Asteroid approaching Earth, artwork. Asteroids are large rocks left over after the solar system formed. They mostly have a diameter of less than a hundred kilometres. Most of the thousands of asteroids in the solar system lie in the asteroid belt between the orbits of the planets Mars and Jupiter. At least 50 asteroids have orbits that intersect with that of Earth and the Moon (upper centre). Earth has been struck by asteroids in the past, causing mass loss of life such as the extinction of the dinosaurs 65 million years ago. It is a statistical certainty that Earth will be hit by another large asteroid in the future.