Proposed by NASA scientist Donald J. Kessler in 1978, The Kessler Syndrome is a theoretical (but very possible) scenario in which the density of objects in low Earth orbit (LEO) due to space pollution is high enough that collisions between objects could cause a cascade in which each collision generates space debris that increases the likelihood of further collisions. This would make space activities very difficult, or even impossible until the debris cleared, which could take a few decades of time.