Being genetically identical will allow twins Mark and Scott Kelly to do something entirely unique. The UK's Sky News has reported that Scott will spend a year aboard the International Space Station while his brother stays on Earth.Once Scott returns Nasa will measure the effects long-duration space flight has had on his body, using Mark as a control.Said Scott : "This will help us learn a lot about the adverse effects of space on human physiology and how to mitigate them."The purpose of the experiment is to determine the feasibility of a manned flight to Mars, which would take between six and eight months.Being removed from Earth's gravitational pull can affect bone density, blood circulation and the astronauts' sense of balance.Dr John Charles, head of Nasa's human research programme, told Sky News: "It's a way to start using 21st-century medical technology to understand what happens to people in long-duration space flight."One of the effects of his time in almost zero gravity will be that Scott is taller when re-united with his brother. Back vertebrae separate slightly in greatly reduced gravity and astronauts grow as much as 5cm taller in space. The effect is temporary and astronauts return to their usual size once back on solid ground.Said Scott: "This will last long enough so that when I get back I can look down on Mark a little."The mission is to launch from Kazakhstan on March 27...