Why Astronauts' Faces Puff Up in Space — and Push on the Eyes of 7 in 10 Long-Term Crew
On Earth, fluid pools in your legs. In microgravity, it has nowhere to go but up, which is why astronauts get puffy faces and skinny legs. The real trouble is what that fluid does to the eyes: on missions of four to six months, about 70% of crew show some sign of optic nerve swelling.