More likely, asteroids are material that never coalesced into a planet. Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids . When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.
Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.
Because asteroids are material from the very early solar system, scientists are interested in their composition. Spacecraft that have flown through the asteroid belt have found that the belt is really quite empty and that asteroids are separated by very large distances. 951 Gaspra and 243 Ida . The following pages contain information on several asteroids that have been studied during the last few years. The Galileo spacecraft flew past Gaspra in October 1991 and Ida in August 1993.