What's a gravitational slingshot?

Astronomer Matt Bothwell helped Ken out with this one...

Matt - So it's a manoeuvre that people use in astro-engineering for spacecraft to gain a velocity boost by bypassing an astronomical body, so normally like a planet or a moon or something. So if you think about a spacecraft flying through space in order to get it to speed up or slow down, it needs some energy and energy can't be created or destroyed, right? That's one of the most fundamental things about physics. But it can be moved around from place to place. And so if you do a fly-by of a planet, it's possible to steal some of the energy, some of the momentum from that planet and transfer it to the spacecraft.

Chris - I get the point that if I aim myself at a distant body, let's say I'm pointing towards Jupiter, I'm going to feel the gravitational acceleration that Jupiter has on me, so I'm going to speed up towards Jupiter a bit. But then when I go past Jupiter, why don't I lose the nudge that has given me again? Why don't I have to give it back? Because obviously I don't want to follow Jupiter around forever. I want to go where I want to go. So why don't I lose the energy again?

Matt - It's all a matter of getting the angles right. So you are exactly right, as you go towards Jupiter for example, you'll speed up and speed up as you're pulled in by its gravitational fields. And under normal circumstances you would swing around and then lose all this energy. But by carefully working out the angles, and this is why we call it a gravitational fly-by, you can kind of calculate it just right, so you actually leave with a bit more energy than you gained. Often the analogy that gets used is a bit like bouncing a tennis ball off a speeding train, right? If you see a train coming towards you at the station, you can gently throw a tennis ball at it and then it will hit the train at a hundred miles an hour and go flying off right? And that's stolen some energy from the train.

Chris - So the ball would not only bounce back towards you, but it would also be going sideways with the train. Is that what you're saying?

Matt - Exactly, yes. It's the gain in momentum basically. Obviously nothing comes for free. So as the spacecraft has gained a bit of energy, the planet also has to lose that energy, right? So just as the spacecraft leaves going faster, the planet is left going a tiny bit slower after the spacecraft has left.

Chris - So you basically plan your journey so that you would aim towards Jupiter and as Jupiter's in the right position, it's accelerating you, but then Jupiter on it's orbit moves out of the way, but you're by then moving on on a new trajectory, but you've gained a bit of a push in the right direction? And you keep repeating that cycle in various ways to just gain more and more free energy, as it were, off of the planets that give it to you, to give yourself ultimately more speed?

Matt - Exactly, yes. And planning the journey is actually very, very important, because you obviously need a planet to be in the way of your trajectory in order to get one of these fly-bys. There was a very nice fortuitous one in the late 1970s when the Voyager missions got sent off. There was what was called the Grand Tour. So the four outer planet, Jupiter, Saturn, Uranus, and Neptune were all in a nice line. And so we could gravitational slingshot through all of them and get a very, very nice boost into the outer solar system.