340 Days in Space Turned Scott Kelly’s Body Into Something Measurably Different

When NASA sent astronaut Scott Kelly to the International Space Station in March 2015, his identical twin brother Mark stayed home in the United States. That genetic coincidence gave researchers something almost impossible to arrange: a living, breathing control subject with the same DNA, the same upbringing, and a lifetime of shared biology. What 10 research teams across 12 universities found over 25 months of data collection was, by their own account, the most comprehensive molecular portrait of a human in space ever assembled.

Scott Kelly's genome, telomeres, and gene expression shifted in orbit

The headline findings, published in the journal Science in April 2019, reach across almost every system in the body. More than 50,000 gene expression changes were detected in Scott during his year aboard the station. His DNA methylation rate, which regulates how genes are switched on and off, increased measurably. His telomeres, the protective caps on the ends of chromosomes that typically shorten with age and stress, did something counterintuitive: they lengthened while he was in orbit. Researcher Chris Mason described the situation plainly: "There are over 50,000 genes in the human genome, and when floating in zero gravity, the body is trying to manage that situation in new ways."

The telomere finding drew particular attention. New research from a Twins Study investigator later identified chronic oxidative stress during spaceflight as a likely driver of telomere elongation, a finding with direct implications for future long-duration travelers. The elongation did not last: after Scott returned to Earth, his telomeres shortened to below his pre-mission baseline, suggesting the body's rebound overcompensated.

Long-duration spaceflight health risks that linger after landing

Most of Scott's biology snapped back faster than many researchers expected. According to the NIH, roughly 91.3 percent of his gene expression levels returned to pre-flight baseline within six months of landing. But "mostly returned" is not the same as "fully returned." About 7 percent of gene expression changes persisted six months after he touched down, concentrated in genes related to immune function, DNA repair, bone formation, and the body's response to hypoxia, meaning low oxygen conditions.

Changes in lipid profiles, insulin sensitivity, and liver function were logged during the mission. Heightened cytokine levels, proteins that signal inflammation and immune stress, were found in Scott's blood. His gut microbiome shifted substantially in space, though it largely normalized after landing. UC San Diego researchers examining the proteomic and metabolic data also flagged implications for cardiovascular health and a condition called Space-Associated Neuro-ocular Syndrome, a swelling of structures around the eye caused by fluid redistribution in microgravity. Cognitive performance held steady during flight but showed decreased speed and accuracy in the period immediately after landing, when the body is readjusting to gravity.

Scott also grew two inches taller in space. Spinal discs decompress without gravity's constant compression, a well-documented effect that reverses on return.

How the multi-omics design made the Twins Study credible

The study's design set it apart from earlier spaceflight research. NASA's summary of the Twins Study results emphasizes that the genetically matched control was unprecedented: no previous human spaceflight study could hold genetics constant while varying the environment so completely. Biological samples collected from both brothers included blood, saliva, urine, and stool, gathered before, during, and after Scott's mission across a 25-month window. The analysis covered metabolomics, proteomics, epigenomics, microbiome composition, telomere dynamics, and cognitive assessment simultaneously, a so-called multi-omics approach that let researchers see how different body systems interacted rather than examining each in isolation. Geneticist Mike Snyder called it "probably the most in-depth study, certainly at the biochemical level, that's ever been done in people in space."

The single-subject problem and what it means for interpreting results

The study's authors were candid about its central limitation: one astronaut in space is one data point. The Science paper explicitly frames the work as hypothesis-generating rather than definitive, meaning the findings point researchers toward questions that larger future studies will need to answer. Every percentage cited, every gene expression shift, every telomere measurement comes from a single flight subject. Whether these patterns generalize to other astronauts, to women, to people of different ages or fitness levels, remains genuinely open. The NASA Twins Study FAQ notes that the 91.3 percent gene expression recovery figure is striking, but underscores that more extensive studies are needed before the field can treat any of these findings as predictive baselines.

There is also the question of which spaceflight stressor is doing the most damage. Microgravity, cosmic radiation, disrupted circadian rhythms, psychological isolation, and shifts in diet and exercise all operate simultaneously on the ISS. The twin design controls for genetics but cannot untangle these environmental variables from one another.

What NASA plans to do before sending humans to Mars

Scott Kelly himself drew the practical conclusion: "The bottom line is, from all these studies, would be that there's nothing that we saw that would prevent us from going to Mars." That framing matters because a crewed Mars mission would last roughly three years, nearly three times the duration of his ISS stay. The Twins Study was explicitly designed to inform that planning.

Researchers are now looking at genetic engineering tools, including CRISPR-Cas9, as potential countermeasures against the detrimental gene expression changes identified in Scott. If specific expression patterns can be linked to health risks, the logic goes, they might eventually be corrected or suppressed before or during a long mission. That is speculative for now, but the Twins Study gave the field a molecular map detailed enough to start drawing targets on it. The human body, the study concluded, is considerably more plastic and reactive to spaceflight than previous predictions assumed, which cuts both ways: more adaptable than feared, but also more altered than hoped.

Evidence rating

INVESTIGATIONAL — 65/100

Caveats:

• This study had a sample size of one astronaut — no statistical generalizations to broader populations are possible.
• The gut microbiome changes are descriptive and correlational; the specific microbial mechanisms driving observed shifts have not been isolated.
• The source article is a secondary synthesis, not the primary peer-reviewed paper; the original study was published in Science (April 2019).
• Spaceflight is a highly unusual environment combining microgravity, radiation, diet change, and psychological stress simultaneously — isolating any single cause of microbiome change is not possible from this design.

Single-subject longitudinal multi-omics study comparing one astronaut before, during, and after 340 days in orbit against a genetically identical Earth-based twin. The design cannot support population-level inference, and the simultaneous exposure to multiple spaceflight stressors prevents attribution of microbiome changes to any single cause. The core findings have been published in a peer-reviewed journal and are broadly consistent with smaller prior spaceflight microbiome observations, though independent large-scale replication has not yet occurred.

References

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2. Francine E. Garrett-Bakelman, Manjula Darshi, Stefan J. Green, et al. (2019). The NASA Twins Study: A multidimensional analysis of a year-long human spaceflight. Science. https://doi.org/10.1126/science.aau8650
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