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Studies of astronauts on long missions have shown that prolonged exposure to weightlessness weakens muscles and bones, prompting scientists to seek ways of generating artificial gravity aboard spacecraft. Even before the first space flights, visionaries such as Werner Von Braun suggested making spacecraft spin to create a centrifugal effect that feels like gravity. But experiments in the 1960s and 1970s revealed that rotation rates greater than around 2rpm tended to make people feel nauseous. This was bad news; such a slow spin rate meant that to generate an effect that could mimic Earth-like gravity, a spacecraft would need a diameter of around 450m. Engineers are still trying to solve the problem, but have had limited success. A team at the Massachusetts Institute of Technology has experimented with a ‘gravity gym’ – essentially a man-sized spin-dryer that astronauts can climb into to experience a short period of artificial gravity. The rotation rate must- be higher than the spaceship – 23rpm – so nausea is still a problem. There are other ways, such as making the spacecraft permanently accelerate at 1G or building a vehicle so large that it naturally generates its own gravity. But as yet both methods are far beyond our technical abilities.