Asteroid passing the Moon. Computer artwork of an asteroid passing close to the Moon. This asteroid is an example of a near-Earth object (NEO), an astronomical object whose orbit intersects with that of the Earth. Asteroids are rocky bodies orbiting within the solar system. They vary in size from tens of metres across to over a hundred kilometres across. Large asteroids (over a kilometre across) impacting the Earth can cause mass loss of life. It is thought that this is what caused the extinction of the dinosaurs 65 million years ago. The Sun is at lower left, and the Milky Way is in the background.