What is a gravity assist in spaceflight?

A gravity assist works when a spacecraft flies past a planet or moon and steals a tiny amount of that body’s orbital energy. Because the planet is enormously more massive, the effect on it is negligible, but the spacecraft can gain or lose speed relative to the Sun. Mission planners use this technique to reach distant targets with far less fuel than a direct route would require. Famous missions such as Voyager 1 and Voyager 2 used multiple gravity assists to explore the outer Solar System.