Aerobraking

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), using drag to slow the spacecraft. Aerobraking saves fuel, compared to the direct use of a rocket engine, when the spacecraft requires a low orbit after arriving at a body with an atmosphere.

Method

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 is needed around a body with substantial gravity (as many scientific studies require), 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 either a relatively small science payload or use of a very large and expensive launcher. Provided the target body has an atmosphere, aerobraking can be used to reduce fuel requirements by using a smaller burn to allow the spacecraft to be captured into a very elongated elliptic orbit. Aerobraking is then used to circularize the orbit. Given enough atmosphere, a single pass through the atmosphere can be sufficient to slow a spacecraft as needed. However, to reduce the effect of frictional heating, and because it is difficult to accurately predict the amount of slowing that will take place in any one pass through the atmosphere (because turbulence effects and atmospheric composition and temperature can be somewhat unpredictable), aerobraking is typically done with many orbital passes through a thinner (higher altitude) region of the atmosphere so that after each pass there is time to measure the change in velocity and make appropriate corrections for the next pass. Thus achieving the final orbit 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. During the last aerobraking orbit, 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 and so a spacecraft using the technique needs to be designed to dissipate the heat generated. The spacecraft must also have suitable surface area and structural strength to produce and survive the required drag, but the deceleration and thus temperatures and pressures are not as significant as 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). [cite web |url=http://library-dspace.larc.nasa.gov/dspace/jsp/bitstream/2002/11218/1/NASA-2005-15sfmc-jlp.pdf |title=NASA LANGLEY TRAJECTORY SIMULATION AND ANALYSIS CAPABILITIES FOR MARS RECONNAISSANCE ORBITER |author=Jill L. Hanna Prince and Scott A. Striepe |publisher=NASA Langley Research Center |accessdate=2008-06-09 ] In another article about Mars Observer the force on the whole spacecraft was compared to force of a 40 mph (60 km/h) wind on a human hand at sea level on Earth. [ [http://www.spaceflightnow.com/mars/mro/060310moi.html Spaceflight Now | Destination Mars | Spacecraft enters orbit around Mars ] ] Another [http://www.spacedaily.com/mars/features/aero-97g.html article on MGS] quotes a force of roughly 0.2 N (0.04 lbf) per square meter.

Aerocapture is a related but more extreme method in which no initial orbit-injection burn is performed. Instead, the spacecraft plunges deeply into the atmosphere without an initial insertion burn, and emerges from this single pass in the atmosphere with an apoapsis near that of the desired orbit. Several small correction burns are then used to raise the periapsis and perform final adjustments. This method was originally planned for the Mars Odyssey orbiter, but the significant design impacts proved too costly.

pacecraft missions

Although the theory of aerobraking is well developed, utilising the technique is difficult as a very detailed knowledge of the character of the target planet's atmosphere is needed in order to plan the maneuver correctly. Currently, the deceleration is monitored during each maneuver, modifying future plans accordingly. Since no spacecraft can yet aerobrake safely on its own, this requires constant attention from both human controllers and the Deep Space Network, particularly near the end of the process when the drag passes are only about 2 hours apart (for Mars).

Aerobraking was first used during the extended Venus mission of the Magellan spacecraft to circularize the orbit in order to increase the sensitivity of the measurement of the gravity field. The entire gravity field was mapped from the circular orbit during a 243 day cycle of the extended mission. After the gravity field was mapped, a [http://www2.jpl.nasa.gov/magellan/prmgnwind.html "windmill experiment"] was performed during the termination phase of the mission where atmospheric drag was used to deorbit the Magellan spacecraft.

In 1997, the Mars Global Surveyor (MGS) orbiter was the first spacecraft to use aerobraking as the main planned technique of orbit adjustment. MGS used the data gathered from the Magellan mission to Venus to plan its aerobraking technique. The spacecraft used its solar panels as "wings" to control its passage through the tenuous upper atmosphere of Mars to lower the apoapsis of its orbit over the course of many months. Unfortunately, a structural failure shortly after launch severely damaged one of MGS's solar panels, requiring a higher aerobraking altitude (and hence one third the force) than [http://mars.jpl.nasa.gov/mgs/sci/aerobrake/SFMech.html originally planned] , significantly extending the time required to attain the desired orbit. More recently, aerobreaking was used by the Mars Odyssey and Mars Reconnaissance Orbiter spacecraft, in both cases without incident.

Aerobraking in fiction

In Robert A. Heinlein's fictional 1948 novel "Space Cadet", aerobraking is used to save fuel while slowing the spacecraft "Aes Triplex" for an unplanned extended mission and landing on Venus, during a transit from the Asteroid Belt to Earth. Verify source|date=November 2007 (This is probably aerocapture, not aerobraking).

In the film of 2010, the Alexei Leonov uses aerobraking in Jupiter's atmosphere.

ee also

* lithobraking
* reentry
* aerocapture

References

* [http://mars.jpl.nasa.gov/mgs/sci/aerobrake/SFMech.html JPL aerobraking report for MGS]
* [http://www.inspacepropulsion.com/tech/aeroassist.html NASA article on aeroassist]
* [http://research.nianet.org/~rtolson/documents/MGSAtmosModeling.pdf MGS aerobraking technical paper] (warning: PDF)


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