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Obviously, the benefits of building ring sections on spacecraft are pretty large when you factor in the detrimental effects prolonged stays in zero gravity environments (not accounting for radiation exposure) have on the human body. They also contribute to mechanical complexity and can exert greater mechanical stressors to other parts of a spacecraft under acceleration and deceleration, which would increase the cost of spacecraft designs which include them.

What other benefits are conferred by having an environment wherein crewmembers can operate under gravity? Some I have considered include:

  • We're generally built to do a lot of things under the influence of gravity, which are made a lot harder without it.
    • Eating and drinking.
    • Sleeping.
    • Maintaining hygiene (using the bathroom in general).
  • General familiarity with gravity acting as a psychological comforter.
  • Cleaning. Gravity generally ensures specific surfaces get dirty rather than everything.
  • Having both hands free without being strapped down (though having hooks to slip your feet through would mitigate this somewhat).
W.Asp
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    The ISS was supposed to have a centrifuge module in it - it's now sitting in a parking lot in Japan - although not big enough for crewmembers. An advantage for science of having a centrifuge in a free-fall lab is that theoretically you could dial in any g you wanted and sustain it. – Organic Marble May 05 '21 at 17:21
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    zero-G exercise is difficult as well. – Carl Witthoft May 06 '21 at 13:07
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    In case of a physical injury, damage mitigation and surgery are easier if you have gravity.

    (My cite is Fallen World, episode 11 of season 3 of The Expanse. )

     – Nimloth May 06 '21 at 14:04
  • @CarlWitthoft an exercise in a (undersized, sub-100m diameter) centrifuge is at least that bad. – fraxinus May 06 '21 at 14:18
  • @fraxinus Too small a centrifuge also makes exercising difficult? I would hazard that to be the result of differentials in relative gravity felt at floor and head height. – W.Asp May 06 '21 at 16:02
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    I find it important to note that artificial gravity does not require any ring structures. The simplest design is to just tether a habitat to a counterweight (or two habitats together) and have them spin about their common center of mass. – leftaroundabout May 06 '21 at 16:14
  • @leftaroundabout true, but it requires either rotating the whole thing (but you want some place with microgravity, right?), a good ring seal at the gateway (ring seals and deep vacuum dont play well together), or a spacewalk between the rotating and non-rotating parts (spacewalk is an expensive procedure in itself and this one implies the risk to fly away at the rotation speed). – fraxinus May 06 '21 at 16:33
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    @W.Asp The tidal effects are part of the problem. There are coriolis (pseudo) forces that will make every movement quite a fun (to look at). Imagine jumping up and hitting a wall. – fraxinus May 06 '21 at 16:38
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    @fraxinus well, either have a research space station with microgravity everywhere, or a space travel vessel with artificial gravity everywhere. Combining both doesn't really work very well. – leftaroundabout May 06 '21 at 17:10
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    @fraxinus no need for a ring seal, just enclose the entire rotating section in a non-rotating cover. The cover does not even need to be rigid. Some inflatable bag would be enough. –  May 07 '21 at 09:26
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    Didn't think about inflatable, but yes, it is pretty much a valid approach. There is (was?) even an inflatable module at ISS. – fraxinus May 07 '21 at 11:31
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    https://www.nasa.gov/content/bigelow-expandable-activity-module . @laolux Yeah, inflatable parts have a lot of promise. The problem is however that you would need to enclose the entire disk of the rotating part. The advantage of a tether design is that you can get a huge radius with little material effort, and that reduces the coriolis issue. A non-rotating enclosure, even if it's inflatable and low-criticality, would not be feasible in such a large size. Also, rotating in – effectively – an atmosphere would incur drag for the rotating part. – leftaroundabout May 07 '21 at 22:15
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    Alien chase scenes look weird unless there's gravity, ok? – smci May 09 '21 at 01:04
  • @W.Asp interesting question, but what do you mean by "inertial" gravity? In isolation it doesn't mean much to me. Is there some distinction of inertial or not inertial specific to this context? – Puffin Jun 09 '21 at 19:36
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    @Puffin Well, there are two main ways of generating artificial gravity on a spacecraft: accelerating in one direction constantly, or using a ring section or similar to rotate such that the "inertial force" (i.e. the fictitious force generated that seems to pull outwards from a rotating body) can act upon a body on the inner side and create the sensation of gravity. A body that constantly accelerates in a straight line forever is impractical, so the alternative is a ring structure built into the spacecraft, and the latter is what this question primarily concerns itself with. – W.Asp Jun 10 '21 at 21:44

5 Answers5

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The presence of gravity simplifies a number of common processes. Some examples:

  • Separating liquids from gasses, eg. getting the water out of the air after you take a shower, or removing the hydrogen bubbles from water produced in a fuel cell.
  • Cooling hot objects: you can use radiator fins and convective airflow rather than needing to stick cooling fans or coolant loops on everything.
  • Mixing gasses: convection currents will keep you from suffocating on your own exhaled carbon dioxide, so you don't need to sleep in front of a fan.
Mark
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    Also separating solids from gasses. Keeping filters clean becomes more of a problem. http://freefall.purrsia.com/ff900/fv00900.htm – TLW May 07 '21 at 02:48
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    I appreciate all of the points offered by everyone who's answered my question, though I'm picking this answer as the most helpful because it's noting a number of things that will apply to spacecraft travel in general and not just research missions. Thank you for your time and answers, everyone! – W.Asp May 07 '21 at 16:21
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It would help you to learn which plants can grow properly in a greenhouse on the moon or Mars. When a seed germinates, the root starts growing downward with gravity, and the shoot grows upward to the surface of the soil.

Light also helps to orient the plant, but it only works once the shoot has breached the surface of the soil. The angle of sunlight is affected by latitude, seasons, and time of day (as it does here on Earth), so the direction of sunlight is not always straight "up". Plants have therefore evolved a balance between gravity and light clues to point them in the best direction for growth. The gravity of the moon or Mars might not be enough for some plant species to grow properly.

With a centrifuge module on a space station, you could simulate the gravitational and light conditions of the moon or Mars, to see which plants grow best. The centrifuge does not need to be large enough for a person; just big enough to conduct an experiment.

DrSheldon
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Having a facility to investigate the effect of variable gravity levels would be very interesting from a scientific perspective.

Some of the questions that might be worth finding the answers to are: How do various plants and bacteria respond to different gravity fields? How does this effect how they grow and the crop yields for plants?

How does Lunar or Martian gravity effect mammalian reproduction using mice? If mice can't produce viable offspring or have off spring with serious problems under Martian gravity the whole concept of colonising Mars is moot.

Slarty
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    Or for future longer-term spaceflight: at which gravity do we see the most payback? At which point do we get diminishing returns? Because a faster spinning/larger centrifuge is obviously harder to build and maintain, so if we figure out that 1/4 G already gives close to perfect results we can use this instead of going for full or half earths gravity in future spaceships – Hobbamok May 06 '21 at 08:18
  • Wait... so we're planning to take rats to Mars? They won't have to stowaway? – CGCampbell May 06 '21 at 14:37
  • We don't want any rats on Mars thankyou (so long and thanks for all the fish) – Slarty May 06 '21 at 17:08
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I'd assume you could essentially use it as a very large reaction wheel, so use it for station keeping/ fine control of attitude

jk.
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    errr... except that using it (as in entering ang leaving it or moving inside it) will impose additional strain on the station's reaction wheels. And I can only hope it self-balances somehow, or the whole station will get a funny shaking. – fraxinus May 06 '21 at 12:16
  • @fraxinus The logical approach is to have two counter-rotating spinning sections. Mount them concentrically and you do indeed have a reaction wheel. As you say, changes of mass are going to be a problem, but a good control system (and some counterbalances) should be able to figure it out. – Graham May 06 '21 at 13:32
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    Well... a coaxial reaction wheel, a set of pretty damn good bearings, a damping suspension plus two automatically adjusted counterweights on sides will probably do the trick. Ah, and I forgot: positively no jumping inside!. – fraxinus May 06 '21 at 13:43
4

While @DrSheldon's answer points out that a reduced gravity simulator...

...would help you to learn which plants can grow properly in a greenhouse on the moon or Mars.

A access to a long-term reduced artificial gravity field will also yield information on how people might fare on Mars. There is a lot of data on rates of bone loss and deterioration of eyes in microgravity, talk of colonization of Mars makes us wonder about the health effects of living on Mars or even How would travel to Mars without artificial gravity affect a crew's initial experience in Mars gravity?

Experiments exposing astronauts to Mars level of gravity (0.36 of that on Earth) for six months could be compared to ISS data, and let us know if the health effects are two thirds as bad as microgravity, worse, or better.

update:

Hackaday's ISS Artificial Gravity Study Shows Promise for Long Duration Spaceflight links to Off-World Cement Tested for the First Time which explains that there are both advantages and disadvantages to curing concrete in reduced or zero gravity.

Advantages are less or no settling of the aggregate (those bits of rocks, sand, other stuff) to the bottom. Disadvantages are no rising of trapped air bubbles to the top, leaving voids.

Testing these things in a Mars gravity field may be helpful for finding the right mix for Martian concrete, should any be necessary.

Abstract

For the first time, tricalcium silicate (C3S) and an aqueous solution were mixed and allowed to hydrate in the microgravity environment aboard the International Space Station (ISS). The research hypothesis states that minimizing gravity-driven transport phenomena, such as buoyancy, sedimentation, and thermosolutal convection ensures diffusion-controlled crystal growth and, consequently, lead to unique microstructures. Results from SEM micrographs, image analysis, mercury intrusion porosimetry, thermogravimetry, and x-ray diffraction revealed that the primary differences in μg hydrated C3S paste are increased porosity and a lower aspect ratio of portlandite crystals, likely due to a more uniform phase distribution. Relevant observations led by the presence or absence of gravity, including bleeding effect, density, and crystallography are also presented and discussed.

These images compare cement pastes mixed in space (above) and on the ground (below).

These images compare cement pastes mixed in space (above) and on the ground (below). The sample from space shows more porosity, or open spaces in the material, which affects concrete strength. Crystals in the Earth sample also are more segregated. Credits: Penn State Materials Characterization Lab

uhoh
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