It's an odd fact that, for some male astronauts like former Space Station resident Mike Barratt, spending 6 months without gravity produces changes in vision that often are irreversible. He went up nearsighted and needing glasses for distance and came down eagle eyed and needing reading glasses. His condition seems permanent. The phenomenon is under serious study at NASA which is concerned that the possibility of a long trip to Mars might just carry the risk of blindness. Just why it happens and whether or not it can be prevented may be, according to CNN.com, the determining question as to "whether he or any other astronaut ever journeys into deep space or sets foot on other worlds."
Prolonged weightlessness causes papilledema -- a swelling of the optic nerve for about half the male astronauts, some of whom recover and others, like Barratt do not. Women seem to be immune. Of course another question that doesn't seem to be addressed here is why we don't just hire more female astronauts? Sometimes we obsess so much about a puzzling matter that we neglect to look at obvious alternatives.