In a way, Chicken Little was right - the sky is falling, at least a little bit. In its orbit around the sun, the Earth encounters tons of extraterrestrial material each day. Most of this space debris burns up in the atmosphere, treating us to the occasional shooting stars and meteor showers.

Every so often, though, some of this extraterrestrial material survives the forces and temperatures during the trip through earth's atmosphere, and reaches the surface. Then we'd officially call it a meteorite. In very rare cases, these can get pretty big - a few miles across, qualifying them as asteroids - and upon impact, they disintegrate but leave a huge crater. Maybe you've even heard of the theory that the dinosaurs were wiped when earth collided with an asteroid.

The earth is certainly not as cratered as the Moon, but appearances are a bit deceiving. Because the earth is bigger, it's actually been hit by about ten times more objects than the Moon. Tomorrow night, I'll focus on some of these encounters, and explain why there's not much evidence of them left on earth.