It's very common to use gravity assist, especially in outer planets missions. Fuel is very expensive and has diminishing returns (the more fuel you put on, the

It's very common to use gravity assist, especially in outer planets missions. Fuel is very expensive and has diminishing returns (the more fuel you put on, the more fuel you need to lift the extra fuel you put on). After Voyager, Galileo, Cassini, and New Horizons (among others) have used gravity assist to get to Jupiter, Saturn, and Pluto respectively (NH is still en route). Even Stereo, a two-satellite pair in earth-trailing and earth-following orbits looking at the Sun, used gravity assist (with the Moon). Gravity assist is one way to provide "delta-v" (velocity change, the coin of the realm in propulsion). You can provide negative delta-v using aerobraking -- orbiting your destination, Mars say, and slowing down using atmospheric friction. This is used on the Mars orbiters. As usual, see: http://en.wikipedia.org/wiki/Delta-v_budget http://en.wikipedia.org/wiki/Delta-v_budget