After six months in space they will, in technical terms, have aged 0.007 seconds less than someone on earth. But in other respects they’ll have aged five or ten years more, and this is only in the ways they currently understand. They know that the vision can weaken and the bones deteriorate. Even with so much exercise still the muscles will atrophy. The blood will clot and the brain shift in its fluid. The spine lengthens, the T cells struggle
