Scott and Mark Kelly are identical twins with two sets of the same DNA.
While Scott spent a year in space, his brother Mark stayed on Earth, giving NASA a unique opportunity to see how space flight changes the human body and brain.
They're uncovering some fascinating results: about 7% of Scott Kelly's DNA may have permanently changed in space.
When NASA astronaut Scott Kelly stood up last March after spending a year in space, he was two inches taller.
The engineer and veteran of four space flights is part of a long-term NASA study that aims to figure out how being in space changes our bodies and brains.
Scott Kelly is uniquely positioned to give NASA key insight into these changes because he is both an astronaut and a twin. For its research, NASA is comparing Scott Kelly's DNA with the identical DNA of his twin brother, Mark Kelly. Mark stayed on Earth for Scott's 340-day stint aboard the International Space Station, giving NASA the rare opportunity to compare how being space affected his genes.
Although each Kelly brother was born with the same set of DNA, life has exposed each set of genes to a range of divergent experiences — space being one of them. Those experiences affect the way the Kellys' genes are expressed (also known as being "turned on" or "turned off").
Scott's newfound height turned out to be only a temporary result of his spine being physically stretched in a gravity-free environment, and not a tweak to his genes. But it's just one of the many alterations the researchers have documented so far. Deep within Scott's DNA, they are finding a range of tweaks that are not present in his brother Mark. While some were temporary and seemed to occur only while he was in space, others were long-lasting.
"When he went up into space it was like fireworks of gene expression," Christopher Mason, a principal investigator on the NASA twins study and an associate professor at Weill Cornell Medical College, told Business Insider. "But the changes that seem to have stuck around include changes in immune system function and retinal function related to his eye health."
Roughly 7% of Scott Kelly's genes may have permanently changed as a result of his time in space
According to Mason, some 7% of Scott's genes have not returned to normal since he landed back on Earth more than two years ago. Kelly said he was surprised by that change in a Marketplace interview on Thursday.
"I did read in the newspaper the other day … that 7% of my DNA had changed permanently," Kelly said. "And I'm reading that, I'm like, ‘Huh, well that's weird.'"
Those changes appear to have occurred in genes that control functions related to Kelly's immune system, bone formation, and DNA repair, as well as in those involved in responding to an oxygen-depleted or carbon-dioxide rich environment.
"With a lot of these changes, it's as if the body is trying to understand this, quite literally, alien environment and respond to that," Mason said.
In many respects, Kelly's genes display the hallmarks of a body reacting to what it perceives as a threat, he added.
"Oftentimes when the body encounters something foreign, an immune response is activated. The body thinks there’s a reason to defend itself. We know there are aspects of being in space that are not a pleasant experience and this is the molecular manifestation of the body responding to that stress."