From Academic Kids

Lithobraking is the a technique of descent by an unmanned space vehicle to the surface of a terrestrial body by which the vehicle is slowed by the use of friction with the body's surface. Lithobraking is typically accomplished by the shrouding of the probe in sufficient cushioning to withstand an impact with a terrestrial surface such as to allow the probe to come to rest undamaged. The angle is made acute enough such that the impact has the characteristic of a glancing blow, rather than a direct hit bounce from the surface. In the case of bodies with thick enough atmospheres (e.g., Mars), lithobraking is typically accompanied by the use of parachutes to reduce the speed prior to impact.

In the case of bodies without sufficient graviation, such as comets and asteroids, the term is taken to imply penetration of the surface by the probe.

The word was probably coined as a whimsical adaption of aerobraking, which is the process of slowing a space vehicle by the use of friction against a planet's atmosphere. Lithos is a Greek word meaning "rock" or "stone."

Lithobraking as a method for de-orbiting a spacecraft has not been used, and is unlikely to ever become viable. The velocities involved (2.2 kilometers per second to land on the moon) would destroy any spacecraft that attemped it, unless it was of extremely rugged - and impractical - construction.

Certain concepts involve the spacecraft in an orbit tangent to the surface of the body in question, and "docking" with a magnetically levitated (Maglev) train, and the train then slowing. This qualifies as lithobraking, as the reaction mass is the planet itself. This technique requires extremely precise guidance and control, in addition to a large infrastructure, and is thus not yet a viable option - although it may be in the future. An advantage to this method is that it can also launch spacecraft, without needing propellant.

The term is also sometimes used in a humorous context to describe situations in which lithobraking was not the original desired landing method - i.e., crashes.


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