Target Pillar: Astronauts, Training, and Life in Orbit
Last Updated: August 19, 2026 Author: Sarah Mitchell
Introduction
In one of the most detailed human physiology studies ever conducted, NASA had a rare scientific advantage: identical twins, one of whom would spend nearly a year on the ISS while the other stayed on Earth as a genetically matched control. Astronauts Scott and Mark Kelly’s participation in NASA’s Twins Study, published in the peer-reviewed journal Science in 2019, produced what remains one of the most comprehensive maps of how long-duration spaceflight actually changes the human body — down to the molecular level.[^1]
Combined with newer research through 2024 on sex-based differences in astronaut recovery, the picture that’s emerged is more specific and more reassuring than early space-medicine speculation once suggested — while still identifying real risks that matter enormously for future Mars missions.
Fluid Shift: The Root Cause of Several Other Problems
Many of spaceflight’s physiological effects trace back to one underlying mechanism: without gravity pulling bodily fluids downward, blood and other fluids redistribute toward the head and upper body.[^2] This isn’t a minor cosmetic effect — a NASA-funded review found this persistent headward fluid shift is the likely trigger behind one of spaceflight’s more serious risks: changes to eye structure and pressure that can produce lasting vision problems.[^3]
SANS: The Vision Problem Nobody Predicted Early On
Spaceflight-associated neuro-ocular syndrome, or SANS, describes a cluster of eye and vision changes that emerge during long-duration missions — and it’s one of NASA’s most actively studied current spaceflight risks.[^3] The Twins Study documented measurable ocular changes in Scott Kelly, including a flattening of the eyeball itself and a thickening of the retinal nerve.[^4] NASA’s ongoing SANS research program has connected these structural changes to the headward fluid shift described above, theorizing that sustained changes in pressure around the eyes and brain during long-duration microgravity exposure is a likely driver.[^3]
This isn’t a uniform risk across all astronauts either. More recent research led by geneticist Christopher Mason and others, examining a broader set of astronauts beyond just the Kelly twins, found a notable sex-based difference: women’s vision appears to be less affected by microgravity than men’s, a finding researchers are still working to fully explain.[^5]
The Immune System: “On High Alert”
Spaceflight measurably stresses the immune system, though the practical consequences have proven more nuanced than early researchers expected. The Twins Study found small but real changes in Scott Kelly’s gene expression, particularly in genes governing immune function, alongside documented reactivation of normally dormant viruses during his mission.[^4] Separate NASA research has confirmed this isn’t unique to Scott specifically — viral reactivation during spaceflight is a recognized, recurring pattern researchers actively monitor across missions.[^6]
Geneticist Christopher Mason, who worked on expanded post-Twins-Study research, has described the astronaut immune system in spaceflight as being “on high alert, aggravated” — with RNA sequencing from astronaut blood samples showing that spaceflight stress affects the transcription of immune system genes in ways that could plausibly reduce the body’s ability to defend against viral infections.[^5] Yet despite this cellular-level stress signature, Scott Kelly’s actual immune response to a flu vaccine administered during his mission remained normal throughout — a reassuring practical outcome even amid clear molecular evidence of immune system strain.[^7]
NASA’s broader research program notes that despite documented immune system changes, crews don’t tend to actually get sick more often in space than expected — a genuinely encouraging real-world finding that sits somewhat apart from the more concerning molecular data.[^8]
Telomeres and DNA: The Finding That Surprised Researchers
One of the Twins Study’s more unexpected findings involved telomeres — the protective caps on the ends of chromosomes that typically shorten as cells divide and are closely associated with cellular aging.[^1] Rather than shortening, as researchers might have predicted given the stress of spaceflight, Scott Kelly’s telomeres actually lengthened during his mission — a genuinely surprising result that additional research since has confirmed occurs in other astronauts as well, not just as an anomaly specific to Scott.[^9] The reassuring part of this finding: telomere length, along with global gene expression changes and Scott’s gut microbiome composition, returned to near-preflight levels within about six months of landing back on Earth.[^1]
The study also documented genome instability and DNA methylation changes in immune and oxidative-stress-related cellular pathways during Kelly’s mission.[^1] Crucially, though, researchers emphasized that Scott remained in low Earth orbit throughout his mission, where Earth’s Van Allen belts provide meaningful shielding from the most extreme space radiation.[^7] Astronauts traveling to Mars would be expected to experience roughly eight times the radiation dose Scott received on the ISS — a substantially higher exposure with correspondingly higher DNA damage risk that current low-Earth-orbit research can only partially predict.[^10]
Cardiovascular Changes: A Real, Documented Risk
The Twins Study identified genuine cardiovascular changes in Scott Kelly during his mission, including carotid artery distension and increased intima-media thickness — a marker associated with arterial stiffening.[^1] This aligns with broader spaceflight research documenting cases of atherosclerosis, the buildup of fatty deposits that can clog arteries, in astronauts following long-duration missions.[^2] Cardiovascular deconditioning is a well-established risk of extended time in microgravity, tied partly to the heart no longer having to work against gravity’s normal resistance the way it does on Earth, and is part of why ISS crews maintain such a demanding daily exercise regimen.
Cognitive Effects and the Encouraging Overall Picture
The Twins Study documented some cognitive decline in the postflight period, alongside the physiological changes described above.[^1] Encouragingly, though, Scott’s spatial orientation and motor accuracy — both measurably disrupted by his time in space — returned fully to normal after his return, as did his body mass.[^7] NASA researchers involved in the study were notably direct in their overall framing: the results demonstrated the human body’s real resilience and adaptability in the face of the many changes spaceflight induces, and researchers concluded the findings pointed to no major medical obstacles standing in the way of long-term space missions.[^7]
That said, NASA scientists were equally careful to flag the study’s real limitations. A single pair of twins represents too small a sample to draw firm, generalizable conclusions on its own, and Scott’s mission stayed within low Earth orbit — meaning the radiation exposure, isolation duration, and specific stressors of an actual Mars mission remain, in important respects, still unmeasured territory that current research can only estimate rather than directly confirm.[^7]
Frequently Asked Questions
Do all these physical changes from spaceflight reverse after returning to Earth?
Most of what NASA’s Twins Study documented did reverse — telomere length, gene expression, gut microbiome composition, immune response, spatial orientation, and body mass all returned to near-normal within about six months of landing. Some effects, however, particularly certain vision changes associated with SANS, have been documented as more persistent in some astronauts.
Is spaceflight equally risky for men and women?
Not identically — more recent research beyond the original Twins Study has found real sex-based differences, including that women’s vision appears less affected by microgravity-related SANS than men’s, and that certain immune cell types (monocytes) return to normal faster in women after landing. Researchers are still working to fully explain the underlying mechanisms behind these differences.
Why did Scott Kelly’s telomeres get longer instead of shorter in space?
This was a genuinely unexpected result that ran counter to what researchers might have predicted given the physical stress of spaceflight. Subsequent research on other astronauts has confirmed this lengthening isn’t unique to Kelly, though the underlying biological mechanism remains an active area of study rather than something fully explained.
Would a Mars mission be riskier than what Scott Kelly experienced on the ISS?
Yes, substantially — Scott’s ISS mission stayed within low Earth orbit, partially shielded from extreme space radiation by Earth’s Van Allen belts. Astronauts on a Mars mission would be expected to receive roughly eight times Scott’s radiation dose, with longer mission duration and no possibility of a quick return to Earth if a serious medical issue emerged.
Does time in space actually weaken astronauts’ immune systems in a way that makes them sick more often?
The evidence is more nuanced than a simple yes — cellular and genetic markers do show real immune system stress and activation during spaceflight, including viral reactivation, but NASA’s broader research notes astronauts don’t actually tend to get sick more often than expected despite this molecular-level evidence of immune strain.
Sources
- Science (AAAS) — The NASA Twins Study: A Multidimensional Analysis of a Year-Long Human Spaceflight
- ScienceABC — How Does Space Travel Affect the Human Body?
- NASA Human Research Program — Risk of Spaceflight Associated Neuro-ocular Syndrome (SANS)
- CBS8 — NASA’s Twin Study Sheds Light on How Long-Term Spaceflight Affects the Human Body
- Science/AAAS — Astronauts Face Health Risks, Even on Short Trips in Space
- Frontiers in Microbiology — Herpes Virus Reactivation in Astronauts During Spaceflight and Its Application on Earth
- Space.com — Landmark NASA Twins Study Reveals Space Travel’s Effects on the Human Body
- NASA — The Human Body in Space
- CBS News — Astronaut Twins Study Finds No Major Medical Obstacles to Long-Term Space Missions
- Space.com — Landmark NASA Twins Study (Mars radiation comparison)
Note on methodology: findings above are drawn primarily from NASA’s peer-reviewed Twins Study (Science, 2019) and subsequent published research through 2024, along with NASA’s own published human research program materials as of mid-2026. Long-duration and deep-space spaceflight health research remains active and ongoing — consult NASA’s Human Research Program directly for the latest findings.