Space Jam.

When you look at the clear night sky, it's hard to imagine that space is so ... jammed.

Did you know the U.S. Space Command tracks nearly 9,000 human-made objects orbiting Earth (see chart, right)? Of these, more than 6,000 objects, collectively known as space debris, no longer serve any purpose at all--they're just junk!

Now the U.S. is working with the United Nations and countries like Russia, France, and Japan, to slow the growth of space debris. But where does all that debris come from? Try leftover materials from space ventures such as shuttle or satellite launches; old booster rockets that thrust satellites into space, for example, linger in orbit. But most debris results from space explosions, which can occur when excess rocket fuel becomes overheated or batteries are over-pressurized. Explosions can shatter a rocket into millions of tiny pieces.

The U.S. Space Command tracks objects that measure at least 10 centimeters (4 inches) in diameter. But experts estimate the number of space-junk objects between 1 and 10 cm exceed 100,000, and those that measure less than 1 cm add up to tens of millions. Most debris lies between 600 and 2,000 kilometers (373 and 1,243 miles) above Earth, where many satellites orbit. Whizzing by at about 8 km (5 mi) per second, a 1-mm aluminum sphere can damage a spacecraft as badly as a .22-caliber long-rifle bullet. In fact, last October the International Space Station (ISS) had to be boosted an additional 1.6 km (1 mi) to avoid a possible space-crash with debris from an old rocket!

To prevent explosions, the U.S. government requires all its spacecraft to vent or burn off excess fuel and drain extra energy from batteries. Other countries, as well as private companies, are considering the same guidelines. But there's still time to remedy the problem, claims Nicholas Johnson, program manager for NASA's Orbital Debris Program Office. "The current threat to spacecraft from orbital debris remains relatively low," he says. Are you in danger of being struck by falling space debris? Hardly. On re-entry to Earth's atmosphere, almost all space debris burns up. Poof!