As of March, if retired astronaut Mark Kelly wants to talk with his identical twin, he will have to make a long, long-distance call—to the International Space Station (ISS). His brother, Scott Kelly, age 51, chose the same profession and will be living there as part of a yearlong study of space travel's effect on human health. The Kellys' clonal DNA provides a unique opportunity to observe changes in genetic expression in a zero-gravity environment and compare them with a simultaneous quasi-control case on Earth. nasa scientists will also compare fluid flow in the twins' brains, the composition of their microbiomes, and rates of decay of their telomeres—the protective caps on the ends of chromosomes that signal cellular aging.

Repeat fliers Scott and Mark Kelly are among more than 200 humans to have floated around all day on the ISS. Ever since the station's first crew arrived in November 2000, researchers have monitored how astronauts there respond to long-term spaceflight. Michael Barratt, who spent 199 days in space in 2009, says “it feels like you're hanging upside down.” He adds: “We train really hard so we know every task—especially in the first couple of weeks so you rely on your training and checklist even if you're not feeling very well.” nasa scientists already know a great deal about how an extended celestial sleepover affects the body in space and will monitor the twins for such symptoms, too.