NexFuture (June 13, 2026) — When we look up at the night sky, it's easy to get lost in the romance of space travel, but we often overlook the biological toll that leaving Earth takes on the human body. To solve that mystery, NASA turned to a unique opportunity back in 2015 when they launched a year-long experiment involving identical twins, Scott and Mark Kelly. While Scott spent 340 days aboard the International Space Station, Mark stayed here on Earth, giving scientists a perfect control group to see exactly how spaceflight shapes a person.
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| An astronaut in space (Illustrative photo) |
The results, which were published in 2019, showed us that while Scott remained healthy overall, space travel triggers some pretty profound changes at both the physiological and molecular levels. Perhaps the most headline-grabbing discovery was how Scott's genes behaved; even six months after he landed, about 7% of his gene expression—how his genes were being used—had still not returned to their pre-flight levels. Researchers identified 811 genes, many linked to DNA repair and immune function, that remained altered long after he returned home.
Then there was the mystery of his telomeres, those protective endcaps on chromosomes that usually shorten as we age. In a completely unexpected twist, Scott’s telomeres actually lengthened while he was in space, only to shorten again once he got back to Earth.
While we know shorter telomeres are typically associated with aging, scientists aren't entirely sure what this fluctuation means for long-term health, though it definitely highlights that space travel messes with our biological clocks in ways we're still figuring out. It wasn't just biological cells that were affected, though, as Scott also saw a dip in his cognitive skills regarding speed and accuracy that persisted even after his return. When you combine this with the muscle loss and physical shifts astronauts already deal with, it paints a clear picture: long-duration spaceflight is incredibly taxing on the human machine.
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| Astronaut Scott Kelly and twin brother Mark (Image credit: Robert Markowitz/NASA) |
This doesn't mean space travel is impossible, but it does show that if we want to send people to Mars or beyond, we need to implement much better safety measures to keep them healthy. This study was just the first of its kind to focus on twins, and scientists are the first to admit we need more data to know if these findings apply to everyone or if they were unique to Scott's specific physiology.
Ultimately, the mission highlights that while humans are remarkably adaptable, we are also deeply sensitive to the cosmos, and understanding these changes is the only way we can safely plan the future of human exploration.
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