Asteroid, illustration. Asteroids are rocks left over from the formation of the solar system. Most of the thousands of asteroids in the solar system lie in the asteroid belt, a region between the orbits of the planets Mars and Jupiter. The largest asteroids are 100 kilometres or more across, but most are much smaller. The asteroids in the asteroid belt are spread over a large volume, with an average separation of around 6 million kilometres. The bright band of stars (background) is the Milky Way galaxy.Detlev Van RavenswaayScience Photo Library