For feeling nausea, nobody knows as we have never done the experiment. We just haven’t spun astronauts fast enough for long enough in zero g for them to feel sick, though we do have some very limited data that suggests that they don’t feel nauseous after spinning motions in zero g that would make them feel nauseous on Earth. Also don’t know what level of AG is needed for health. But if we need full g, then if you go by the actual experimental data from space, we can’t rule anything out. All we know is that you don’t get sick after a few minutes of spinning to achieve full g. Anything else on this subject is hypothesis at present. We can work out the physics. However that’s not enough. It is currently impossible to know for sure how a human being would respond to that physics.

We have some data from experiments to study space sickness rather than artificial gravity. These experiments on Skylab involved spinning the astronauts for a few minutes only (as they weren’t meant as experiments in AG).

They showed that the astronauts did not get nauseous or even disorientated, during experiments which made them nauseous both before and after the flight. Tim Peake also demonstrated this anecdotally on the ISS during his recent visit there, though not part as a planned experiment, just in his recreation time. Here he is spinning at about 60 rpm in the ISS. for a couple of minutes, no nausea, only momentary dizziness when he stops.

Only a professional ice skater or similar could do that on Earth, probably. Or people who have a natural tolerance to fast spins (some have a defective vestibular system and are not affected in any way by spins, however fast).

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? As far as I know, nobody has tested to see how long astronauts on the ISS can spin like this without ill effects.

Note that this also shows that we can tolerate large gravity gradients in our body from zero g at his stomach to 1 g at his head and feet.

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.

The experimenters for the Skylab litter chair experiment hypothesized that the reason their subjects could tolerate back and forth spins and continuous spins without getting nauseous in space was because the otoliths aren't stimulated, because there is no gravity acting in the direction of the spin axis. (See chapter II, Chapter 11, Experiment M131. Human Vestibular Function in Biomedical results from Skylab)

Our otoliths are separate from the vestibular system. Instead of sensing turning motions, they sense linear acceleration.

In any spin on Earth you have these two things at once -

• The spin sensed by the vestibular system, which is a spin around the rotation axis
• The linear acceleration due to gravity sensed by the otolith system. For slow spins this is mainly along the rotation axis. As the spin gets faster this is at an angle to the rotation axis.

Our otoliths tell us that down is in the direction of the spin axis, offset slightly, depending on the rate of spin.

Meanwhile our eyes and our vestibular system tell us that we are spinning in a different direction. It seems to be a conflict between these.

But in space,

• Our ostoliths will sense no linear acceleration at all in the direction of the spin axis.
• The sensation from the otoliths is only a sensation of an apparent linear acceleration away from the spin axis (the “centrifugal force” due to momentum causing our body to “try” to continue in a straight line when accelerated in a circle towards the center)

So there is no conflict between the ostoliths and our eyes and vestibular system (which senses spins).

That's what the experimenters hypothesized. But they didn't do any more experiments after that to try to follow it up to see if this was correct or not.

For more details see my Robert Walker's answer to Why not simply create a ship with artificial gravity for the journey to Mars - this is an extract from that which in turn is from the section Need for adventurous experiments in life support and artificial gravity in LEO first from my new book OK to Touch Mars? Europa? Enceladus? Or a Tale of Missteps?