Most people are unaware of it, but our planet is under a constant barrage by the cosmos. Our galactic neighborhood is littered with comets, asteroids and other debris left over from the birth of the solar system. Most of the space detritus that strikes the earth is interplanetary dust, but a few of these cosmic projectiles have measured five kilometers (about 3.1 miles) or more across. Based on the number of craters on the moon, astronomers estimate that about 60 such giant space rocks slammed into the earth during the past 600 million years. Even the smallest of those collisions would have left a scar 95 kilometers (about 60 miles) wide and would have released a blast of kinetic energy equivalent to detonating 10 million megatons of TNT.