How do spacecraft use gravity to change course?

A gravity assist, or slingshot maneuver, uses a planet’s motion around the Sun to boost or redirect a spacecraft. The spacecraft falls through the planet’s gravitational field and leaves with a different velocity relative to the Sun, gaining or losing energy depending on the trajectory. This technique has been crucial for missions such as Voyager, Cassini, and many others because it saves enormous amounts of propellant. The craft does not get free energy; instead, the planet loses an unimaginably tiny amount of orbital momentum.