Spacecraft use planets as 'gravitational slingshots' to gain speed and save fuel.

The gravity assist maneuver, or 'slingshot,' is a fundamental technique in orbital mechanics used to change the velocity of a spacecraft. When a craft enters a planet's sphere of influence, it is pulled by the planet's gravity. As it swings around, it gains a portion of the planet's massive orbital momentum. While the spacecraft gains significant speed, the planet loses an infinitesimal amount of energy, slowing down by only about one trillionth of a millimeter per year.The first successful application of this technique occurred in 1959 when the Soviet Union's Luna 3 used the Moon's gravity to photograph its far side. However, the first interplanetary use was NASA's Mariner 10 mission in 1974, which used Venus to reach Mercury. This maneuver was mathematically refined by Michael Minovitch at the Jet Propulsion Laboratory in the early 1960s, proving that gravity could propel missions further than chemical rockets alone.NASA's Voyager 1 and Voyager 2 missions famously used a rare alignment of the outer planets in the late 1970s to perform a 'Grand Tour.' By hopping from Jupiter to Saturn, Uranus, and Neptune, they shaved decades off their travel time. More recently, the Juno mission used an Earth flyby in 2013 to gain over 16,330 kilometers per hour of speed to reach Jupiter. Without these maneuvers, spacecraft would require massive, heavy fuel tanks that would make launch costs and engineering requirements nearly impossible.