One of the scientists prodding and poking the Kelly brothers is Prof Christopher E Mason, the lead geneticist on the Twins Study. Mason’s lab at Cornell University is nothing if not ambitious. Its work centres on a “500-year plan for the survival of the human species on Earth, in space, and on other planets.”

As well as studying what happens to astronauts, it involves laying the genetic groundwork for humans to live among the stars. Mason envisions a future in which the human genome can be bioengineered to adapt to almost any environment, augmented with genes from other species that allow us to explore and settle the farthest corners of the Universe.