of material containing various carbon-rich or water-rich compounds (carbonaceous) and occupy the outer part of the belt.
Collisions between two asteroids may produce effects ranging from craters (if a small one collides with a large one) to fragmentation of the two asteroids (if they are of comparable size). For example, if the body producing crater Stickney on Mars's satellite Phobos had been a little larger, Phobos might have been broken into many small pieces. As it is, the grooves on Phobos may be large cracks produced by the impact.
