Space debris

Objects in low orbit fly at about 28,000 km per hour. That is several times faster than a bullet, so even a very small piece can wreck a satellite. Objects that are low eventually brush against the thin air, fall and burn up, but the higher an object is, the longer it stays.

The biggest worry is a chain of collisions. Objects hit each other and make fragments, and those fragments hit other objects and make even more, until the fragments multiply out of control. This is called Kessler syndrome. So rules have been made that require dead satellites to be brought down on purpose, and technologies that catch and clear debris are being tested.

A small screw dropped on a highway can cause a big accident, and in orbit a small piece can smash a satellite. On top of that, this road has no cleanup truck yet.

It is a big problem in an age when satellites are growing by thousands. It is why you hear that a space station moved to dodge debris, or that two satellites passed close to each other.