Space junk
There is a growing amount of junk floating around Earth. Thousands of pieces of debris from launches, nonfunctional spacecraft and fragmentation debris are all orbiting Earth.
Today one of those pieces, a rocket part which weighs around three-tonnes, is predicted to have smashed into the moon at around 5,500 miles per hour (8,851 kilometers per hour). The impact was expected to take place on the far side of the moon, so we don't know for sure if it's happened yet.
Kessler Syndrome
Space junk hitting the moon isn't likely to cause us any problems here on Earth, but the growing body of debris is a cause for concern because of something known as "Kessler Syndrome".
Kessler Syndrome is a predicted phenomenon in which the amount of junk in orbit around Earth reaches a point that creates a cascade of collisions, in which each collision generates space debris that increases the likelihood of further collisions. Basically a vicious cycle in which more debris creates more collisions, which makes more debris.
As Earth's orbit gets increasingly crowded (there are now 6,500+ satellites in space), Kessler Syndrome gets more relevant.
