Why do spacecraft use gravity assists to travel through the Solar System?

A gravity assist, or slingshot, works because planets are already moving around the Sun. As a spacecraft passes near a planet, the planet’s gravity bends the spacecraft’s path and exchanges a small amount of energy and momentum with it. In the planet’s frame, the spacecraft may leave at nearly the same speed, but in the Sun’s frame it can gain or lose speed significantly. This technique is essential for deep-space missions because chemical rocket fuel is limited. The Voyager missions used multiple gravity assists in the outer Solar System, enabling their historic grand tour.