A gravity assist, also known as a gravitational slingshot or a swing-by, was when a ship traveling at sublight speeds used the natural gravitational forces of planets, moons and other celestial bodies to allow the ship to change speed or trajectory. It was a practical way to conserve energy, fuel expenses and time. Gravity assists were commonly used for decelerating when arriving into planetary orbit and also to decelerate through a planet's atmosphere.

Stars can also be used for gravity assists to greatly magnify a ship's thrusting power. This solar slingshot however, was dangerous, as the ship would need to divert all its power to the deflector shields, lest the spacers expose themselves to extremely high levels of radiation.