How do Planetary Flybys Work?

Gravity assists -- flybys -- are pretty neat. These precision maneuvers that involve harnessing and using the gravity of a planet to accelerate and direct a spacecraft to its destination. It’s often described as a slingshot maneuver as though the planet grabs and flings a passing spacecraft along its way. But really, a flyby is more like throwing a ping pong ball into the blades of a ceiling fan. The blades will hit the ball and send it flying away faster and in a different direction. Now imaging throwing a ping pong ball into a ceiling fan in such a way that the ball then hits a marker on the wall next to you. That’s a planetary flyby.

A planet’s gravity is far stronger than a spacecraft’s, meaning that when a spacecraft flies past a planet, the planet exerts a far stronger gravitational pull on the spacecraft than the spacecraft does on the planet. The planet gravitationally pulls in and tosses away the spacecraft, transferring some of its momentum to the passing vehicle in the process. And at the same time, the spacecraft actually robs the planet of a little bit of its own momentum. But that’s not all. Planets aren’t static; they orbit around the Sun and rotate around their own axis. So when a spacecraft passes by the planet, the planet’s rotation helps bends the spacecraft’s trajectory.

Flybys are essentially used increase the energy of a spacecraft's solar orbit beyond the velocity afforded by its launch vehicle. The Voyager missions, which had Saturn as the target planet in both cases, are a perfect example. The Titan-III/Centaur rockets that launched these twin spacecraft only had enough energy to get them to Jupiter. Had the planet not been there, both spacecraft would have entered into a permanent oval-shaped solar orbit coming as close to the Sun as Earth's orbit and getting as far as Jupiter's orbit. But the giant planet was there with the spacecraft crossed its orbit. The spacecraft were pulled in by the planet's gravity. Without slowing down enough to stay at Jupiter, the spacecraft instead gained momentum from the gas giant and began the trip to Saturn. The Voyager spacecraft diverged after reaching the ringed planet because their fly bys were slightly different. Voyager 1 passed through the system such that its trajectory sent it flying out the plane of the ecliptic while Voyager 2 passed through the system such that its trajectory was bent in the direction of Uranus.

The opposite works as well. A properly aimed flyby could decelerate a spacecraft, slowing it down enough for it to be captured by the gravity of another body. NASA's Galileo spacecraft flew by Jupiter's moon Io to lose some speed, meaning the mission had to carry slightly less fuel for the retrofire burn that would put it in orbit around Jupiter.

Gravity assists are extremely useful in deep space robotic missions, and in the 1960s it was something NASA explored (sadly only as concept missions) as a means to send astronauts visiting both Venus and Mars on one flight. It would have been a long, but very interesting mission.

Sources: NASA