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No doubt about it, identical twins are remarkable. And being identical twin astronauts makes Scott and Mark Kelly even more special.

Identical twins often look EERILY the same, they tend to share many of the same mannerisms, and they’re born genetically identical—a duplicate of the other. These similarities make identical twins great subjects for studies, where you treat one of the twins differently than the other and then compare the two, measuring their differences and similarities.

And that’s what NASA did with these extra special astronauts. Scott Kelly, who stayed up in the International Space Station starting in 2015 before returning to Earth 340 days later on March 1, 2016, and his identical twin Mark Kelly, a retired astronaut living in Tucson who stayed on Earth for the same period, are the subjects in this amazing NASA Twins Study. The study looks at how living in space can affect a person’s genes. And the latest results were just announced on Oct. 24.

Things like living in weightlessness, breathing a different atmosphere and being exposed to much more cosmic radiation can genetically change a space traveler! Many of these changes could affect our ability to endure long missions in space or even living on another planet or celestial body.

“That study is looking at a lot of important things between my brother and I and our physiology and how it changes over the course of the time that I’m here,” Scott Kelly said during an interview with Bear Essential News on his final day aboard the ISS. “One part (studies) the effects of spaceflight on a genetic level. That’s something I’m pretty excited about for personal reasons but also for the research to try and have a better understanding of how this microgravity environment and the radiation environment affects us genetically. There’s a lot to learn.”

Besides Scott having grown two inches taller than Mark, the study reveals that tens of thousands of Scott’s genetic switches—how genes express themselves—were turning off and on in Scott.

“Looking at gene expression in space…we really see an explosion, like fireworks taking off, as soon as the human body gets into space,” Twins Study Principal Investigator Chris Mason notes. “We’ve seen thousands and thousands of genes change how they are turned on and turned off. This happens as soon as an astronaut gets into space, and some of the activity persists temporarily upon return to Earth.”

November 2017