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 the top of a planet's atmosphere. Lithos is a Greek word meaning "rock" or "stone."
The technique has been used both successfully and unsuccesfully on recent unmanned missions to the Moon and Mars.
