Spacecraft steal a little of a planet's motion to speed up
Throw a tennis ball at 30 kilometres an hour toward a train coming at 50, and it bounces back at 130. Spacecraft pull the same trick with planets: swinging close past a moving world, they borrow some of its orbital momentum and leave much faster, all without burning fuel.
In the train's frame, the ball arrives at 80 and leaves at 80; from the platform, the train's speed gets added on. A planet does the same to a passing craft. Relative to the planet, the spacecraft enters and leaves at the same speed, just in a new direction, but relative to the Sun the planet's own motion is added or subtracted. Energy is conserved: whatever momentum the craft gains, the planet loses, though for a body that massive the change is immeasurably small.
The trick works in reverse too. Because Earth-to-inner-planet flights naturally speed up as they fall sunward, Mariner 10 and MESSENGER used flybys to shed speed on the way to Mercury. When more change is needed than a flyby supplies, firing the engine at the moment of closest approach gets the most out of every kilogram of propellant. That matters because every bit of fuel must itself be lifted into space, so missions run on a tight delta-v budget.
Yuri Kondratyuk sketched the idea in a manuscript from 1918 to 1919 that went unpublished until 1938, and in 1956 Italian engineer Gaetano Crocco worked out a journey using several flybys. The Soviet Luna 3 first used the manoeuvre in 1959 to photograph the Moon's far side, relying on research led by Mstislav Keldysh. At NASA's Jet Propulsion Laboratory, the graduate student Michael Minovitch refined the method in 1961, and in 1964 Gary Flandro spotted a rare lineup of Jupiter, Saturn, Uranus and Neptune that could cut an outer planet tour from forty years to under ten.
The Voyager probes seized that alignment in the late 1970s; the next comparable one arrives in the middle of the 22nd century. Planets are seldom placed conveniently, and how much a flyby helps depends on how closely a craft can skim past. The Sun cannot serve as a slingshot, since it sits essentially still at the centre of the Solar System.
Source: Gravity assist
