People often say, "What goes up must come down." But the phrase isn't always true, not in space at least.

There are millions of things that humans have shot up into Earth's orbit, and they are still there.

Most of them are tiny, but because there are so many of them, these pieces of space junk add up to a big problem. The biggest pieces of space junk are old satellites that don't work anymore.

However, those inactive satellites are only some of the 30,000 pieces of junk up there bigger than 10 centimeters across.

There are also smaller pieces that are bits of debris from broken equipment, satellites destroyed by weapons tests, or even chips of paint. In total, there may be more than 100 million pieces of space junk flying around our planet.

If the junk's orbit is high enough, it stays in space.

Because it's still traveling as fast as a rocket, a collision between space junk and a satellite, spacecraft, or space station could be disastrous.

The International Space Station (ISS) sometimes has to move itself to avoid hitting pieces of garbage. Some experts fear what's called Kessler syndrome, a situation where a chain of collisions creates so much debris that more collisions are inevitable, making it impossible to put more satellites in orbit around Earth.

Cleaning up these pieces of flying junk is a difficult problem.

People's ideas for doing it range from using lasers to magnets. Space junk hasn't caused a serious disaster yet, but it might in the future.

In any case, keeping the space near Earth clean will be important for continuing to make use of space technology.