03/27/2015 01:34 pm ET Updated Mar 27, 2015

Astronaut Will Return From A Year In Space Both Older And Younger Than His Twin Brother

Bill Ingalls/NASA via Getty Images

Today NASA’s Scott Kelly, along with two Russian cosmonauts, blasts off for the International Space Station (ISS), where they plan to spend 342 days. Kelly’s identical twin brother Mark, also a (retired) astronaut, will stay on Earth to serve as the other half of an unprecedented experiment.

Researchers at a host of universities and medical schools will be watching how Scott Kelly’s weightless lifestyle affects him differently from his Earth-bound twin. They’ll study whether it increases his risk of atherosclerosis (hardening of the arteries), distorts his perception and reasoning, or alters his gut bacteria.

And among other things they’ll look at whether cosmic rays—radiation from deep space—might shorten his lifespan by speeding up damage to his telomeres.

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