On Wednesday, NASA astronaut Scott Kelly tweeted that he received his last health scans in a series of tests to gauge the health effects from his year in space, as part of NASA’s goal to better understand how long-term space travel impacts the health of astronauts before launching the mission to Mars. One of the effects Kelly is still reeling from after 340 days aboard the International Space Station, besides loss of muscle mass and bone density, is a lesser-known phenomenon called fluid shift.

According to NASA, more than half of American astronauts develop changes in eye and vision structure after a long-duration space flight. These changes in ocular refraction (how the eye processes light) and cranial pressure are believed to be the result of fluid shifts. While short-term flights can also impact vision, fluid shift does not seem to be a factor. However, such suspicions had never been tested before the Kelly medical test.

In order to determine the impact of fluid shift on eye vision and structures, researchers used noninvasive techniques developed by NASA to determine arterial and venous flow parameters, ocular pressure and structure, and changes in intracranial pressure.

Researchers believe that lower-body negative pressure could help negate some of the effects of fluid shift, but results are inconclusive. The most recent study, which was a partnership between NASA’s Human Research Program and the Russian Space agency, could provide critical insight into keeping astronauts healthy on deep space missions.

In order to gauge the extent of the changes Kelly experienced, researchers performed the same test on Kelly’s twin, former astronaut Mark Kelly.

As astronauts suffer from increasing “unforeseeable risks” of space travel, it’s up to NASA to continue pioneering research that will keep astronauts like Kelly healthy after they retire.

Photos via NASA