Scott Kelly spent a year in orbit while his identical twin brother stayed on Earth, and when he came home NASA discovered his gene expression had changed in ways that didn't fully reverse (opens in new tab)

In March 2015, NASA astronaut Scott Kelly launched to the International Space Station for a 340-day stay, the longest single spaceflight ever made by a NASA astronaut. His identical twin brother, Mark Kelly, a retired astronaut himself (and now a US senator), stayed on the ground. That setup gave scientists something they had never had […]