Gravity assist, also known as a gravitational slingshot, is a space maneuver that uses the gravity of a planet or moon to change the speed and direction of a spacecraft. By flying close to a celestial body, the spacecraft can gain energy from the body's motion, allowing it to travel faster and conserve fuel for its journey. This technique is often used in interplanetary missions, such as those involving NASA spacecraft. For example, the Voyager missions utilized gravity assists from Jupiter and Saturn to reach the outer planets and beyond, demonstrating the effectiveness of this method in space exploration.