While Scott was on the ISS, experts were monitoring Mark’s DNA as well as Scott’s so they could compare the two, a groundbreaking experiment known as The Twins Study.

“Some of the most exciting things that we’ve seen from looking at gene expression in space is that we really see an explosion, like fireworks taking off, as soon as the human body gets into space,” said Twins Study Principal Investigator Dr Chris Mason, of Weill Cornell Medicine.

“With this study, we’ve seen thousands and thousands of genes change how they are turned on and turned off. This happens as soon as an astronaut gets into space, and some of the activity persists temporarily upon return to Earth.

“This study represents one of the most comprehensive views of human biology. It really sets the bedrock for understanding molecular risks for space travel as well as ways to potentially protect and fix those genetic changes.”