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Spacecraft rotating to generate artificial gravity through "centrifugal force" are commonplace in science fiction but not in reality. Considering the problems in long missions (among others: bone loss, muscle loss, fluid redistribution, permanent visual impairment, feet disease, lowered immune defence, increased growth of dangerous microbes like ecoli, increased exposure of eyes and skin and lungs to freefloating microbes and dust), why isn't this method used?

Also, wouldn't artificial gravity (radial acceleration) simplify the design and increase the reliability of technical equipment which use moving parts, flows of fluids or gasses or need to distribute heat? Wouldn't eliminating microgravity eliminate the cause of all these severe and very diverse problems which today have no other working solution?

I understand that a large structure with moving parts as in 2001 is currently unfeasible, but how about a simple tether with the craft and a counterweight at the ends, like bolas? Does it interfere with communications or require the craft/tether to be much stronger?

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    $\begingroup$ 1) Because the movements of a centrifuge would disturb the microgravity experiments aboard. 2) Because simulated gravity is not considered necessary for a human mission to the Moon or to Mars or its moons. It's a luxury we can simply skip to begin with. Astronauts will suffer a bit, but they are jolly go along anyway so that doesn't matter. $\endgroup$
    – LocalFluff
    Aug 26, 2015 at 14:33
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    $\begingroup$ "I'm wondering why artificial gravity is not included on the ISS": Maybe because the idea of the ISS is to use it for micro-gravity experiments. $\endgroup$
    – mins
    Aug 26, 2015 at 17:01
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    $\begingroup$ @LocalFluff Can confirm I would "suffer" through microgravity for a chance to go to the ISS. $\endgroup$
    – corsiKa
    Aug 26, 2015 at 20:55
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    $\begingroup$ Don't forget that Coriolis effects are large on small rotating bodies. If we stick to the recommended 2rpm, or (6pi/60) radians per second, then omega^2=0.1/s^2, so to get to one gee we need a radius of 100 meters. Kinda large for the existing lifting capacity I think. $\endgroup$
    – Jyrki Lahtonen
    Aug 27, 2015 at 8:13
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    $\begingroup$ Is artificial gravity the right term to use here? That suggests to me a sci-fi style gravitational field rather than simulated gravity through rotation. $\endgroup$
    – Tim B
    Aug 27, 2015 at 9:45

14 Answers 14

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First off, it really isn't artificial gravity, it is radial acceleration (often known as centripetal force) - AlanSE's answer on the 'Size and rotation of a station' question describes how radial acceleration is very different to gravity.

The major reason this is unlikely for some time is that in order for radial acceleration to feel like gravity, you need a very large diameter. Over 200 metres according to his calculations. This presents a couple of major problems:

  • Getting all the materials to orbit. A 200m diameter station or spacecraft will weigh a lot!
  • Building it securely so punctures from space debris don't cause major problems
  • Building a stationary (or contrarotating) hub for comms antenna/docking etc will require spinning seals...complex and likely to fail

Basically, we could do it - it would just be incredibly expensive and dangerous.

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  • $\begingroup$ 200 meters is just a shade more than a fully equipped Saturn V at 111 meters height. $\endgroup$
    – user
    Jan 27, 2016 at 12:37
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    $\begingroup$ @MichaelKjörling But Saturn V doesn't go in one piece into orbit. $\endgroup$
    – Gp2mv3
    Dec 6, 2016 at 9:30
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    $\begingroup$ The OP never said 1G, just not 0G, so 200m is not necessarily required. They also called for a simple tether with a counter weight, ie not a huge structure. $\endgroup$
    – Brooks Nelson
    Aug 26, 2017 at 15:53
  • $\begingroup$ See comments under Robert's answer for why you do not want to use a tether for this! $\endgroup$
    – Rory Alsop ♦
    Aug 26, 2017 at 15:55
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    $\begingroup$ A 200m tether would not be too heavy, but docking would be problematic $\endgroup$
    – ikrase
    May 25, 2020 at 4:10
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It is worse than that. In order to really plan for future human habitation elsewhere, you do not want just 1G fields, you want 1/6th to mimic the Moon, and 1/3 to mimic Mars.

You need to be able to run long term studies comparing similar samples at 0G, and other G levels.

There was planned to be a Centrifuge Accomodations Module, on the sky facing port of Kibo (JAXA Science module) but it was stripped due to funding.

Secondarily, a centrifuge large enough for people would need to be very large, and that is basically out of the question for something as small as ISS. A centrifuge for running experiments on mice, or small items introduces another problem. Vibration. This can affect other experiments on the ISS relying on 0G. The ISS is not a great 0G facility, too many darn humans moving around and bumping the walls all the time, and all the altitude adjustments.

The Space Sciences Institute headed by Gary Hudson (Of Roton fame) is pushing for funding for a space based centrifuge facility.

Short answer to why? Money.

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    $\begingroup$ The CAM now sits outside in the rain :( en.wikipedia.org/wiki/Centrifuge_Accommodations_Module $\endgroup$
    – Organic Marble
    Aug 26, 2015 at 20:08
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    $\begingroup$ I don't think the first point is really valid. A station spinning fast enough to have 1g on its outside would have lunar/martian levels of artificial gravity closer to the hub. At a minimum those levels would be accessible via the spokes that held it together. If more volume was needed additional rings could be built at those levels; or in a solid cylinder design it'd just be inner vs outer "floors'. $\endgroup$
    – Dan Is Fiddling By Firelight
    Aug 27, 2015 at 17:54
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    $\begingroup$ @DanNeely That is a design for a station that is hugely larger than the ISS. $\endgroup$
    – geoffc
    Aug 27, 2015 at 21:59
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I feel like the other answers are missing one fundamental design driver of the ISS. Unlike space stations like the rotating von Braun station or the Stanford Torus, the ISS was primary designed as a scientific space station and not as a leisure outpost in space.

Microgravity (in fact there are not absolute zero-G on the ISS, but a very small force differential - see MrPaulch comment) is the big unique characteristic, which differentiates the ISS from every other human laboratory. No microgravity would more or less render the ISS unnecessary from a scientific point of view.

Therefore it was never intended to build a whole rotating space station. Of course money is a second reason (it always is a reason), but if money is a real concern, the ISS would've never been built. Building an international space station was not an economic or scientific decision, it was mainly a political decision.

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    $\begingroup$ ISS as a diplomatic project. In that respect it maybe has been very successful? Now that there is this conflict between Russia and the West over the Crimea, the ISS has been exempt from any sanctions. Both sides are crucially dependent on each other. One might boycott food imports and stuff. But cutting off the space station would mean death to the Russian as well as NASA's and ESA's space programs. So they find a way to get along after all. That's a good thing. $\endgroup$
    – LocalFluff
    Aug 27, 2015 at 10:19
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    $\begingroup$ There is a centrifuge on every second funfair, so the scientific insights on another artificial gravity would not justify the costs as far as I can tell. You are right about your comment on the central module. The other answers here are not wrong. But they do not mention that an artificial gravity was never a design driver of the ISS. Also you could debate whether insight of human body's behaviour in microgravity is more important than saving time and money by not doing any exercises. $\endgroup$
    – wheeler
    Aug 27, 2015 at 10:28
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    $\begingroup$ @AdamSmith It's not as easy as it might sound - friction would be a real factor, so the two parts of the station would tend to try and rotate at the same rate - you really need at least two wheels spinning in opposite directions, and even then it's not as trivial as it sounds - even with very high quality bearings etc., there would be considerable vibration in the "static" part of the station, disturbing the experiments. It really isn't worth it for a station designed to study microgravity. $\endgroup$
    – Luaan
    Aug 27, 2015 at 12:25
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    $\begingroup$ This is why I dislike the term Microgravity. You write that "...there still exists a small gravitational force. [On the ISS]". That is a misleading statement. The gravitational force excerted on the ISS by Earth is about 90% of what is excerted on anyone on the surface. Microgravity describes the Weightlessness bodies experience on the ISS due to it's orbital motion. I would suggest you clarify that in your answer. $\endgroup$
    – MrPaulch
    Aug 27, 2015 at 15:41
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    $\begingroup$ @MrPaulch That's why I try very hard to use freefall environment rather than weightlessness or microgravity. "Freefall environment" is far closer to what it actually is, without getting into all of the details; "weightlessness" or "microgravity" is the colloquial way of describing the resulting behavior of objects in that environment. You can get exactly the same behavior of objects, generally speaking briefly, in environments people generally don't associate with weightlessness at all, such as inside an elevator or aircraft. $\endgroup$
    – user
    Nov 15, 2016 at 15:31
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What hasn't been mentioned so far are the extra problems caused by gravity: it's not just that gravity is an expensive and inconvenient luxury, it also brings problems. (In this answer I will just say "gravity" to refer to a constant force along one axis, whether that's real or simulated.)

Accessibility

In zero-g, you can get anywhere on the ship by pushing/pulling/drifting there. In gravity, you can't, unless it's all so compact that you can reach anything by hanging off a ladder or just standing and reaching up. Even if you can reach everything, it might be the case that you can't reach it from quite the right angle to use it most easily. In zero-g you can cover every surface with "stuff" and not worry about what angle people are at when they use it, since they can always just rotate themselves round.

Accidents

All of a sudden you can fall down a ladder, or drop something on another astronaut. This is not a problem in zero-g.

Consistency

Most space stations are sort of long cylinders, maybe with a few branching cylinders. Where would you have the gravity in this situation? Would it be parallel to the long axis of the ship, so you climb up and down to traverse the ship? Or, parallel to the side axis, so that you stand on the side of a cylinder, and walk around it as if it was all laying flat on the earth? Either way, it's hard to achieve a consistent centrifugal (or is it centripetal? I can never get that) force over the whole space station. The further you get from the axis of rotation, the more it's going to feel like you're falling towards the walls.

Docking

Docking with spinning things is so much harder. The docking point can only work if it is in the middle of the axis of rotation, which brings in some extra constraints to the design and layout of the station. Also, the need to match rotation also adds extra complexity to the docking, which is probably dangerous enough as it is.

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There are paper missions that include a tether and simulated gravity in the hab, but the only long-term stays in space have been in LEO. The ISS was designed to be a microgravity research facility, so there's little motivation for a rotating design. There was a Japanese centrifuge module that was developed but never launched (Centrifuge Accommodations Module). There was also an inflatable sleeping centrifuge that has been talked about.

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    $\begingroup$ Mars Direct is one famous mission design that comes to mind as including a rotating tethered spacecraft. If only such missions had been pursued in the 1990s. $\endgroup$
    – areologist
    Aug 17, 2013 at 6:04
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Because adding centripetal force makes everything more complicated and more expensive, and it's already so difficult and expensive to put anything, and especially people, in space that it's just been a non-starter. Since people can survive a relatively short stint in zero-gee with few permanent health effects, it's just easier to rotate out the crew than worrying about a trillion-dollar space station breaking in half because some bearing in the artificial gravity system seizes up.

I remember reading about a very few experiments along these lines. In The Case for Mars, I think the author suggested that we could attach the ship to a counterweight with a long tether, and spin that assembly for gravity. I think he cited some previous mission where they'd tried that as an experiment, and the cable melted or something, possibly because they were sending too much electricity across it.

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    $\begingroup$ That latter part would be Zubrin's idea of using a spent upper stage as a counterweight. The tether part you refer to was done in LEO. Electric current that finally snapped it was an environmental effect (plasma buildup), not something deliberate. $\endgroup$
    – TildalWave
    Aug 26, 2015 at 14:59
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    $\begingroup$ @TildalWave Wow. After following your link, it sounds like one of actual goals of the mission was seeing if you could use the tether as an electrical generator. That seems like putting the cart before the horse to me. It's like a group of 19th century scientists trying to determine the feasibility of the internal combustion engine, but for some reason they insist on doing all their experiments on submarines. $\endgroup$
    – Doug Warren
    Aug 26, 2015 at 19:07
  • $\begingroup$ @DougWarren I think the reason for doing it in space is that since orbital velocity is so fast, the tether could be used to generate a lot more electricity than on earth. $\endgroup$
    – Erik Engheim
    Aug 27, 2015 at 10:11
  • $\begingroup$ @DougWarren It's not that weird. Sure, it would be more understandable before photovoltaics were a thing (that was one big surprise for sci-fi writers :)), but even so, it could be very useful for generating electricity for a probe around Jupiter, for example. RTGs have their issues - longetivity, and using radioactive materials (rocket launches fail once in a while, you know), for example. $\endgroup$
    – Luaan
    Aug 28, 2015 at 8:17
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Actually it's not dangerous if the tether breaks, at least: the spacecraft would fly apart at only a few mph and would be easy to get back together again. And the tether only needs to be strong enough to hold the weight of the habitat under gravity; not a super material, just like the sort of cables you have on cranes.

Plus you could have multiple smaller stays like a suspension bridge, I suggested idea here of a tube you can roll up with internal stays so also doubles as ways to connect the two modules and perhaps also if wide enough, greenhouse space for plants.

You might like my blog post about all this.

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    $\begingroup$ If a tether breaks, one danger is the tether itself whipping back and damaging the spacecraft. $\endgroup$
    – Hobbes
    Feb 12, 2014 at 12:00
  • $\begingroup$ @Hobbes But the rotating speed would just be a few meters per second, and the tether is light weight.Also, one could use something like a air beam tether. A collision with an object large enough to cut a wider tether is extremely unlikely, and would kill a microgravity space station anyway. $\endgroup$
    – LocalFluff
    Nov 25, 2014 at 10:12
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    $\begingroup$ The tether is under tension equal to the weight of the station at 1G. There's potential energy stored in the cable due to its elasticity. This means a cable under several tons of tension will snap violently, independently of the rotation speed of the station. $\endgroup$
    – Hobbes
    Nov 25, 2014 at 15:09
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    $\begingroup$ For 200m radius and 2 rotations pr minute I get; (2*200*pi)m/rotation*2rotation/min*60min/hr=150km/hr. I would not want to do EVA from this. $\endgroup$
    – Taemyr
    Apr 17, 2015 at 10:27
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    $\begingroup$ You absolutely would get a whip back, as @Hobbes commented. Physics doesn't stop just because you are in space. $\endgroup$
    – Rory Alsop ♦
    Jan 9, 2018 at 10:40
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First, let's not erect a strawman argument where it either has to be 1g gravity or 0-g. The major drawbacks of 0-g as a living-work environment are circulatory muscle atrophe (and body fluid problems) and time lost fastening every object down. Maybe there is a sweet-spot at 0.5-g, 0.3-g, etc. Let's call this gravity factor f.

The formula which determines the radius r of a drum-like structure (given f and T, the period of rotation) is:

9.8 f = V^^2 / r (V is tangential velocity at outer rim of drum)

9.8 f = (2 pi r /T)^^2 / r

9.8 f = 39.5 r / T^^2

r = 9.8 f T^^2 / 39.5 = .2482 f T^^2

or, if we want to specify radius and solve for rotation period T:

T = sqrt ( r / .2482 f)

Let's imagine a drum structure with a radius of 30m, and 0.5-g artificial gravity. How long does it take for one rotation? 15 seconds.

The "view" out the window would be too disorienting to look at, so the view from the bridge would be presented on large-screen displays, where the rotation is cancelled out in the graphics.

A big challenge would be getting all the structural materials up into orbit...for instance a fiberglass-epoxy hub and spokes. A second big challenge would be keeping the drum in perfectly balanced rotation (as is done when balancing a tire). If unbalanced, illusion of constant gravity would be broken, and astronauts would feel their weight constantly wobbling (cycling every 15 sec).

Docking would require halting rotation during the docking (then rebalancing), or an anti-rotating docking port on the central axis. Docking this way would be uncomfortable for astronauts, who would experience vertigo. There are other docking designs that involve a tangential capture which would be possible under algorithmic control, but would require the drum to have a dynamic ballast system for rapid rebalancing.

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  • $\begingroup$ to "circulatory muscle atrophe (and body fluid problems)" should bone mass loss be adde? $\endgroup$
    – uhoh
    Feb 6, 2021 at 22:53
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I think the Eu:CROPIS Mission of German Aerospace Center DLR might be the first which really utilize the rotation as artificial gravity.

http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-17874/#/gallery/23027

And until that link is repaired, hat-tip to @ PM2Ring for finding The Eu:CROPIS mis­sion

The mission name, Eu:CROPIS, stands for 'Euglena and Combined Regenerative Organic food Production in Space'. The mission is intended to demonstrate that urine can be converted into fertiliser for plants under lunar and Martian gravitational conditions. [...]

The urea converted into nitrate in the trickle filter serves as fertiliser for the tomato plants; hence, the plants also act as biosensors. They indicate whether the urine has been successfully converted into a fertiliser solution. Cameras record everything and the data is sent to the DLR control centre in Oberpfaffenhofen (German Space Operations Center; GSOC) and the DLR Microgravity User Support Center (MUSC) in Cologne. GSOC controls the satellite and payload, while MUSC receives the data from the greenhouses. LED light is controlled by a timer to provide a regular diurnal rhythm and a pressure vessel provides an atmospheric pressure of one bar, corresponding to the pressure at Earth’s surface.

Experiments in two phases

During the mission, the satellite will rotate about its longitudinal axis, and the strength of the resulting artificial gravity will depend on the rate of rotation. In the first experimental phase, lasting approximately 23 weeks, gravitational conditions equivalent to those on the Moon will be created. In this phase, the first greenhouse will be in operation. In the second phase, the gravity will replicate that of Mars. This is when the experiments in the second life support system will be conducted. The mission will last 62 weeks. The satellite will remain in space for approximately a further 18 years, and will then burn up during re-entry into Earth's atmosphere.

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  • $\begingroup$ In the unofficial Skylab 500, astronauts ran around a storage ring. That gave them 0.5 gee at the feet and at least 0.25 gee at the head. The resultant torque interfered with pointing of the solar telescope, so they did not continue the practice. Attach a BA330 to ISS for a similar opportunity, and there is also room for a 3 m radius centrifuge. It could also be a good place for toilet facilities and other plumbing. JAXA completed a 1 gee mouse study in KIBO nature.com/articles/s41598-017-10998-4 and plans partial gee experiments. $\endgroup$
    – MBM
    Mar 23, 2018 at 2:40
  • $\begingroup$ I can't get this link to work, and without it, there is no answer. Sorry, but -1 for a link-only answer that is now no longer an answer. $\endgroup$
    – uhoh
    Feb 6, 2021 at 22:54
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    $\begingroup$ @uhoh Here's an intro page dlr.de/content/en/articles/missions-projects/eucropis/… $\endgroup$
    – PM 2Ring
    Mar 17, 2021 at 14:18
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    $\begingroup$ @PM2Ring Great, thanks! I've added some quotes back into the answer, I think it's much more useful now. $\endgroup$
    – uhoh
    Mar 17, 2021 at 14:24
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Actually, the rotating space station in "2001" is a pretty accurate rendition of the size and rotation speed needed. No center hub around which to rotate - hence no seals or bearings to go bad - and a large central docking bay. Of course, it should be noted that the fictional space station is part of a larger transportation system, to and from the moon; so there's that. As far as trying to use centripetal force to simulate gravity in a long-term space mission, that's a little trickier - for all the reasons already listed - but it seems to me that there's actually no reason the whole spacecraft couldn't rotate, making it, in essence, a real "flying saucer".

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Microgravity is not harmful during shorter periods, like a few weeks. No space agency has any plans to send humans on a long term mission, such as a mission beyond the Moon. Nor any plans to let humans stay a long time in lunar gravity. So no one has any use for simulated gravity.

The ISS is a microgravity lab and deliberately exposes astronauts for it during 6-12 months at a time. It is a medical experiment with humans in order to find out exactly how the health of the astronauts is destroyed and if they can recover from it when back on Earth. Even though microgravity is the one health issue which does not plague people on Earth, the medical community hopes that this kind of experiments one day will discover something which is useful for human medicine in general. (The ISS only costs about as much as all cancer research ever undertaken, so it maybe is worth it in the hunt for a serendipitous discovery?)

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    $\begingroup$ I assume you're not counting, say, NASA's planned Mars mission in the mid 2030s here. Also, the ISS is not primarily a medical experiment. $\endgroup$
    – Nathan Tuggy
    Apr 17, 2015 at 17:55
  • $\begingroup$ @NathanTuggy I haven't heard of any NASA mission to Mars in 20 years. Do you have a name of that mission or of the principal investigator of that mission and what budget he/she disposes of? The ISS as primarily a microgravity station competes only with its use as an Earth observation station. $\endgroup$
    – LocalFluff
    Apr 17, 2015 at 18:47
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    $\begingroup$ Microgravity, yes. Medical experiments on humans, not so much. (Not that that's not a small part of it, just that it's nowhere near the main focus.) As far as the manned Mars mission: nasa.gov/content/… $\endgroup$
    – Nathan Tuggy
    Apr 17, 2015 at 19:00
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The use of centripetal force is a very feasible solution. The reason that no current spacecraft is using rotation for artificial gravity is because no space travel to date, or currently planned, is of long enough duration to require it.

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have a look at these:
1966, first radial acceleration / centripetal force experiment by Gemini XI (11) using a tether (youtube reference)
1996, Tethered Satellite System experiment during STS-75 (youtube reference).

I have read the issues but are still not sure why they they did not try to improve this. Cost may be higher due to primary & secondary backup modules, the need to duplicate life support, docking issues, ... However, alternative features to improve may be: (dynamic/variable) counterbalanced module, strutted modules with center section for docking, ...

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  • $\begingroup$ Your first unnamed youtube reference has been deleted $\endgroup$
    – uhoh
    Feb 6, 2021 at 22:55
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Balance issues would constantly change depending on who/what was where and when. It would eventually stop rotating because of this, further its orbit and or path through space would constantly change because of the imbalance.

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    $\begingroup$ Some control would need to keep it from precessing, but internal movements can't change it's orbit. $\endgroup$
    – Steve Linton
    Mar 22, 2018 at 22:16

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