NASA is studying twin astronauts Mark and Scott Kelly to understand the affects of long space missions on the body and brain.
Scott will spend a year on the International Space Station while Mark stays on Earth as the control in the experiment. From Smithsonian:
The twin study will help NASA prepare to take humans farther than ever before. "The horizon destination is Mars," says NASA scientist Craig Kundrot. The idea to study the twins came after the agency first announced Scott's ISS mission in late 2012. While preparing for a press conference following that announcement, Scott asked how he should address reporters' questions about having a twin brother who has also traveled in space. That got NASA thinking and the agency put a call out for study ideas. In March 2014, NASA chose 10 of the proposals from researchers at the agency and universities, and the twins signed on.
Before, during and after Scott's flight, the Kellys will undergo physical and cognitive tests. "The weightlessness, the radiation, the isolation and confinement, there are a number of things that are really affecting the space traveler," Kundrot says. Mark will undergo six tests; Scott will have about 11. One will assess how fluid in the body gets redistributed in microgravity. Another will gauge changes to blood cells and the immune system. The twins will provide blood, urine and fecal samples, have blood pressure and other measurements taken and use laptops to answer arithmetic and risk-taking questions.