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The Kessler syndrome (also called the Kessler effect, collisional cascading or ablation cascade), proposed by NASA scientist Donald J. Kessler in 1978, is a scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade – each collision generating debris which increases the likelihood of further collisions. One implication is that the distribution of debris in orbit could render space exploration, and even the use of satellites, unfeasible for many generations.
Every satellite, space probe, and manned mission has the potential to create space debris. As the number of satellites in orbit grows and old satellites become obsolete, the risk of a cascading Kessler syndrome becomes greater.
Fortunately, the most commonly used orbits for both manned and unmanned space vehicles are Low Earth Orbits, which cover an altitude range low enough for residual air drag to be sufficient to help keep the zone clear. Collisions that occur in this altitude range are also less of an issue, since the directions into which the fragments fly and/or their lower specific energy often result in orbits intersecting with Earth or having perigee below this altitude.
At altitudes above the levels where atmospheric drag is significant, the time required for orbital decay is much longer. Slight atmospheric drag, lunar perturbation, and solar wind drag can gradually bring debris down to lower altitudes where fragments finally reenter, but at very high altitudes this can take millennia.
The Kessler Syndrome is especially insidious because of the "domino effect" and "feedback runaway". Any impact between two objects of sizable mass spalls off shrapnel debris from the force of collision. Each piece of shrapnel now has the potential to cause further damage, creating even more space debris. With a large enough collision or explosion (such as one between a space station and a defunct satellite, or the result of hostile actions in space), the amount of cascading debris could be enough to render low Earth orbit essentially impassable.
To minimize the chances of damage to other vehicles, designers of a new vehicle or satellite are frequently required to demonstrate that it can be safely disposed of at the end of its life, for example by use of a controlled atmospheric reentry system or a boost into a graveyard orbit.
One technology proposed to help deal with fragments from 1 cm to 10 cm in size is the laser broom, a proposed multimegawatt land-based laser that could be used to target fragments. When the laser light hits a fragment, one side of the fragment would ablate, creating a thrust that would change the eccentricity of the remains of the fragment until it would re-enter harmlessly.
The Envisat satellite is a very large target with a mass of 8.211 metric tons, now drifting at 785 km (488 mi), an altitude where the debris environment is the greatest and likely to increase. In the next 150 years that the inactive satellite will remain in orbit, it will become a significant debris source and could easily become a major debris contributor from a collision. Envisat is currently drifting in an environment where 2 catalogued objects can be expected to pass within about 200 meters of Envisat every year.