• 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 than his identical twin brother Mark.

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.

"," 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 Kellys 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 an interview on Marketplace.

"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.

The full results of NASAs twin study arent public yet — but here are some interesting findings

The full results of NASA's twin study haven't been released yet, but the preliminary data is already giving scientists a lot to ponder.

Some of those findings build on what we already knew, like the fact that being in space stretches your spine, shrinks your muscles, and messes up your sleep cycle.

But the long-term effects of taking our bodies for a jaunt outside Earth's protective atmosphere are much less understood. Here's a quick look at what the researchers have uncovered so far:

• Scott's telomeres got longer, then shrunk back to normal.
• told Nature last year
• statement
• According to NASA
• The twins hosted different gut bacteria.
• Scientists are looking for what they're calling a "space gene."

NASA is still combing through the results of the study and expects to release the full set later this year. That research will inform space missions — including potential trips to Mars — for years to come.

Dina Spector contributed to an earlier version of this story.