NASA and SpaceX on Friday will count down to a long-awaited crewed rocket launch that will allow them to bring home U.S. astronauts Butch Wilmore and Suni Williams, who have been stuck on the International Space Station for nine months.

According to the Baylor College of Medicine's Center for Space Medicine, long durations in space can change a person's eyes, including globe flattening, optic disc edema, and vision changes. The Space-Associate Neuro-ocular Syndrome (SANS) is a recognized condition which causes swelling in the back of the eye after a duration in space.