Asteroid approaching the Moon, artwork. This asteroid is an example of a near-Earth object (NEO), an astronomical object whose orbit intersects with that of the Earth. If an object approaches Earth closely (within 7.5 million kilometres), it is further classifed as a Potentially Hazardous Object (PHO). The maximum Earth-Moon distance is 407,000 kilometres. 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.