HOUSTON, March 12 (UPI) -- NASA Astronaut Scott Kelly announced he will retire on April 1, nearly two weeks after returning from a year-long mission in space.
Kelly, 52, announced his retirement on Friday in a Facebook blog post titled "A thousand miles begins with a single step."
"On April 1, I will retire from NASA. While I am leaving NASA, the journey is not over," he wrote. "I remain ever committed and dedicated to the service of human exploration and advancement whether in space or on Earth."
Kelly took part in four space flights since being selected by NASA in 1996 and has spent more time in space than any other American, at 520 days.
A few days after his return from his most recent 340-day mission, Kelly expressed muscle pain an soreness 'a lot higher' than previous missions.
"This year-in-space mission was a profound challenge for all involved that also gave me a unique perspective and a lot of time to reflect on what my next step should be on our continued journey to help further our capabilities in space and on Earth," he wrote. "I am very proud of what my NASA colleagues and our partners around the world have accomplished together."
Kelly said that he will continue to participate in research related to NASA's Twins Study alongside his brother Mark.
He concluded his letter by thanking NASA for the opportunity to serve for 20 years and expressed excitement about the next phase of his life.
"I am humbled and excited by new opportunities for me to support and share the amazing work NASA is doing to help us travel farther into the solar system and work with the next generation of science and technology leaders," he wrote. "I look forward to continuing my 30 years of public service in a new role."