By the 1990's more than 7000 had been observed at two or more oppositions, and 5000 had been assigned numbers, which is done as soon as accurate orbital elements have been determined.
Asteroids are smaller than any of the nine major planets of the solar system.
Aproximately 250 asteroids have a diameter of at least 62 miles. It is estimated that millions of asteroids of boulder size exist in the solar system. These smaller objects are probably formed when larger asteroids collide. A few of them strike the earth's surface in the form of meteorites. The largest asteroidsare massive enough for their gravity to have molded them into spheres during formation.