gravity assist

A gravity assist — also called a gravitational slingshot — exploits the fact that a spacecraft's speed relative to a planet is unchanged by a flyby, but the direction of its velocity vector rotates. In the planet's reference frame the encounter is elastic: the spacecraft swings around and leaves at the same speed it arrived. But the planet is moving relative to the Sun, so when you transform back to the heliocentric frame the spacecraft's speed has changed. A trailing flyby (approaching the planet from behind in its orbit) adds the planet's orbital velocity, boosting the spacecraft. A leading flyby subtracts it, slowing the spacecraft down.

The energy comes from the planet's orbital kinetic energy. The planet slows down by an imperceptibly tiny amount — its mass is so large that the momentum transfer is negligible for it but transformative for the spacecraft. Conservation of energy and momentum are satisfied exactly; no propellant is consumed.

Gravity assists are essential to deep-space mission design. Voyager 2 used assists at Jupiter, Saturn, and Uranus to reach Neptune. Cassini flew past Venus twice, Earth once, and Jupiter once to reach Saturn. The Parker Solar Probe uses repeated Venus flybys to shed angular momentum and tighten its orbit around the Sun. Without gravity assists, most outer-planet missions would require prohibitively large rockets or impossibly long flight times.