When Canadian Astronaut Chris Hadfield was in space, on the International Space Station (ISS), he stayed there for six months, the standard length of stay for an astronaut.  On the ISS, three of the six-person crew are replaced every three months.  After returning to Earth, Hadfield could barely walk.  He had lost bone density and muscle mass, his immune system weakened, cardiovascular functions slowed, and he produced less red blood cells.  The lack of gravity is bad for humans,  The longest a human being has ever been in space was Russian cosmonaut Valeri Polyakov, who was in space for a staggering...