A direct hit by a comet or an asteroid is thought to be responsible for wiping out the dinosaurs on Earth. Such collisions were in fact thought to be quite common during the first few hundred of million years of Earth's infancy. Such "killer asteroids" (also known as Near-Earth Objects; e.g. 2004FH, Toutatis) are still found, but they are much less common than before -- at least a part of the reason why life still exists on Earth. Why is that?