First identical twin astronauts

Identical twins Mark and Scott Kelly (USA, b. 21 February 1964) are the only identical twin astronauts and the only siblings to have both flown in space. The twins were part of a group of 2,400 candidates for two of 35 positions in NASA’s 1996 class. Scott first flew into space as the pilot of the space shuttle Discovery on the STS-103 mission on 19–27 December 1999, which replaced the gyroscopes and computer on the Hubble Space Telescope. Mark’s first spaceflight was as pilot of the space shuttle Endeavour on the STS-108 mission on 5–17 December 2001, which carried three astronauts and supplies to the International Space Station (ISS). As of 2014, Mark had been on four space flights and had a record of over 2,000 flight hours and 54.09 days in space, while Scott had been on three space flight and had a record of 1,800 flight hours and 180.08 days in space. By March 2016, Scott had spent another 340 days in space, bringing his total flight time to 520.43 days. In March 2015, Scott Kelly was set to spend one full year in the International Space Station (ISS) while his twin brother, Mark, would remain on Earth. This experiment, proposed by the twins themselves, would allow scientists to uniquely compare Scott’s psychological and medical functioning with Mark’s – his perfect genetic control. Ten out of 40 proposed research projects were approved for implementation by NASA.