Small objects frequently collide with Earth. There is an inverse relationship between the size of the object and the frequency of such events. The lunar cratering record shows that the frequency of impacts decreases as approximately the cube of the resulting crater's diameter, which is on average proportional to the diameter of the impactor. Asteroids with a diameter strike Earth every 500,000 years on average. Large collisions – with objects – happen approximately once every twenty million years. The last known impact of an object of or more in diameter was at the Cretaceous–Paleogene extinction event 66 million years ago.