Scott Kelly's DNA changed during his year in space as part of NASA's study that included his Earth-bound twin brother Mark Kelly to determine how the body is affected in long periods of zero gravity.
The research, called the Twins Study, will play a huge role in NASA's hope one day to send astronauts to Mars, reported USA Today. Scott Kelly returned to Earth last March after nearly a year in space on the International Space Station.
In testing both Kellys over that period of time, NASA scientists found that Scott Kelly's level of chemical DNA modification slowed while in space.
"In the DNA within Scott's white blood cells, we found that the level of methylation, or chemical modifications to DNA, decreased while inflight – including a gene regulating telomeres, but returned to normal upon return," said NASA.
"On the ground, Mark's level of methylation in the DNA derived from his white blood cells increased at midpoint in the study but returned to normal in the end. Variability was observed in the methylation patterns from both twins; however, this epigenetic noise was slightly higher in Scott during spaceflight and then returned to baseline levels after return to Earth. These results could indicate genes that are more sensitive to a changing environment whether on Earth or in space."
In other findings, NASA said it found that Scott Kelly had a slight decrease in speed and accuracy post-mission but the research did not support a relevant change in cognitive performance inflight by increasing the mission duration from six to 12 months.
NASA also found that Scott Kelly's telomeres on the ends of chromosomes in his white blood cells increased in length while in space. Researchers suggested the change could be connected to increased exercise and reduced caloric intake during the mission.
Those changes, though, also returned to normal once he returned to Earth, NASA said.
Time magazine said the Kelly brothers will be followed by researchers for the next four years as part of the NASA's Human Research Program.