Scott Kelly Spent a Year in Space, and Now He Has Different DNA Than His Identical Twin Brother

Scott and Mark Kelly are identical twin brothers — at least, they were until Scott spent a year living in space.

When Scott Kelly returned to Earth after a 340-day voyage aboard the International Space Station (ISS) two years ago, he was 2 inches taller than he'd been when he left. His body mass had decreased, his gut bacteria were completely different, and — according to preliminary findings from NASA researchers — his genetic code had changed significantly. (Interestingly, Scott Kelly has since shrunk back down to his initial prespaceflight height.)

A new NASA statement suggests the physical and mental stresses of Scott Kelly's year in orbit may have activated hundreds of "space genes" that altered the astronaut's immune system, bone formation, eyesight and other bodily processes. While most of these genetic changes reverted to normal following Scott Kelly's return to Earth, about 7 percent of the astronaut's genetic code remained altered — and it may stay that way permanently. [Seeing Double: 8 Fascinating Facts About Twins]

The changes are "thought to be from the stresses of space travel, which can cause changes in a cell's biological pathways," the NASA statement said. "Such actions can trigger the assembly of new molecules, like a fat or protein, cellular degradation, and can turn genes on and off, which change cellular function."

Two Brothers
Scott spent nearly a year aboard the ISS as part of a unique NASA project called the Twins Study, which aims to reveal the long-term effects of space travel on the human body and mind. In March 2015, he flew up to the ISS to begin what would become the single longest space mission any astronaut has ever accomplished. Most astronauts stay aboard the ISS for six months at a time. Scott Kelly...