Gravitational Assist

Deep-space missions some times use close gravity-assist 'swingbys' of planets and moons to gain or lose velocity. These maneuvers increase the amount of mass that can be delivered and/or decrease mission flight times. The two Voyager spacecraft used gravity assists to leave the solar system. The Galileo spacecraft is using gravity assists to move among the various moons of Jupiter and the Cassini spacecraft will do similar maneuvers around Saturn.