How does artificial gravity work?

Factual Questions
21

It is rarely or never presented like that in fiction. (1) It’s usually not “one big room”. (2) Rotational artificial gravity is constant – they don’t start and stop it. It would take a lot of energy to start and stop the rotation, so why do it?

Even in the movie 2001 the astronauts were depicted as weightless in the hub and they acquired “gravity” gradually as they descend toward the floor:

NASA initially planned on launching a 2nd Skylab space station, and there were plans to experimentally spin it at 4 rpm to produce roughly lunar gravity, about 0.18g. Skylab “B” was cut from the program so this was never done.

Even without spinning the vehicle, Skylab astronauts experimented with running around the interior wall, which produced “centrifugal gravity”: https://www.youtube.com/watch?v=iQ2LgUV2Dic

22

This scene from Babylon 5 gets it pretty right. Don’t click on it if you are concerned about spoilers in a 25 year-old TV series as it has one of the biggest reveals of the whole arc.

Ivanova isn’t correct when she says he’ll hit the ground at sixty miles an hour; more like he’ll smack into the side of a building when he gets close enough.

23

I don’t see how it was “pretty right”. The train runs down the axis of the rotating cylinder, right? Where does the gravity for the people inside the train come from? They seems to have normal gravity as they were walking inside the train and out into the station.

24

Would you get the same effect if, rather that having an entire section of a station rotate, you simply had a rotating cabin similar to the drum inside a clothes dryer?

25

The train isn’t quite on the axis, so it has a little gravity. Obviously there are limits to TV special effects, but they at least convey the hint that gravity is diminished, with everyone using the handholds more than usual and walking more slowly (not to mention the announcer saying “this is a low gravity area”).

That’s not what she says: she says the ground is rotating at 60 miles per hour, and that he’ll be killed by the impact. It’s a little ambiguous but it can certainly be interpreted as meaning he’ll be hit by a building or hillside.

I do wonder, though: the air is also moving at 60 miles per hour. He’ll enter this area of increasing wind speeds and be gently accelerated to that speed. The centrifugal acceleration will also increase gently. It’s just possible that he could land slowly enough to survive. With a perfect combination of factors, it could be as gentle as stepping off an escalator.

26

Basic Physics: circular motion is created by acceleration constantly perpendicular to the direction of motion. For a satellite, this is caused by gravity that pulls the satellite toward the center of the earth as it goes around the earth.
For a brick in a bag with a rope (or on a platform, with a rope) you are pulling on the rope. The other trick is to spin a bucket full of water on a rope. For this trick, the problem is a bit more complex - apparent gravity wows as the bucket hits top - centrifugal minus gravity - and bottom - centrifugal plus gravity. spin the same bucket in a circle over your head, the water level will stay slightly tilted - downward gravity plus apparent (“centrifugal”) gravity.

But, you say, where’s the acceleration? The acceleration is the force you exert to stop the bucket from flying off in a straight line, David-and-Goliath-like. You pull on the rope. with a heavy bucket, you even lean. that’s force. you are exerting force.

As you spin the bucket or bag, the air inside travels along - just like air in a car or train. As the station spins up the first time, it may take some slight breeziness until the inertia of the air is overcome, but air with low mass has low inertia - plus, most space stations will not be an open complete donut. There will be doors - so the air does not blow through. Think like a train accelerating - the air inside moves too.

However, air is a tricky subject. the air in a space station, moving or not, has 14psi.(assuming normal air). Here on this big ball of dirt, it takes a column of air about 100 miles or more high, all being pulled down by a 1G force, to stop air from flying off into space. (And notice too that, minus some weather effects, this air too rotates along with the dirt and water.) So a space station will be a pressurized container, and if a hole appears, that measly 100 feet or so of rotational “centrifugal” force will not have an appreciable effect on its desire to seek to equalize pressure with an area that is leaking into space. Close the intermediate hatches!

Also a note about that carnival ride. The fun part is to try moving your hand or head while the gizmo is spinning. You will experience Coriolis “force”. You are trying to move something in a straight line while the frame of reference is turning. It will flop in a different direction. Move your head, and hope you don’t regurgitate the cotton candy and hot dogs. Your inner ears are not prepared for rapidly changing frame of reference. (Ever been an a whirly ride and felt like upchucking? That’s the tiny hairs in your inner ear that contribute to balance. Shake them too badly, and they think you’re being poisoned and make you remove your stomach contents.)

In fact, Arthur C. Clarke said in designing 2001, they acknowledged that to get 1G in the Discovery spaceship, without Coriolis forces mucking up your inner ear to cause severe nausea whenever you turned your head, the wheel would have to be 300 feet in diameter. Since it obviously is not, they simply fudged/ignored this point. (It obviously is over 300 feet on the space station.)

27

Yep. Although, the smaller the cylinder, the more pronounced Coriolis effect will be. In a clothes dryer, you’d have trouble balancing objects, drinking from a glass, and so on.

28

I don’t think they ever said it generated 1G. I always assumed it was much less.

The Leonov from 2010 is a more sensible design in this respect. It has a whole large-diameter rotating section, while the Discovery had a small squirrel cage inside the spherical hull. The Omega-class destroyer in Babylon 5 was inspired by this design.

29

That is correct. Either the 2001 book or other production documentation stated it was 1/6 G, or lunar gravity. The habitation module in this fictional vehicle had a band-like carousel which supposedly spun at about 5 rpm and the diameter was about 38 feet.

For filming the movie, a slowly rotating room was built with an interior that replicated the spinning Discovery habitation module. This allowed cameras to film the actors running around the interior: http://3.bp.blogspot.com/-KoYwG8T4MRA/UT1hGqOP8lI/AAAAAAAAANQ/sDMEs7wetVk/s1600/2001+Space+Odyssey+(20).jpg

Of course they were always oriented vertically with respect to local gravity – the room rotated around them: http://www.qanvas.co/uploads/4/8/1/0/4810826/2330122_orig.jpg
Tumblr: Image

As already mentioned, NASA planned on experimentally spinning the 2nd Skylab station at about 4 rpm to produce approximately lunar gravity. It was not designed for this but studies indicated the mechanical and control systems would tolerate it.

Early studies suggested that 3 rpm was the limit for human adaptation, but more recent studies have shown that some individuals can acclimate to 7.5-10 rpm rotational speeds: Artificial gravity as a countermeasure in long-duration space flight - PubMed

The crew either 2001 or a hypothetical Mars mission would be very small and highly selected for various criteria, so adding one more selection criteria doesn’t seem impossible. E.g, the Mercury astronauts couldn’t be over 5 ft 11 inches tall. There were (and would be) adequate candidates to fill the needed pool.

30

Low budget-itis. If you look at the scene again you’ll see all the yellow Low Gravity warning signs. Not having enough money in the budget to build a set in the Vomit Comet, they made the actors fake it by walking slowly and always having a hold of something. What I meant was the part after Sheridan leaped.

Judging by where the window of C&C was from the outside of the station, the crew was held down by about 2/3s g and that effect was totally ignored. Hell, most of the time when they’re looking out of the window in C&C, unless it’s at something specific like a Raider ship headed right at them, the stars don’t rotate. JMS said something about the CGI was too expensive so they’d tried a rotating wheel with lights but that was unsatisfactory so they gave it up.

31

Even better fictional example, because the author actually did the math.

For context, shortly before that scene, from Brad’s point of view. From the readers’ point of view, it’s about three weeks, but it’s worth it.

32

The Math used in 2001 is actually quite simple (but still a plot device that would be impractical)

Gravity = radius * (radians/second)^2

So with the assumption that someone is 1.8 M tall and the spaceship in 2001 is ~10M in radius, it would need to spin at a rate that would cause serious issues.

To make the math easy lets round the earths gravity to 10m/sec^2

squrt(10 M /10 M radians ) = 1 radian/sec = ~47 degrees/sec = ~7.9 RPM

The problem here is that if you are 1.5 meters tall, and you stood up your head would feel ~15 percent slower and less force. The coriolis force would start to cause serious issues.

Think about the running scene, if you ran one direction you would feel heavier and the other you would almost seem to become weightless. But the real issue here is with your balance.

When you stand up you are moving at about 1 M/Sec, and due to the Coriolis effect, the rough math would make you feel like you were falling forward at about 1M/Sec or like you were trying to stand up at a 45 degree angle.

This becomes even more of an issue if you consider turning your head as one side would feel “heavier” and the other would feel “lighter”. This would actually be worse at about 1/10th the Earths gravity for nausea or by causing odd behavior in moving limbs/objects.

Really you would have to resort to scales like in Ringworld , or Halo to use rotation for artificial gravity that would be mostly undetectable. But then you run into issues with the limitations of material strength etc…

For the most part artificial gravity is purely a plot device, or a budgetary requirement and does not reflect practical solutions under our current understanding.

33

“Dave” jogs around the squirrel cage in the Discovery during one of the scenes. I don’t think jogging at 1/6G would look anything like 1G jogging. But yeah, it was filmed on Earth in a giant rotating set… fairly spectacular effects for the time. The floating pen in zero G on the shuttle was actually stuck to a rotating plastic disc in front of the camera, and when the stewardess in zero G turns 90 degrees in Velcro shoes, they again actually rotated the whole set.

I suspect Clarke threw in the bit about 1/6G for the book because he recognized the movie was fudging it.

34

There’s an unfairly-excluded middle between “small enough to be undetectable” and “small enough to not cause any significant problems”. We could certainly make rotating spacecraft with a radius of, say, 100 m with current materials, which wouldn’t be too bad for 2 m humans.

One tip that might help is that you don’t need to construct a full circle. You just need two pods (one of them can be uninhabited, carrying only cargo, if you’d like) tethered together with a cable. Making a cable strong enough to hold the weight of a spacecraft under 1 g isn’t too hard, especially since the cable only needs to be that strong at the ends, and can be thinner in the middle.

35

There are some medical issues with extended weightlessness. What we don’t know is what level of gravity (Moon, 1/6G? Mars, 40%G?) is sufficient to counter this problem longer term. I guess some experiments are needed before we build a giant wheeled space station. But yes, a pair of spinning pods is sufficient. (Plus an elevator or something to run up and down the cable from the center. However, a cable under tension feels the same tension the entire distance.

36

You’re right, of course (assuming that the cable’s own mass is small compared to the end loads, which it probably is in this case): That was a mistake on my part.

37

As I stated above, while not articulated in the movie, the production design stated 1/6 g and 38 ft diameter, which translates to 5 rpm. The latest research indicates certain individuals can adapt to this (or even higher rotation rates): Artificial gravity as a countermeasure in long-duration space flight - PubMed

NASA planned on spinning Skylab B at various rates up to about 6 rpm, which is faster than the 2001 Discovery carousel. This was real world, not a movie. It would not have continuously spun but done so for significant periods as part of biological testing to compare the astronaut’s health to those in the non-spun Skylab A. Skylab B was constructed but never flown, and the actual flight article is sitting in the Smithsonian.

The movie’s larger “Space Station V” was 300 meters in diameter and rotated a 1 rpm, which also produced 1/6 g.

It’s just a movie but these elements are within current physiological science.

Re math, anyone curious about this can use the on-line spin calculator: SpinCalc

38

Mass and motion.
Your mass wants to continue in that motion in a straight line, which is outwards from center basically.

And it wont be real gravity.
At the outer ring where your mass is moving a lot more, you could walk around, long as the floor faces outwards.

Try going anyplace else, and you’re probably going to take a fast trip to the outer ring.

Dead in the center, i think you would still be weightless.

And the reason you can not just sit still in the middle of the ring and float there is everything in the station begins moving including the air, which will eventually gain momentum and shove you along too.

If is was an absolute vacuum, i think you could just remain in place.
You’d probably have to strap in until you got into motion.

Outside of scifi, ive no idea if it actually works past the theory.
It works on earth but thats different

39

IIRC, there was a scene or two in The Martian, where the transition between the hub and the spokes of the Hermes wasn’t handled very well. I remember seeing one of the crew floating in the hub, as one would expect, but then getting magically “sucked” into the tube that went “down” to the bottom of the rotating section. A more realistic scene would have required them to do a lot of grabbing and swinging to enter the top of the tube.

Given how fabulous the rest of the movie was, I’m willing to forgive a bit of slightly lazy wire work.

40

Your first link, from 17 years ago, so we have differing connotations on the "latest. But most importantly it is is covering “fatal problems of bone loss, cardiovascular deconditioning, muscle weakening, neurovestibular disturbance, space anemia, and immune compromise” and not the effects of the coriolis effect etc…

As for the second refrence let me quote the paper related to that link.
[

](http://www.artificial-gravity.com/AIAA-2006-7321.pdf)

or to quote the referenced material in the refrences:

[

](NATURAL AND INDUCED ENVIRONMENTS)

Neither of these cites contradicts my post and the science is unsettles, but here is a modern report calling out the concerns.
[

](Human Health Countermeasures - Partial-Gravity Analogs Workshop - NASA Technical Reports Server (NTRS))

But there have been no studies on long term adaptation, and it is absolutely not going to end up in Keir Dullea jogging around the ring while shadow boxing.