Gravity assist
A gravitational slingshot, gravity assist maneuver or swing-by is the use of the relative movement and gravity of a planet to alter the path and speed of a spacecraft. This helps the spacecraft to save fuel, time, and expense. Gravity assistance can be used to speed up, slow down and/or change the path of a spacecraft.  The "assist" is provided by the motion of the gravitating body as it pulls on the spacecraft. The Soviet Union used it first for the Luna 3 probe that photographed the far side of the Moon. The later Pioneer program used gravity assist in several missions.