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, and it requires less fuel than does the direct use of a rocket engine.
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 around a body with substantial gravity (as is required for many scientific studies) is needed, 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 the spacecraft is limited to a relatively small science payload and/or the use of a very large and expensive launcher. 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 be captured into a very elongated elliptic orbit. Aerobraking is then used to circularize the orbit. If the atmosphere is thick enough, a single pass through it can be sufficient to slow a spacecraft as needed. However, aerobraking is typically done with many orbital passes through a higher altitude, and therefore thinner, region of the atmosphere. This is done to reduce the effect of frictional heating, and because unpredictable turbulence effects, atmospheric composition, and temperature make it difficult to accurately predict the decrease in speed that will result from any single pass. When aerobraking is done in this way, there is sufficient time after each pass to measure the change in velocity and make any necessary corrections for the next pass. Achieving the final orbit using this method takes a long time (e.g., over six months when arriving at Mars), and may require several hundred passes through the atmosphere of the planet or moon. After the last aerobraking pass, the spacecraft must be given more kinetic energy via rocket engines in order to raise the periapsis above the atmosphere—unless, of course, the intent is to land the spacecraft.
The kinetic energy dissipated by aerobraking is converted to heat, meaning that a spacecraft using the technique needs to be capable of dissipating this heat. The spacecraft must also have sufficient surface area and structural strength to produce and survive the required drag, but the temperatures and pressures associated with aerobraking are not as severe as those of reentry or aerocapture. Simulations of the Mars Reconnaissance Orbiter aerobraking use 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). The force density, of roughly 0.2 N (0.04 lbf) per square meter, that was exerted on the Mars Observer, during aerobraking is comparable to the force of a 40 mph (60 km/h) wind on a human hand at sea level on Earth.
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