What is the Kessler syndrome and how close are we to triggering it?

The answer

The Kessler syndrome, proposed by NASA scientist Donald Kessler in 1978, describes a scenario where the density of objects in low Earth orbit becomes high enough that collisions generate debris causing further collisions in a cascading chain reaction. As of 2025, over 36,500 tracked objects larger than 10 centimeters orbit Earth, along with an estimated 130 million pieces between 1 millimeter and 1 centimeter. SpaceX's Starlink constellation alone comprises over 6,000 active satellites. The 2009 collision between Iridium 33 and Cosmos 2251 created over 2,000 tracked fragments. China's 2007 anti-satellite test generated 3,500 pieces of debris still orbiting. Companies like Astroscale are developing active debris removal spacecraft, and the ESA's ClearSpace-1 mission is planned to demonstrate debris capture. Without intervention, some orbital zones may become unusable within decades.