Record-setting NASA astronaut Scott Kelly retired from NASA on Friday, after spending more than 520 days in outer space, spread across four space missions â" two space shuttle flights and two recent missions on the orbiting International Space Station. The 52-year-old astronaut's last visit to the space station lasted more than a year, "a profound challenge for all involved," Kelly says, "and it 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." Kelly's time in space started with a 1999 mission on the space shuttle to NASA's Hubble Space Telescope, and he's since spent more time in space than any other American. According to Space.com, NASA and Kelly have both said that his long-duration visits are the first baby steps towards a manned missions to Mars.
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