Scott Kelly Retires From NASA After 20 Years

Today, after 20 years with NASA, astronaut Scott Kelly retires.

— -- Astronaut Scott Kelly retired today after 20 years with NASA and spending a year in space.

During his career, Kelly completed four separate missions in 1999, 2007, 2010 and 2015, accruing 520 days in space. He retires with the American record for most cumulative time in space.

Kelly's final excursion included participating in NASA’s twin study.

Kelly spent 340 consecutive days floating in the International Space Station, while his identical twin brother, Mark Kelly, remained on Earth. Since Scott’s return on March 1, he and his brother have been the subjects of extensive medical testing.

NASA is comparing them to see how the human body responds to extended time in space.

ABC News' Erin Dooley contributed to this report.