Asteroids are small rocky bodies that orbit the sun in a belt (Kuiper belt) between orbits of Mars and Jupiter. Not all the asteroids orbit in Kuiper belt. Some are between orbit of Jupiter and some are beyond the orbit of Saturn. More that 100,000 asteroids have been found with well charted orbits, out of which 2000 follow the orbits that lead them into inner and outer solar system. Earth has been struck many times by an asteroid.
About 200 asteroids are more than 100 km (60 mi) in diameter, and tens of thousands are estimated to be more than 10 km (6 mi) in diameter. There are probably a million or more that are larger than 1 km (0.6 mi) and billions that are smaller.