• what if we can manage just fine with a 4 meter diameter centrifuge?
• Or what if humans simply can't tolerate sleeping in a centrifuge the size of the Nautilus, at any spin rate large enough for useful gravity?
  
• Or what if we need gravity 24/7 to stay healthy and continually going back and forth between zero g and full g is worse than zero g?
• Or what if we need gravity while exercising, and it is of little value in sleep? (The Nautilus centrifuge is designed to be just large enough for an astronaut to crawl into wearing a spacesuit - for initial testing safety, not designed for exercise).
  
• Or what if, or what if, ... you can go on with an endless list of questions like that, which nobody can answer yet, at least not definitively.
  

• On the Earth you  always have full Earth gravity operating. Any spinning motion increases that so all artificial gravity experiments on Earth involve hypergravity.
• Generally have full Earth gravity along the rotation axis for the spin - in space there will be no gravity along the spin axis
• Can't experiment with true partial gravity. It might be that the optimal gravity level for human health is less than full g. We have no way of finding this out on the Earth.
• We can't experiment with temporary gravity - i.e. gravity for only part of the day - in space you could get into a centrifuge and experience artificial gravity for just a few minutes (say while using the toilet) or half an hour or so (meal times say, and to help with digesting the meals) or several hours (for exercise and while resting) or eight hours (while asleep). Even if you can only tolerate a few minutes of full g a day, that might be beneficial. We can't test for this on Earth.
• Gravity gradients differ. On the ground, in any centrifuge experiment, the gradients shade from full gravity to hyper gravity. In space they shade from partial gravity to full gravity
• Coriolis effect acts in a different direction. It's awkward to walk in straight lines in centrifuges on the ground, or to move your hand horizontally. This would not be an issue in space, because the axis of rotation is above your head. Instead you'd feel the Coriolis effect when you stand up suddenly or sit down suddenly or move your hand vertically.
  
• Felt gravity levels depend on the direction you walk - heavier when you walk with the spin, and lighter when you walk against the spin.
• Spin reversal. In space, the spinning sensation in your ear will reverse direction if you turn your head around (spin reversal). So for instance depending on which direction you are facing compared with the direction of the spin axis, your head tumbles in a forwards vertical tumble, a backwards tumble, a sideways tumble to the left or a sideways tumble to the right, or intermediate tumbles - the direction continually changes. The anterior and posterior canals in the vestibular system in your ears should be able to detect this.
  
  This effect never happens in the usual experiments on the ground if you keep your head vertical and parallel to the spin axis, though they can happen if you lean your head sideways, or if you turn your head upside down, as you spin, or in experiments with reclining volunteers with head towards the spin axis.
  
• The direction of the spinning motion  affects a different part of the ear. On the ground, in the usual experiments, your horizontal canals (in your vestibular system in your ear) get stimulated.  With artificial gravity in space, the spin axis is overhead, rather than to one side. So it's a tumbling sensation and your vertical canals get stimulated (depending on orientation)
• The otolithic organs (the utricle and saccule) will respond differently in space conditions without the full Earth gravity along the axis. These help us sense linear accelerations. They are implicated in space sicknesss as the body adjusts to different ways of interpreting the sensations from our otoliths. They would surely make a difference to the sensations we feel in artificial gravity.
  
  Experiments with the Skylab litter chair suggested that we may tolerate back and forth spinning motions more easily in space. And the otoliths were thought to be responsible for this increased tolerance (even though they don't detect spinning motions at all, they are stimulated differently which seems to improve tolerance of spinning motions for some reason)
  
  

  Spinning motions in space artificial gravity stimulate the posterior and anterior canals instead of the horizontal canal because the axis of rotation is above your head, and the Utricle and Saccule are stimulated differently as well.

  
  
  (for background on structure of the ear and the motion sensitive organs see Otoliths - and the limited data we have from space is based on the Skylab litter chair experiments, discussed in papers such as: The relative roles of the otolith organs and semicircular canals in producing space motion sickness. These experiments were designed to study motion sickness rather than artificial gravity).

"If, as has been suggested by previous flight research, microgravity actually provides an even less nauseating environment for centrifugation, then vestibular problems should certainly no longer remain an excuse that stands in the way of flight-testing an SRC [Short Radius Centrifuge] countermeasure. An orbiting test platform would allow not only definitive answers to the integration of otoliths and canals in the process of vestibular adaptation, but would also provide the first solid data beyond bed rest analogues about the efficiency of AG [Artificial Gravity] against musculoskeletal and cardiovascular losses. Furthermore, only in microgravity does the opportunity arise to examine the physiological effects of partial-g load, those between microgravity and Earth-normal 1-g."

"In order to truly address the operational aspects of short-radius AG, a centrifuge must be made available on orbit. It's time to start truly answering the questions of "how long", "how strong", "how often", and "under what limitations" artificial gravity can be provided by a short radius device.

2002 ESASP.501..151H Page 155