Upcoming NASA mission is a twin thing

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Mark and Scott Kelly/Getty Images

CAPE CANAVERAL, Fla. – On Friday, NASA astronaut Scott Kelly will blast off into space where he will spend an entire year at the International Space Station. His identical twin brother, retired astronaut Mark Kelly, will remain on earth, and as a result, NASA plans to conduct a study comparing the two.

Mark Kelly is married to former Congresswoman Gabby Giffords.

The study will focus on how space affects different human organs, decision making, organisms in the gut, and the way genes react.

Scott Kelly will be  traveling with two Russian cosmonauts. They have been training for the mission for more than two years. It will be the longest space mission ever for NASA, and the longest in almost two decades for the Russian Space Agency, which holds the record at 14 months.

Their Soyuz rocket is scheduled to blast off from Kazakhstan on Friday afternoon in the U.S. (early Saturday morning in Kazakhstan.)

The world’s space agencies want to know how the body adapts to an entire year of weightlessness before committing to even longer Mars expeditions. The typical stint at the space station is six months.

Both CNN and the Associated Press contributed to this report.