Summary
Aerobraking is a spaceflight 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). The resulting drag slows the spacecraft. Aerobraking is used when a spacecraft requires a low orbit after arriving at a body with an atmosphere, as it requires less fuel than using propulsion to slow down. When an interplanetary vehicle arrives at its destination, it must reduce its velocity to achieve orbit or to land. To reach a low, near-circular orbit around a body with substantial gravity (as is required for many scientific studies), the required velocity changes can be on the order of kilometers per second. Using propulsion, the rocket equation dictates that a large fraction of the spacecraft mass must consist of fuel. This reduces the science payload and/or requires a large and expensive rocket. Provided the target body has an atmosphere, aerobraking can be used to reduce fuel requirements. The use of a relatively small burn allows the spacecraft to enter an elongated elliptic orbit. Aerobraking then shortens the orbit into a circle. If the atmosphere is thick enough, a single pass can be sufficient to adjust the orbit. However, aerobraking typically requires multiple orbits higher in the atmosphere. This reduces the effects of frictional heating, unpredictable turbulence effects, atmospheric composition, and temperature. Aerobraking done this way allows sufficient time after each pass to measure the velocity change and make corrections for the next pass. Achieving the final orbit may take over six months for Mars, and may require hundreds of passes through the atmosphere. After the last pass, if the spacecraft shall stay in orbit, it must be given more kinetic energy via rocket engines in order to raise the periapsis above the atmosphere. If the craft shall land, it must loose kinetic energy, also via rocket engines. The kinetic energy dissipated by aerobraking is converted to heat, meaning that spacecraft must dissipate this heat.
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