It’s not exactly well advertised by the National Aeronautics and Space Administration on LinkedIn, but extended stays on the International Space Station — or in micro-gravity in general — may irreparably damage your vision. Two-thirds of astronauts spending six months or longer on the ISS now suffer from permanently blurry vision. Unsurprisingly, this “blindness” is fluid-related; surprisingly, it’s from brain fluids, not … idle hands.
We already know that fluid behaves differently in zero-G. What we didn’t realize is that, given enough time, the cerebrospinal fluid in the skull’s brain cavity flows willy-nilly and can eventually press on the backs of the eyeballs enough that they flatten a bit. And changing the shape of the eye even a little is enough to impair the ability to focus.
At the moment, NASA has no plan to prevent this as there’s no way to control where fluid flows within the skull. The only current possibilities are either shorter stays or inventing artificial gravity — which introduces its own problem: