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Kessler syndrome

Kessler syndrome — the scenario in which collisions between objects generate debris that goes on to cause further collisions, in a self-sustaining cascade — is often invoked in popular coverage as an imminent, sudden event. Most specialists in orbital debris describe it more accurately as a slow-building risk that increases gradually as object density rises in particular orbital bands, rather than a single catastrophic tipping point that occurs on a specific date. Certain congested altitude bands are of greater concern than low Earth orbit as a whole, and the risk is unevenly distributed rather than uniform.

• Roughly 30,000-plus objects are large enough (generally above about ten centimetres) to be individually tracked and catalogued.
• Untrackable fragments below that threshold are estimated to number from the hundreds of thousands into the millions.
• A 2007 Chinese anti-satellite weapons test and a 2009 collision between an active and a defunct satellite each generated large, long-lived debris fields still being tracked today.
• Large satellite constellations have substantially increased the number of active objects sharing low Earth orbit and, correspondingly, the number of collision-avoidance manoeuvres operators must perform.