In March 2015, NASA astronaut Scott Kelly launched to the International Space Station for a 340-day stay, the longest single spaceflight ever made by a NASA astronaut. His identical twin brother, Mark Kelly, a retired astronaut himself (and now a US senator), stayed on the ground. That setup gave scientists something they had never had before: two people with the same DNA, the same training, and most of the same life history, with one sent to orbit for almost a year and the other kept on Earth as a comparison.
The result was the NASA Twins Study, one of the most detailed looks ever taken at what long-duration spaceflight does to the human body. Here is what it actually found, including the part that got widely misreported.
Why identical twins make a near-perfect experiment
The hardest part of studying spaceflight is separating the effects of space from everything else that makes one person different from another. Genetics, diet, and lifestyle all muddy the picture. Identical twins remove most of that noise. Because Scott and Mark started from the same genetic baseline, researchers could treat Mark as a living control and attribute differences that showed up in Scott, and not in Mark, to the year in orbit.
Ten research teams collected blood, urine, saliva, and stool samples from both brothers before, during, and after the mission, along with cognitive tests and physical measurements. They tracked everything from gut bacteria to gene activity to the structure of Scott’s eyes and arteries.
The headline finding: gene expression, not DNA
The result that made news was about gene expression. When Scott returned to Earth, NASA reported that about 93 percent of his gene expression went back to normal within roughly six months, while around 7 percent stayed altered well after he landed.
This is the point where most of the coverage went wrong, so it is worth slowing down. Gene expression is not the same thing as DNA. Your DNA is the fixed genetic code in your cells. Gene expression is which of those genes are switched on or off, and how strongly, at any given time. Expression changes constantly in everyone, in response to stress, sleep, diet, illness, and environment. A year in microgravity, bombarded by radiation and cut off from a normal day-night cycle, is about the biggest environmental change a body can experience, so it is no surprise that it shifted which genes were active.
The genes that stayed altered were linked to things like immune function, DNA repair, bone formation, and the body’s response to low oxygen and high carbon dioxide. NASA nicknamed them Scott’s “space genes.”
No, the twins did not stop being identical
In 2018, a wave of headlines claimed that Scott’s “DNA had changed” and that he and Mark were “no longer identical twins.” That was false, and NASA publicly corrected it.
Scott’s underlying DNA sequence did not fundamentally change. He and Mark remained identical twins. The “7 percent” figure referred to gene expression that had not returned to baseline, not to alterations in his genetic code. The confusion came from collapsing two different ideas, expression and sequence, into one scary-sounding sentence. If you take away one thing from this story, make it that distinction.
The other changes the study found
Gene expression got the attention, but the study turned up several other effects, some of which reversed and some of which lingered:
- Telomeres: The protective caps on the ends of chromosomes, called telomeres, unexpectedly got longer while Scott was in space. Researchers had expected the stress of spaceflight to shorten them. After he returned, they shrank quickly, and he ended up with more unusually short telomeres than before, which is generally associated with aging.
- Gut bacteria: The balance of microbes in Scott’s gut shifted noticeably in orbit, then largely returned to normal once he was back on Earth.
- Cognition: Scott’s thinking speed and accuracy held up well in space but dipped after landing, likely from readjusting to gravity and a packed post-mission schedule.
- Body and eyes: He lost body mass, his arteries showed some thickening, and he experienced changes in his eyes and vision, a known issue for long-duration astronauts.
What it actually means for space travel
When the full study was published in the journal Science in 2019, the overall message was more reassuring than the early headlines suggested. The majority of the changes returned to baseline, and the researchers concluded that human health can be largely sustained over a year in space. There was no evidence of catastrophic or wholesale damage to the body.
That does not mean spaceflight is harmless. The lingering changes, the short telomeres, the altered immune-related gene activity, and the eye and vascular effects, are exactly the kind of thing NASA needs to understand before sending crews on a multi-year round trip to Mars, where the exposure would be far longer. The Twins Study is best understood as a roadmap of where to look, not a clean bill of health.
The bottom line
A year in space measurably changed Scott Kelly’s body, mostly at the level of gene expression rather than his actual DNA, and most of those changes reversed after he came home. The study is a landmark not because it revealed something alarming, but because it showed, in unusually fine detail, how adaptable the human body is, and which specific systems we will need to watch closely on the way to Mars.
