# Why isn't a centrifuge used for astronauts on the space station?

One thing I've always wondered is why don't astronauts sleep in a kind of rotating bed that spins creating force? This would allow them to sleep and would be able to simulate earths gravity. Why don't they do this considering the impacts zero g has on the human body?

• – uhoh Jul 30 '17 at 12:56
• Judging from the number answers and amount of joint effort and discussion put into them, you seem to have posed quite an interesting question! +1! – uhoh Jul 30 '17 at 13:44
• @uhoh Here is a list en.wikipedia.org/wiki/… it looks complete to me, as far as I can tell – Uwe Jul 30 '17 at 17:03
• @uhoh This is anecdotal and a sample size of one, but my old professor was an astronaut and said sleeping in microgravity was the best sleep he's ever had. – Arthur Dent Aug 1 '17 at 21:24
• – hobbs Aug 2 '17 at 3:59

The short answer is it would cost a lot of money.

In order to get a 1G force, you'd either need something really big, or rotating very fast. For example, the reference design for the space colonies I'm working on calls for a structure with a 900 meter radius rotating once a minute. For something the size of the ISS, it would have to be rotating much faster. (I will get the actual numbers in a bit, when I'm not in the middle of another project.)

In addition to the rotational speed problem, you have to also take into account that the structure would have a lot of mass in order to be strong enough to support all that (centrifugal) weight - and the more mass you put into orbit, the more it costs.

On top of that, since you probably don't want to make the entire ISS rotating that fast (to keep the mass - and costs - down), you'd need to have a set of bearings between the rotating and the non-rotating parts of the station, preferably one that's big enough to provide a passageway for the crew to go through (so they don't have to put on their space suits to get to bed) - and that bearing is going to - guess what - have a lot of mass that needs to be launched - which means it would cost more money.

Oh, and you'd also have to make sure that bearing isn't leaking, or you'd have to send up more air to replace what got lost - which would cost more money.

There are a host of other issues, but I'm guessing the list I've given already made the ISS designers realize a centrifugal gravity sleeping chamber was probably not something that would fit the project's budget.

EDIT

OK, I did some calculations. If your centrifuge is 5 meters in diameter, it's got to be rotating at 18.9 RPM for a 1G acceleration at the rim which will be moving at 17.82 kph (11 mph).

Since you don't want the centrifuge to be torquing the station around it, you're actually going to need two counter-rotating centrifuges of equal mass, and both arms of each centrifuge must have the same mass being rotated so everything is in balance. That's not impossible, you could, for example, have a system that pumps a balancing amount of water into each of the four ends - but that adds system complexity, weight and cost. I'm open to suggestions for a better solution.

As Russell Borogove has pointed out, this could be done in an enclosing compartment to eliminate the seal issue, but now you've got to build a vessel 5.5 meters or so in diameter that's twice the width of a centrifuge pod, plus clearances, in length, figure 3 meters. That's a larger diameter but about half the length of the Unity module (4.57 m dia x 5.47 m long), so it's not entirely out of the question. The noise of the equipment and the pods passing each other at 22 mph relative speed would be pretty substantial.

Speaking of the equipment, the centrifuges are going to need motors to start and stop them every time an astronaut goes to bed or gets up. If you don't want to spend all night getting up to speed, you will need a bigger motor, along with a beefier power system to run it. Then, when you're slowing the centrifuge down so the astronauts can get in or out, you don't want to throw away all the energy that was used to accelerate it, so you need an energy storage system. Batteries may come to mind first, but rapid-cycle batteries that could repeatedly store and release enough energy across many cycles would be really heavy and expensive. An alternative would be winding up a flywheel for energy storage, but again, that's going to be heavy and expensive.

Oh, and if you're going to have more than one of those four sleeping pods occupied at a time, make sure the astronauts all have the same sleep cycles: We wouldn't want an early riser having to lie in bed awake waiting for the other guy to get back from dreamland, or astronauts cranky from having been awakened too soon because the centrifuge stopped to let the other one off.

...and make sure there aren't any emergencies that require getting out of bed on short notice - yeah, you could jump out of a pod moving at 11 mph without too much danger of hurting yourself - but make sure you get out of the way before the next one comes along a second and a half later and bops you in the head!

The math:

\begin{align} a &= v^2 / r = 1G = 9.8\:\mathrm{m/s^2}\\ d &= 5\:\mathrm m\\ r &= 2.5\:\mathrm m\\ \end{align}

\begin{align} v^2 &= 9.8\:\mathrm{m/s^2} \cdot 2.5\:\mathrm{m} = 24.5\:\mathrm{m^2/s^2} \\ v &= \sqrt{24.5\:\mathrm{m^2/s^2}} = 4.95\:\mathrm{m/s} = 17.82\:\mathrm{kph} = 11\:\mathrm{mph} \end{align}

$$\text{circumference} = \pi \cdot d = \pi \cdot 5\:\mathrm{m} = 15.71\:\mathrm{m}$$

$${15.71\:\mathrm{m} \over 4.95\:\mathrm{m/s}} = 3.17\:\text{sec per rotation} = 18.9\:\text{RPM}$$

Centrifugal Acceleration

• A centrifuge sleeping compartment contained entirely within a non-rotating pressure vessel would eliminate your sealed-bearings problem, as well as potentially being useful at a substantially smaller scale than a general centrifuge habitat. This doesn't eliminate the space/mass/power problems, though. – Russell Borogove Jul 30 '17 at 0:54
• That's great thanks I've always wondered why there wasn't something @RussellBorogove said some kind of spinning centrifuge inside to help with bone and muscle decay by adding Gs while they slept. That's what I was thinking and only a small solo one but as you say it will still be creating a lot of noise! Thanks for the response! – Callum K Jul 30 '17 at 7:48
• You should do some reading about short arm centrifuges, some links: dlr.de/envihab/en/desktopdefault.aspx/tabid-8667/#gallery/23780 medes.fr/en/the-space-clinic/the-equipments/… – Uwe Jul 30 '17 at 11:43
• @Zaibis I'm using interia in the physics sense (specifically rotational inertia), so there is no suitable alternative word. If I was using it in one of the other senses I would happily use a synonym. In fact I try not to use non-physical meanings of technical terms in the first place, when there's potential for confusion. – Chris H Jul 31 '17 at 10:47
• @JollyJoker I was imagining a clear axis through the centrifuge and the outer chamber because of the need for hatches. Certainly a non-rotating pole run through the middle could help a lot. So the centrifuge(s) in an outer drum suggested somewhere here would be compatible with this. – Chris H Jul 31 '17 at 12:43

The point of the ISS is to study 0G. 1G sleeping bags defeat the purpose... The humans are experiment subjects too:)

• I almost down-voted with some comment about there being no need of additional studies on performance degradation due to difficulty sleeping, or the inevitability of bone loss, until I realized that the logic behind your fifteen-word zinger is inescapably correct. :) +1 – uhoh Jul 30 '17 at 11:02
• @uhoh I can see some value in asking 'what happens if we put people in 0G, but with shorter periods in 0.3-1G'. Particularly if looking at long-term transits. – SomeoneSomewhereSupportsMonica Jul 30 '17 at 12:14
• @SomeoneSomewhere I've been interested in this as well and see value there also, see for example In what way is artificial gravity expected to avoid/reduce bone loss? You can suggest to the OP to add "scientific study" to the question. One might ask why the answer would be interesting or useful and to whom (possibly the crazy-rich guy who wants to move a million people to living on low-gravity mars). Otherwise who needs to know this any time soon, and badly enough to pay for it? – uhoh Jul 30 '17 at 12:57

OK let's build a hypothetical cylindrical sleeper system that could fit inside the crewed area of the current ISS for example, and look at some of the issues you would need to address. We'll name it after the famous Bill Haley and the Comets song: Shake, Rattle and Roll.

You can also apply what's learned here to a futuristic, much larger structure for a higher g-force system to produce skeletal stress with hopes of reducing calcium loss.

Find a spare, or currently empty, unused module on the ISS and build a 2 meter diameter, 2 meter long rotating cylindrical "astronaut tumbler". The astronauts sleep along the inside walls, laying parallel to the cylinder's axis, around which it rotates.

Using $$\mathbf{a}=-\omega^2 / \mathbf{r}$$ the speed required to obtain a modest 1/6 of Earth gravity in order to provide a small but meaningful experience of "laying down" rather than floating is $$\omega=1.3 \text{s}^{-1}$$ which works out to one revolution every 5 seconds, or a rotation frequency of 0.2 Hz.

There may not be room for six of these, so it's going to be a shared space and astronauts will still need their cubby-holes for personal space, and a separate allotment of time to spend in it. Alternatively, they could pick up and move their personal cubby-holes and either attach them to this rotating frame, or move it back to the wall.

No matter how you look it it, it's more stuff shipped from Earth, which is OK if it offers a significant enhancement to the astronauts' well being or contribution to the science of living in space.

Balance is critical. If one astronaut wants to sleep, a "dummy astronaut" needs to be placed opposite in order not to unduly shake the ISS with 0.2 Hz mechanical oscillation. If the sleeping astronaut moves, the dummy needs to move accordingly, or a servo mechanism at each end of the cylinder must automatically and constantly translate the rotation axis of the cylinder back to the center of mass. More stuff to break and mass to ship from Earth. If there are two people arranged opposite each other and a third wants to join, one person must "re-azimuth" themselves by 60 degrees (or if they are sleeping soundly, be re-azimuthed by their fellow astronaut), or the dummy astronaut could be added opposite the third person.

If someone wants to "get on" or "get off" the whole thing must be stopped and started. That may wake up anyone already "on". Where does that angular momentum come from? If it stopped and started on a regular schedule with a fixed duty cycle, perhaps it could be balanced out by a tiny counter-rotation of the ISS, and each major stop/start cycle would alternate direction so that the net rotation of the ISS were minimal.

The alternative is to build a counter-rotating flywheel either coaxially, or at least nearby. As the load (number of real + dummy astronauts) on the astronauts' cylinder changed, the load on the flywheel would have to be adjusted as well. The flywheel could also have servos to better null out some components of the structural vibrations as long as it were spinning synchronously. You could null out angular momentum at any frequency, so you wouldn't have to change the mass, but if it's not synchronous you are now adding a second exciting frequency to your vibrations, doubling the chances you might hit a particularly dangerous one!

The ISS is in no need for a periodic source of vibration. Unless that servo-system that constantly re-aligned the rotation axis of the cylinder to pass through the instantaneous center of mass of the astronauts in the drum, a cyclic vibration will be transmitted to the ISS frame. This is a problem that must constantly be battled, and has to be dealt with every time an astronaut begins or ends a sleep period, or rolls around too much.

Low frequency periodic vibrations are the bane of large mechanical structures not previously designed for them.

From The International Space Station (ISS) Researcher’s Guide International Space Station Acceleration Environment:

Vehicle Structural Modes

Vehicle structural modes reside at the low-frequency end of the vibratory portion of the acceleration spectrum. These vibrations fall within the frequency range from about 0.1 hertz to about 5 hertz. These vibrations arise from the excitation of natural frequencies associated with large components of the space station structure, such as the main truss, and with fundamental appendage modes, such as solar arrays. These structures are typically excited by relatively large magnitude, relatively brief impulsive events like during a reboost or by crew locomotive events like push-offs. The driving excitation of such events results in response vibrations as structural ringing damps out. Also, relatively small magnitude vibrations at just the right frequency will give rise to structural resonance. (emphasis added)

above: Cropped from Figure 4 of The International Space Station (ISS) Researcher’s Guide International Space Station Acceleration Environment. "Figure 4. Spectrogram Showing Mode One with Crew Slow Transition to Sleep." This suggests there are several structural resonances in the 0.1 to 1.0 Hz region. See original document for further discussion, and a list of about 20 different known resonant frequencies on page 12.

A very frightening and dangerous event happened on board the ISS in 2009 when a mis-programmed servo on a booster engine started tweaking the booster engine's thrust direction at about 0.5 Hz.

But during the Jan. 14 firing, something went seriously awry. The station’s solar power wings began swaying back and forth alarmingly. More dramatically, an interior camera captured views of wall-mounted equipment and cables flopping back and forth to a two-second beat, as the camera itself swayed on its mounting bracket.

Buildup of gyrations

It was quickly apparent that some periodic force had excited the space station’s structure at one of its resonant frequencies, leading to a buildup of gyrations rather than a damping down. As with the traditional "soldiers marching across a bridge" story, and the all-too-real Tacoma Narrows bridge collapse in 1940, resonance buildup in a large structure can quickly lead to serious consequences. (emphasis added)

• under no circumstances should you seek out a copy of the 1990 second installment of the Filipino horror film series "Shake, Rattle and Roll" and start watching from here. – uhoh Jul 30 '17 at 10:05
• Those vibrations are utterly terrifying. – Magic Octopus Urn Aug 28 '18 at 19:12

In addition to the other answers: a small structure (like a single module on the ISS) needs to rotate really fast to create 1G. This has undesirable side effects:

• Coriolis forces make moving around inside the module non-intuitive. There's an old Soviet experiment where people lived inside a centrifuge for a while, on the film (haven't found this online, it's in the BBC documentary 'Cosmonauts: How Russia won the space race') you can see them stagger and lurch along a corridor as if they're drunk. Another segment has someone throw darts at a dartboard, with the darts flying in a 90º horizontal arc.

• in a small centrifuge, there's a significant difference in the gravity levels between your head and feet, again making movement inside this module non-intuitive.

• if you use the centrifuge module only to sleep in, the astronauts have to get used to 0 G every morning. This would mean adapting fully to 0 G (takes about 2 weeks in the current situation) takes much longer, and you lose valuable time to space sickness.

• Presumably one would have to be laying coaxial to the rotation if it's going to fit inside the ISS or be a reasonably sized add-on module. But I think you have them sleeping standing up in order to increase loading on the major bones of the skeleton, e.g. spine, pelvis, legs? I wonder if there are any down-sides to sleeping "standing up" - like falling down for example? That's going to go over really well with the astronauts I'd imagine! :) – uhoh Jul 30 '17 at 11:30
• Hadn't thought about orientation during sleep. Sleeping upright would be very uncomfortable, I should think. Even with a harness to keep you upright. – Hobbes Jul 30 '17 at 11:37
• The part of the question "Why don't they do this considering the impacts zero g has on the human body?" suggests that the proposed "Centrifugal force sleeping" might be a solution to some of the zero-gee problems. The only four I could think of are bone loss, fluids in the head, eye shape changes, and insomnia. Wouldn't artificial gravity address the first three only when received in a standing position? And the bone loss only if one were actively standing, bearing the load on the bones (rather than in some kind of sleeping sling or jumpsuit)? – uhoh Jul 30 '17 at 11:57
• @uhoh Good points about sleeping horizontal not really achieving the intended goal. – FKEinternet Jul 30 '17 at 12:23
• I misremembered the documentary title a bit, see bbc.co.uk/programmes/b04lcxms – Hobbes Jul 30 '17 at 12:44

Offered as an addendum: a Centrifuge Module was planned for the ISS, and was partially built. Its centrifuge was for science experiments, not sleeping though. Budget problems doomed it, and it now sits in a parking lot in Japan.

Source

last image from here

• Any idea what was planned to go on the inside? Could one of the science experiments have been astronauts "centrifugally sleeping"? – uhoh Jul 30 '17 at 12:54
• There's an artist's concept of the module's inside in the Wiki article. It looks like the actual centrifuge was only a few feet wide. So sadly, it looks like there would have been no centrifuge rides for the crew. I'll link to that image. – Organic Marble Jul 30 '17 at 12:59
• Other places show the centrifuge "payload container" as a small box a couple feet on a side. forum.nasaspaceflight.com/… It's possible that crew could have fit in a centrifuge this size but the planned one was not designed for that. – Organic Marble Jul 30 '17 at 13:10
• OK so the big drum may be the external containment for the rotor. That keeps the swishing air from building a vortex in the module, reduces noise, and other things. OK this makes more sense. Oh, your comment on the astronaut safety review issues makes complete sense as well. – uhoh Jul 30 '17 at 13:15
• Also safety in case it flies apart. – Organic Marble Jul 30 '17 at 13:16

### Is it comfortable to sleep on the ISS?

It is a whole new type of comfortable to sleep in weightlessness. Even on the most expensive Earth mattress you occasionally have to roll over or adjust your pillow. In orbit, you can relax every muscle in your body. At bedtime you ﬂoat into your sleeping bag (which is loosely tethered to the wall with a couple of shoelaces), do up the long zipper and shut off the light. Because there's no effect of gravity pushing you into your mattress, you are perfectly relaxed and your whole body can go deliciously limp. Your arms and leg joints bend a bit and ﬂoat up, your neck slumps forward like a napping passenger on a plane; every muscle takes a rest. You can feel the slow pulse of your heartbeat, moving you slightly against the nothingness. When space travel eventually becomes inexpensive enough, it may well be the “space sleep spa” that attracts the largest crowd.

Chris Hadfield. An astronaut's guide to life on earth. --- Pan Books Ltd. 2015

So astronauts would probably vote for sleeping in 0 G.

• Relaxing every muscle in your body is not possible, all the muscles needed for blood circulation and oxygen/carbon dioxide exchange should do their work during sleep too. The muscles used for digestion also got work to do. Only the skeletal muscles could rest, but the intercoastal muscles are used for breathing. An astronaut should be interested in keeping their muscle mass and bone density, but sleeping both in zero or artificial gravity is not effective to prevent loss of muscles and bones. – Uwe Aug 1 '17 at 18:43
• @aguadopd this answer is really informative, thanks for adding it. Chris Hadfield is an excellent "explainer" of life and experiences aboard the ISS. – uhoh Aug 2 '17 at 4:06
• @Uwe I think the context clearly shows that Chrid Hadfield didn't include the vegeatitve nervensystem. – peterh - Reinstate Monica Apr 5 '18 at 18:30

A rotating structure big enough to accomplish that would be bulky, heavy, and very power-hungry to operate.

Space, mass, and power are all at a sharp premium on spacecraft and space stations such as the ISS, so a centrifugal bed isn't remotely within the budget.

• You typed faster than I did ;) – FKEinternet Jul 30 '17 at 0:47

Even if a 1g sleeping compartment were feasible, which the other posts have demonstrated it is not, the health issues associate with micro gravity would not be alleviated. Just sleeping in full gravity but working and waking in micro gravity would still have significant health effects.

In particular calcium depletion would still occur. Skeletons grow in response to compression stress (on the bones), which is usually caused by weight which is the effect of gravity on body mass. In the ISS this compression stress is simulated with ample exercise which when combined with a calcium and vitamin D enriched diet provides an element of compensation.

There are many other health issues associated with microgravity and I have only focussed on one to illustrate the issue, but many of the others would also not be solved by a specially engineered sleeping chamber.

One reason is that rotation at the kind of speeds practical in a spacecraft would cause nausea and dizziness, if not vomiting. Not conducive to rest. Also rotating machinery would cause a large number of risks of various sorts, and the need for a maintenance program. The more I think about this the more reasons I can think of.

A rotating space station, or one with a rotating gallery, is practical, if large enough that the rotation rate doesn't cause nausea (after all that's what Earth is).

• How do we know that rotating structures would cause nausea and vomiting? I thought it had never been tried. – Mike H Aug 1 '17 at 4:14
• @MikeH It's not been tried in space, but if you want a simple test, go to a playground and try riding on a merry-go-round - one of the "kid powered" ones you can spin up to a good speed – FKEinternet Aug 1 '17 at 16:42
• Skylab astronauts did not complain about the effects of running around the ring of lockers, 1/4 to 1/2 gee. youtube.com/watch?v=7ZPVg3qD07g . Or Google "motorcycle sphere cage" to see high-rpm small-radius high gee activity. Some people can adapt. Japan plans partial-gee experiments with mice on ISS, so there should be some indications coming soon. – MBM Apr 5 '18 at 19:11

There is no reason to do this for the sleeping quarters, as lying in bed at normal Earth gravity does not reduce the effects of weightlessness on the human body - in fact, it is has been used in multiple experiments to study the effects of weightlessness on bone and muscle loss:

In a recent review on bed rest studies of the past 20 years, it was concluded that head-down bed rest has proved its usefulness as a reliable simulation model for most physiological effects of spaceflight.

So the difference would be between sleeping in a level bed and sleeping with your head a few degrees down. It would be better to use centrifugal gravity for zones where the astronauts where performing load bearing activities.