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How Do Gravitational Slingshots Work?

Have you ever heard that spacecraft can speed themselves up by performing gravitational slingshot maneuvers? What’s involved to get yourself going faster across the Solar System.

Let’s say you want to go back in time and prevent Kirk from dying on the Enterprise B.

You could use a slingshot maneuver. You’d want to be careful that you don’t accidentally create an alternate reality future where the Earth has been assimilated by the Borg, because Kirk wasn’t in the Nexus to meet up with Professor Picard and Sir Iandalf Magnetopants, while they having the best time ever gallivanting around New York City.

*sigh* Ah, man. I really love those guys. What was I saying? Oh right. One of the best ways to increase the speed of a spacecraft is with a gravitational slingshot, also known as a gravity assist.

There are times that fantasy has bled out too far into the hive mind, and people confuse a made up thing with an actual thing because of quirky similarities, nomenclature and possibly just a lack of understanding.

Simulator view of Rosetta's first Earth flyby / gravity assist in March 2005. The probe flew by Earth three times and Mars once to conserve fuel and send it beyond the asteroid belt to rendezvous with Comet Churyumov-Gerasimenko. Credit: INOVE

Simulator view of Rosetta’s first Earth flyby / gravity assist in March 2005. The probe flew by Earth three times and Mars once to conserve fuel and send it beyond the asteroid belt to rendezvous with Comet Churyumov-Gerasimenko. Credit: INOVE

So, before we go any further a “gravitational slingshot” is a gravity assist that will speed up an actual spacecraft, “slingshot maneuver” is made up bananas nonsense. For example, when Voyager was sent out into the Solar System, it used gravitational slingshots past Jupiter and Saturn to increase its velocity enough to escape the Sun’s gravity.

So how do gravitational assists work? You probably know this involves flying your spacecraft dangerously close to a massive planet. But how does this help speed you up? Sure, as the spacecraft flies towards the planet, it speeds up. But then, as it flies away, it slows down again. Sort of like a skateboarder in a half pipe.

This process nets out to zero, with no overall increase in velocity as your spacecraft falls into and out of the gravity well. So how do they do it? Here’s the trick. Each planet has an orbital speed travelling around the Sun.

As the spacecraft approaches the planet, its gravity pulls the much lighter spacecraft so that it catches up with the planet in orbit. It’s the orbital momentum from the planet which gives the spacecraft a tremendous speed boost. The closer it can fly, the more momentum it receives, and the faster it flies away from the encounter.

To kick the velocity even higher, the spacecraft can fire its rockets during the closest approach, and the high speed encounter will multiply the effect of the rockets. This speed boost comes with a cost. It’s still a transfer of momentum. The planet loses a tiny bit of orbital velocity.

If you did enough gravitational slingshots, such as several zillion zillion slingshots, you’d eventually cause the planet to crash into the Sun. You can use gravitational slingshots to decelerate by doing the whole thing backwards. You approach the planet in the opposite direction that it’s orbiting the Sun. The transfer of momentum will slow down the spacecraft a significant amount, and speed up the planet an infinitesimal amount.

Messenger's complicated flyby trajectory. Credit: NASA

Messenger’s complicated flyby trajectory. Credit: NASA

NASA’s MESSENGER spacecraft made 2 Earth flybys, 2 Venus flybys and 3 Mercury flybys before it was going slowly enough to make an orbital insertion around Mercury. Ulysses, the solar probe launched in 1990, used gravity assists to totally change its trajectory into a polar orbit above and below the Sun. And Cassini used flybys of Venus, Earth and Jupiter to reach Saturn with an efficient flight path.

Nature sure is trying to make it easy for us. Gravitational slingshots are an elegant way to slow down spacecraft, tweak their orbits into directions you could never reach any other way, or accelerate to incredible speeds.

It’s a brilliant dance using orbital mechanics to aid in our exploration of the cosmos. It’s a shining example of the genius and the ingenuity of the minds who are helping to push humanity further out into the stars.

What do you think? What other places is the general comprehension between actual facts and fictional knowledge blurring, just like the “slingshot maneuver” and “gravitational slingshot”?

And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

Comments on this entry are closed.

  • Gnark1ll July 28, 2014, 2:16 PM

    Hey Fraser and UT team,…great video as always..these videos should be required for all the kids at school who might be doing a class on orbital dynamics (or any other space based course that is covered in this great series)..

    well actually not just the kids…and not just the videos… the WHOLE SITE should be in everyones bookmarks, due to its quality and endlessly fascinating subject matter…..

    Enough fawning …though I cant help it…but Ive been lurking around for at least 5 or 6 years now..I was at University when I discovered the site (Aeronautical Engineering, which is relevant to what Im about to ask) and it became immediately, and ever since, my main ‘feed’ for the latest goings on in Space Exploration of all forms. Sure I do use other sites an awful lot..but UT is my first stop every morning….I just love how when things are happening ‘live’, such as Chelyabinsk, when no firm info is available..the story is updated, and any erroneous data is corrected, and importantly, with a complete explanation. Unlike Space News in the ‘normal’ media that suffers terribly from false information.

    This site has always been an antidote to that..(along with Bad Astronomy’ too!) and of course the multitude of various space agency sites sites (what a gift to the world NASA, JPL, ESA etc online really are…we truly are privileged to be around at this time in history, with such rare info at all of our fingertips)

    So…Thanks Guys, I mean it.

    Anyway, the question I really wanted to ask Fraser, and thus, in an attempt to get an answer I wrote a glowing recommendation of his site (:p), was this,

    ‘With all this talk of Orbital mechanics, and the obvious expertise Fraser displays in the subject (still fawning a bit, sry), has he ever tried any of the sums ‘For Real’?

    By that, I mean…has he ever put a Kerbal on the line in a quest for a perfect parabola, in order to leave Kerbin far behind him (or rather his ‘test subject’), while using the least amount of fuel as possible?’

    Or to put it another way…is the UT staff as hooked on ‘Kerbal Space Program’ as the staff are at my office?

    If they are, and Im sure it would have been mentioned at some point, and Ive somehow missed it….but if NOT…..well I advise you to get the ‘game’ immediately, for a true treat is in store…(this goes for anyone that loves the subjects covered so expertly in UT..and have never heard of the majesty that is ‘KSC’)

    Im bringing it up now, a couple of years after its ‘Early Access’ release, because they have added ‘Missions’ to the game…and they truly offer a good challenge, even to those that can build extraordinary vehicles in the sandbox mode, and seemingly fulfill any self chosen goal…

    Its also another E-Item (like UT itself) that should be in every school in the world, for the kids interested in science and aeronautics to enjoy and fiddle about with..the way proto-engineers should..

    Anyway…thanks again, and if you ARE all Kerbal botherers at UT…Id love to see your creations : D

    • Fraser Cain July 31, 2014, 12:18 AM

      I’m a gigantic Kerbal fan. In fact, I’d say playing the KSP has taught me more about spaceflight than years of reporting and my educational background.

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