Astronauts in microgravity for extended periods experience a number of maladaptations, including bone loss, muscle atrophy and muscle mass loss, redistribution of fluids, and reduction in immune function. Many proposals have been put forward to use centripetal acceleration to simulate gravity in long duration flights, thereby reducing the astronauts' physical deterioration. Most of these proposals are built around the assumption of simulating 1g, earth surface gravity. The resulting designs are heavy and large enough to eliminate them from serious contention for near-term human spaceflight.

This thinking seems oversimplified to me. Why a full 1.0g? My intuition tells me there may be a step function involved. i.e., bone loss is minimal from 1g down to a threshold value, below which some mal-adaptation trigger point is reached and bone loss jumps up to the high levels we see from 0g.

My question is two-fold.

1) Do we have any data (I'm thinking from mice on a centrifuge in space) correlating mal-adaptation versus artificial gravity in values below 1.0g?

2) How could we most cheaply collect this data, if it does not exist?

Expanding on my second question, my first thought is to put mice into a centrifuge at 0.5g and monitor their urine for calcium. Are there protein biomarkers for space mal-adaptation? Could you collect a sample of urine or blood from a mouse and tell quickly if that mouse is losing bone, losing muscle mass, redistributing fluids, or suffering a loss of immune function? I would love to see a "cheap" research mission with sealed cages of mice arranged along a long rotating tether like a string of pearls, the habitats closest to the counterweight experiencing the least artificial gravity but all other parameters identical, with their collected urine telling a story of how much gravity it takes to keep them healthy. Or have these questions been answered already to our satisfaction?

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    $\begingroup$ I'm not trying to start a discussion about the viability of artificial gravity in manned spacecraft or colonies. We currently have a lot of discussion about colonizing the moon and mars and zero data about how our bodies would fare in those gravity fields. The same as zero-g? Half as bad? No impairment? Space Adaptation Syndrome: Step or continuous function? And since there are multiple aspects to SAS, each may have a different gravity response curve. This seems like basic medical data that's 30+ years overdue. $\endgroup$
    – Kengineer
    Jun 6 '18 at 22:53

The formula for artificial "gravity" due to centrifugal action is simple:

g = rω²

where g is the gravitational acceleration, r is the radius of the centrifuge, and ω is the angular velocity. So the "gravity" is proportional to the radius. This is a problem - attempting to scale the centrifuge down for lower-mass mice is not helpful - mass is not a variable in the above equation. So regardless of the species of meat popsicle that you want to run this experiment on, you'll need a pretty big centrifuge.

How big? This youtube clip attempts to answer this by comparing several sci-fi centrifugal gravity schemes. It turns out that there is a trade-off:

  • If the centrifuge is too small (e.g. r=8m as per Space Odyssey), the coriolis effects will be huge and make it extremely uncomfortable (nauseating) to live on such a centrifuge for any period of time.
  • Conversely, if the centrifuge is too big (e.g. r=93 million miles as per Ringworld), then the structural integrity of the centrifuge is impractical.

The clip posits that Babylon 5 strikes a pretty good balance of these two at r=8km for 1g.

So even if we only want to produce 0.5g, we'd need a 4km centrifuge for humans. Perhaps mice can handle the nausea due to coriolis effect better than humans can. The largest man-made structure in space is the ISS, with a length of 108.5m. In order to use the ISS for this experiment, we'd have to:

  1. assume mice can handle 40x the coriolis effect that humans can
  2. vacate it of all humans

At this point, this seems like an unlikely set of assumptions.

How does this answer the OP questions?

  1. Given the analysis above, I think we can be fairly confident that there currently is no such data for gravitational acceleration between 0 and 1g. Given the scale of centrifuge required, it might be technically possible today, but there is no evidence I'm aware of of an orbital centrifuge on this very large scale.
  2. I can only conjecture the answer to the second part of the question. I would guess that spinning the existing ISS might possibly be the cheapest way to do this, despite the need to vacate all human crew. I don't know if the strength of the structure would be up for this sort of motion though. Alternatively, it might be possible to have two crew modules connected by a several km cable and spin it. I have no idea how this would look cost-wise, though.
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    $\begingroup$ It's an important point - the "artificial gravity" in small rotating crafts would be unpleasant to say the least. The video - whew - that was actually worth it. I like the part where he says "...and also look for books, and read!" I remember those - I'm going to look for some now... $\endgroup$
    – uhoh
    Jun 4 '16 at 5:40
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    $\begingroup$ This doesn't address the question at all $\endgroup$ Jun 5 '16 at 7:44
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    $\begingroup$ @pericynthion It does address the practicality of testing mice in artificial gravity, which is part of the question. (I'm not sure mice can't be tested even if they are constantly nauseous due to Coriolis Effect, but that is a separate issue.) However let me take this opportunity to ask that if you feel an answer doesn't answer the question, please flag it. Perhaps you wished to give the OP time to edit, but if such a case isn't flagged follow-up is harder - people can forget to check back, for instance. Putting things in the Review queue always gets them more attention. $\endgroup$
    – kim holder
    Jun 5 '16 at 22:31
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    $\begingroup$ @kimholder You had posted this in chat: DESIGN CONCEPTS FOR A MANNED ARTIFICIAL GRAVITY RESEARCH FACILITY I have only skimmed the paper, but I do note the author discusses Coriolis effect. $\endgroup$ Jun 6 '16 at 12:19
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    $\begingroup$ @JerardPuckett yes, and i'd also point to Al Globus's paper on rotation tolerance. It may be that higher rotation rates can be adapted to. My point above was only that important data on bone and muscle loss could perhaps be gotten even if the mice are always woozy (poor things). $\endgroup$
    – kim holder
    Jun 6 '16 at 14:45

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