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When you ask about rotating artificial gravity (yes, yes, technically radial acceleration), most answers boil down to "you'd need a 200m diameter spacecraft, and we can't build that yet"
2001 torus

But, of course, why would you need the entire circle to be a continuous space station? Just tie two modules together with a cable and extend, right?

Are the engineering problems just too difficult and numerous to even consider that? I can see a few already:

  1. Moving people between the modules, or to a stationary part of the station, would require an EVA. Not only that, but an EVA from a rapidly moving module.
  2. Making orbital or attitude adjustments would be much more complicated.
  3. Docking to visiting spacecraft would require a stationary module or stopping the rotation entirely.
  4. Moving resources between the modules and/or a stationary section would require long cables and pipes, and possibly seals between moving pieces.

Of course, one can think of solutions to all of these. Most can be addressed by temporarily halting the rotation and maybe winching the modules back together. But none of the solutions seem simple. Is it all just too much to even contemplate?

Edit:
I didn't think I needed to, but I guess I'll clarify the motivation. I've read plenty of people questioning whether Mars gravity (0.38g) is sufficient to ameliorate the many negative effects of long-term weightlessness. It seems reckless to wait until we're there, in the middle of an 18-month mission, to find out.

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    $\begingroup$ What would be the purpose of your ship ? For now, human adaptation is enough/better for all practical needs; for the foreseeable future. $\endgroup$ – Antzi Oct 28 '16 at 1:21
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    $\begingroup$ @Antzi Testing the effects of Martian gravity on human health. It's one of the biggest unknowns about a Martian mission, and one that I think is too important to find out about once you're already there. $\endgroup$ – Nick S Oct 28 '16 at 1:54
  • $\begingroup$ Nasa plan is to do it in situ. $\endgroup$ – Antzi Oct 28 '16 at 2:22
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    $\begingroup$ Significantly long tethers have an extremely checkered history in space. en.wikipedia.org/wiki/STS-46 en.wikipedia.org/wiki/STS-75 as well as the Gemini 9 experiment mentioned by Russell Borogove $\endgroup$ – Organic Marble Oct 28 '16 at 16:14
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    $\begingroup$ yep, we tried it, we got burned badly, nobody wants to risk it again anytime soon. With no air drag to dampen them, oscillations become a very ugly problem as they can easily accumulate and reach destructive levels... $\endgroup$ – SF. Mar 19 '18 at 8:51
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Tethering two habitable modules together splits your already cramped living space into two smaller spaces that can't be easily traversed; this means a lot of equipment like life support and bathroom facilities need to be duplicated (presumably power and comms could be on an umbilical alongside the tether.) That's an unacceptable efficiency/mass hit, so let's reject it.

You could put an inert mass on a long tether; this would involve launching a lot of dead weight, which is also unattractive.

Gemini XI did some tether experiments with an Agena spacecraft, with some odd "jump-rope" oscillations and jerkiness; I don't know if that's an insurmountable problem. The Gemini experiment achieved only milli-gee levels of acceleration. http://www.spacesafetymagazine.com/space-exploration/gemini/m-equals-1-all-up-mission-gemini-xi-part-2/

You'd have to reel in the weight and de-spin every time you wanted to dock a visiting spacecraft.

Finally, one of the most significant purposes of having a space station is to do experiments in zero g. Spinning the station defeats that purpose.

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    $\begingroup$ As a counterweight, one could use uninhabited mass such as a spent upper stage, the solar array, fuel for the arrival, surface equipment. But one would like to have that mass near the astronauts to provide some radiation shielding. With two habitats on a string, crew could transfer between them along the circle of rotation, rather than radially through the center, with little effort. That would improve mass utilization, but it still lacks the shielding benefit of putting those two modules next to each other, and introduces potential failure points. $\endgroup$ – LocalFluff Oct 28 '16 at 8:08
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    $\begingroup$ "one could use uninhabited mass such as a spent upper stage" Proposed for astronautix.com/s/spacestation1970.html And since your comment includes wording implying a planetary mission ("fuel for the arrival, surface equipment") which wasn't in the (direct) scope of the original question, I should note that the empty-upper-stage scheme was also part of the Mars Direct proposal. $\endgroup$ – Tristan Klassen Oct 28 '16 at 14:29
  • $\begingroup$ Could the docking be done in the center of the spinning wheel (with the visiting spacecarft spinning, too). $\endgroup$ – ypercubeᵀᴹ Dec 23 '18 at 21:44
  • $\begingroup$ @yper-crazyhat-cubeᵀᴹ If you're using tethered spin, there isn't anything but a cable to dock to in the center, unless you add a (massy, inconveniently located) docking module to the complex. Rotating docking is possible, but probably best left to robots rather than humans ("come now, do you really expect me to do coordinate substitution in my head while strapped to a centrifuge?"). $\endgroup$ – Russell Borogove Dec 23 '18 at 21:55
  • $\begingroup$ @RussellBorogove: Docking is probably best left to robots anyway. And a cable could provide a compliant, robust shock-absorbing structure to dock to, leading the craft straight to a docking port. (Realistically, you'd have multiple cables for stability against twisting, likely interlinked for redundancy.) $\endgroup$ – Christopher James Huff Jan 5 at 18:43
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There's no need for simulated gravity in human spaceflight the next several decades. It might be of interest to basic biological research, but it's up to them to buy a biological research station in LEO, it is of no concern for agencies and companies planning human space flight.

It will be some time before humans go further than Mars, and it only takes a 6-9 months economical Hohmann transfer to get to Mars. Hundreds of astronauts have spent that much time in microgravity and enjoyed it. No one has ever been hurt by microgravity, and reduced gravity isn't exactly epidemic on Earth, so simulated gravity has very low priority in both the space and the medical communities.

Microgravity has the benefit of increasing human utilization of spaceship space. One can squeeze more people into a weightless room. It helps reducing the size and mass of a spaceship. Structures such as solar arrays and antennas can be made lighter and more delicate in microgravity. A spinning spacecraft suffers a mass penalty throughout the design. Also, gravity hurts. It breaks backs, it kills people falling, you drop things on your toes. It is a blessing for human health to get rid of the great cause of accidents, wear and toil which gravity is.

Reduced gravity, such as 16% or 38% as on the Moon and Mars, should take care of several of the problems experienced in microgravity. It should be enough to somewhat normalize the fluid pressure in the upper body, to give load on muscles and skeleton, to greatly increase the effect of exercise, to make dust fall down instead of floating in everybody's faces (reducing the need and noise of ventilation), to deactivate microbes which in microgravity seem to react as if they were buoyant in water which activates them, to sit or sleep without strapping down.

The resources for a simulated gravity station are better needed on a real mission to Mars to lower real risks of launch, landing and any kind of hardware or software failure. The best place to find out about the effects of reduced gravity is on the Moon and on Mars! We don't need to recreate that which we already have got handy by nature. Gravitational effects interact with radiational, chemical and psychological effects. No examination could be more complete than actually spending a long time on the Moon or Mars. If people can stay on the Moon for a year, they certainly can stay on Mars with more than double the gravity for a year too. And actually, people have spent a year in microgravity, so there's the question about what purpose simulated gravity would have.

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    $\begingroup$ "No one has ever been hurt by microgravity." Define 'hurt'. Muscle atrophy, loss of blood volume, loss of bone density and ocular impairment come instantly to mind. $\endgroup$ – RonJohn Mar 19 '18 at 0:47
  • $\begingroup$ @RonJohn I was thinking that, but in fairness, most of those become issues only when returning to the surface. $\endgroup$ – Gargravarr Mar 21 '18 at 16:09
  • $\begingroup$ @LocalFluff I think my main concern is that while we can guess that Mars gravity is enough to prevent the effects of microgravity, we don't know because we've never tested it! And it's not fun to find out when you're trying to lift off from Mars. So I thought it'd be a pretty good idea to set up a rotating space station to find out. Of course, your idea of finding out on the Moon is a nice alternative. $\endgroup$ – Nick S Jan 7 at 20:22
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In a space ship, if an astronaut doesn't touch any part of the inside of the ship then they will not feel centrifugal force after rotating the ship and hence no gravity will be established for them.

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  • $\begingroup$ You don't feel the gravity either when you jump from a skyscrapper. Until you hit the ground ;) $\endgroup$ – ypercubeᵀᴹ Dec 23 '18 at 21:41
  • $\begingroup$ Arvind, not spinning a single ship - the idea being discussed is connecting two ships/modules/habitats with cables and swinging them around their common centre of gravity, a bit like (2 ended) Bolas - en.wikipedia.org/wiki/Bolas $\endgroup$ – Ken Fabian Dec 25 '18 at 0:03

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