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.
