NASA's only identical twin astronauts are planning to serve as guinea pigs for studies investigating the genetic impacts of long-duration spaceflight.
Astronaut Mark Kelly, who commanded four space shuttle missions including the final flight of Endeavour, is set to be a test subject on Earth while his twin undergoes studies in orbit.
Kelly left NASA in 2011 to care for his wife, former U.S. Representative Gabrielle Giffords. She was shot through the head in January 2011 when a gunman opened fire at a political event in Arizona, killing six people and wounding 26 others.