The slingshot effect is an effect where an object that travels through the universe can be accelerated by other objects, by transferring some their momentum to the traveling object through gravitational interaction with it. This is often used by spacecraft as it reduces the fuel they require. When a probe is passing a planet in direction of movement of this planet, the gravitational interaction between the probe and the planet accelerates the probe (pushes the probe forward) and the probe gains kinetic energy during the encounter with the planet. This results in an increase in the probe's speed without using any fuel. Similarly if the probe passes behind the planet, it will be slowed down.
The slingshot effect is used in practice to put cosmic probes on higher orbits but can also be used to push them out of the Solar system. An example of this is the Voyager 1 mission. When used to alter the orbit of a spacecraft, the manoeuvre is called a gravity assist.
- A Gravity Assist Primer from solarsystem.nasa.gov