What We’ve Really Learned From the Twin Astronauts

Scott Kelly’s DNA did change during his year in space—but not in the way you think.

Astronauts Capt. Scott Kelly and Capt. Mark Kelly speak onstage.
Astronauts Capt. Scott Kelly and Capt. Mark Kelly speak onstage at LocationWorld 2016 at the Conrad on Nov. 2, 2016, in New York City.
Brian Ach/Getty Images for LocationWorld 2016

When researchers compared the DNA of twin astronauts Scott and Mark Kelly after Scott had spent a year in space on the International Space Station, they found something remarkable. They also found something rather mundane. Guess which one ended up making headlines this week?

Let’s first get the mundanity out of the way, because yes, that’s the one that lit up the internet. Reports informed readers, inaccurately, that Scott’s DNA had become “7 percent different” from the DNA of his earthbound twin. Many outlets have since updated their pieces, but originally, headlines asserted that “Astronaut’s DNA Changes After Spending a Year in Space” and “Astronaut’s DNA No Longer Matches His Twin’s.”

That’s not what researchers found. It is, however, what NASA’s own Jan. 31 news release initially implied. In the seventh paragraph of the garbly statement, the agency says that one of the “interesting” findings concerns what some call the “space gene.” The awkward paragraph then rushes from “the” space gene to saying that “Researchers now know that 93% of Scott’s genes returned to normal after landing. However, the remaining 7% point to possible longer-term changes in genes related to his immune system” and some other processes.

Any reporter in a hurry to publish these fun twin findings from spaaaaace could easily have construed that to mean that some of Scott Kelly’s gene sequences literally changed. Except that the news release had already said a few paragraphs earlier that these changes were in gene expression, not the genes themselves. That means that the twins didn’t differ in their sequences but in how they used them. Space Kelly had changes in how his cells were using genes related to bone turnover, low oxygen, high carbon dioxide, and inflammation. These factors are all what you’d expect to change when you’re living in spaaaaace, where gravity doesn’t stress your bones sufficiently, oxygen isn’t as bountiful as on Earth, and you’re living in a tiny space where everyone’s exhaling carbon dioxide all the time. There are no surprises here.

NASA added a sort of mea culpa on Thursday to its original statement after so many outlets misreported the gene expression findings. The update says clearly that Scott Kelly’s DNA “did not fundamentally change.” It then clumsily says that “what the researchers did observe are changes in gene expression, which is how your body reacts to your environment.”

Um, sort of? Your body reacts to your environment in all kinds of ways, and how it uses genes is one of those ways, so sure. The release also notes that the reactions of Scott Kelly’s cells are quite similar to how humans respond to other low-oxygen stress conditions like climbing Everest or scuba diving.

In other words, this “7 percent different from his twin” news was not … news. All pairs of twins use their (largely) identical DNA sequences differently, in fact, increasingly so as time passes, something that’s been on the research radar for years. It’s one explanation for why, within twin pairs, one twin might develop a disease with strong ties to lifestyle, like Type 2 diabetes, while the other twin doesn’t. “Identical” means only that they’re using a largely identical genetic dictionary as a starting point. But the cells of identical twins are choosing selections from that dictionary differently even at birth.

Having buried the lede just like many outlets did, let’s look at what is genuinely intriguing about the differences between the Kelly twins. Their DNA did diverge, but not in the coding sequences. They became different in their telomere lengths. Telomeres are the tips of DNA strands, areas meant to withstand some wear and tear, just like the plastic-protected end of a shoelace. Telomeres help buffer the rest of the DNA from damage, keep chromosome tips from bumping up against each other and fusing, and engage in a host of other important cell activities.

Telomeres are implicated in cancer, aging, and death. These DNA end bits mark time for human cells—and lives. An enzyme called telomerase has the task of maintaining these ends, but in mortal cells, telomeres eventually become so short that cells can no longer rely on their DNA to be stable. Eventually, cells divide wildly out of control because of the unstable DNA, or they get old and die. And so do we.

But some of our cells don’t go through this aging and death process. Their telomerase keeps up the telomeres, conferring on them a kind of immortality. They are stem cells, which rely on this service to be stemmy, to stay fresh and young and constantly resupplying their associated tissues.

So the news that Scott Kelly’s telomeres became “significantly longer in space” is intriguing and a genuine DNA sequence difference that arose between the twins because of space vs. Earth environments. The NASA statement calls it “unexpected,” and the research team has confirmed it using various tests.

Something about living on the space station triggers a fountain-of-youth cell effect, and many outlets missed the opportunity to highlight that. Scott Kelly’s gene expression response to his year in space might provide some hints at factors driving the telomere elongation. That, in turn, could lead to insights about cancer, aging, and death. Quite a lede to bury, yes?

Coming back to Earth, though, was a bit of a dampener on the “Was Scott Kelly becoming immortal in space?” question. Most of his telomeres shortened up again within a couple of days of his return to the home planet.