Asteroid approaching Earth. Artwork of an asteroid (at centre left) approaching Earth. Asteroids are minor planets which orbit the Sun and have diameters which are typically less than 100 kilometres. There are thousands of asteroids in the solar system and they lie mainly between the orbits of the planets Mars and Jupiter. Asteroids in this belt may have their orbits perturbed so that they approach Earth. About 50 bodies, known as the Apollo asteroids, have orbits which intersect with that of Earth. It is a statistical certainty that Earth will be struck by an asteroid at some point in the future.