Scott will spend a year on the International Space Station while Mark will stay here on Earth. Tests and measurements on the two will be conducted before, during, and after Scott's time in space. The tests will likely include blood and urine samples, ultrasounds and CT scans.
The experiment will try to measure the genetic effects of long-term space travel as a stepping stone to completing NASA's mission to send humans to Mars by 2030. Space travel has been known to dramatically decrease bone and muscle mass and increased exposure to radiation can raise the risk of cancer. Astronauts have to exercise several hours each day to counteract the loss of bone and muscle mass.
"It was kind of ironic or interesting -- maybe serendipitous -- that after this flight I will have flown about 540 days compared to [Mark's] 54, so an order of magnitude more," Scott told NPR's Eric Westervelt.
Mark will be remaining on Earth since he retired from NASA in 2011 after his wife, former Rep. Gabrielle Giffords, D-Ariz., was shot in the head. He will not have to exercise like his brother every day, but it might encourage him to get a mile or two in during the year-long experiment. He will undergo the same experiments and tests as the control subject.
After 15 years of being a NASA astronaut, Mark said, "I've had a lot of science done on me already, so I kind of know what I'm getting myself into."