Rocks and Hard Places

Asteroids are composed of stony as well as metallic materials—mostly iron—and are basically tiny planets without atmospheres. Some asteroids have a good deal of carbon in their composition as well. These, called carbonaceous chondrites, are thought to represent the very first materials that came together to form the objects of the solar system. Carbonaceous chondrites are truly the solar system’s fossils, having avoided change for billions upon billions of years.
Earlier astronomers surmised that asteroids were fragments resulting from various meteoric collisions. While some of the smaller meteoroids were likely produced this way, the major asteroids probably came into being at the time of the formation of the solar system as a whole. Theoretical studies show that no planet could have formed at the radius of the asteroid belt (at about 3 A.U. from the sun). The region between Mars and Jupiter is dominated by the gravitational influence of the giant planet Jupiter. These forces stirred up the potential planet-forming material, causing it to collide and break up instead of coming together to create a planet-sized object. The smaller asteroids come in a wide variety of shapes, ranging from nearly spherical, to slab-like, to highly irregular.
During 1993–1994, the Galileo probe passed through the asteroid belt on its way to Jupiter and took pictures of an asteroid orbited by its own miniature moon. Potato-shaped, the asteroid was named Ida, and is about 35 miles (56 km) long, orbited at a distance of roughly 60 miles (97 km) by a rock less than 1 mile in diameter. This little moon is the smallest known natural satellite in the solar system.