After spending a year in space, NASA astronaut Scott Kelly is not the man he used to be - at least genetically speaking.
His genetic expression has changed, according to preliminary results from a NASA study that compared the bodily changes between the astronaut and his identical twin, who stayed on Earth while Kelly was aboard the International Space Station.
About 7 percent of Kelly's genes have yet to "return to normal" ― almost two years after his yearlong expedition came to an end. Kelly has since retired from NASA.
The Twins Study, as it's been dubbed, looked at what happened to Kelly - both from a physiological and psychological perspective - before, during and after his trip in space, and then compared that data to Kelly's twin brother, Mark.
Mark Kelly is also a retired NASA astronaut. Unlike his brother, however, who spent months at a time in space, Mark's missions were on the shorter side. His last ― and longest ― mission, which took place in 2011, lasted 15 days.
NASA described the research as a "perfect nature versus nurture study" ― one that could provide important insights into the effects of long-term spaceflight on the human body.
"By measuring large numbers of [the brothers'] metabolites, cytokines and proteins, researchers learned that spaceflight is associated with oxygen deprivation stress, increased inflammation, and dramatic nutrient shifts that affect gene expression," NASA said of the study's initial findings.
Although most of the biological changes Kelly experienced in space disappeared in the hours and days (and in some cases, weeks) after his return to Earth, NASA said some alterations appear to have persisted.
While 93 percent of his genetic expression has returned to normal, several hundred "space genes" have remained altered, the data suggests. NASA said this could indicate "longer term changes" in the genes caused by the stresses of spaceflight.
NASA said it would be releasing more findings from the Twins Study in the coming months. This research, they added, will inform their planning for a mission to Mars, which would see astronauts spending some three years in space.
Reacting to the news of the study results this week, Kelly expressed amazement at his body's changes ― and also used the opportunity to poke fun at his brother.
"This could be good news," he joked on Twitter. "I no longer have to call [Mark] my identical twin brother anymore."