Asteroids approaching Earth. Computer artwork of two asteroids (left and lower right) approaching the Earth, the visible side of which is mostly in darkness, with a sunlit crescent arc at lower left. The Milky Way is in the background. These asteroids are examples of near-Earth objects (NEOs), astronomical objects whose orbits intersect 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.