It has been found that micro-gravity adversely affects blood flow to the human eye. How will this problem be treated on flights to Mars, for instance? Rotating spacecraft, perhaps?
Rotating for now seems to be the only viable option for artificial gravity, and alternative is keeping a constant acceleration of g halfway through the journey, and reversing it to slow down with g on the other half, which unfortunately requires a lot of fuel.
And there are a lot of effects the different gravity has on the human body, which also won't end after the trip is done, since Mars' gravity is also smaller than Earth, there is no telling just how much our physiology would be affected in the long term!
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