slingshot effect

[tolbiny]

Lots of sci-fi books/space travel stories talkabout using planets' gravitational pull for a "slingshot" efect to pick up speed. How does this work? How much speed can be gained (i am sure this is dependant on how big the mass is) and how close do you have to be for this to take effect? (in other words could you slingshot from just outside the horizon of a black hole?). Any cool comments on space travel also welcome.

[MidGe]

This is used "routinely" by NASA where useful/required. Here (http://www.tao.ca/~slingshot/displaybi.php?0059008) is a 1999 article about the Cassini mission which used such a technique. There have been many others.

Here (http://www2.jpl.nasa.gov/basics/bsf4-1.htm#gravity) is Nasa own explanation of the technique.

[chief444]

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How much speed can be gained

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The planet's orbital velocity relative to the sun is as much as the spacecraft can gain (also relative to the sun). In other words...if the planet itself wasn't in a rotational orbit the spacecraft could change direction but not speed. And if you just look at the planet and spacecraft without consideration of both of them relative to the sun or relative to some other point it will appear that the spacecraft has only changed direction as a net effect. Also...to increase speed the spacecraft must change direction.

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how close do you have to be for this to take effect?

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This is determined by the mass of the planet/magnitude of the gravitational force.