Artwork representing the gravity assist flyby, also known as gravitational slingshot, of the spacecraft Voyager 2 around Jupiter. A gravity assist flyby is used to modify the trajectory of a spacecraft and to either increase or decrease its heliocentric velocity (relative to the Sun). In a gravity assist flyby the heliocentric velocity is increased if the spacecraft passes behind the planet in its motion around the Sun, decreased if it passes in front of the planet. In the case of Voyager 2, the gravitational field of Jupiter (represented by circular shells drawn around the planet) was used to speed up the spacecraft and direct it to Saturn, Uranus and Neptune.