Kessler syndrome

The Kessler Syndrome (or collisional cascading), 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.^[1] One implication is that the distribution of debris in orbit could render space exploration, and even the use of satellites, unfeasible for many generations.^[1]

The term ablation cascade has been used to describe the same scenario. First used in the 1999 fiction novel The Sky Road, it posited a single explosion or crash in LEO as the seed for a cascade of collisions and explosions, leading to a LEO band devoid of working artefacts, and uniformly filled with debris.

Debris generation and destruction

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, at the most commonly used Low Earth Orbits residual air drag helps keep the zones clear. Collisions that occur under this altitude are also less of an issue, since the energy lost in the collision results in fragment orbits 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 re-enter, but at very high altitudes this can take millennia.

Seriousness

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 a war in space), the amount of cascading debris could be enough to render low Earth orbit essentially impassable.^[2]^[3]

Avoidance and reduction

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^[4].

One technology for the bigger fragments that can be tracked is the laser broom, a 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.^[5]