Aerobraking
Aerobraking (sometimes aerocapture) is a technique for reducing velocity relative to a celestial body through the controlled use of atmospheric drag.
Usually, when a craft enters the sphere of influence of a celestial body, its relative speed will be much too high to enter an orbit and without player intervention it will simply fly out the other side of the SOI and return to orbiting Kerbol (or the parent body of the target in the case of a moon). To reduce velocity and be captured into an orbit, the craft can either burn fuel, perform a gravity assist, or, if the target body has an atmosphere, aerobrake.
To perform an aerobraking maneuver, the trajectory of a craft is altered to pass through the body's atmosphere at a high altitude. The atmospheric drag will slow the craft down enough to convert its escape trajectory into an eccentric orbit. After the apoapsis is lowered to the desired height, it is necessary to raise the periapsis back outside the atmosphere to stop losing velocity and enter a sustainable orbit.
Aerobraking is a dangerous maneuver. If the craft dips too deep into the atmosphere, it will lose too much speed and if it cannot accelerate itself to compensate it will fall out of orbit entirely and crash into the surface. If it doesn't dip deep enough, it might not lose enough speed to be captured. Usually it is better to be too high than too low, because even if the orbit's apoapsis is just barely within the SoI of the planet, the periapsis will remain in the atmosphere and allow additional aerobraking maneuvers to further lower the apoapsis.
As of the current version (0.21.1[outdated]) traveling through an atmosphere at high velocity will cause dangerous-looking red flames around your craft, but this is almost completely harmless. The only parts which can be damaged by drag are deployed solar panels; retracting them prior to entering the atmosphere will protect them. However, Squad has announced that in the future reentry heat will be implemented which will be able to cause damage to a vessel. This factor could make aerobraking even more dangerous.
For the ideal height for an aerobraking maneuver, consult the article about the planet. The following planets and moons have an atmosphere which allows aerobraking:
In real world space operations, an aerocapture is a maneuver intended to enter an orbit around a target body, while aerobraking refers to lowering the apoapsis of an existing orbit. However, the terms are generally used interchangeably in Kerbal Space Program. Additionally, in real life aerobraking maneuvers are planned to be extremely conservative and a craft will typically perform many passes at a height at which the atmosphere is very thin, losing small amounts of velocity each time while avoiding the effects of turbulence and frictional heating (neither of which are present in KSP).
See also
- KSP Aerobraking Calculator by Alterbaron
- Aerobraking on Wikipedia
- Aerocapture on Wikipedia
