In microgravity fluids get redistributed throughout the body. The upper body and head get comparatively more (as opposed to normal gravity), which causes immune

In microgravity fluids get redistributed throughout the body. The upper body and head get comparatively more (as opposed to normal gravity), which causes immune problems (swollen sinuses) and increased pressure in the eye and blood vessels around it, sometimes headaches and other issues. Microgravity is not a must for the Mars trip, however.