# Gravitational slingshots: could they be used to decelerate a spacecraft traveling at a small fraction of light speed?

I have heard of gravitational slingshots being used to theoretically accelerate a spacecraft using a planet's gravity and momentum in orbit. But how about decelerating a ship? If it is at all possible, could it be done without g-forces killing the entire crew and destroying the ship if said spacecraft were traveling at a fraction of the speed of light, say .1c?

plot synopsis: a ship traveling at sub-light speed is too low on fuel to decelerate in time to keep from blowing through and past its target solar system. Radical measures are considered.

Any help would be appreciated, thanks!

• I don't mean to discourage you, but you may get better results on the Space exploration SE
– Kys
Jul 20, 2016 at 20:18
• Also, yes. MESSENGER used this maneuver.
– Kys
Jul 20, 2016 at 20:24
• This would work at interplanetary speeds, but at a fraction of c, you would need far more deceleration than a single planetary slingshot could provide. If things lined up right, you might be able to make a series of slingshots around different planets, or even try to slingshot around the Sun, but I am not clear enough on the math to do an estimate. Jul 20, 2016 at 20:33
• G-forces? How could there be G-forces when there's no relative acceleration on the crew? You could accelerate at 1000g and feel nothing at all. G-forces arise from a difference in acceleration of some of your body parts from others - e.g. your legs accelerate at 10g, while your brain accelerates at 0g (and then suddenly gets hit with 1000g or more). The bigger problem is that orbital velocities are tiny compared to any significant fraction of the speed of light. You'd barely get a nudge (though of course, over interstellar distances, that will mean you'll miss your target by quite a bit). Jul 21, 2016 at 8:16
• You may be interested in this question about a similar emergency deceleration.
– Kys
Jul 21, 2016 at 14:10

Yes, in theory.

Gravity assist braking is a thing

The problem is the speed. Going that fast you'd need to do a lot of maneuvers in order to lose enough velocity to enter orbit around the sun, and that means that the planets would need to be in just the right places.

In the book Aurora, a ship traveling around .1c has to do something like this, and it ends up making something like 14 passes over the course of 20 years, gradually slowing until it got to a point where there would be no planet in its path for the next maneuver.

So not impossible.

Edit: getting crazy

OK, say that your crew is really really desperate. There is a very risky way to shed a lot of velocity really fast, but only if you're going really fast to start with...
The trick is to fly through the star.

If the structural integrity is really really good, and all the passengers are protected from the sudden deceleration, and you're going fast enough, you could pass through the sun so fast that the ship wouldn't have time to singe much, while hopefully losing enough speed to stay in the solar system and not just shoot it the other side. This would cut years off of the braking process. And hopefully a few passengers would not be crushed to jelly by the tidal forces.

Edit 2:

The sun isn't very dense near the surface. By aiming off center you'd be able to avoid the worst of the pressure. The heat wouldn't be an issue if you have some ablative material shielding the hull, since the heat would be removed before it can do damage, providing you get through fast enough that it cant all burn away.

Sun grazer comets have been observed passing through the outer layers of the sun and surviving.

• @jorfus You can't Gravity Assist Brake using the star. You always leave the object at the same speed as you arrived at an object (with gravity assists). So your stellar centric velocity remains constant with a stellar flyby. Planetary gravity assist braking works because you shift the frame of reference to the planet. So even though you don't slow down wrt to the planet, your stellar centric velocity is lowered.
– Aron
Jul 21, 2016 at 2:17
• The only way to slow down on a stellar flyby is by Aero/Plasma-braking. With the associated risk...>_<
– Aron
Jul 21, 2016 at 2:25
• Of course, if flying through the star doesn’t help, you can also try flying through a planet. You have to face similar challenges. Jul 21, 2016 at 11:53
• @AndyD273: not every planet is a solid rock, say Jupiter, for example, has even the advantage of not being that hot, but I guess, when hitting Earth, Sun or Jupiter with 0.1c, there’s not much difference… Jul 21, 2016 at 14:22
• Remember, the particles of the star will be hitting you at 0.1c, which gives them a vastly larger kinetic energy than if you passed through the outer layer of the star at speeds more typical of modern space probes...the total energy of a particle at 0.1c is about 1.005 times its mass energy, so the kinetic energy is 0.005 times mass energy, so the energy from collisions is about 1/200th that of colliding with the same number of antimatter particles at low velocity, still huge. Unless you have some science-fictional "force shield" this will probably be enough to destroy the ship. Jul 23, 2016 at 18:36

If you're coming into a star system at 0.1c, with no propulsion, you're screwed. The only way planets can help you actually stop is via lithobraking, which at these speeds amounts to "Maybe they'll name the crater after me?"

You can loose speed via gravitational slingshots, but you can't lose nearly enough. The maximum amount of speed you can loose is twice the planet's orbital velocity. That sounds like a lot, but 0.1c is ridiculously fast, 30,000 kilometres/second. The fastest-moving planet is Mercury, at 47.3 km/sec, Venus is 35 km/sec, and so on. Adding them up, if you could arrange a perfect encounter with all of the Solar System's planets, for the odds of which we need a phrase more extreme than "preposterously unlikely", you could lose about 340 km/sec.

That would let you lose about 1.1% of your speed. That hasn't really done any good, has it? The good news is that the G-forces of these encounters won't do your crew any harm, they're pretty weak. This is limited comfort as you zoom off into interstellar space.

• At one tenth the speed of light, it's entirely likely that there won't be a crater, or much of a planet after lithobraking. Jul 21, 2016 at 2:24
• "It's easier to ask forgiveness than permission", especially if the people whose forgiveness/permission you need are plasma. Jul 21, 2016 at 8:20
• "lithobraking" : sounds a good concept, until you try it. I'm in love with this word now :)
– Benj
Jul 21, 2016 at 8:59
• Does a gravity assist even work at those speeds? You'll be zooming past the planet in seconds, I don't think there is enough time for the planet to exert any significant gravitational attraction on you before you're already far past it... and at 0.1c you are certainly not reentering the solar system for a second pass. Jul 22, 2016 at 0:12
• You can only lose that maximum from a flyby braking pass if you're on a parabolic orbit with respect to the planet. Which you aren't at even mildly relativistic speeds, so even the best possible scenario gives much less than 340 km/s. Jul 22, 2016 at 3:11

## No

• in reasonable time, less then 100 years as example

## But

look at page 42 of manual, there is about emergency decelerating in situation with almost no fuel, I cite:

Press emergency braking system button to deploy Magnetic sail of emergency decelerating system, it's right there where button for Standard decelerating procedure in star systems which are not equipped with standard intergalactic star system braking systems(read as like outpost of humanity, no body home at the moment)

end of cite.

So press the button and RTFM.

• What does RTFM stand for? Read the flipping manual? Aug 8, 2016 at 17:51
• @XandarTheZenon Yes, it is indeed it. There are some difficulties to get that manual, in that case. I have no glue why. But some pages are available now, the rest have to be found in future, I guess. Aug 9, 2016 at 9:42

## Yes, but to do it with a single body in a way that avoided deadly tidal forces, you'd need a black hole with around 10,000 times the mass of the Sun or more, and it would probably have to be orbiting a much larger "supermassive black hole"

First of all, it should be noted that all the theories of gravity that physicists use (both Newtonian gravity and the more accurate theory of general relativity) are time-symmetric, which as discussed in this article means that if you take a movie of some bodies acting under the influence of mutual gravity and run it backwards, a physicist will have no way of telling whether the movie is playing backwards or forwards (or equivalently, time-symmetry implies that it's possible to set up a second system with different initial conditions such that if you calculate the behavior as time runs forwards, using the same laws of gravity, this second system's dynamics running forwards in time will look just like the first system's dynamics played backwards). Some reversed movies might be more unlikely than the forwards version if the forwards version featured a significant increase in entropy, but this would usually require a large number of different objects (like a collection of dust particles collapsing inwards due to mutual gravity), if you're just dealing with two bodies that don't crash into each other or otherwise break apart the change in entropy will probably be small. So, if you can come up with a situation where an object is initially traveling at only a small fraction of light speed relative to a larger body, but uses a gravitational slingshot to increase its velocity relative to the larger body to a much larger fraction of the speed of light, then the reverse scenario should be equally possible.

And the question of using a gravitational slingshot to attain a large fraction of light speed relative to the body used in the slingshot is addressed in the book The Science of Interstellar by gravitational physicist Kip Thorne, who consulted on the movie. In chapter 7, "Gravitational Slingshots", he notes that the ship in the movie (the 'Ranger') did not have sufficiently powerful rockets to accelerate to significant fractions of the speed of light on their own, but that

Fortunately, Nature provides a way to achieve the huge speed changes, c/3, required in Interstellar: gravitational slingshots around black holes far smaller than Gargantua.

Gargantua was the supermassive black hole in the movie (it was supposed to have a mass 100 million times that of the Sun), but Thorne writes that he imagined smaller black holes orbiting Gargantua. He also notes that while a neutron star or stellar mass black hole could possibly provide the required velocity change, doing so would require getting so close to them that the so-called tidal forces--the stretching people would feel due to the gravitational pull being noticeably stronger on the side of their bodies closer to the center of the neutron star or black hole than the side that was just a bit farther--would be deadly for bodies of this mass, so that a much more massive intermediate mass black hole would be needed to avoid being ripped apart by tidal forces (what astrophysicists colorfully refer to as spaghettification).

To change velocities by as much as c/3 or c/4, the Ranger must come close enough to the small black hole and neutron star to feel their intense gravity. At those close distances, if the deflector is a neutron star or is a black hole with a radius less than 10,000 kilometers, the human and Ranger will be torn apart by tidal forces (Chapter 4). For the Ranger and humans to survive, the deflector must be a black hole at least 10,000 kilometers in size (about the size of the Earth).

Now, black holes that size do occur in Nature. They are called intermediate-mass black holes, or IMBHs, and despite their big size, they are tiny compared to Gargantua: ten thousand times smaller.

He also mentions that "A 10,000-kilometer IMBH weighs about 10,000 solar masses", so that would be around the lower mass limit of what could be used to get a change in velocity of c/3 or c/4 without being torn apart by tidal forces, if you only need a change of c/10 it might be a bit smaller but my guess is it wouldn't be more than an order of magnitude.

@Aron also makes an excellent point in a comment on the answer by @AndyD273 -- namely, that gravitational assists can't actually provide a long-term boost in a ship's velocity in the rest frame of the massive body a ship gets the assist from, the velocity boost will only be seen in some other reference frame. The reason for this is that the total energy of the ship in the body's rest frame is just the sum of its potential and kinetic energy, and when the ship is some large distance D away from the body before passing close and getting an assist, its potential energy will be exactly the same as when it is the same distance D away from the body after the assist, so its kinetic energy must be the same too. Thus, a gravitational assist will only boost a ship's velocity in some reference frame where the massive body itself has a large velocity, like boosting one's velocity in the Sun's reference frame by passing close to Jupiter. In The Science of Interstellar Thorne was assuming the IMBHs were in orbit around the supermassive black hole Gargantua, and orbits around a fast-rotating supermassive black hole can reach substantial fractions of light speed, see my answer here for details. So if you want to decelerate relative to the galaxy, and your initial velocity relative to the galaxy is 0.1c, you'll probably need to find an IMBH that is itself moving at somewhere close to 0.1c (or greater) relative to the galaxy, with the most plausible astrophysical scenario for this being an IMBH orbiting a supermassive black hole.

• Best answer so far! Jul 22, 2016 at 13:37

Since you're traveling at speeds far higher than the escape velocity of the solar system relative to a point close to the star, you are not going to be able to stop within the solar system. Instead, you have to get rid of the excess kinetic energy by doing flybys of nearby stars until the velocity is low enough to enter the solar system and then do a flyby of a planet to lose enough energy to be captured in the solar system.

• Note that "low enough to enter the solar system" is going to be pretty darn slow. Max speed you can lose via a gravitational assist is 2x the speed of the planet - and speed of a planet in a circular orbit is 1/sqrt(2) of escape velocity. So your max excess speed is something like 0.6 * the speed of the planet, which is slow in interstellar terms.
– TLW
Jul 21, 2016 at 2:09

A mega-engineering structure could be used to both launch and catch ships moving at relativistic speeds. Using gravity means the acceleration would not be felt by the contents and could be at a much higher level than easier magnetic systems.

But we're talking about toruses made from hyperdense material. The rings are spinning such that a point on the torus traces a path that goes through the hole. The gravito-magnetic forces can accelerate any object that flys through the hole.

Now arrange a series of these in a line, like a gun barrel.

I’ve seen a similar designs described in Robert L. Forward’s Starquake, along with the awsome substance “monopole-stablized black hole dust”. Forward was a real-life physicist who knew gravity.

Well, how much of the ship are you going to decelerate?

Gravitation braking sort of works, in the "slow you down a few percentag points" sort of way. In the meantime, you are entering dense space near a star, so figure that comfortable world of crashing into one hydrogen atom per cubic meter just went wonky. If most of your ship is ablative, you can couple the gravity slingshot with skimming near a planet you never liked anyway.

I never ran the numbers, but skimming around in system should be slowing you down just by running through the heavy particle space. Of course, there are lots of issues about radiation, penetration, etc. Also, forget any blackbody attempts: at that speed, the fuzz will likely pull you over and anything black will be shot.

• Any particular reason people are just randomly down voting this? Not very nice :( May 5, 2018 at 19:57