Nobody knows. Could be anything from a few meters, to tens, or hundreds of kilometers.
Ground experiments suggest 1 to 3 rpm. That would make it somewhere between 200 meters (at 3 rpm, full g) and 1.8 km (at 1 rpm, full g).
But, we can't rely too much on those figures. The problem is that the experiments have only ever been done on the ground, nobody has ever done this experiment in space.
There are various reasons why we can't have high confidence in these experiments.
The main problems are
We also need to recognize, these figures are for general public so including people with high susceptibility to spinning motions.
Some people get sick even just in a rotating observational deck on top of a skyscraper - so for them even 1 rpm may be too much. Other people can tolerate high spin rates for long time periods - and there are a few people who through an inner ear defect are not affected by spinning motions at all.
This is a report of an early study in rotating rooms
If you know the permitted spin rate - and the level of artificial gravity needed - then the calculation is easy, using e.g. SpinCalc.
So - is not the physics that is in question - but human biology - what can we tolerate and what do we need for health?
The Skylab experiments with the rotating litter chair suggested that in space conditions, that we can withstand higher spin rates than on the Earth.
They did several experiments that made the astronauts nauseous on the ground before the experiment - and they were symptom free in space. And nauseous again on the ground when they repeated the same experiments after the flight.
Sadly they weren't really testing artificial gravity - could easily have designed that chair to test constant spins - but didn't. Instead the experiment tested short bursts of rapid spins, and confusing changes of orientation in different directions, including rocking motions also.
But - is closely enough connected to artificial gravity type spins to be relevant. Basically, they did their best to make the astronauts sick in the spinning litter chair - but they just didn't get nauseous at all, surprisingly, in experiments that did make them nauseous on the ground.
This may be because on the Earth - then you always have gravity operating along the spin axis. So is an extra g force on the inner ear that you don't have in space, in that direction.
In space - then all the artificial g operates in the direction away from the spin axis, and there are no conflicting sensations along the spin axis.
In particular - in space - then the Otolithic Organ which senses linear accelerations will behave differently from the way it does in the ground experiments. The Skylab experimenters in their papers postulated that this might be the reason for the different results from the ground experiments.
Also - on the ground, they learn to adapt. Experiments at MIT showed that many people can adapt to 24 rpm for an hour, after three sessions in a centrifuge.
They did this step by step so they spent an hour each day at gradually increasing spin rates.
Might well be that if you do it that way - you can learn to tolerate spin rates that you can't tolerate if someone just puts you into a rotating room rotating at say 10 rpm without any previous acclimatization.
The MIT experimenters concluded that the next step should be to do the experiments in space - because of these differences uncovered in Skylab - they made this point, that ground experiments are only of limited validity until we have some experimental data from space to relate them to.
Also the spinning is in a different direction in ground experiments - that is - if you sit upright or stand upright and then spin around a vertical axis - like a merrygoround - then - the coriolis effect influences sideways motions. But in space - you would be orientated with head towards the axis and coriolis effects only make a difference to what you perceive as vertical motions.
And if you lie down with head towards the centre of the merrygoround - as it were - then you have the full g along the axis - making it very different from space experience.
And - other effects include the difference in gravity between head and feet - something hard to simulate on the ground again, at all realistically, without "ground truth" from space.
Is this something humans are very sensitive to - or bad for our health long term - or something we hardly notice, even if head is almost at zero gravity? Again nobody done the experiment - though the Skylab astronauts running around their jogging track show no obvious discomfort with their head at a far lower level of artificial g than their feet, so it may not be as much of an issue as some have suggested.
So - might be like seasickness -that most people eventually adapt to withstand high spin rates even for a long time in space.
And - some individuals may be far less susceptible. And some people are born with a defect in their ear which means that though they can hear etc perfectly fine - they don't become sick or nauseous at all when spun.
And - is a human thing. Rats are not susceptible to spinning for instance and won't get sick - so they have had rats in space in tiny fast rotating centrifuges no problem at all for months at a time, a Russian experiment.
So - upshot is -that we just don't know.
We need to do experiments in space before we can tell. And have to do the experiments with humans.
And - might well be that it helps to acclimatize - e.g .slowly spin up over some months.
Until then 1 rpm is just a guess.
Might as easily be 0.1 rpm or 10 rpm or even 30 rpm for all we know.
If you want to give it a go, there's this handy on-line tool where you just feed in the figures and it calculates dimensions, artificial gravity or whatever for you
SpinCalc
And for the literature on the subject
Artificial Gravity
Another factor is, that we have no idea what the "gravity prescription" is.
Does it need to spin fast enough for full gravity? Or is, say, lunar gravity okay.
Can get dramatically different numbers there.
E.g. feed in, say 9 rpm, and lunar gravity, both of which are within the range of possibility for human health and tolerance of spins on our knowledge so far, and you get a radius of less than 2 meters.
Feed in 0.1 rpm and full gravity (for all we know, maybe even Mars gravity is so bad for your health you won't want to live like that long term), and you get a radius of 90 Km.
So - I'd say - the answer is probably somewhere between 2 meters and 90 km for the radius, so between 4 meters and 180 km for the diameter.
Both are within the range of plausible figures I think.
But conceivably could be outside that range also. E.g. less than lunar gravity okay for health, or higher than 9 rpm okay.
Or in the other direction - that many people are so susceptible they can never adapt even to 0.1 rpm - something that makes us super susceptible to spinning motions in space conditions and need, say, 0.01 rpm or whatever.
This table may help give an overview, notice how quickly the radius goes up for a small decrease in spin rate.
from
Could Spinning Hammocks Keep Astronauts Healthy in Zero g?
BTW a tether based habitat is far easier to build than a spinning station.
If we do need say 1 rpm or less and kilometers length tethers - it can be done - not even that hard to do. We could do that even this year, set up a couple of very small habitats - say two Soyuzes - joined together by a kilometers long tether and spinning at a fraction of an rpm.
See also, my article about Joe Carroll's ingenious suggestion of an easy way to test this - indeed don't even need two Soyuzes.
You can have a Soyuz spinning tethered to its final stage (which also goes into orbit).
That could be done even on perhaps the next but one crewed Soyuz flight to the ISS if there was the political interest and will to do this experiment and it was fast tracked. Then we might have a few answers to these questions at last.
You can do it on the way to the ISS, the Soyuz has enough provisions to last the crew for some days - long enough for the experiment (used to take several days to reach the ISS).
Crew Tether Spin - With Final Stage - On Routine Mission To ISS - First Human Test Of Artificial Gravity?
Ground experiments suggest 1 to 3 rpm. That would make it somewhere between 200 meters (at 3 rpm, full g) and 1.8 km (at 1 rpm, full g).
But, we can't rely too much on those figures. The problem is that the experiments have only ever been done on the ground, nobody has ever done this experiment in space.
There are various reasons why we can't have high confidence in these experiments.
The main problems are
- There are many factors that make spinning motions in space different from ground spins in their effect on a human being. We could be either more, or less tolerant to spins in space.
- It assumes we need full gravity for health, which is unknown at present.
We also need to recognize, these figures are for general public so including people with high susceptibility to spinning motions.
Some people get sick even just in a rotating observational deck on top of a skyscraper - so for them even 1 rpm may be too much. Other people can tolerate high spin rates for long time periods - and there are a few people who through an inner ear defect are not affected by spinning motions at all.
BASIS FOR THE 1 - 3 RPM FIGURES OFTEN QUOTED HERE
This is a report of an early study in rotating rooms
In brief, at 1.0 rpm even highly susceptible subjects were symptom-free, or nearly so. At 3.0 rpm subjects experienced symptoms but were not significantly handicapped. At 5.4 rpm, only subjects with low susceptibility performed well and by the second day were almost free from symptoms. At 10 rpm, however, adaptation presented a challenging but interesting problem. Even pilots without a history of air sickness did not fully adapt in a period of twelve days.That sort of research is the basis for the present day recommendations of 1 to 3 rpm.
The Architecture of Artificial-Gravity Environments for Long-Duration Space Habitation
If you know the permitted spin rate - and the level of artificial gravity needed - then the calculation is easy, using e.g. SpinCalc.
So - is not the physics that is in question - but human biology - what can we tolerate and what do we need for health?
SKYLAB ROTATING LITTER CHAIR EXPERIMENTS SUGGESTING EASIER TO TOLERATE SPINNING SENSATIONS IN SPACE
The Skylab experiments with the rotating litter chair suggested that in space conditions, that we can withstand higher spin rates than on the Earth.
They did several experiments that made the astronauts nauseous on the ground before the experiment - and they were symptom free in space. And nauseous again on the ground when they repeated the same experiments after the flight.
Sadly they weren't really testing artificial gravity - could easily have designed that chair to test constant spins - but didn't. Instead the experiment tested short bursts of rapid spins, and confusing changes of orientation in different directions, including rocking motions also.
But - is closely enough connected to artificial gravity type spins to be relevant. Basically, they did their best to make the astronauts sick in the spinning litter chair - but they just didn't get nauseous at all, surprisingly, in experiments that did make them nauseous on the ground.
WHY THIS MIGHT BE - THE OTOLITHIC ORGAN
This may be because on the Earth - then you always have gravity operating along the spin axis. So is an extra g force on the inner ear that you don't have in space, in that direction.
In space - then all the artificial g operates in the direction away from the spin axis, and there are no conflicting sensations along the spin axis.
In particular - in space - then the Otolithic Organ which senses linear accelerations will behave differently from the way it does in the ground experiments. The Skylab experimenters in their papers postulated that this might be the reason for the different results from the ground experiments.
ADAPTATION
Also - on the ground, they learn to adapt. Experiments at MIT showed that many people can adapt to 24 rpm for an hour, after three sessions in a centrifuge.
They did this step by step so they spent an hour each day at gradually increasing spin rates.
Might well be that if you do it that way - you can learn to tolerate spin rates that you can't tolerate if someone just puts you into a rotating room rotating at say 10 rpm without any previous acclimatization.
The MIT experimenters concluded that the next step should be to do the experiments in space - because of these differences uncovered in Skylab - they made this point, that ground experiments are only of limited validity until we have some experimental data from space to relate them to.
CORIOLIS FORCES DIFFERENT
Also the spinning is in a different direction in ground experiments - that is - if you sit upright or stand upright and then spin around a vertical axis - like a merrygoround - then - the coriolis effect influences sideways motions. But in space - you would be orientated with head towards the axis and coriolis effects only make a difference to what you perceive as vertical motions.
And if you lie down with head towards the centre of the merrygoround - as it were - then you have the full g along the axis - making it very different from space experience.
DIFFERENCE IN GRAVITY FROM HEAD TO FEET
And - other effects include the difference in gravity between head and feet - something hard to simulate on the ground again, at all realistically, without "ground truth" from space.
Is this something humans are very sensitive to - or bad for our health long term - or something we hardly notice, even if head is almost at zero gravity? Again nobody done the experiment - though the Skylab astronauts running around their jogging track show no obvious discomfort with their head at a far lower level of artificial g than their feet, so it may not be as much of an issue as some have suggested.
LIKE SEASICKNESS - SOME MAY BE LESS SUSCEPTIBLE OR ADAPT MORE QUICKLY AND MIGHT BE THAT MOST GET OVER IT AFTER A FEW DAYS
So - might be like seasickness -that most people eventually adapt to withstand high spin rates even for a long time in space.
PEOPLE WHO ARE NOT AFFECTED AT ALL - AND RATS DON'T HAVE THE ISSUE
And - some individuals may be far less susceptible. And some people are born with a defect in their ear which means that though they can hear etc perfectly fine - they don't become sick or nauseous at all when spun.
And - is a human thing. Rats are not susceptible to spinning for instance and won't get sick - so they have had rats in space in tiny fast rotating centrifuges no problem at all for months at a time, a Russian experiment.
So - upshot is -that we just don't know.
NEED FOR EXPERIMENTS IN SPACE CONDITIONS
We need to do experiments in space before we can tell. And have to do the experiments with humans.
And - might well be that it helps to acclimatize - e.g .slowly spin up over some months.
Until then 1 rpm is just a guess.
Might as easily be 0.1 rpm or 10 rpm or even 30 rpm for all we know.
SPIN CALC
If you want to give it a go, there's this handy on-line tool where you just feed in the figures and it calculates dimensions, artificial gravity or whatever for you
SpinCalc
And for the literature on the subject
Artificial Gravity
Another factor is, that we have no idea what the "gravity prescription" is.
Does it need to spin fast enough for full gravity? Or is, say, lunar gravity okay.
Can get dramatically different numbers there.
LET'S TRY SOME FIGURES
E.g. feed in, say 9 rpm, and lunar gravity, both of which are within the range of possibility for human health and tolerance of spins on our knowledge so far, and you get a radius of less than 2 meters.
Feed in 0.1 rpm and full gravity (for all we know, maybe even Mars gravity is so bad for your health you won't want to live like that long term), and you get a radius of 90 Km.
So - I'd say - the answer is probably somewhere between 2 meters and 90 km for the radius, so between 4 meters and 180 km for the diameter.
Both are within the range of plausible figures I think.
But conceivably could be outside that range also. E.g. less than lunar gravity okay for health, or higher than 9 rpm okay.
Or in the other direction - that many people are so susceptible they can never adapt even to 0.1 rpm - something that makes us super susceptible to spinning motions in space conditions and need, say, 0.01 rpm or whatever.
This table may help give an overview, notice how quickly the radius goes up for a small decrease in spin rate.
Could Spinning Hammocks Keep Astronauts Healthy in Zero g?
TETHER APPROACH
BTW a tether based habitat is far easier to build than a spinning station.
If we do need say 1 rpm or less and kilometers length tethers - it can be done - not even that hard to do. We could do that even this year, set up a couple of very small habitats - say two Soyuzes - joined together by a kilometers long tether and spinning at a fraction of an rpm.
See also, my article about Joe Carroll's ingenious suggestion of an easy way to test this - indeed don't even need two Soyuzes.
You can have a Soyuz spinning tethered to its final stage (which also goes into orbit).
That could be done even on perhaps the next but one crewed Soyuz flight to the ISS if there was the political interest and will to do this experiment and it was fast tracked. Then we might have a few answers to these questions at last.
You can do it on the way to the ISS, the Soyuz has enough provisions to last the crew for some days - long enough for the experiment (used to take several days to reach the ISS).
Crew Tether Spin - With Final Stage - On Routine Mission To ISS - First Human Test Of Artificial Gravity?
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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