There is something unbearable about the body as evidence. When it changes — not through accident, not through illness in the ordinary sense, but through the systematic, documented, cellular-level effects of an environment it was never designed to occupy — the change forces a confrontation that philosophy usually allows us to defer. We are used to saying that the self is continuous, that the person who wakes this morning is the same person who went to sleep, that the I who remembers is the I who acted. The NASA Twin Study, published in Science in 2019, does not destroy that continuity. But it makes it strange. It puts the question of identity under a pressure that Dostoevsky would have understood immediately: not the external pressure of society or poverty or cruelty, but the internal pressure of a self that has been changed by forces beyond its choosing and must reckon with what it now is.
Scott Kelly spent 340 days aboard the International Space Station from 2015 to 2016. His identical twin brother Mark Kelly — same genetic starting point, same developmental history, same decades of astronaut training — remained on Earth. When Scott returned, the study that followed produced a dataset of unusual intimacy: two men, genetically identical at birth, measured simultaneously across hundreds of biomarkers, separated by nothing but 408 kilometers of altitude and the difference between orbital velocity and standing still.
The body had confessed.
What the Cells Did
The NASA Twin Study, led by a consortium of researchers and published in Science in April 2019, examined changes across multiple biological systems simultaneously. The scope was intentionally broad: cognitive performance, gene expression, telomere dynamics, microbiome composition, immune response, cardiovascular and ophthalmological changes, epigenetic modifications. The intention was to characterize, as comprehensively as existing measurement technology permitted, what a year in space does to a human body.
The telomere findings drew immediate attention. Telomeres are protective caps on the ends of chromosomes, analogous to the plastic tips on shoelaces; they shorten with each cell division, and their length is a rough proxy for cellular aging. The expectation was that space’s stressors — radiation, fluid shifts, sleep disruption, confinement — would accelerate telomere shortening, aging Scott faster than Mark. The opposite happened. Scott’s telomeres lengthened during spaceflight.
This result surprised the researchers. The leading hypothesis is that the extreme exercise regimen required to prevent muscle and bone loss in microgravity — two hours of vigorous exercise daily — stimulated telomerase activity, the enzyme responsible for maintaining telomere length. But the lengthening did not persist: after Scott’s return to Earth, his telomeres shortened rapidly, and within months were actually shorter than before the mission. He had aged, in telomere terms, past his pre-flight baseline. The space-induced lengthening may have masked a net loss.
The gene expression data revealed another layer of complexity. Approximately 93% of Scott’s genes that had changed expression during spaceflight returned to preflight levels after his return. But approximately 7% did not. These are sometimes called “space genes” in the popular press — a phrase imprecise enough to require unpacking. The study found that genes involved in DNA repair, the immune response, bone formation, hypoxia response, and cognitive function showed altered expression patterns that persisted for six months after return. Whether these alterations are permanent, or merely slow to reverse, could not be determined within the study’s timeframe.
The Microbiome of a Stranger
The gut microbiome change is in some respects the most philosophically vertiginous finding of the Twin Study.
The human microbiome — the community of bacteria, archaea, fungi, and viruses inhabiting the gut — is not, in ordinary experience, part of what we mean when we say “I.” We do not think of the bacteria in our intestines as constituents of the self. They are, in the conventional view, passengers: useful, even necessary, but separate from the host in some fundamental way. The study of the microbiome over the past two decades has complicated this picture considerably. The gut microbiome affects mood, cognition, immune function, and metabolism through pathways that are increasingly understood to be bidirectional — the gut affecting the brain as significantly as the brain affects the gut. The microbiome is not a passenger. It is, in a significant functional sense, part of how the person works.
Scott Kelly’s gut microbiome changed substantially during spaceflight. The ratio of the two dominant bacterial phyla in the human gut — Firmicutes and Bacteroidetes — shifted significantly during the mission. The causes are multiple and difficult to disentangle: altered diet aboard the ISS, the microgravity environment’s effect on gut motility and fluid dynamics, changes in immune surveillance, the closed microbial ecology of the station itself. When Scott returned to Earth, his microbiome shifted back toward its preflight composition. But not entirely. Not immediately.
For some period after his return, Scott Kelly carried, in his intestines, a microbial community that was neither what it had been before the mission nor what it would eventually become after sustained ground living. It was a transitional microbiome. A self in process. A person returning to Earth whose gut was still, in part, elsewhere.
The underground man of Dostoevsky inhabits a consciousness in permanent tension with itself — aware of its own contradictions, unable to synthesize them into a stable identity. Scott Kelly’s return presented a different version of this: not a psychological tension but a biological one, a body whose systems were disagreeing with each other about where they were and what they required, recalibrating on different timescales in ways that produced, at the phenomenological level, the experience of a self that had been changed and was in the process of negotiating what it now was.
Cognitive Changes and the Performance Cliff
Scott Kelly’s cognitive performance, measured before, during, and after the mission, showed changes that he discusses in his memoir Endurance with characteristic directness. Speed and accuracy on certain cognitive tasks declined during the mission and remained below preflight baseline for some time after return. Spatial awareness tasks, abstract problem-solving, and cognitive flexibility assessments all showed slight but measurable decrements.
The cause is likely multifactorial: the radiation environment of low Earth orbit, which is insufficiently shielded by the station’s hull, produces chronic low-level damage to neural tissue. The fluid shift caused by microgravity — bodily fluids migrating toward the head in the absence of gravity’s downward pull — alters intracranial pressure in ways that may affect neural function. The stress of prolonged confinement, disrupted circadian rhythms from sixteen sunrises daily, and the sustained cognitive load of operating in a dangerous environment all likely contribute. The specific mechanism behind the cognitive changes observed in Scott Kelly has not been isolated.
What is notable is the asymmetry. Mark Kelly, on Earth, showed no comparable cognitive changes. The twin who left came back with a brain that performed differently than the one he carried into orbit. Not catastrophically — Scott Kelly remained a highly functional, highly capable individual throughout. But differently. Measurably differently. In ways that his identical twin, living the same year on Earth, did not replicate.
The philosophical question that this raises is not new, but space makes it unusually sharp: if the brain that processes experience has been altered, what has happened to the experiences processed? Not to the memories — those are relatively intact. But to the person who is currently doing the processing, now doing it with a slightly different instrument than before? The question of self is not merely about memory and continuity. It is about the organ of consciousness and what happens when that organ has been, without the person’s consent and in ways beyond the person’s awareness, modified.
The Doubling
In The Double, Dostoevsky’s novella of 1846, the protagonist Yakov Golyadkin encounters an exact copy of himself — same face, same clothes, same position, same history — and watches the double succeed where he cannot, navigate society with a facility he lacks, insert itself into his relationships and his career with a smoothness that destroys him. The horror is not that the double is different. The horror is that the double is the same and yet manages to do what the original cannot. It is the self, performing better.
The NASA Twin Study is not exactly The Double. Mark Kelly did not replace Scott Kelly. Neither is the successful version of the other. But the comparison between their biological states after the year is a form of doubling that is, at the level of philosophical weight, close to what Dostoevsky was reaching for: two identical men, measuring each other across a biological divide they did not choose and cannot fully interpret. The measurable differences between them — Scott’s altered gene expression, his transient telomere changes, his shifted microbiome, his cognitive performance data — are not Scott’s choices. They are the record of what the universe did to him when he went to a place the body was not designed for.
Scott Kelly chose to go to space. He did not choose what space would do to him. This is the underground man’s condition inverted: where the underground man cannot act, cannot commit, cannot enter the world cleanly — Scott Kelly committed entirely, entered the most extreme environment available to a human being, and returned to find that the commitment had changed the instrument of his identity in ways he had not authorized. He had chosen to go. The cells had gone somewhere else without asking.
What Endurance Actually Means
Scott Kelly’s memoir is titled Endurance, and the word is usually read in its heroic sense: the capacity to persist through difficulty, to absorb punishment without breaking, to remain recognizably oneself across conditions that would unmake a lesser person. The reading is not wrong. Kelly is, by any measure, a person of extraordinary psychological and physical endurance.
But the Twin Study suggests a second meaning of the word, darker and more interesting: endurance not as resilience but as transformation. To endure space is not to remain unchanged by it. The body that endures a year in microgravity is not the same body at the end of the year. The genome has expressed differently. The gut has shifted. The telomeres have moved. The brain has processed sixteen thousand sunrises and the resulting data shows that it has been, in some small but measurable ways, reconfigured.
Endurance, in this reading, is not the preservation of the self against change. It is the continuation of the self through change — the maintenance of a narrative thread, a sense of ongoing identity, even as the substrate of that identity has been rewritten by forces that care nothing for the narrative. This is perhaps the truest endurance available to a person: not to resist the world’s reshaping of the body, because it cannot be resisted, but to sustain the sense of self across the reshaping, to continue to function and to mean and to choose in a body that is not quite the one you started with.
Dostoevsky’s characters are always being broken by the world. The great ones do not break; they transform, and they live with what they have become. Scott Kelly returned to a brother who had not changed. He looked at that brother and saw, with perfect clarity, what space had cost him — because the cost had a face, and the face was his own. He kept going.
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- Descartes’ Error by Antonio Damasio — The Body Was Never Separate from the Mind
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Sources
- Garrett-Bakelman, Francine E. et al. “The NASA Twins Study: A multidimensional analysis of a year-long human spaceflight.” Science 364, no. 6436 (2019): eaau8650. https://www.science.org/doi/10.1126/science.aau8650
- Kelly, Scott. Endurance: A Year in Space, A Lifetime of Discovery. Alfred A. Knopf, 2017.
- Terada, M. et al. “Characterization of changes in the gut microbiome in long-duration human spaceflight.” npj Biofilms and Microbiomes 7, 18 (2021).
- NASA Human Research Program. “Human Research Program Highlights.” https://www.nasa.gov/hrp/
- Stahn, A.C. et al. “Cognitive performance in deep space exploration.” npj Microgravity 5, 13 (2019).
- Ade, N. et al. “Telomere dynamics in the context of human spaceflight.” Summary from NASA Twins Study consortium data.
