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In zero gravity, the vestibular system (which is responsible for feeling the balance) doesn't work since the fluids in it don't move properly in absent of significant gravitational force. Does it also mean that we can't feel dizzy when we are spinning around (either vertically or horizontally)?
Motion sickness also based on that the vestibular system signs that we are moving but we can't see it - which definitely occurs in space, where Earth and its horizon move slowly because of its size and the distance. It may mean that if we are in space and the vestibular system works, then those who are sensitive to it, they should be sick all time - or, in my theory, at least they should feel themselves like on a ship. Fix me if it's false.