If a spacecraft is not to land on the surface of a planet, but is rather required to go into orbit around it, then aerobraking can be of significant benefit. Aerobraking significantly reduces the amount of onboard propulsion needed to slow down and be captured into orbit, but it does not eliminate it. To understand why this is so important, it is useful to discuss how an orbiter is captured around a planet using only rocket-based propulsion.

A typical mission without the use of aerocapture would go something like this: (1) launch (using some sort of rocket to go from the surface of the Earth into space); (2) Earth escape (using a propulsion system on board the spacecraft to provide enough velocity to escape the gravitation attraction of the Earth); (3) braking or orbital insertion (using the same or a different

2 Spencer, D.A., Blanchard, R.C., Braun, R.D., Kallemeyn, P.H. and Thurman, S.W., "Mars Pathfinder Entry, Descent, and Landing Reconstruction," Journal of Spacecraft and Rockets, vol. 36, no. 3, May-June 1999.

onboard propulsion system to shed energy and slow down the spacecraft so that it can be captured into orbit by the target planet's gravitational field; (4) circularization (meaning that that spacecraft uses its onboard propulsion system to place it in a useful orbit, typically a circular one).

The difference between this scenario and one that makes use of the atmosphere for aerobraking occurs in the very last phase. Instead of using an onboard chemical propulsion system to modify its final orbit, the spacecraft would use the friction between its solar arrays and the planet's atmosphere to slow down in small increments—a little every orbit until the final desired orbit is achieved. A spacecraft is typically captured into some sort of elliptical orbit and then performs a series of propulsive maneuvers to circularize it. These maneuvers are performed at periapsis (the portion of the orbit where the spacecraft is closest to the surface) because any thrusting there will counter-intuitively lower the spacecraft's apoapsis—or high point of the orbit. With aerobraking, the spacecraft encounters the planetary atmosphere during periapsis, using the slight friction between its solar arrays and the atmosphere to slowly bring down the ellipse toward a more circular orbit. Instead of using propellant, repeated encounters with the atmosphere can perform the same function. The Mars Global Surveyor mission—while reducing the need for significantly more propellant, used this approach in lieu of a rocket.

The primary benefit of aerobraking is the reduction in the amount of fuel needed. Without it, some missions would require larger and more expensive rockets simply to launch the spacecraft with all of the additional propellant on board. A drawback is that several orbital passes are required before the spacecraft can be where it needs to be—sometimes taking months to accomplish! This means that the spacecraft will not be able to achieve its objectives as quickly and it also introduces additional mission risk. Each time an additional maneuver is added, the risk of something going wrong increases.

Aerobraking can take several months to perform, as was the case of the Mars Global Surveyor, which launched in November 1996.3 The spacecraft's mission was to analyze and send data back to Earth about the planet's magnetic field, atmosphere, and surface. The Mars Global Surveyor made a series of aerobraking maneuvers over a nine-month period to gradually reduce its altitude and achieve its intended orbit. The Mars Odyssey spacecraft, launched in 2001, made a series of aerobraking

3 Beerer, J., Brooks, R., Esposito, P., Lyons, D., Sidney, W., Curtis, H.L. and

Willcockson, W., "Aerobraking at Mars: The MGS Mission,'' Journal of Spacecraft and

Rockets, vol. 33, January 1996.

maneuvers over a period of 77 days to gradually reduce its altitude and attain its final orbit around Mars.

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