Mark and Scott Kelly, the only identical twins who are also astronauts, have volunteered themselves for study, creating a unique opportunity to disentangle the health effects of space from those of genetics.

"We have the best ground control you could dream of," says Graham Scott of the Baylor College of Medicine in Houston, Texas, who is helping NASA with the experiment. The question of space health is especially timely as several human trips to Mars are currently being discussed.

Last year, Scott Kelly was chosen to take part in the first one-year mission aboard the ISS, double the usual stay, along with Russian cosmonaut Mikhail Kornienko. Then last week NASA announced a twist: his brother Mark will be monitored on Earth throughout.