The Kessler Syndrome is a theoretical scenario or concept that describes a self-sustaining, cascading chain reaction of collisions in Low Earth Orbit (LEO). It was first proposed in 1978 by NASA scientists Donald J. Kessler and Burton G. Cour-Palais in their paper, "Collision Frequency of Artificial Satellites: The Creation of a Debris Belt". The problem it sought to address was the growing risk of space debris, which they predicted would become the primary ablative risk to spacecraft by about 2000.
The mechanism works when the density of objects in LEO reaches a critical density threshold. At this point, a single collision between two objects creates numerous fragments, which then collide with other satellites or debris, exponentially increasing the amount of space junk. This process, also called collisional cascading, continues until the production of new debris outpaces its decay due to atmospheric drag, potentially rendering certain orbital regions unusable for generations.
The syndrome connects directly to the issue of space debris and the operations of large satellite constellations like SpaceX's Starlink. Real-world events like the 2009 collision between the Iridium 33 and Kosmos-2251 satellites are considered harbingers of the syndrome. To mitigate the risk, the Inter-Agency Space Debris Coordination Committee (IADC) works on policy. A recent change in mitigation strategy occurred in 2026, when SpaceX announced plans to lower the altitude of nearly half of its Starlink satellites from 550 kilometers to approximately 480 kilometers to increase atmospheric drag and reduce collision risk.