IN Greek mythology, twins Castor and Pollux are so tightly bound that when mortal Castor dies, divine Pollux surrenders half his immortality to stay with him, and the pair are transformed into the constellation Gemini. In a modern reversal of the myth, twin astronauts Scott and Mark Kelly were reunited in March after Scott returned from a year’s stint on the International Space Station – an event eagerly awaited by medical researchers who saw their chance to conduct tests on the twins to assess the effects of space on the human body…

This article first appeared in Aeon on 18 August 2016. To continue reading, click here.