The solar system has a large number of rocky and metallic  objects that are in orbit around the Sun but are too small to be  considered full-fledged planets. These objects are known as  asteroids or minor planets. Most, but not all, are found in a band  or belt between the orbits of Mars and Jupiter. Some have orbits  that cross Earth's path, and there is evidence that Earth has been  hit by asteroids in the past. One of the least eroded, best  preserved examples is the Barringer Meteor Crater near Winslow,  Arizona.  Asteroids are material left over from the formation of the  solar system. One theory suggests that they are the remains of a  planet that was destroyed in a massive collision long ago. More  likely, asteroids are material that never coalesced into a planet.  In fact, if the estimated total mass of all asteroids was gathered  into a single object, the object would be less than 1,500  kilometers (932 miles) across -- less than half the diameter of our  Moon.  Thousands of asteroids have been identified from Earth. It is  estimated that 100,000 are bright enough to eventually be  photographed through Earth-based telescopes.   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. One of the best places to look for meteorites  is the ice cap of Antarctica.   Of all the meteorites examined, 92.8 percent are composed of  silicate (stone), and 5.7 percent are composed of iron and nickel;  the rest are a mixture of the three materials. Stony meteorites  are the hardest to identify since they look very much like  terrestrial rocks.   Since 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.