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As the month of November 2020 has marked 2 decades of the ISS being operational as well as the first operational flight of a next-generation spacecraft to it, I felt it fit to ask this question today. I will give sources to claims made in this question as desired.

From spaceflights to earlier stations like Skylab, the Salyut series, and Mir, it was known that prolonged exposure to microgravity is very damaging to human health, causing massive bone and muscle loss as well as blood loss and blood redistribution among numerous other ill effects, to the extent that a crewed Mars mission would be barely recoverable from without mitigating technology. While not fully foreseeable by the original developers of the ISS, development of this technology is happening extremely slowly, with non-negligible progress only in the field of muscle loss (the ARED is still being tested on more than a decade after launch).

Even discounting health issues, microgravity has effects that make comfortable life challenging. Because of the lack of settling, the air volume has to be kept extremely clean, necessitating loud fans that disrupt sleep. These are also necessary to even carbon dioxide and heat distribution due to the lack of convection, which also makes cooking extremely difficult—the first attempt in December 2019 taking a whole 6 times longer than in 1 g. Food consumption is very problematic in general. Even something as simple as a soft drink isn't enjoyable in space, due to eructation often resulting in acid reflux. These began to be known even before the medical effects were understood.

Attempts to ameliorate issues like these—not to mention simply designing systems to operate in the alien environment of zero-g—had cost billions in R&D even before the ISS was launched, and they knew it would continue to cost billions. On the other hand, a minimal artificial gravity station could be constructed using as little as 1 largely-conventional module with an equally-massed inert counterweight. To ease docking and attitude control a connecting module and another contra-rotating couplet could be added, for a total of 3–5 modules plus 4 trusses, 2–4 of which are pressurized. Optional additional modules could be symmetrically added to fill out the torus. While initially at 1 g, the gravity could be reduced to determine a cutoff value, allowing the future design of (or modification of the ISS into) smaller, more mass-efficient stations. With all that in mind, why was the ISS constructed as a zero-g station at all?

My guess is that they (mostly NASA and what later became Roscosmos) did so mainly because it was the result of a combination of the much older Space Station Freedom and Mir-2 programs that faced cancellation, but only one module was completed before the ISS program was announced, which was not the first one to be launched.

IMPORTANT NOTE: I know one of the primary objectives of the ISS is the study of non-human subjects and materials in microgravity, but that research could be carried from modules docked to the central one, with the crew living in the centrifugal modules.

This doesn't seem more costly than the ISS at "completion" in 2010, or even significantly more so than it at modular operation in 2002. Correct me if I'm wrong.

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    $\begingroup$ "necessitating loud fans that disrupt sleep" isn't really that big a deal. I used to live literally around the corner from an oil drilling rig where the drill ran day and night, 24/7 for weeks on end. It wasn't a problem. Many people live in houses where they need an air conditioner in the bedroom in the summer time. A loud, humming and blowing window AC unit isn't a problem - I've slept in plenty of rooms like that and it's ok. A bigger problem comes from intermittent noises - other people banging around and equipment that loudly (and randomly) cycles on and off. $\endgroup$ – JRE Dec 1 '20 at 13:28
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    $\begingroup$ None of your examples of earlier stations rotated, meaning that artificial gravity was (and still is) an idea without long-term flight heritage. What makes you think that risk-averse NASA could have pursued a rotating design 20 years ago, even politically? $\endgroup$ – Bear Dec 1 '20 at 13:42
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    $\begingroup$ Re With all that in mind, why was the ISS constructed as a zero-g station at all? (1) That was the explicit premise for building it. (2) It is so very easy to build a rotating space station in sci-fi movies and novels. In reality, it is anything but easy. Much of the needed technology is at Technology Readiness Level (TRL) 3, at best. (3) A rotating space station would need to be immense, much larger than the ISS, to avoid physiological problems. $\endgroup$ – David Hammen Dec 1 '20 at 14:04
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    $\begingroup$ In addition to the IMHO blindingly obvious fact that the purpose of the ISS was to study life (and do science) in microgravity, the "negative" impacts only show up in the long term. In the short term, they're either fun, or tolerable. It's like going on a long camping trip, or even say a research mission to Antarctica or an Everest climb: a great adventure, but not the way you'd want to live your entire life. FWIW, I haven't noticed that NASA has ever faced a dearth of astronaut applicants :-) $\endgroup$ – jamesqf Dec 1 '20 at 17:12
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    $\begingroup$ Why do oceanographers study the sea when they know it is so wet? $\endgroup$ – Dave Gremlin Dec 1 '20 at 18:48
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Reliability.

Any rotating station needs non-rotating components: solar panels need to face the Sun, radiators need to be shadowed, docking points need to be non-moving, and so on. Making a rotating joint that can last decades is hard; if the hub of a rotating station seizes up, the resulting accelerations are likely to tear the station apart and kill everyone aboard.

Making a rotating airtight joint is even harder, and you'll need one if you want both a rotating living space and a non-rotating laboratory.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage Dec 3 '20 at 15:20
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I'll add one or two more items to Mark's excellent list.

  1. Stability - large rotating platforms (and they have to be large to produce useful artificial gravity) are subject to all sorts of precession.

  2. Cost. The ISS was not cheap. Now imagine just getting maybe 50 ISS' worth of mass into orbit, assembled, and then enough fuel to spin it up.

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    $\begingroup$ Can you elaborate on why precession is a problem? I'd have thought that a spinning platform would be more stable than a stationary one, due to its higher rotational inertia resisting any changes in orientation. A spinning top, for example, remains relatively stable because it's spinning, not in spite of it. $\endgroup$ – Nuclear Hoagie Dec 1 '20 at 19:58
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    $\begingroup$ @NuclearHoagie, if you have a rotating object, there is a center of rotation, based on where the center of the mass is. In order to have a perfectly non-moving docking port on a rotating joint, you'd need to keep the space station perfectly balanced around that center of mass - even people moving around inside the station's artificial gravity would disrupt that center of mass. It may not be much imbalance for a single person, but equipment, waste (even tanks of fuel/oxygen/water) would need to be balanced around the center of rotation at all times. $\endgroup$ – Milwrdfan Dec 1 '20 at 21:39
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    $\begingroup$ 50 ISS is worth of mass seems like a bit much. Are you talking about an entire wheel style station? a tether based tumbling pigeon seems like it would probably be more like two or three ISS's if even one. $\endgroup$ – ikrase Dec 2 '20 at 7:52
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    $\begingroup$ @rtaft ISS weighs over 400 tons. If you want to produce 1g gravity, you'll need enough cables so that they can hold the weight of 420 tons in Earth's gravity without snapping, or a tad over a fully loaded Boeing 747... or a tube pressurized to 1ATM that can suspend half a Boeing while there are astronauts bouncing around inside of it. And any structural failure means loss of the entire station and crew. And you still haven't solved the issue of docking, precession, orbital maneuvering of a large rotating dumbbell or how you are going to spin it up in the first place. $\endgroup$ – John Dvorak Dec 2 '20 at 22:23
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    $\begingroup$ @NuclearHoagie : Rotating around the long axis of a generally prolate object (more precisely, around the objects first or third principal axes) can be stable, otherwise not. See the Dzhanibekov effect or watch a video demonstrating quasistable tumbling. The ISS was assembled over years, so its principal axes were going to be changed abruptly by the addition of modules whose detailed mass distributions were not known and not knowable at design time... $\endgroup$ – Eric Towers Dec 3 '20 at 10:22
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It's a good question, followed by many relevant responses so far. I'll focus on the physiology aspects.

Research had been conducted for decades prior to ISS launch on creating artificial gravity through spinning. The short of it is: the human balancing system (inner-ear plus brain) cannot not handle the spinning motion on the scales of what humans could build at the time (or even now for that matter). As I recall, in order to not screw up the human motion/balancing sensory system, the radius of rotation would have to be huge, even for as little as a quarter "G" environment. Something well over a hundred meters in the radius of the spin.

The space "ring" in earth orbit from 2001 Space Odyssey, as I recall, was likely not even large enough for a near 1 "G" environment that did not screw up the human balancing system, let alone the smaller "Odyssey" ship sent to Jupiter with "HAL" on board (Can we talk about this Dave?). There are formulas for calculating the percent of artificial gravity created, using the radius, rate of spin versus the human balance sensory capabilities. If a given human has a functioning or near functioning vestibular nerve, the radius of the spin must be huge, or otherwise they would experience constant vertigo.

Experiments have been performed on humans who have damaged or 100% non-functioning vestibular nerves. The short of that is: only people with 100% non-functioning vestibulars could survive in a relatively small spinning artificial "G" environment, since those people have no sense of motion or "falling".

It's a good question you asked, worth refreshing our understanding of... every generation or so.

Related links:
https://www.artificial-gravity.com/sw/SpinCalc/
https://www.space.com/558-artificial-gravity-spin-idea.html
Why are there no spacecraft rotating for artificial gravity?
https://phys.org/news/2019-07-artificial-gravity-free-science-fiction.html
https://www.popularmechanics.com/space/rockets/a8965/why-dont-we-have-artificial-gravity-15425569/

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    $\begingroup$ I found a 1998 experiment that's related, sciencedirect.com/science/article/abs/pii/S0165017398000393. Can you cite any published results? $\endgroup$ – Camille Goudeseune Dec 1 '20 at 21:13
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    $\begingroup$ Couldn't you have two stations connected by a kilometer long cable? That would have a large rotation radius. $\endgroup$ – PyRulez Dec 1 '20 at 23:42
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    $\begingroup$ @uhoh: Note that the claim here isn't that the gravity wasn't strong enough, but that at that radius (and spinning fast enough to make 1G) the rotational effects would be noticeable to humans standing up. I left a similar comment under your question, replies should probably go there. $\endgroup$ – Peter Cordes Dec 2 '20 at 3:33
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    $\begingroup$ @PeterCordes I somehow missed the whole second paragraph and started at the 3rd. Thank you for pointing this out, I'm going to make adjustments to the question so as not to misrepresent this answer which is far more on the money than I'd originally realized. Thank you for bringing this to my attention! $\endgroup$ – uhoh Dec 2 '20 at 3:48
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    $\begingroup$ @uhoh: I have a bad habit of skimming quickly, and then sometimes writing a long comment without checking that the post actually said what I though >.< So yeah, happens to me, too, and glad I could help. $\endgroup$ – Peter Cordes Dec 2 '20 at 3:53
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There was a proposal to add an experimental rotating habitat: Nautilus-X. One of those wonderfully tortured backronyms: Non-Atmospheric Universal Transport Intended for Lengthy United States Exploration - eXperimental. Its primary purpose was to extend how long humans could live in space not only through artificial gravity, but also storage for consumables, radiation shielding, micro-meteoroid defense, and self-sealing hull.

The idea was to build a spacecraft in orbit for long term journeys, but first they were going to test the idea on the ISS.

enter image description here

To save weight and launch costs, it would use inflatable structures pioneered by Bigelow Aerospace. It would provide partial-gravity for sleeping quarters, food prep, eating, and toilets; the last possibly being most appreciated.

One of the design goals was to not mess up the micro-gravity environment of the ISS. That means no vibration from the rotating ring and flywheel. Minimal leaks from the ring seals.

The demonstration ring would be only 30 feet in diameter with a 50 inch cross section. That's not quite big enough to stand up in, but it's just a demonstrator and standing in a ring that small might make you sick. At 10 RPM it would provide .5g.

It never happened. The proposal to build a demo is from 2011, 13 years after the ISS was launched, should give you some idea about our capability to sustain gravity in space. There is a lot of basic science and engineering we still have not done. The proposal lists them...

  • Bearing and hub design
  • Seal design
  • Effects on guidance, navigation, control, and station keeping
  • Centrifuge startup
  • Centrifuge operation
  • Effects on the human body
    • Repeatedly moving between partial and zero-g
    • Emotional
    • Vascular
    • Digestion (with the delicious note "tendency to vomit during transition")
    • Excretion
    • Vision
    • Bones
    • Sleep
  • Load distribution (what happens if the wheel is out of balance?)
  • Thermal management
  • Load transmission to the ISS

No space station will incorporate gravity into its design until we've gained enough experience through trial and error to answer those questions and mitigate the problems.

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    $\begingroup$ @ThatCoolCoder : except it's not a retronym. A retronym is when we need a new term to differentiate something that's existed for a while. (like "snail mail", when all "mail" used to mean postal mail before e-mail) This is a case of a backronym. (an acronym in which you're specifically trying to match an existing word) $\endgroup$ – Joe Dec 2 '20 at 16:40
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    $\begingroup$ I believe the word you're looking for is backronym. $\endgroup$ – zephyr Dec 2 '20 at 22:02
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    $\begingroup$ @Joe Sorry, I wasn't clear. It's a Reverse Explanation To Rationalize One's Name Yet More. $\endgroup$ – Schwern Dec 2 '20 at 22:06
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    $\begingroup$ @Schwern : I'm not sure if I should laugh or cringe at that $\endgroup$ – Joe Dec 3 '20 at 1:08
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The other answers good, but I think we miss something.

Imagine a spacewalk around a rotating space station. With the current technology, we do need regular spacewalks.

  1. Everything dropped doesn't just float around. It is lost for good.
  2. The peripheral surfaces are hard to access from the outside - it would need mountain climbing equipment and skills. Astronauts are easy to get lost as well.

... or the station to be stopped for maintenance?

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  • $\begingroup$ We already have a solution for point 1 - when astronauts are doing spacewalk repairs, any tools and loose parts that are involved are generally tethered so they can't get lost. $\endgroup$ – Darrel Hoffman Dec 2 '20 at 14:29
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    $\begingroup$ One cannot tether every bolt and nut. Or probably can, but the spacewalks will become even less efficient. And they are expensive to start with. $\endgroup$ – fraxinus Dec 2 '20 at 15:49
  • $\begingroup$ @fraxinus : Correct, you can't tether them individually. Instead, they use fastener capture plates : spacetelescope.org/images/hst3-venice2010 $\endgroup$ – Joe Dec 2 '20 at 16:45
  • $\begingroup$ @Joe And even then, they lose stuff from time to time. It's just not that big of a problem. The ISS is at a relatively low altitude, the orbits of any loose parts or tools will decay within a few months. $\endgroup$ – Ryan_L Dec 3 '20 at 4:22
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O'Neil cylinders have a very large minimum radius of several kilometers for a reason, or several reasons in fact.

One can't just spin up a small station to simulate gravity and expect a person to be comfortable when the acceleration at their feet is so much greater than the acceleration at their head, and it gets worse as soon as they start moving due to Coriolis effects.

Making a spinning space station increases complexity by enough, then it has to be ENORMOUS to achieve your objective.

From O'Neill cylinder; Artificial gravity:

The cylinders rotate to provide artificial gravity on their inner surface. At the radius described by O'Neill, the habitats would have to rotate about twenty-eight times an hour to simulate a standard Earth gravity; an angular velocity of 2.8 degrees per second. Research on human factors in rotating reference frames indicate that, at such low rotation speeds, few people would experience motion sickness due to coriolis forces acting on the inner ear. People would, however, be able to detect spinward and antispinward directions by turning their heads, and any dropped items would appear to be deflected by a few centimetres. The central axis of the habitat would be a zero-gravity region, and it was envisaged that recreational facilities could be located there.

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  • $\begingroup$ +1 This is a really good point and surprisingly the first time on the page that the Coriolis effect is mentioned and described explicitly. Welcome to Space! $\endgroup$ – uhoh Dec 2 '20 at 23:12
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    $\begingroup$ @uhoh Thanks for the edits. $\endgroup$ – DKNguyen Dec 2 '20 at 23:12
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To Study Zero G

It's pretty hard to study zero-g if you're not in a zero-g environment.

Now the question implies that we could mitigate some of the effects of zero-g, say on the crew's health, by simulating gravity. However, this ignores that the effects of zero-g on crew health is part of the ISS's mission:

Research on the ISS improves knowledge about the effects of long-term space exposure on the human body. Subjects currently under study include muscle atrophy, bone loss, and fluid shift. The data will be used to determine whether space colonization and lengthy human spaceflight are feasible.

https://en.wikipedia.org/wiki/Scientific_research_on_the_International_Space_Station

Or to put it another way, the ISS and Astronauts that crew here are not just there to conduct scientific experiments. They are part of the experiment.

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    $\begingroup$ This is the most correct answer imho. $\endgroup$ – DarcyThomas Dec 4 '20 at 7:38
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    $\begingroup$ Indeed, it's by design basically, although they also recognize the technical difficulties of implementing it, as explained in other answers. From the NASA itself: "NASA and others prefer to work in microgravity -- or free-fall -- where many amazing experiments and processes can be carried out. The Space Station is the world's only large, long-term, gravity-free science laboratory" Source: web.archive.org/web/20060929044226/http://liftoff.msfc.nasa.gov/… $\endgroup$ – Quora Feans Dec 4 '20 at 11:30
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To add yet another facet to a question which has provoked such excellent answers:

WEIGHT

A space station that is not spinning weighs nothing, however massive it is. To a first approximation, it could be held together with string.

A space station which provides a 1g centripetal force means that the floor under your feet weighs 1 ton per ton. Which is a lot of weight. Which means strong structure to hold it together. Which is massive. Which, therefore, weighs. Which, therefore, needs more strength…

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