Aerobraking

Aerobraking is a technique used by spacecraft in which it uses drag within a planetary atmosphere to reduce its velocity relative to the planet.

Aerobraking has been used extensively over the decades by spacecraft orbiting Earth as a means to remove their orbital velocity and return to the surface. This removes a large amount of kinetic energy over a short period of time, most of which is converted into heat by shock heating the air ahead of the spacecraft. Spacecraft require a sturdy heat shield to survive the maneuver, as well as an aerodynamic shape and good resistance to acceleration. Aerobraking has also been employed in a similar manner by landers sent to Mars, Venus, and Jupiter.

Aerobraking can also be used in a less extreme manner, to adjust the velocity of a spacecraft without permanently reentering the atmosphere. For example, the Mars Global Surveyor Mars orbiter used its solar panels as "wings" to control its passage through the tenuous upper atmosphere of Mars to lower the apoapsis of its orbit over the course of many months. This sort of aerobraking does not result in as extreme temperatures or pressures, and so does not require as many design considerations.

Lithobraking

A related concept sometimes used in a humorous context is "lithobraking," the use of friction with a planetary crust to slow a spacecraft. This is perhaps better known as "impacting" or "crashing." However, it is not entirely without serious application; many landers destined for airless bodies such as Earth's Moon have employed lithobraking to various degrees and with some success. Many proposed probes to small asteroids or comets include penetrators intended to impact at high speed.