For much of history, the solar system was thought to consist of large objects: Earth, the Moon, the Sun, and other planets. However, within the last two centuries astronomers have discovered a menagerie of small objects throughout the solar system, including asteroids, comets, Kuiper belt objects, and many small moons orbiting planets. Studies of these objects, from both telescopes and spacecraft, have provided new insights into the formation of the solar system and its true nature.

There is no good definition for what constitutes a "small body" in the solar system. Nevertheless, one definition used by some planetary scientists considers objects less than 400 kilometers (250 miles) in diameter to be small bodies. When objects are larger than 400 kilometers, they have enough mass so that their gravity is powerful enough to shape the object into a roughly spherical form. For smaller objects, their gravity is not powerful enough...