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The American astronaut Scott Kelly and his Russian counterpart, Mikhail Kornienko, are due to return to Earth after nearly a year on the International Space Station. Why was it so important for the pair of spacemen to spend three hundred and forty days in space and what will be the likely impact on their bodies when they return? Dr Kevin Fong is author of Extremes: Life, Death and the Limits of the Human Body. (Photo: An astronaut participates in a spacewalk. Credit: NASA/Getty)