Wisconsin astronaut, Jeff Williams, has set the American record for the most hours logged in Space.

On Wednesday, Wisconsin Public Radio (WPR) reported that Williams had surpassed astronaut Scott Kelly's record of 520 days. He will return form the International Space Station (ISS) on September 6 and will have logged a total of 534 days in space.

Williams has been conducting research on how the lack of gravity affects heart functions. NASA is learning more about how long missions in space affect the body, including vision.

NASA spokeswoman Brandi Dean told WPR, "As your body doesn't have to fight against the gravity, you end up with a lot of fluid in your head. That's affecting the shape of astronaut's eyes, so that affects their vision while they're in space. We're trying to figure out why that is and how we might counteract it before we send people on longer missions."

Williams is currently the first NASA astronaut to spend three long-duration stays in space; this is the fourth mission aboard the ISS.

The Wisconsin astronaut's title hangs in the balance with astronaut, Peggy Whitson having logged 377 days already and is setting out on a six-month mission in November.