Asteroid approaching Earth. Composite image of a 2 kilometre wide asteroid over Los Angeles, USA. Asteroids are large rocks left over after the solar system formed. 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. 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.