NASA is pointing to the "stresses of space travel" as the factor behind a fascinating phenomenon involving the only pair of identical twin astronauts in history. Per Live Science, even though Scott and Mark Kelly once boasted the same DNA makeup, Scott's nearly yearlong trip on the International Space Station in 2015 and early 2016 changed all of that—leaving him with a lower body mass, altered gut bacteria, and a seeming growth spurt of 2 inches, among other shifts. Although some of Scott's changes could be tied to his body acclimating to the low-gravity environment with little oxygen, a NASA statement notes that Scott's journey into the cosmos may have triggered chemical changes in his RNA and DNA that resulted in "space genes," which affected his immune system, eyesight, bone formation, and other bodily functions.
"Oftentimes when the body encounters something foreign, an immune response is activated. The body thinks there's a reason to defend itself," Christopher Mason, one of the study's lead investigators, tells Business Insider. NASA calls its "Twins Study," in which more than 200 researchers from 30 states took part, the "perfect nature versus nurture" experiment, with Mark used as the control subject back on Earth. The study was broken down into what NASA calls three "acts": a "peek" into preliminary findings, released in January 2017; the corroboration of those findings now being offered, plus some additions; and a more comprehensive report due later this year. Although most of Scott's genetic alterations reverted back (he even lost the extra 2 inches to his height) within hours, days, and months after his return to Earth, about 7% haven't, and it's possible they may never do so. ("Fake news" on this astronaut's growth spurt.)