Less than two weeks after returning to Earth after nearly a year in space, astronaut Scott Kelly is hanging up his spacesuit.

NASA announced Kelly's retirement Friday, which begins April 1. The 52-year-old Kelly holds the American record for most time in space: 520 days over four missions.

Kelly spent 340 days in orbit on the International Space Station to see how the human body holds up for long periods of time in space. His results are being compared to his twin brother, Mark, who is a retired astronaut.

NASA said Kelly will undergo periodic medical tests as part of that mission.

In a statement, Kelly said his time in space allowed him to reflect on what his next step on Earth would be. No future plans were announced.