Aerobrake
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Aerobraking is an advanced interplanetary space navigation technique, whereby the velocity vector of a space vehicle is modified by interaction with a target body's atmosphere. The primary effect is dissipation of the vehicle's velocity via drag; however, it is also possible to add a lift component to the vehicle's interaction with the atmosphere in order to effect orbital plane changes or cross-range maneuvers.
All interplanetary vehicles must change their velocity in some way upon arriving at the destination planet, and the required velocity changes are usually on the order of several kilometers per second. The primary appeal of aerobraking is that the vehicle can save rocket fuel by using aerobraking to perform much of the necessary velocity change. Two caveats apply: the fuel savings are offset somewhat by the extra mass of the required heat shielding; and it may still be necessary to use some rocket power to achieve the desired maneuver.
This is the case for Aerocapture, in which the spacecraft's trajectory intersects the target planet's atmosphere, and the velocity dissipation is enough to capture the spacecraft into an elliptical orbit around the planet, but not so great that the vehicle falls out of orbit. In the case of aerocapture, the spacecraft will enter an orbit which has its apoapsis in space and its periapsis in the target planet's atmosphere. As the spacecraft takes one or more passes through the target atmosphere, the apoapsis is gradually lowered, until the vehicle's rockets are fired at apoapsis to raise the spacecraft's periapsis out of the planet's atmosphere. At that point the orbit has been stabilized.
Although the theory of aerobraking is very well developed, it is still a difficult maneuver to carry out. It is necessary to have detailed knowledge of the character of the target planet's atmosphere in order to plan the maneuver correctly, and even then success is not guaranteed. The 1999 loss of the Mars Climate Orbiter was due to an error in the execution of the aerobraking maneuver. However, since that time, the Mars Odyssey spacecraft and the Mars Global Surveyor have successfully performed sophisticated aerocapture maneuvers. The Mars Reconnaissance Orbiter is scheduled to attempt the maneuver in March - November 2006.