NASA meticulously tracks pieces of space junk that are in orbit around the Earth. So far, it's identified more than 20,000 pieces that are larger than a softball, and more than 500,000 pieces larger than a marble. If it can predict a collision with a larger piece of debris, NASA instructs the spacecraft to perform avoidance maneuvers. In addition to the visible debris, however, there are millions of smaller pieces that are too hard to track. These tiny bits of space junk pose the most risk to spacecraft and astronauts. Keep scrolling for more on this deadly cosmic litter.