Astronaut Twin Returns To Earth With Altered Genes, Leaving Scientists Stunned

How space rewrote the DNA of an identical twin.

Jesse

• Published in News
• December 31, 2024

Imagine this: two identical twins, born with the same genetic code, grow up to achieve the same extraordinary career—both becoming astronauts.

As twins, they’re not just regular siblings; they’re mirrors of one another, right down to their DNA. But what happens when one twin spends nearly a year zipping around Earth at 17,400 mph aboard the International Space Station while the other stays firmly planted on the ground? Does the twin in space come back the same person—or does something deeper, something embedded in their very biology, change?

This isn’t science fiction. This is the true story of Scott and Mark Kelly, NASA astronauts whose unique relationship made them perfect candidates for one of the most ambitious biomedical studies ever conducted: NASA’s "Twins Study." This study aimed to unlock the mysteries of long-term space travel and its effects on the human body.

Scott’s year-long mission aboard the ISS in 2015 wasn’t just a test of endurance; it was a groundbreaking opportunity for scientists to study the effects of space on the human body in ways they’d never been able to before. Mark, serving as the Earth-bound control subject, allowed researchers to compare changes in the twins’ physical and genetic makeup, giving humanity a glimpse into what it might take to survive a trip to Mars—or beyond.

But what NASA discovered was far more than anyone expected. By the time Scott returned to Earth, the once-identical twins were no longer 'identical.'

NASA uncovers genetic shifts in twin astronauts after space mission

The Kelly twins’ story begins like many others, with shared DNA and matching childhood memories. But their parallel paths diverged in 2015, when Scott launched into orbit for 340 days.

While Scott was adapting to life in microgravity, his body began to adapt in ways that would astonish scientists. Mark, on the other hand, stayed on Earth as the perfect control for the experiment.

One of the first noticeable differences was Scott’s height. When he returned, he was 5 centimeters taller than his brother. The lack of gravity allowed his spine to stretch, but the change was temporary as gravity on Earth quickly brought him back to his original height.

Both Scott (right) and Mark (left) have now earned their cosmic stripes, each having ventured to space

Then there were the changes you couldn’t see. Scott’s telomeres—the protective caps on the ends of his DNA—became longer while he was in space. This was unexpected, as telomeres usually shorten with age.

The researchers linked this anomaly to Scott’s carefully controlled diet and exercise routine in orbit. Yet, after his return to Earth, his telomeres began shortening again, some even faster than before.

Perhaps the most mind-boggling discovery was the alteration in Scott’s gene expression. While his DNA sequence remained the same, the way his genes were "switched on" or "off" changed significantly.

These changes persisted for months after his return, suggesting that the stressors of space travel—radiation, microgravity, and isolation—had left a lasting imprint.

Scott's body experienced a lot of interesting changes.

Even Scott’s cognitive abilities weren’t immune. Although his mental sharpness held steady in space, researchers observed a dip in speed and accuracy once he was back on Earth. Adjusting to gravity, coupled with a packed schedule, likely contributed to these changes.

And then there were his eyes. Fluid shifts caused by zero gravity reshaped his eyeballs, resulting in vision changes. This is a common issue for astronauts that scientists are still trying to fully understand.

Some of Scott's genetic changes refused to return to their original "Earth settings

The Kelly twins’ story offers a glimpse into the resilience and fragility of the human body. As we dream of venturing deeper into the cosmos, their experience reminds us that space is as much about pushing our physical and mental limits as it is about exploring the stars.

Jesse