What is a gravity assist or a swingby for a spacecraft? I've heard about spacecraft gaining velocity from passing close to planets and being 'sling shot'-ed into a higher orbit. Where does this energy come from?

Gravity assist is a technique used to speed up, slow down or change the direction of a spacecraft as it approaches a planet within our solar system. This technique saves fuel that would otherwise have to be used to make these adjustments.

The basic principles behind gravity assist have to do with the relative velocities of the spacecraft and of the planets involved and the conservation of angular momentum. The spacecraft approaches the planet. Because the planet is moving too (while orbiting the Sun), it actually transfers some of its angular momentum to the spacecraft. What the craft gains in velocity, the planet loses. The mass of the planet is huge compared to that of the spacecraft so the loss is imperceptible. For example, Galileo increased its speed at Earth by 11,620 mph in 1990 and 8,280 mph in 1992. The Earth slowed down in its orbit by a speed of about 5 billionths of an inch per year from the two events.

The technique of gravity assist or swingby approach for a spacecraft was used as early as the Mariner 10 mission in 1973-1974 and has been used recently to create unique orbits such as with the Ulysses spacecraft.

Submitted by Mark (Maryland, USA)
Submitted by Jean-Marc (Oullins, France)
(September 3, 1998)

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