A three-year mission to Mars will have profound effects on bodies—and brains. The recent NASA study of twin astronauts Scott and Mark Kelly provides a new understanding of how life in space can alter cognition, heart health, and even gut bacteria. Dr. Christopher Mason of Weill Cornell Medicine and Dr. Mathias Basner of the University of Pennsylvania School of Medicine reveal their findings about long-term space flight and explain why using a "free-range astronaut" as a control was uniquely helpful to their work.