Space junk could trigger a chain reaction that closes off orbits for generations.
In 1978 NASA scientist Donald Kessler warned that if enough debris built up in low Earth orbit, collisions would create more debris, causing more collisions, in a runaway cascade. Decades later, with thousands of satellites launched, he and others think parts of that process may already have begun.
After Sputnik in 1957, the US military began cataloguing everything reaching orbit: satellites, spent rocket stages, protective shields. Trackers soon noticed many extra fragments, some from rocket stages exploding as leftover fuel burst their tanks. When the catalogue became public in the 1970s, Donald Kessler applied a technique used to study collisions in the asteroid belt. In 1978 he and Burton Cour-Palais showed that the same grinding process could happen in low Earth orbit, not over billions of years but within decades.
At the time most people assumed the thin upper atmosphere would drag debris down faster than it formed. Kessler identified three regimes. At low density, the atmosphere wins. Beyond a critical density, each collision creates fragments that cause more collisions. Beyond that, the cascade feeds itself, grinding the orbiting population down to hazardous small pieces. The name 'Kessler syndrome' was coined by John Gabbard, a NORAD analyst who studied orbital explosions.
The physics is brutal. A one-kilogram object hitting at 10 kilometres per second may be able to shatter a 1,000-kilogram spacecraft, producing many new fragments. By 2021 there were estimated to be about 600,000 pieces of junk between one and ten centimetres across and 23,000 larger ones. Two satellites collided with each other in 2009, and deliberate anti-satellite tests have added large debris clouds that may last decades or centuries.
In 2009 Kessler wrote that models suggested the debris environment had already become unstable in places, so even stopping new debris would not halt growth. The number of active satellites has surged since, passing 11,800 in 2025. A 2025 analysis warned that a severe solar storm disrupting collision-avoidance manoeuvres could leave only days before serious collisions began.
Source: Wikipedia — Kessler syndrome · Text summarised from Wikipedia (CC BY-SA 4.0)
