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Why doesn't the ISS (International Space Station) spin to imitate Earth gravity?

I remember reading somewhere that centrifugal force isn't enough to help people avoid things like osteoporosis, etc., but I can't imagine why not. I have thought about this with all of my brains and still don't get it. It seems to me that it's doable -- you'd have to have like a ball-bearing docking center (or whatever -- we'll work on that later) so folks could get on/off, but assuming that problem's solved, what are the other problems? Getting the spin started might take extra weight/fuel, but once it starts spinning won't it spin forever? I'm not mathy so consider NOT citing advanced physics in your answer, but I'm picturing like a[t least a] two-armed ship where the dock is the center (i.e., no "gravity") and is stationary (on spaceship-equivalent ball bearings) and the arms get progressively more gravityish until you get to the tips, wherein are the work/living areas. Tips are bulbous or otherwise larger than the arms, which are long and...arm-y.

Can't we make this happen?

EDIT: I shouldn't have specifically mentioned the ISS since my question was just as much about a theoretical spinning ship's effects on its inhabitants. In putting this question here (in Worldbuilding), I was imagining a ship or structure, in space, meant for long-term living. I'm new on StackExchange, too, and am therefore an idiot w/regard to posting in wrong areas. :)

I did search all over the place for a similar question/answers, but didn't find one (was probably not doing the search right). I'm sorry if I goofed the placement.

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closed as off-topic by sphennings, L.Dutch, JBH, MichaelK, Mołot Nov 15 '17 at 10:27

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question does not appear to be about worldbuilding, within the scope defined in the help center." – sphennings, L.Dutch, JBH, MichaelK
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ I'm glad you want to participate, but this is an engineering question. $\endgroup$ – Spencer Nov 14 '17 at 23:19
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    $\begingroup$ I would recommend space.se, and someone asked a milder version $\endgroup$ – user25818 Nov 14 '17 at 23:22
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    $\begingroup$ In the sci-fi stories that try to do this, the space ships are a huge ring (huge) that spin around a central hub. ISS has a different design, presumably it was more cost effective, and perhaps practically speaking it is better to rotate the crews every few months anyway. $\endgroup$ – DPT Nov 14 '17 at 23:47
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    $\begingroup$ artificial-gravity.com/sw/SpinCalc/SpinCalc.htm $\endgroup$ – Ville Niemi Nov 15 '17 at 1:03
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    $\begingroup$ “At a conference a few years back I quizzed a NASA official who advocated a multi-decade program investigating zero-gravity health effects on humans prior to a piloted Mars mission. ”Why not just employ artificial gravity?” I asked. “We can’t do that,” he said, “all our data is going to be for zero gravity”. - Robert Zubrin The Case for Mars p129 $\endgroup$ – Slarty Nov 15 '17 at 11:31
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ISS is too small and its modules are not balanced to sustain a meaningful spin.

If we want to have a space station with artificial gravity, it has to be balanced, like a wheel, and have thrusters to control rotation. Also, having a static, non-rotating segment would help with things like docking and solar panel orientation. The station should not rotate too quickly because it can cause motion sickness.

It is considered sufficient for human comfort to create an artificial gravity as little as 0.15g, however, this is still not enough to eliminate harmful health effects like osteoporosis.

You can check out Centrifugal force calculator. To get 0.15g with 4 RPM, a station needs to have a radius of 8.4m. To get full 1g, the radius needs to be 56m.

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The problem here is not the approach, it's the scale. Using Centrifugal force to simulate gravity via angular momentum can work, but it on smaller scales introduces other problems. For one, the simulated 'gravity' on a small scale would be measurably different between the head and the legs, making movement clumsy and difficult. The ISS is just too small to make rotation practical, and given that it's in a low earth orbit the practicalities of maintaining that orbit AND angular momentum would just add to the complexity of an already complex system. If the ISS had been placed in the L5 Lagrange point, perhaps this would be easier but it would still have to be a lot bigger.

Even on a large scale, it's not a panacea. Today, we have warnings on international flights about Deep Vein Thrombosis (DVT) because sitting around all day, even in a gravitational field, is not good for us. Having simulated gravity would only help if we did regular exercise that actually puts some stress on our bones and joints.

You really need a very large ring of some kind, where everyone would live on the inside of it to make it work. I don't know the correct minimum viable scale, but I suspect you're talking a minimum of half the diameter of the moon, for instance.

[EDIT] @Sugarbat;

What you're saying is true; the slightly lower pressure amplifies the problem to dangerous levels, as I understand it. But, it's the sedentary nature of being a passenger over an extended period that is the root cause. Incidently, British soldiers on guard duty who had to stand at attention and completely still out the front of places like Buckingham Palace were always trained to wiggle their toes while standing so as not to let their legs fall asleep can cause similar concerns; this is standing at more or less sea level pressure for an extended period. The real killer here is lack of circulation; sedentary office workers who don't exercise have (according to some studies) double the health risks of active smokers.

So, yes; you're right Sugarbat and I should have made it clear how I was using DVT as an example in this case, namely that lack of physical activity even in a conventional gravity well can cause problems under a range of other conditions.

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    $\begingroup$ You'd need to be a little bigger than the ISS (though not necessarily more massive as most of the size could be the arms described in the question), but certainly not Moon-scale. See the calculator linked in Alexander's answer. $\endgroup$ – Elukka Nov 15 '17 at 9:57
  • $\begingroup$ Yes, that's partly why I was describing a theoretical ship with arms instead of one shaped like a wheel. I.e., roughly the shape of a dumbell, if the handle (arm, whatever) of the dumbell were a pencil and the weights on the end were gigantic. It would help if the living/working quarters (the "weights") actually were ring-shaped, themselves, since that would allow for maximum "floor" space. But I did totally realize the point made by several commenters about how the ISS is used primarily for tests in micro-/zero-gravity, which eliminates the need for artificial gravity. $\endgroup$ – sugarbat Nov 15 '17 at 17:04
  • $\begingroup$ Tim -- I thought the DVT warning for airplane travelers was more about the pressure changes than the sitting? I.e., low pressure makes body think it's losing blood, so initiates clotting factors? $\endgroup$ – sugarbat Nov 15 '17 at 17:17
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Theoretically a space station could be made to spin to simulate Earth gravity. Theoretically this could keep astronauts healthier longer (though they still would get increased radiation). Theoretically we could build a spinning dock or a gigantic flywheel to assist non-spinning ships docking with such a station, or commit to spinning all ships trying to dock.

"In theory, theory and practice are the same, in practice that ain't true." But Disregarding all the engineering of could, would we want to?

No.

ISS is made to play with microgravity. There is a centrifuge to allow them to start exploring the difference between spinning and microgravity, but most of the science needs weightlessness: if we wanted gravity we'd do it on the ground.

ISS uses much of its internal surface area, and with spin it could only use two walls, something like the difference between living in a shipping container standing on it's bottom or standing its door.

ISS has antennas and solar panels outside that need to stay oriented. People go outside to work, this would be more dangerous with a force trying to throw them off the station.

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    $\begingroup$ Even if they did get to spin at sufficient speed it is too small to have an even gradient of artificial gravity, (literally your head and your feet would be at significantly different gravities) so the poor astronauts would spend all their time nauseous or puking their guts out. $\endgroup$ – John Nov 15 '17 at 0:44
  • $\begingroup$ Thank you both. I actually wasn't remembering about astronauts needing (for whatever reason) to crawl around on the outside of the ship. I'm now trying to imagine a way for them to do this on a spinning ship without having to fight the problems you describe. Really strong magnets/special boots? Tethers w/carabiners and metal loops built all over the surface of ship/structure? Don't address those questions if they make the original post even more wrong-area-posted. :) $\endgroup$ – sugarbat Nov 15 '17 at 17:20
  • $\begingroup$ @sugarbat I think that would make a good space.se question, but I think the answer is that we already worry about people falling off, and take all the steps we can afford to minimize that risk. $\endgroup$ – user25818 Nov 15 '17 at 17:28
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There is also a major issue with angle. A spinning station throws the occupants toward the outer edge of the station, so the outer walls would in fact, have to be the floor. IN other words, your head would be pointed sideways toward the central axis, and your feet would be pointed outward. That means all the usable equipment on such a station would have to bolted on the outer wall, so you could walk along that wall to use it. Obviously, ISS is totally not designed that way. The most sensible design is indeed, a huge ring.

Physiologically, gravity on Earth affects arms, legs, head, heart, circulatory system, digestive system, skeletal system, nervous system, etc. A spinning centrifugal force station could somewhat simulate that, but it is simply not the same thing. Your bones in your legs would only maintain their strength if you used them to move around the station, but bones in your arms, fingers, neck, spine, etc. would not have the same load. I suspect the station would have to be very massive, as others have suggested, to properly load your body.

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  • $\begingroup$ Why woud the bones in your arms, fingers, neck, spine ect not have the same load? $\endgroup$ – Slarty Nov 15 '17 at 10:30
  • $\begingroup$ @Slarty -- I think if we're talking about size (something approximately the same size/weight as the ISS, even if engineered to spin), the smaller the spinning ship the bigger the "gravity" difference is between your feet, which are at the outer edge (floor) of the spin and your head, which is further away from the outer edge. In order for your whole body equally to feel the same, gravity-wise, several commenters are pointing out that a spinning ship would have to be huge (see Tim B.: "minimum of half the diameter of the moon") $\endgroup$ – sugarbat Nov 15 '17 at 17:00
  • $\begingroup$ fsgregs: Yes, I do understand that the outer edge would be the floor, and of course the build of the ship/structure would have to be built to comply with that. But can one of you explain why it would have to be a ring instead of an arm-structure? I get that whatever shape it is, it would have to be huge (or long/heavily weighted) to make the spin-gravity affect whole bodies of inhabitants equally, but I don't see how that can't be effected by building a structure shaped like a dumbell (see my other comment here). Assuming hugeness, why can't spin-gravity equal earth gravity? $\endgroup$ – sugarbat Nov 15 '17 at 17:25
  • $\begingroup$ @sugarbat I think the commentators you are referring to are simply mistaken. artificial-gravity.com/sw/SpinCalc/SpinCalc.htm#comfort All authors used in the above calculator believe that a spinning radius of 250m should provide a comfortable environment. Some suggest that even a 25m spin radius might be acceptable with acclimatization. The 5 sets of authors data used in the above calculator seem to have a good pedigree for example Stone, Ralph W. (1973). An Overview of Artificial Gravity. (NASA SP-314, p. 23-33). NASA Scientific and Technical Information Division. $\endgroup$ – Slarty Nov 15 '17 at 18:02
  • $\begingroup$ @sugarbat you are correct to say that the structure does not have to be a ring and it is possible to have an artificial gravity of 1g. In Zubrin’s book (the case for Mars) he proposes a cylindrical habitat tethered to a spent upper stage rocket for the Mars trip. But several configurations are possible see marsforthemany.com/news/technology/… $\endgroup$ – Slarty Nov 15 '17 at 18:15

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