You’ve in all probability watched this kind of science-fiction scene greater than as soon as: some stalwart starship captain and their crew are fleeing from aliens/escaping a supernova /operating out of gasoline and are seemingly out of choices, about to get eaten/vaporized/caught. However then, simply forward, they spot a planet! In order that they head proper for it, rockets blazing, then dive down and use its gravity to slingshot to security. Hooray! Cue the triumphant music.
So it goes on the silver display screen, at the very least. However does this maneuver work in actual life?
Sure! Effectively, not a lot the best way it’s completed in films—however it’s an precise factor. It’s broadly often called a gravitational slingshot, although most scientists check with it as a gravitational help, and it’s a vital software for many interplanetary missions.
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The thought appears easy sufficient. As a spacecraft approaches a large object, say, a planet, the gravity of the planet bends its trajectory, altering the spacecraft’s course. However there’s extra to it than that: the spacecraft can truly use the planet’s gravity to hurry up or decelerate after this maneuver, permitting simpler voyages to the outer or internal planets, respectively.
Whereas the trajectory-bending half appears apparent sufficient, that speed-up-or-slow-down half is fairly counterintuitive. It’s associated to the symmetry of gravity.
In the event you maintain a rubber ball a long way from the bottom and drop it, the ball will speed up because it falls, dashing up till impression. Then it bounces, transferring upward and decelerating because it does so. It should ultimately come to a cease, whereupon you may catch it or let it fall once more. However both manner, it could possibly’t bounce any larger than the peak from which you dropped it. It gained kinetic power—the power of movement—because it fell however then misplaced it as soon as once more postbounce because it slowed on its manner again up. This motion is symmetric, so at greatest (when you had a wonderfully elastic ball and did this experiment in a vacuum), it might bounce to the identical top from which you dropped it.
The identical is true for a spacecraft approaching a planet. The world’s gravity will speed up you as you fall in, you’ll whip round at closest method (that’s the “slingshot” half), and then you definately’ll lose that additional velocity as you progress away as a result of the planet’s gravity continues to be pulling on you. As that gravitational grip slips away, the spacecraft can be transferring relative to the planet on the identical velocity at which it initially approached.
So if all of the bonus velocity is misplaced on the best way out, how can this maneuver be used to speed up a spacecraft? The bottom line is within the phrase “relative to the planet.” In the event you method the planet at, say, 20 kilometers per second (km/s), you’ll go away with the identical velocity. However that’s your velocity measured in opposition to the planet.
At that very same time, crucially, the planet can be orbiting the solar. In the event you method the planet from behind (that’s, within the course of its movement), then, because the planet’s gravity offers you a lift, it additionally, in a heliocentric sense, pulls you alongside, including a few of its orbital velocity to yours. That provides you a kick relative to the solar, dashing you up in your strategy to your vacation spot. In essence, the spacecraft will get a web acquire in velocity by stealing just a little little bit of the planet’s orbital kinetic power.
In flip, this implies the planet truly slows a bit in its orbit across the solar—which sounds harmful! However concern not: the planet slows in proportion to how way more large it’s than the spacecraft. Given a typical one-ton probe in contrast with a multisextillion-ton world, the planet doesn’t noticeably sluggish in any respect. You would launch one million probes at it and by no means be capable of inform the distinction in its orbital velocity. A bacterium bouncing off you when you’re out strolling would have a far bigger impact on you.
The explanation it’s worthwhile to go to the difficulty of gravitational assists is that spacecraft are launched by rockets, which may solely speed up to some high velocity. For our present rocketry, these speeds are so low and the interplanetary distances so nice that even the quickest, most direct voyages take years (and even many years for locations within the outer photo voltaic system). You may load the spacecraft with extra gasoline to burn to go sooner, however there’s a restrict to that, too. Gas has mass, and also you’d have to speed up that additional mass, which takes extra gasoline, which has extra mass. This catch-22 is described by what known as the rocket equation, and it means the quantity of gasoline you need to add to maneuver even barely sooner reaches prohibitive scales very shortly.
So shaving day off your voyage requires another technique—akin to siphoning velocity from a giant, juicy planet alongside the best way! For instance, the Cassini probe to Saturn, which launched in 1997, was an enormous spacecraft, the dimensions of a college bus, and had a mass of two.5 metric tons with out gasoline. (The addition of the gasoline it wanted to meet its mission at Saturn, together with the launch automobile and different gear, tipped the scales to five.7 metric tons.) It will’ve taken virtually perpetually to get to Saturn with the rockets we had then. So the mission planners took benefit of Jupiter, sending the spacecraft previous it on a speed-boosting slingshot maneuver that shaved important day off the journey. The truth is, simply to get out to Jupiter within the first place, Cassini additionally carried out two fuel-saving flybys of Venus and one among Earth, stealing planetary orbital power each time.
A gravitational help works the opposite manner, too. Earth orbits the solar at greater than 30 km/s, so firing a probe on the solar or the internal planets is extraordinarily exhausting due to all that sideways velocity. As an alternative mission planners favor a extra circuitous route. They launch the spacecraft with sufficient velocity in the other way of Earth’s path across the solar to drop in entrance of, say, Venus, the place it could possibly then donate a few of its orbital power to the planet to drop towards the solar much more. BepiColombo, a joint European House Company and Japan Aerospace Exploration Company mission to Mercury, did precisely this, passing Earth as soon as and Venus twice to get in Mercury’s neighborhood. Even then, it needed to do a complete of six gravity assists previous Mercury to match the planet’s orbital velocity across the solar. The final help was in January 2025, and it’ll enter Mercury orbit in November 2026.
Gravitational assists are an emblematic instance of why area journey is exhausting—it is precisely rocket science, in any case. Gravity is the most important wrongdoer; simply getting away from Earth within the first place is the most important a part of the issue. It’s ironic, then, that gravity could make reaching a lot of the remainder of the photo voltaic system a lot simpler.