Lithobraking is a landing technique used by uncrewed space vehicles to safely reach the surface of a celestial body while reducing landing speed by impact with the body's surface. The word was probably coined as a whimsical adaptation of aerobraking, which is the process of slowing a space vehicle by the use of aerodynamic drag in a planet's atmosphere. Lithos is a Greek word meaning "rock" or "stone," similarly used in words such as lithosphere or lithography.
Successful lithobraking requires either reducing the velocity of the lander prior to impact or protecting the probe with sufficient cushioning to withstand an impact with the surface undamaged. The velocity of a lander can be reduced using retrorockets or parachutes, and it can be protected from the force of impact by cushioning air bags or shock absorbers. The first successful lithobraking was achieved by the Soviet Luna 9 probe resulting in the first soft landing on the Moon using a combination of retrorockets and gas-filled cushioning bags.
When landing on bodies with an atmosphere, lithobraking can be combined with aerobraking instead of relying on retrorockets and air-bags. For bodies with a significant yet insufficiently thick atmosphere (e.g., Mars) all of these may be used together. The Mars Pathfinder and Mars Exploration Rover programs have used this approach successfully. The Russian Mars 96 mission would have attempted a similar landing had it not been lost in Earth's atmosphere shortly after launch.:193–194 For bodies such as Venus with an exceptionally thick atmosphere, the combination of lithobraking and aerobraking can be sufficient. The Soviet Venera landers descended using parachutes high in the atmosphere before falling freely through the dense lower atmosphere until finally dissipating their remaining velocity (between approximately 7.5 and 8 m/s) on impact.:150–157
In the absence of a thick atmosphere, lithobraking is difficult due to the extremely high orbital velocities of most bodies. However, the orbital velocity of small moons (e.g., Phobos), asteroids, and comets can be sufficiently small for this strategy to be feasible. For example, Rosetta's lander, Philae, passively landed on the comet 67P/Churyumov–Gerasimenko after separating from the orbiter, dissipating energy only through impact with the surface of comet. The MASCOT lander from Hayabusa2 landed on asteroid 162173 Ryugu in a similar manner. Rather than risking a large monolithic lander in a lithobraking maneuver, a proposed alternative is to instead attempt lithobraking with a swarm of much smaller landers at the same time. If there are many opportunities for a successful landing, then the chance of success on any single landing does not need to be high.
Instead of attempting to slowly dissipate the incoming velocity, it can be used to enable the probe to penetrate the surface. This can be tried on bodies with low gravitation, such as comets and asteroids, or on planets with atmospheres (by using only small parachutes, or no parachutes at all). Several such missions have been launched, including penetrators on the two Phobos probe landers targeted for Mars' moon Phobos and ones for Mars itself on Mars 96 and Deep Space 2, but so far none have succeeded. The cancelled LUNAR-A probe would have carried penetrators to the Moon.
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.
Lithobraking is also used as a humorous euphemism for the result of a spacecraft crashing into the surface of a body with no measures to ensure its survival, either by accident or with intent. For instance, the term has been used to describe the impact of MESSENGER into Mercury after the spacecraft ran out of fuel. This usage is popular among fans of the game Kerbal Space Program, where unintentional use of lithobraking is a common gameplay experience, and is referenced in the Kerbal Space Program 2 announcement trailer, where the tagline is "Lithobraking near you in 2020."
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