In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement and gravity of a planet or other celestial body to alter the path and speed of a spacecraft, typically in order to save propellant, time, and expense. Gravity assistance can be used to accelerate, decelerate and/or re-direct the path of a spacecraft.

The "assist" is provided by the motion (orbital angular momentum) of the gravitating body as it pulls on the spacecraft. The technique was first proposed as a mid-course manoeuvre in 1961, and used by interplanetary probes from Mariner 10 onwards, including the two Voyager probes' notable fly-bys of Jupiter and Saturn.

A gravity assist or slingshot maneuver around a planet changes a spacecraft's velocity relative to the Sun, though the spacecraft's speed relative to the planet on effectively entering and leaving its gravitational field, will remain the same (as it must according to the law of conservation of energy). To a first approximation, from a large distance, the spacecraft appears to have bounced off the planet. Physicists call this an elastic collision even though no actual contact occurs. A slingshot maneuver can therefore be used to change the spaceship's trajectory and speed relative to the Sun.