Astronauts returning from long-duration missions aboard the International Space Station carry back more than scientific data. They come home with swollen optic discs, weakened hip bones, and redistributed bodily fluids that can take months or longer to resolve. These physiological changes, documented across multiple peer-reviewed studies and NASA research programs, represent one of the most pressing barriers to deep-space exploration, including planned missions to the Moon and Mars. As of spring 2026, the accumulated evidence underscores how much remains to be solved before crews can safely venture beyond low Earth orbit for extended periods.
What the evidence shows
The clinical case for spaceflight-related eye damage is built on more than a decade of imaging data. A foundational study published in Ophthalmology documented optic disc edema, globe flattening, choroidal folds, and hyperopic shifts in astronauts after long-duration ISS missions. Optical coherence tomography (OCT) confirmed thickening of the retinal nerve fiber layer in the same group, and several crew members reported blurred near vision, a direct result of the eye reshaping. NASA and the broader aerospace medical community now classify these symptoms under the label Spaceflight-Associated Neuro-ocular Syndrome, or SANS, a term that replaced the earlier designation Visual Impairment and Intracranial Pressure (VIIP).
The leading explanation points to microgravity-driven headward fluid shifts. Without gravity pulling blood and interstitial fluid toward the legs, pressure builds in the head and around the optic nerve. NASA has linked these fluid shifts directly to SANS and continues testing countermeasures aboard the ISS, including lower-body negative pressure devices designed to simulate gravity’s downward pull on circulation. The agency has noted that a significant proportion of long-duration crew members show at least some ocular changes, making SANS one of the most common medical findings in returning astronauts.
A separate case report in Ophthalmology tracked an astronaut who developed optic disc edema after a repeat long-duration mission. Pre-flight, in-flight, and post-flight imaging using fundus photography, OCT, ultrasound, and MRI revealed that the edema recurred and worsened compared to the same astronaut’s first flight. That pattern raises pointed questions about whether crew members who fly multiple long missions face compounding eye damage with each trip.
Bone loss runs on a parallel track. NASA’s Human Research Program monitors astronaut bone density before and after flight using dual-energy X-ray absorptiometry (DXA) and identifies microgravity as a clear bone loss risk factor. A study published in Osteoporosis International found that combining the Advanced Resistive Exercise Device (ARED) with bisphosphonate medication attenuated bone mineral density losses as measured by DXA and quantitative computed tomography (QCT). That study remains one of the most cited countermeasure trials, though researchers have continued refining exercise protocols in the years since.
Even with those interventions, vertebral strength is a concern after landing. Research published in the Journal of Bone and Mineral Research used CT-based analysis and strength modeling on long-duration crew members and found that vertebral strength deficits warranted caution during physical activity in the first days and weeks back on Earth, when astronauts are readjusting to gravity and their skeletons are at their most vulnerable.
NASA’s Twins Study added another layer of confirmation. The integrated investigation, published in Science, compared Scott Kelly’s physiology during his year-long mission against his twin brother Mark on the ground. Among dozens of findings spanning genomics, cognition, and cardiovascular function, the study documented fluid shifts and eye structure changes consistent with SANS. It reinforced a broader conclusion: microgravity triggers multi-system adaptations, from vascular remodeling to shifts in gene expression, that do not all snap back to baseline once an astronaut touches down.
Where the science is still catching up
Despite the strength of the clinical observations, the precise mechanism behind SANS remains unresolved. A systematic review published in npj Microgravity examined competing hypotheses, including fluid shift models, venous outflow obstruction, and intracranial pressure theories, alongside alternative explanations such as individual anatomical variation and changes in cerebrospinal fluid dynamics. Post-flight lumbar puncture data from some affected astronauts have shown elevated opening pressures, but the sample sizes remain small, limiting the conclusions researchers can draw.
One of the most significant gaps is the absence of direct intracranial pressure measurement during spaceflight. A study in the FASEB Journal assessed noninvasive intracranial pressure indicators across pre-flight, in-flight, and post-flight phases using ultrasound-based methods and modeling. The researchers acknowledged that invasive monitoring in orbit has not been feasible, forcing the field to rely on proxy measures. That constraint makes it difficult to determine whether elevated intracranial pressure is a cause of SANS or simply a correlated finding, and whether individual susceptibility can be predicted before launch.
Recovery timelines add another layer of uncertainty. Some vision changes resolve within months of landing, but the case of the astronaut whose optic disc edema worsened after a second mission suggests damage may accumulate. No published study has yet identified a threshold beyond which structural changes to the optic nerve or posterior globe become irreversible. Whether personalized fluid management protocols, informed by biomarker data from investigations like the Twins Study, could prevent SANS recurrence in repeat flyers remains an open question.
The bone picture is similarly unfinished. Resistive exercise and bisphosphonates have demonstrated measurable benefits, but the degree to which vertebral strength fully recovers after long-duration missions has not been definitively established. Fracture risk modeling from the Journal of Bone and Mineral Research points to lingering deficits in some crew members even months after return, raising questions about how long those vulnerabilities persist and whether subsequent missions reset or worsen an astronaut’s skeletal risk.
Compounding the problem is the small size of the study population. As of April 2026, the number of people who have completed multiple missions of six months or longer remains limited, and follow-up periods vary widely. That makes it hard to determine whether repeated microgravity exposure accelerates age-related bone loss or interacts with factors such as sex, baseline bone density, or nutritional status. For SANS, there is similarly limited data on how pre-existing conditions like optic nerve anatomy, refractive error, or cardiovascular health might influence who is most at risk.
What this means for the Moon and Mars
These gaps carry direct consequences for mission planning beyond low Earth orbit. Artemis lunar missions will expose astronauts to partial gravity on the Moon’s surface, but transit legs will still involve microgravity. Mars expeditions would require many months of weightless travel in each direction, bracketing surface operations in roughly one-third Earth gravity. If SANS is driven primarily by fluid shifts that persist despite current countermeasures, crews on multi-year journeys could face progressive vision degradation that impairs their ability to read instruments, pilot spacecraft, or perform precision tasks.
Incomplete bone recovery between missions could also constrain how often an individual astronaut can safely fly and how physically demanding their surface assignments can be. Habitat design, exercise hardware, and drug regimens may all need to sustain skeletal integrity over years rather than the six-month windows currently standard on the ISS. Mission planners will have to weigh the benefits of extended surface stays against the cumulative musculoskeletal and neuro-ocular toll of transit and habitation in reduced gravity.
Research priorities in the near term include in-flight monitoring tools capable of tracking intracranial pressure and optic nerve status in real time, along with imaging and modeling approaches that quantify bone strength rather than relying on density alone. Prospective studies following astronauts across multiple missions, with standardized baselines and long post-flight follow-up, will be essential for distinguishing reversible adaptations from permanent injury. Concepts such as onboard short-arm centrifuges, which could provide intermittent artificial gravity during transit, are under study but have not yet been tested on crewed missions.
A slow-building dataset that shapes every future crew assignment
For now, space agencies are taking a cautious path: refining countermeasures, limiting cumulative exposure for the most affected crew members, and building conservative safety margins into mission timelines. Each ISS rotation and each returning astronaut adds to a dataset that, while still small by terrestrial clinical standards, grows more detailed with every flight. The emerging picture of spaceflight-induced vision and bone changes does not make deep-space exploration impossible. But it makes clear that pushing farther from Earth will demand not just better rockets and life support systems, but a far deeper understanding of what months and years without gravity do to the human body.
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*This article was researched with the help of AI, with human editors creating the final content.
