Asteroids, also known as minor planets, are large rocks left over after the solar system formed. They mostly have a diameter of less than a hundred kilometres. There are millions of asteroids in the solar system and a large fraction of them lie in the asteroid belt between the orbits of the planets Mars and Jupiter. Many 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 approximately 65 million years ago.