When the Russian space station Mir plummeted back to earth a few weeks ago, it was a reminder of just how much man-made stuff - much of it not functioning anymore - is up there orbiting the earth. Old weather satellites are a small part of this space junk.

Weather satellites are placed much higher up than Mir was, so they can get a wider view of the earth. Many reside about 22,000 miles up, over the equator. In that position, a satellite can orbit the earth at the same rate that the earth rotates, so the satellite essentially remains fixed above the same point. This altitude of 22,000 miles is popular because it's a balance point between two opposing forces: the tendency for a satellite to fall back to earth due to gravity and the tendency for the satellite to fly off into space simply because objects naturally want to move in a straight line. Satellites in this kind of orbit are called "geosynchronous" - meaning "in synch with earth."

The first such satellites were launched back in the 1960s, and over time the altitude around 22,000 miles has become rather crowded, relatively speaking. In fact, earlier this month, U.S. scientists had to do a little space-cleaning of their own, moving one of their older weather satellites - one launched in 1977 - to an orbit about 200 miles higher just to make room for future spacecraft.