Kessler Syndrome, proposed by NASA scientist Donald Kessler in 1978, describes a cascading scenario in which the density of objects in orbit becomes high enough that collisions between them generate debris faster than it naturally deorbits, triggering a self-sustaining chain reaction. In the worst case, entire orbital bands could become unusable for decades or centuries.
While a full Kessler cascade has not occurred, modeling suggests that certain LEO altitude bands (particularly 700-1,000 km, where atmospheric drag is weak) may already be in an early stage of instability. This has made active debris removal, responsible satellite design (including built-in deorbit capability), and international debris mitigation guidelines urgent priorities for the global space community.