Countermeasures for Visual Impairment in Astronauts

This study is being done to prevent changes in the vision of astronauts during prolonged residence on the International Space Station or a mission to Mars. This project will provide data about the efficacy of lower body negative pressure to mimic daily upright posture by intermittently lowering central venous pressure while in simulated microgravity (3 days of bed rest). The investigators expect that 3 days of bed rest will increase choroid thickness, and choroid area. The investigators hypothesize that nightly LBNP (-20mmHg) will prevent these changes.