In 1978, the NASA scientist Donald Kessler predicted that a collision between two pieces of space junk could trigger a cascade of further impacts, creating dangerously large amounts of debris.
Kessler pointed out that when the rate at which debris forms is faster than the rate at which it de-orbits, then the Earth would become surrounded by permanent belts of junk, a scenario now known as the Kessler syndrome.
By some estimates, the Kessler syndrome has already become a reality. In January 2009, a collision between the Iridium 33 and Cosmos 2251 satellites created just this kind of cascade. Two years earlier, the Chinese military tested an anti-satellite weapon by destroying one of its own satellites called Fengyun 1C. Both incidents took place at altitudes of about 800 km.
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