Space debris is tracked by the United States Space Surveillance Network that uses radar that can detect an object the size of a pea at 400 miles.

BBC: By BBC News Online Science Editor Dr David Whitehouse

The U.S. military's Space Surveillance Network tracks roughly 22, 000 pieces of orbital debris larger than 4 inches (10 centimeters), which include broken satellite parts and spent rocket bodies.

MSN: Space Junk Hazards Force International Response