Kessler syndrome

The prediction that as satellites and fragments multiply in orbit, debris from one collision will trigger more, possibly making the orbit unusable.

In 1978 Donald Kessler and Burton Cour-Palais of NASA calculated that as objects in low orbit increase, collisions rise roughly with the square of their number, until a debris belt keeps growing on its own even without new launches^[1].

In 2009 an active communications satellite and a defunct one actually collided. With satellite numbers now rising fast, deorbiting spent satellites and technologies to remove debris are in demand.