| Astrophysics (Index) | About |
Gravity assist, aka gravitational slingshot or swing-by is the use of an interaction with an astronomical body to boost the speed of a spacecraft. Gravity assists can be very effective, vastly reducing the amount of fuel needed for a space mission, in turn, reducing the rocket power needed to begin an interplanetary space flight, and they are now commonly used and make otherwise impractical missions possible. Compared with alternatives, they can require more travel time, though in some instances can shorten it. The earliest space flights outward from Earth simply used the fuel necessary to shift to the outward trajectory of a more eccentric orbit, then when the spacecraft approaches the target orbit, to shift into it, the amount of fuel necessary to do these requiring a larger rocket to launch off Earth, in turn, requiring yet more additional fuel. Gravity assists are also used to reduce the speed of a spacecraft, and have been useful slowing down spacecraft traveling from Earth closer to the Sun, for example, to match the speed of Mercury's orbit. Gravity assist can produce extremely fuel-efficient travel throughout the inner solar system, certainly out into the Kuiper Belt, but assists can require particular launch windows, and the highest efficiencies can require a lot of travel time.
The assist uses the fact that a spacecraft undergoing a gravitational interaction with a body that is moving in some direction invariably picks up some velocity along that direction. For example, if the flight of a spacecraft leaving Earth orbit (achieving some orbit of the Sun) can be arranged so that it interacts with Venus in such a way that the enhanced shift in velocity further increases its orbital speed, such that it enters an eccentric orbit that ranges to a further distance from the Sun that it later achieves. Such an eccentric orbit (like those of comets) can travel outward among outer planets yet also return inward, allowing the same planet to be so-used more than once. Current missions may use multiple such assists, using the same and/or additional planets.
The gravitational interaction is (somewhat) analogous to bouncing a ball off a vertical surface. If the surface is simultaneously moving toward you, the ball can return after the bounce at an increased speed. The interaction between the spacecraft and the planet that consists of the spacecraft approaching the planet nearly head-on, being drawn around behind it and sent forward again (tracing a hyperbola), is much like such a bounce. Using the spacecraft's booster during the interaction can adjust the resulting velocity, both to further increase the speed-change and to place the spacecraft on the desired trajectory (i.e., in the desired subsequent orbit).