For decades, scientists have studied twins and triplets to measure how human characteristics are influenced by heredity and to what degree they can be altered by the surrounding environment.
But never have they tested an environment so unusual as the one described in a new study published Thursday: 340 days aboard the International Space Station.
Astronaut Scott Kelly was subjected to a battery of biological tests before, during, and after his voyage while his identical twin, Mark, a retired astronaut, underwent the same tests on Earth.
Ten teams of researchers detected a variety of subtle changes in the physiology of the space-bound twin, they reported in the journal Science. Most of Scott Kelly’s test results returned to the level of his twin brother after the mission ended in March 2016, and he remains in good health — offering hope that astronauts someday could endure a much longer mission to Mars.