It's a bird! It's a plane! No, it's space junk!

Houstonist had never thought about it before, but our apartment is a lot like Earth's orbit: Both are full of junk, and the amount will likely increase in the near future. Though we can do a quick clean-up by shoving everything in the closet, though, NASA might have a harder time when it starts trying to clean up space.

There are more than 13,000 man-made objects bigger than 4 inches in diameter circling the Earth right now — 95 percent of the white dots in the graphic at left represent human-produced space junk. The debris is everything from flecks of paint to old rocket bodies to functioning spacecraft (and maybe even a bag or two of Soviet Puppy Chow). A Houston-based NASA team came up with a computer model to predict how the amount of junk will change in the future; they found that, even if no more rockets or satellites are launched, there'll begin to be more junk in about 50 years as the result of deterioration and collisions. That can cause some big problems:

These objects travel at speeds over 22,000 miles an hour (35,000 kilometers an hour). At such high velocity, even small junk can rip holes in a spacecraft or disable a satellite by causing electrical shorts that result from clouds of superheated gas.

Three accidental collisions between catalogued space-junk objects larger than four inches (ten centimeters) have been documented from late 1991 to early 2005.

The most recent collision occurred a year ago. A 31-year-old U.S. rocket body hit a fragment from the third stage of a Chinese launch vehicle that exploded in March 2000.

Nations with space programs know space junk is a problem — they've even banded together to form the Inter-Agency Space Debris Coordination Committee — but so far, work has focused on preventing new debris, not removing what's already there. Figuring out how to remove junk is the hard part: Proposals have included using lasers to shoot debris out of orbit and sending up spacecraft that would grab debris and bring it back to Earth. We can imagine NASA's itching to start the cleanup — they'll undoubtedly make a killing selling old rocket bits on eBay.