Asteroids are chunks of rock and metal that orbit the sun, mainly in an area between Mars and Jupiter called the Asteroid Belt.   The Asteroid belt contains thousands of asteroids. Most asteroids are only a few miles in diameter, but a few are from three hundred to six hundred miles across.   Although asteroids usually stay in their orbit around the sun, sometimes one will spin out of its orbit and crash into a planet.  A huge asteroid impact is believed to have caused the dinosaurs to become extinct.