Gravity assist
A gravity assist uses a planet's gravity during a flyby to change a spacecraft's speed and direction without fuel.
A gravity assist, sometimes called a slingshot, is a manoeuvre in which a spacecraft flies close to a planet and uses the planet's gravity to change its path. Relative to the Sun, the craft can gain or lose speed, because it borrows a tiny amount of the planet's orbital momentum.
Gravity assists make many missions possible. Rosetta used three Earth flybys and one Mars flyby to reach comet 67P. OSIRIS-REx used an Earth flyby in 2017 to tilt its orbit to match Bennu's. Lucy is using several Earth flybys to reach the Trojan asteroids near Jupiter's orbit.
The effect on the planet is far too small to measure, but the gain for the spacecraft can be enormous. Without gravity assists, reaching distant asteroids and comets would require much more fuel, which means bigger rockets and much higher costs.
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