How do spacecraft use gravity assists to save fuel?

A gravity assist, or slingshot, uses a planet's motion around the Sun to alter a spacecraft's trajectory. As the craft swings by, it gains speed relative to the Sun by borrowing a minute amount of the planet's orbital energy. Mission planners use this technique to reach faraway targets with far less fuel than a direct flight would need. Voyager 1 and Voyager 2 famously used gravity assists from the giant planets to extend their missions into deep space.