Asteroid approaching Earth, artwork. Hypothetical image of an asteroid aproaching the Atlantic Ocean, 30 seconds prior to impact. 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. Asteroids measuring 15 kilometres across are believed to strike the earth every 50-100 million years and this is one theory for the sudden mass 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.