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#Countdown 2 days & a wake-up! Getting closer to Earth & hope Mars too. #GoodNight from @space_station! #YearInSpace pic.twitter.com/9x3ERJgAvc

— Scott Kelly (@StationCDRKelly) February 28, 2016

This is Kelly's fourth mission, bringing his lifetime total to 520 days in space. NASA hopes the medical tests conducted on Scott Kelly and his twin brother, retired astronaut Mark Kelly, over the course of the mission will help them understand the effect of space travel on the body.

Scott Kelly's journey home started Monday when he handed over command to fellow astronaut Tim Kopra.

On his last day onboard, @StationCDRKelly hands over command of the @Space_Station to @astro_tim. https://t.co/Atodu2N2F0

— Intl. Space Station (@Space_Station) February 29, 2016

Kelly is expected to land in Kazakhstan tonight and return to Houston Wednesday.

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