Aerobraking is when a spacecraft uses the atmosphere of a celestial body to reduce its speed relative to the body.

This is useful in a number of situations:

  1. when the spacecraft is in a hyperbolic orbit (i.e. an escape orbit) and it is wished for the spacecraft to be captured by a planet, for example Mars. This can be achieved by arranging for the orbit to just touch the martian atmosphere. By entering the highest levels of the atmosphere it can achieve capture, without any use of fuel. This technique requires a reasonably powerful heat shield.

  2. a spacecraft is in an already in an elliptical orbit, by modifying the orbit so that the perigee (the closest point of the orbit) is just inside the atmosphere, the spacecraft will fall down into a closer orbit. Note that aerobraking in this way does not modify altitude at perigee very much- it mostly reduces the altitude at apogee, this leads to the orbit being more and more circular. Also, little or no heat shield is needed, as the braking is very slow. When the orbit has decayed enough, a short rocket burn at apogee (the furthest point of the orbit) is sufficient to stabilise the orbit by moving perigee above the atmosphere.

  3. a spacecraft wishes to land on the planet. By entering the thickest parts of the atmosphere the spacecraft can burn off all its orbital speed and return to the surface. This is the Apollo technique. Note that it is normal for returning spacecraft to do a deliberate 'bounce', i.e. they enter the atmosphere, burn off some speed and generate enough lift to leave it again for a short time, before reentering and landing- this reduces the load on the heat shields and allows the heat to dissipate somewhat in between.

  4. The Lawrence Livermore National Laboratory (better known for their work on nuclear weapons) has proposed aerobraking to land on airless asteroids. The technique involves sending a nuclear weapon on ahead, exploding it, and hence generate a temporary atmosphere, and aerobraking on that. "Some people only know one good trick".