Medical science loves twins—the near-perfect physical and genetic match makes for ideal conditions to compare and contrast human responses to environmental change. And when the twins also happen to be astronauts, it’s like striking research gold. That’s why NASA leapt at the opportunity when U.S. astronaut Scott Kelly suggested that he and his identical twin brother, Mark, could be test subjects for an investigation on the health effects of long-term space flight.
A first-of-its-kind study was hatched: Scott would travel to the International Space Station and stay there for one year, working and living the life of an astronaut in the confines of microgravity. Meanwhile, back on Earth, Mark would serve as a genetically identical ground control, working and living the