Aerobraking
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Aerobraking is a spacecraft maneuver that reduces the high point of an elliptical orbit (apoapsis) by flying the vehicle through the atmosphere at the low point of the orbit (periapsis), using drag to slow the spacecraft. Aerobraking saves fuel, compared to the direct use of a rocket engine, when the spacecraft requires a low orbit after arriving at a body with an atmosphere.
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[edit] Method
When an interplanetary vehicle arrives at its destination, it must change its velocity to remain in the vicinity of that body. When a low, near-circular orbit is needed around a body with substantial gravity (as many scientific studies require), the total required velocity changes can be on the order of several kilometers per second. If done by direct propulsion, the rocket equation dictates that a large fraction of the spacecraft mass must be fuel. This in turn means either a relatively small science payload or use of a very large and expensive launcher. Provided the target body has an atmosphere, aerobraking can be used to reduce fuel requirements by using a smaller burn to allow the spacecraft to be captured into a very elongated elliptic orbit. Aerobraking is then used to circularize the orbit. Due to the small effect of atmospheric drag, achieving the final orbit takes a long time (e.g., over 6 months when arriving at Mars), and may require several hundred passes through the atmosphere of the planet or moon.
The kinetic energy dissipated by aerobraking is converted to heat and so a spacecraft using the technique needs to be designed to reject the heat generated. The spacecraft must also have suitable surface area and structural strength to produce and survive the required drag, but the deceleration and thus temperatures and pressures are not as significant as reentry or aerocapture. Simulations of the Mars Reconnaissance Orbiter aerobrakinguse a force limit of 0.35 N per square meter with a spacecraft cross section of about 37 m², and a maximum expected temperature as 340 °F (170 °C). In another article about Mars Observer the force on the whole spacecraft was compared to force of a 40 mph (60 km/h) wind on a human hand at sea level on Earth. Another article on MGS quotes a force of roughly 0.2 N (0.04 lbf) per square meter.
Aerocapture is a related but more extreme method in which no initial orbit-injection burn is performed. Instead, the spacecraft plunges deeply into the atmosphere without an initial insertion burn, and emerges from this single pass in the atmosphere with an apoapsis near that of the desired orbit. Several small correction burns are then used to raise the periapsis and perform final adjustments. This method was originally planned for the Mars Odyssey orbiter, but the significant design impacts proved too costly.
[edit] Spacecraft missions
Although the theory of aerobraking is well developed, utilising the technique is difficult as a very detailed knowledge of the character of the target planet's atmosphere is needed in order to plan the maneuver correctly. Currently, the deceleration is monitored during each maneuver, modifying future plans accordingly.
Aerobraking was first used during the extended Venus mission of the Magellan spacecraft to circularize the orbit in order to increase the sensitivity of the measurement of the gravity field. The entire gravity field was mapped from the circular orbit during a 243 day cycle of the extended mission. After the gravity field was mapped, a "windmill experiment" was performed during the termination phase of the mission where atmospheric drag was used to deorbit the Magellan spacecraft.
In 1997 the Mars Global Surveyor orbiter was the first spacecraft to use aerobraking as the main planned technique of orbit adjustment. MGS used the data gathered from the Magellan mission to Venus to plan its aerobraking technique. The spacecraft 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. Unfortunately, a structural failure shortly after launch severely damaged one of MGS's solar panels, requiring a higher aerobraking altitude (and hence one third the force) than originally planned, significantly extending the time required to attain the desired orbit.
The technique has also been used by the Mars Odyssey and Mars Reconnaissance Orbiter spacecraft.
In Arthur C. Clarke's fictional book 2010 (and in the film), aerobraking is used to slow a spacecraft when moving into Jupiter's orbit.
