Scott Kelly Retires From NASA After 20 Years

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

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.

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

ABC News' Erin Dooley contributed to this report.