You’ve probably watched this sort of science-fiction scene more than once: some stalwart starship captain and their crew are fleeing from aliens/escaping a supernova /running out of fuel and are seemingly out of options, about to get eaten/vaporized/stuck. But then, just ahead, they spot a planet! So they head right for it, rockets blazing, then dive down and use its gravity to slingshot to safety. Hooray! Cue the triumphant music.
So it goes on the silver screen, at least. But does this maneuver work in real life?
Yes! Well, not so much the way it’s done in movies—but it is an actual thing. It’s widely known as a gravitational slingshot, though most scientists refer to it as a gravitational assist, and it’s an essential tool for most interplanetary missions.
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The idea seems simple enough. As a spacecraft approaches a massive object, say, a planet, the gravity of the planet bends its trajectory, changing the spacecraft’s direction. But there’s more to it than that: the spacecraft can actually use the planet’s gravity to speed up or slow down after this maneuver, allowing easier voyages to the outer or inner planets, respectively.
While the trajectory-bending part seems obvious enough, that speed-up-or-slow-down part is pretty counterintuitive. It’s related to the symmetry of gravity.
If you hold a rubber ball some distance from the ground and drop it, the ball will accelerate as it falls, speeding up until impact. Then it bounces, moving upward and decelerating as it does so. It will eventually come to a stop, whereupon you can catch it or let it fall again. But either way, it can’t bounce any higher than the height from which you dropped it. It gained kinetic energy—the energy…
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