Space is full of junk like used rocket parts and dead satellites. These objects continue to orbit Earth at about 17,500 mph — 10 times as fast as a speeding bullet.

The Space Surveillance Network (SSN) keeps track of how much debris is out there and who's responsible for it. The SSN is currently tracking 23,000 objects larger than a softball.

Space junk is dangerous because one collision could trigger a chain reaction of objects hitting each other, resulting in a thick cloud of debris that would make space travel extremely dangerous. This kind of disaster was the premise of the 2013 movie "Gravity" starring Sandra Bullock and George Clooney.

The US is responsible for the most space junk with 3,990 pieces of debris as of March 28, 2018. Russia is responsible for 3,959 pieces of debris, followed by China with 3,893.