First, nobody knows what effect lunar gravity has on the human body. The visits by the Apollo astronauts were far too short to notice anything. You can even suppose it is better for health than Earth gravity. And it could also be better for health for some people, e.g. older people with weak bones or failing hearts, or medical conditions, or depending on genetics.

The numbers here are arbitrary. I just used 100 for zero g health, 500 for Earth gravity, and various numbers in between to make a nice graph, with the online Line graph maker. (If you want to duplicate it, I used 100 48 520 500, 100 650 400 500, 100 350 200 500, 100 150 600 500, and 100 40 50 500).

Also the graph may depend on your age, sex, health conditions, genetics, etc. Indeed each person might have a slightly different graph here and different optimal partial gravity levels for health.

Lines 1 and 5 show the possibility that some levels of partial gravity could be even worse for health than zero g, and lines 1, 2 and 4 show the possibility that some levels of partial gravity could be better for health than full g. Lines 2, 3 and 5 shows the possibility that lunar gravity could be better than Mars g for health, perhaps for some people or health conditions.

Can we rule out any of these possibilities yet?

If we do need more than lunar gravity, there could be many ways to deal with it. I cover this in more detail in my Will Astronauts Need To Augment Lunar Gravity? If So, How? and in the two books I link to at the end, but here are some ideas from it:

One idea is to build a velodrome style track on the Moon for cyclists to use to keep healthy.

Dunc Gray Velodrome in the City of Bankstown, Australia, cycling venue for 2000 Olympic games. Photo by Adam.J.W.C. It bends at a maximum angle of 42°. A more steeply banked velodrome on the Moon, banking to a maximum angle of 80° would let cyclists generate between half and full g by cycling round and round. For normal cycling speeds of 10 to 15 meters per second, or 22 to 33.5 miles per hour, the lunar velodrome could be 50 meters in diameter, see Human Powered Centrifuges on the Moon or Mars.

Then, it seems that we can tolerate faster spins in zero g than on Earth. Here is Tim Peake spinning at about 60 rpm in the ISS. for a couple of minutes, no nausea, only momentary dizziness when he stops.

He says he is pretty sure he couldn't tolerate that on Earth. So anecdotally it suggests that we can tolerate very high spin rates in zero g. Taking the radius as 0.25 meters at a guess, his head and feet will be both under full g, his torso around zero g as he spins. Could he spin like this indefinitely? If so, it's very promising I think for the use of a short arm centrifuge to counteract health issues of humans in zero g.

One theory is that it's because the ostoliths that sense linear acceleration along the rotation axis on Earth due to Earth's gravity are not stimulated in space. Perhaps after a while they “tune out”. You can read more about this idea here: chapter II, Chapter 11, Experiment M131. Human Vestibular Function in Biomedical results from Skylab

So, we don't have too much data. There's the series of experiments on Skylab, which found that astronauts spinning in space can tolerate spinning motions they couldn't tolerate before, and then could tolerate in space and for a while after (even two months after returning to Earth). Then there’s the anecdotal experience of astronauts on the ISS who are not measured or monitored in any way as they spin. Both of those were very short duration, a few minutes at most. It would be easy to get at least some preliminary data by just asking astronauts to tumble head over heels in the ISS with other astronauts keeping them tumbling, as with Tim Peakes demonstration. No need for a “litter chair” to do this. think myself on the basis of the very limited data we have from zero g, as well as experiments in adaptation on Earth, that a simple lightweight internal centrifuge might well be all that is needed to counteract many, and perhaps even all the ill effects of zero g on the human body. It might even be that you only need to do it for a few hours a day. Perhaps sleeping for 8 hours in a centrifuge + eating in centrifuge conditions - and it would also be useful for the toilet facilities - and for exercise - that's a lot of AG. Maybe that would be more than enough to stay healthy. If you make that hypothesis - well nobody has any experimental data to date that can prove you wrong, so it is an untested theory, and on the basis of what we know, the truth could be either way.

Perhaps when eventually someone does these experiments, maybe a private aerospace company, then everyone will be amazed that it took us several decades to test out this simple measure for counteracting effects of zero g. There is no way to know how well it will work until we try. The physics is easy to understand but a human body with all the interacting processes going on is far too complex to simulate adequately in computational physics modelling.

If that’s so - well what about lunar gravity? Do you need the ostoliths to be under full Earth g along the spin axis to get this conflict that leads to nausea, or will a sixth of Earth g also cause it? Again if you ask questions like that, nobody can give you a definitive answer. If they say something definitive sounding, they are just guessing as we have no data on it.

My rough diagram of a small version of this inside a space station is like this:

Sketch to show two possible orientations for a spinning hammock inside a large space station module. To prevent this from spinning up the station, then there'd be a counter rotating weight automatically spun in the opposite direction to the astronaut, perhaps attached to the "floor". The motors would not need to be powerful in zero g. It's like spinning a cycle wheel - easy to spin up, and you could stop it just by putting out your hand.

Note, you don’t need big heavy centrifuges to test it. We do on Earth because it has to be able to hold the weight along the axis as well as along the arm. But in space, your weight only acts along the cable, as for a swing or a hammock. Setting yourself in motion is like setting a swing in motion. You could do it by hand with a hammock attached to pivots as shown in the diagram, or you could get another astronaut to push you and keep pushing you from time to time as for Tim Peake, gentle pushes. Or you could have a small motor to do this for you.

For a very small one meter diameter centrifuge like this, you can achieve full g at 30 rpm and with the astronauts moving at only a little over 3 meters per second so it is very safe. That's around seven miles per hour - faster than a jog, but easy running speed so it's not that fast (a little faster than the average speed for the London marathon). For more about it see my Could Spinning Hammocks Keep Astronauts Healthy in Zero g?

On the Moon maybe they can be attached somewhat more like a carousel?

Swing carousel, photo by Wittkowsky, mediawiki commons

But of course on the Moon you’d be suspended almost horizontally for full g, in the weak gravity. Remember that if the ostolith conflict with the vestibular system doesn’t happen with the weak lunar gravity, you wouldn’t feel any dizziness or nausea at all. We just don’t know yet if you would on the Moon.

Solution for large diameter centrifuges on the Moon

For the larger diameter centrifuges, then simplest idea is to just build a train going round and round a circular track. Then there's no limit, you can have circular tracks kilometers in diameter if you so wish, either in the lunar caves or on the surface. Also you don't have to think of narrow carriages as on trains on Earth. As we saw in the section on Lunar railways, lunar gauges could be wide gauge, perhaps twice as wide as most track gauges, perhaps three meters between the tracks, or more. Your "carriages" could also be multistory, with no height restrictions, so in principle they could be as big as a 747 or larger.

If you use circular railways, you don't have any engineering problems of torque on a central axle or pivot. Whatever the diameter of the track, the force outwards is only your that exerted by the train itself and its passengers under 1 g. This is no more of a problem for wheels to support than a conventional train on Earth. This idea is often discussed online, but not so much in the academic literature. But there is a 1996 paper: Artificial Gravity Augmentation on the Moon and Mars

"One method of augmenting gravity is a extraterrestrial railroad. A vehicle on a circular track banked with respect to the horizon creates centripetal accelerations related to the speed of the vehicle and the diameter of the track. Incremental accentuation of gravity may be accomplished by switching the vehicle to a track of larger diameter and steeper bank. Rotation creates accelerations on the vestibular canals of the inner ear that will limit the angular velocity of the vehicle. Colonists would have the opportunity to work part of each day in simulated Earth gravity and easily access the planet's surface. The magnitude of gravity that will protect us is unknown, as is the frequency and duration of exposure. This must be investigated. An extraterrestrial railroad, as one solution to this problem, does not involve exotic technology and is readily expanded."

So the suggestion is that you have banked tracks for the train to run on. As you transition from lunar gravity to full gravity the train would move to steeper and steeper banked tracks so that the floor always feels level to the passengers. The transition would go in the opposite direction when the passengers want to leave the train, the train would move to the less steeply banked tracks first.

Another idea suggested in the forums is a tilting train:

A JR Hokkaido KiHa 283 series tilting DMU on a Super Hokuto limited express service on the Hakodate Main Line, photo by Japanese wikipedia user: 出々 吾壱

For full g it would need to be tilted by 80 degrees instead of the 8 degrees shown in in this photo. For large amounts of tilt, perhaps this would work best if the carriage is in an inside compartment only indirectly coupled to the outside while the train is moving. This idea could also be used with a smaller amount of tilt to keep the floor of a train level while transitioning between tracks with different degrees of bank, and then finally to a stop.

To deal with issues of friction between the trains and the tracks we can

• Use maglev trains so that there is no physical contact with the tracks (though with safety mechanisms to make it impossible for them to leave the tracks)
• Run them in high vacuum - would be the default situation on the surface. In lava tube habitats, then you don't fill the entire tube with air, just the habitats.
• Convert the regolith to glass below the track and for a few meters to either side to reduce dust problems and regularly clear the tracks of dust (not needed very often probably as the dust does get levitated but probably not in huge quantities.

In some ways it is better than for Earth, with no earthquakes, or weather hazards, no snow on the tracks or fallen leaves.

Also bear in mind that in a future where we can build large circular tracks like this, it's also gong to be easy to lay out large areas of solar panels on the surface and we can also design power storage during the lunar night, and by then we may have small nuclear power stations too. So, though it's not going to be as efficient as a spinning habitat in space which spins pretty much endlessly once you set it spinning, it's probably not going to be a huge power drain on a working habitat. See Power during the night.

SUMMARY

We might need nothing. We might need a short arm centrifuge, light construction, built like a swing or carousel, if very short arm then it’s moving only at 3 meters per second like a fast jog just breaking into a run. We might just need exercise facilities where we can do sports that involve cycling around a velodrome, or running round a small circular track just perhaps six meters diameter like Skylab. Or we might need to build a big construction such as a lunar maglev train on a circular track.

There is no way to know until we try.

NOT TIME TO DRAW UP DESIGNS FOR BIG SPINNING CONSTRUCTIONS ON THE MOON

But we might not need any of this. For all we know, astronauts will be in 100% good health on the Moon, even better health than on Earth. It’s not impossible. Certainly I think we shouldn’t devise any spinning habitats for the Moon except as a way to experiment, until we have experience of humans living and working there. As it is one possibility that we don’t even need them and that we do just fine in lunar g.

GROWING CROPS ON THE MOON

As for growing crops, then there’s every reason to suppose we can do it and lots of places we can build on the Moon.

An Astronaut Gardener On The Moon - Summits Of Sunlight And Vast Lunar Caves In Low Gravity

NO HURRY

Now I don’t think we should rush to settle the Moon as quickly as possible. It’s going to be a major issue to have millions of people with space technology living peacefully - a social rather than a technological issue. We can’t have wars in space in the conventional sense, because with spacecraft traveling at thousands of miles an hour and the fragile habitats and nowhere to survive except in a habitat or spacesuit, any “all out” war, especially with automated missiles, but even just with suicide bombing or simpler, just throwing stones at each other from passing spacecraft, would end quickly with everyone who was in space at the time dead.

I think we will have plenty to cope with if we have thousands of people in space. Let’s take it one step at a time.

Also nowhere is nearly as hospitable as Earth. The harshest deserts, Antarctica, or cities floating on the sea using only sea water and air in a minimal impact sustainable way - no fishing or exploitation of the sea in any way apart from the sea water - all of those are far far easier places to live than any space colony. Get many more people living there for much less cost and less use of Earth’s resources and less impact on Earth.

So I don’t see it as a way to expand the regions that we “just colonize” with no other reason to be there. But there can be many good reasons to have people on the Moon such as scientific research like Antarctica, commercial reasons (if that works out, a lot better case than Mars for sure) - I think actually that a lot of the mining if we have mining would be done using robots for economical reasons as they don’t need food or spacesuits or air or water - autonomous or controlled from Earth, but there may be some people needed in situ to maintain and overhaul them and for tele-operations.

Maybe tourism, adventure, retirement to the Moon for the wealthy (especially if it turns out to have health value for older people to live in low gravity conditions).

And maybe some people just trying to set up a colony / settlement. If so I think the lunar poles and the lunar caves are amongst the top spots to try in the solar system, because they are surprisingly advantageous, better than Mars in just about all the comparisons I tried. I don’t konw if it would work. But it could be worth a try.

The early habitats are bound to be harder to maintain. But if you can somehow make a major part of a lunar cave habitable, kilometers in diameter (as they may be in the low lunar gravity and possibly supported by the Grail data) and over 100 km long - that’s a lot of habitable volume. Once we have easy transport into space by whatever reason, if we do, and in situ mining on the Moon - it might then be possible to make an area of that habitable. It would be a bit like an O’Neil cylinder - pre-built. If then it can also be low maintenance, like a few hundred dollars per year per inhabitant, then once you have the place built, it might actually be easier to live in such a place than on Earth with its weather which you don’t have on the Moon.

I explore this in my An Astronaut Gardener On The Moon - Summits Of Sunlight And Vast Lunar Caves In Low Gravity

And the whole thing in a lot of detail in my

MOON FIRST Why Humans on Mars Right Now are Bad for Science.President Obama, if you love science, Please protect Mars life from contamination from Earth (I should change this title now that Trump is about to become president)

Case For Moon First

Both available to read free online and also on kindle.