Neil deGrasse Tyson Explains Gravity Assists

What is a gravity assist? How does it work? On this explainer, Neil deGrasse Tyson and comic co-host Chuck Nice discuss how spacecraft use gravity and orbital speed to slingshot to their destination. How do you speed up or slow down without fuel?

Take a trip with Voyager 1 and 2 and how they used gravity assists to generate enough energy to leave the solar system. We uncover the mystery of its effectiveness: shouldn’t the same gravity that pulls you in and speeds you up, pulls you back as you exit? Discover the art of the slingshot, orbital speed, and “planetary mugging.” What happens to a planet’s orbit if you slingshot enough? What’s the official past tense of slingshot? All that, plus, more enlightened analogies from Chuck.

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Science meets pop culture on StarTalk! Astrophysicist & Hayden Planetarium director Neil deGrasse Tyson, his comic co-hosts, guest celebrities & scientists discuss astronomy, physics, and everything else about life in the universe. Keep Looking Up!

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[RPIdemon]

I had to go back to this video because I recalled the mugging analogy, and I had to listen to it again. It's hysterical!

[hoarder]

"Neilsplaining" 😂🤣😭 this is why we love u Chuck

[einarhornraiser]

These explainers are always a lot of fun, but this one was particularly enjoyable. Chuck is ridiculous - I love it!

[mrcet]

Apollo did this “inverse mugging” as it entered lunar orbit, in order to reduce the amount of propellant it needed to carry to brake into Lunar orbit. That’s why it orbited the moon in retrograde, essentially.

[Odin]

"I never thought to analogize a gravity assist with a mugging"... This what happens when you get a brother from New York and a brother from Philly together at the same time.

[peterlo]

I just about died when they were role playing the planet mugging scene

[sirjamesonsmith]

I actually really like these at home calls...
Neil just randomly calls up Chuck:
"Listen up, I got something for you today"...

[DanielLee-wcgb]

Love the chemistry and energy between Neil and Chuck!

[davetoms]

This makes so much sense. I'd always wondered and assumed it had something to do with entry-vs-exit angles. This was a great explanation. Thanks Chuck and Neil!

[paulm]

'Slangshit' brilliant. Love these two guys, they work together really well. Not only are we wrecking our planet, we've also slowed Jupiter's orbital speed lol.

[jamesharrison]

Startalk is awesome, but the 1.50 minute advert sucks. I wondered if I had the wrong video playing.

[rhydlew]

I hope that one of my favourite scientists won't mind a respectful little correction. The fall and escape aren't mirror images. The reason for this is the non-uniform gravitional pull. If uniform then yes it would work like a pendulum on Earth where for small distances we can ignore differences in gravitational pull (no need to invoke GR or SR) as described by our hero in the video. However, the gravitational pull is non-uniform. The first half of the (simplified 4 section model, each section roughly 50km long), bear with me) approach phase STARTS at v1 and ends at a slightly faster v2 at say 50km from the second body (planet). The second half of approach however takes place (within 50km) in a stronger gravitational field (Newton's inverse Square law is all we need). So the gain from v2 to vmax is greater (than v1 to v2). Nothing unexpected so far, compared to the video. Now this is the important part, with higher velocity STARTING the escape phase, less DURATION is spent in that 3rd phase leaving the higher gravitational field (within 50km), so less deceleration occurs, so we leave this third phase having decelerated for the same/similar distance but a much shorter duration. The duration is important in a non-uniform field. Then we enter the lower gravitional field phase...where the DURATION will also be shorter than for the first phase. Hopefully I've explained that well enough, but some folks love analogies: so imagine you get in your car and accelerate at 0.1G for 20metres/yards, then (moving at speed) decelerate at 0.1G for 20metres/yards. You'll still be moving. Why? because the 0.1G happened for a symmetrical DISTANCE, not a symmetrical DURATION. Once this is understood, it becomes possible to grav slingshot with a body even if it is moving towards you. As long as its mass is considerably greater, you will leave with a higher delta v than you approached. This can be proven by simply changing frame of reference to that of the planet, the physics doesn't care if the common frame of reference of both bodies has some other velocity vector (SR might help there). If you chose a second body that happens to be going in the direction you approach from, the bonus is simply that this effectively reduces the relative (for the planet's frame of reference) approach velocity and so increases the DURATION of the first half of the grav assist. Also remember that this 'bonus' (of Chuck mugging some orbital velocity) applies only to the approach, you can escape/depart in any direction you like, and the mugging bonus isn't lost. Polite request, please avoid commenting until you at least understand the car analogy/calculation.

[rayzorrayzor]

So it shouldn’t be called Gravity Assist, it should be called Orbital Assist .

[moogfooger]

I love your informative science Neil and Co. This time the 3 minute commercial was a bit like a punch in the head cause I want to find out about science!

[chasepalmer]

Thank you so much for explaining this. I always wondered how you gain speed from slingshotting off of other planets.

[EdwardChan.]

Watched some of Scott Manley's videos on Gravity Assist, but Neil's explanation is much clearer!

[kevinflorence]

I love Dr.Tyson and Chuck, please keep making these !

[bladesolar]

I've been a fan for a long time now. You guys are amazing. I'm giving both of you the flowers. I saw 2 videos today and was just enjoying the talk and thought to myself, Phenomenal.
I have to say this. My 2 yr old son soon to be 3, knows the planets and dwarf planets by heart. I want to continue to fan the flame of his curiosity. I'll take any advice you have to see him grow. He loves space just like his daddy.
Also, thank you Star Talk gave me the chance to reignite my love for space, science and the want to learn. I'm a big fan.

[Kismile.Farms.Uganda]

I did biological science. I missed physics and astronomy until I met Neil in Cosmos and star talk. Now I have access to full science package

[joeward]

Always wondered about this! Finally looked it up! Closest approximation I can think of is drafting or Marty McFly on a skateboard grabbing the back of the Jeep for a speed boost- the vehicles grab onto the trailing gravity well of a planet for a speed boost from the orbital velocity!