cokada/Getty Images During my years working with astronauts, it was routine to hear about returning crew being carried from their re-entry capsules on stretchers. Even last year, we saw this when NASA astronauts Suni Williams and Butch Wilmore finally came home after their unexpectedly long nine-month stint on the International Space Station. Despite the top-notch health and fitness required to become an astronaut and the hours spent exercising while on missions, after several months in space, some astronauts can be more frail, less able to walk and more prone to injury than many older people back on Earth. In fact, what an astronaut’s body goes through in orbit is, in some ways, like an accelerated preview of human ageing. The way that a space mission affects the spine, weakens muscles and detunes the balance system is a fast-forward version of what many of us experience when recovering from certain injuries, after a spell of hospital bed rest or simply due to years of sitting around.…