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May 09, 2019 6 min read

It is a staple of science fiction, in print and on screen – the hazards of growing up in low gravity.   Whether the emaciated build of Niven’s Belters, or the bone problems of The Expanse.   Now that we are reaching the point where by commercial and government efforts, settlements in space, or on the Moon or on Mars might become a reality, I see a raft of articles talking about the hazards of Lunar or Martian gravity over long-term exposure.   As science fiction, that’s certainly an appropriate topic – but in press or policy articles, a higher standard is called for – and the truth is, we know almost nothing about the long-term risks to human health in partial gravity environments. These hazards are just made up.

Of course we evolved a 1-‘g’ environment (where ‘g’ is Earth’s gravity, 9.8 m/s^2).   And we now have decades of data on microgravity, or ‘zero-g’ environments.   What we know is that long-term exposure to zero-g isn’t very good for people.   The initial concern, which turned out to be valid, was that without the resistance for maintaining muscle tone, muscles would atrophy. Skylab, Mir, and the International Space Station have given us a lot of data on that, and over time we’ve developed exercise equipment and regimens for zero-g that overcome much of that particular risk.  

Other issues are not so easily dealt with. Without gravity, the human body does not retain the same density of bone – producing symptoms similar to osteoporosis. While there has been speculation on mechanisms which might overcome that and trick the body in to maintaining bone, those have not yet been successfully demonstrated in space.   Recently it has been found that some astronauts experience long-term problems with their vision after prolonged exposure to zero-g; there is speculation that this might be an effect of the headward fluid pressure shifts in zero-g, but that is as yet unproven and no countermeasure has yet been found.   Human immune function in zero-g is depressed as well.

Rather than finding countermeasures for each and every effect of zero-g, the obvious thing to do is provide enough gravity, through centrifugal force or a planetary surface, to maintain long-term human health.   But how much is “enough?”.   That question is sometimes called the “gravity dose” or “gravity prescription”.   The curve of a human’s response to some environmental factor is summarized in a dose-response curve.   Say the amount of gravity is on the X-axis and human health (however measured) is on the Y.   A “linear, no threshold” curve would look like this:

Of the very limited work done on gravity levels between zero and one, much of it involves unicellular organisms centrifuged in the space environment.   These organisms do have behavior, and some of it involves response to gravity (for example, sinking ‘down’ below the surface of the water at certain times of day).   In broad outline, such organisms start exhibiting their “normal” gravity orientation behavior in the 0.12-0.3 “g” range. The Russians also flew some satellites with various small animals in a small centrifuge, and the experiment allowed the animals to select the gravity level they seemed to prefer by moving in and out of the centrifuge radius (unfortunately, there was little other instrumentation of the animals and these were automated experiments). The animals seem to prefer 0.3-0.5 “g”.   All of these seem to indicate that there is some “lower threshold” of “enough” gravity.   So perhaps the curve looks like this … linear, with a threshold.

On the face of it, such a curve seems unlikely. For one thing, we are able to simulate higher gravity exposures and some work has been done on such things (the “Great Mambo Chicken” of Ed Regis’ book, for example). We certainly don’t expect increasing gravity to increase human health indefinitely, so one would expect the curve to flatten out as approaching 1 “G”.   The *ONLY* human exposure data we have in between 0 “g” and 1 “g” is the Apollo astronauts on the Lunar surface – and there, they reported that they felt better and, for those who had sleeping arrangements on later missions, they reported sleeping better and more deeply at 0.16 “g” than at 0 “g”, suggesting that the curve of “goodness” is rather steep.   Plants also respond to gravity (for example, by sending roots “down”), and because their processes are rather slow, zero-gravity and partial gravity can be simulated with a ‘clinostat’ – a device that keeps rotating the plant so that the Earth’s gravity points in all directions equally (or, for partial gravity, that some small portion is left over in a preferred direction).   Plants seem to behave “normally” above ~0.05 “g” and then there’s very little effect.   All of these suggest a curve more like this:

Now the lower threshold for unicellular organisms might then be ~0.12-0.3 “g” and for plants ~0.05 “g” … and the curve might be quite steep, so that, for example, by 0.3 “g”, the interesting part of the curve is over.   Again, that’s speculative, but it is a least speculation that is consistent with some of the limited data.

But only some. Because recall that human immune function is suppressed at 0 “g”. While we haven’t exposed humans to varying gravity levels, human immune “T” cells have been cultured in vitro at 0, 0.5, and 1 “g”.   And what they found was that they were indeed suppressed at 0 “g” – but also, that the activity level at 0.5 “g” was HIGHER than at either 0 or 1 “g”. Three points don’t make much of a curve, but it suggests a shape like this:

That’s especially intriguing because it suggests that there is a second “normal activity” level somewhere BETWEEN 0.5 and 0 “g” … perhaps around 0.3. Recall those animals in a satellite centrifuge? They also seemed to prefer something in the 0.3-0.5 “g” range.

Now none of this is proof of anything about human health.   But it all certainly suggests that the simple picture of “Mars gravity is bad for your health” may not be true.

On the other hand, it is staggering that humans have been going in to space since 1961 (57 years as I write this), and we STILL KNOW ALMOST NOTHING about the effect of gravity on human health other than “zero is bad”.   To me this is one of the biggest questions a space agency ought to be answering.   If 1/3 “g” is enough for human health, Mars looks great.   If 1/6 “g” is enough, both the Moon and Titan are interesting.   If it takes something very close to 1 “g”, the “cloud city” approach of Venus looms larger. How can we not know this?

Studying this was one of the justifications for the ISS, but the human centrifuge module was canceled (AFTER it had been built) to save money.   Still, today, there is at least a rodent centrifuge on the ISS, and we could learn a LOT about this by studying rats. And yet … so far, all the published data is on … you guessed it … 0 “g” and 1 “g”.   Won’t *someone* pay the Japanese to turn their centrifuge to some of the numbers in between?  

It’s worth noting that planets are hardly the only game in town, and this question is also critical for free-space habitats or long-duration spacecraft.   While recent research has expanded the range of rotation velocity that we think people can tolerate, it still takes a rather large radius to produce a 1 “g” environment at comfortable rotation rates (At 6 RPM, or 0.628 radians/s, 1 “g” takes a rotation radius of ~25 meters). If that can be dropped to 1/3 “g” (Martian gravity), that’s a rotation radius of just over 8 meters, and you can’t go a whole lot shorter than that for 1.8 meter tall humans anyway.   If 1/3 “g” is enough to support long-term human health, there’s really no reason to ever build a long duration free-fall space habitat or spacecraft again – because artificial gravity would be easy.

It’s hard not to be frustrated, at times, that we have spent far more on studying the deleterious effects of zero-gravity on long-term health than we ever have in figuring out how much gravity would make those effects go away.

(To read further, G. Diement & K. Slenzka, “Fundamentals of Space Biology”, and Clement & Bukley, “Artificial Gravity” are good references to start with)

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