The truth about astronaut Scott Kelly's viral 'space genes'

After 340 consecutive days in space, Scott Kelly's journey comes to an end.

Here's what you need to know. After 340 consecutive days in space, Scott Kelly's journey comes to an end.

Here's what you need to know. Photo: Kirill Kudryavtsev, AFP/Getty Images Photo: Kirill Kudryavtsev, AFP/Getty Images Image 1 of / 44 Caption Close The truth about astronaut Scott Kelly's viral 'space genes' 1 / 44 Back to Gallery

In March 2016, NASA astronaut Scott Kelly returned from an unprecedented 340-day mission on the International Space Station. A year later, his doctors released preliminary results of an extensive study comparing his health to that of his earthbound identical twin brother, Mark. And this past January, NASA announced that attendees at a recent scientific workshop agreed on the initial medical conclusion - that space travel takes a significant toll and can result in changes at the molecular level.

At no point in that process was Scott Kelly zapped by an alien laser beam, attacked by a xenomorph or otherwise transfigured into a previously unknown mutant variety of human.

But you wouldn't know that from reading some of the coverage the January announcement inexplicably spawned this week - articles claiming that the mission activated Kelly's "space genes," that 7 percent of his genes didn't return to normal post-spaceflight, and that he and Mark are no longer identical twins.

Aside from being about old news, these stories are biologically impossible. If 7 percent of Kelly's genome was altered, he would be about as different from a human as a rhesus monkey.

The actual transformation is much subtler. According to NASA's research - which is still preliminary, with the agency expecting to publish a more complete study this year - it's not Kelly's genes that changed but how they were expressed.

Your genome dwells inside the nuclei of your cells. Think of it as an instruction manual: It is the complete set of DNA that describes the form and function of every aspect of your being, with each gene pertaining to a particular task life requires. But this manual is also like a rare book that can't be taken out of the library. It must be transcribed by enzymes, resulting in a copy of the sequence known as RNA. That RNA is then translated into proteins, the molecules that do the actual work of keeping you alive.

When Scott Kelly went into space, his DNA remained fundamentally the same. What changed was the way his DNA was transcribed and translated into functional products; the study of such shifts is called epigenetics. These epigenetic changes were likely the body's way of responding to the low gravity, oxygen deprivation, increased inflammation and diet challenges of spaceflight.

According to NASA, Johns Hopkins researcher Andy Feinberg, one of 10 investigators on the Twins Study, observed variability in patterns of methylation - the process by which genes are chemically turned on and off. Chris Mason of Weill Cornell Medicine reported epigenetic changes in five biological pathways, including those related to oxygen deprivation, DNA repair and bone formation. This alterations may point to "space genes," the ones in which function is affected by time off Earth. Other researchers noticed changes in Scott's body mass, telomere length and cognition over the course of the mission and after it; most shifts were not lasting.

In a statement Thursday, NASA clarified that Scott and Mark Kelly are still identical twins. Their DNA does differ - but so does the DNA of all humans, even twins, thanks to mutations that accumulate normally over the course of a lifetime. Scott didn't have to spend a year in space to establish his uniqueness.

And the changes in gene expression that scientists observed during Scott's time in space are within the range of what they'd expect to see in a mountain climber, scuba diver or other human under stress. About 93 percent of the changes reverted to preflight levels within six months of Scott's return to Earth.

The 7 percent that persisted is "minimal," the space agency said. But it suggests that spaceflight does induce longer-term changes at the molecular level. This is important because NASA plans to someday send astronauts on a three-year mission to Mars. If and when that day comes, it would be good to make sure they arrive at the Red Planet as healthy humans.