<P> The One - Year Mission was a scientific research project aboard the International Space Station, which studied the health effects of long - term spaceflight . Scott Kelly and Mikhail Korniyenko spent 342 days (11 months, 3 days) in space, with scientists performing medical experiments . Kelly and Kornienko launched on 27 March 2015 on Soyuz TMA - 16M along with Gennady Padalka . The mission encompassed Expeditions 43, 44, 45 and 46 . The pair safely landed in Kazakhstan on March 2, 2016, returning aboard Soyuz TMA - 18M with Sergey Volkov . The mission supported the NASA Twins study, which helps shed light on the health effects of long duration spaceflight, which is of interest for Mars missions especially . </P> <P> In November 2012, NASA, the Russian Federal Space Agency (Roscosmos), and their international partners selected two veteran spacefarers for a one - year mission aboard the International Space Station in 2015 . The mission included collecting scientific data important to future human exploration of the Solar System . Kelly and Korniyenko already had a connection; Kelly was a backup crew member for the station's Expedition 23 / 24 crews, where Korniyenko served as a flight engineer . </P>

Who were the two astronauts who stayed a year on the iss