The Impact of Space on Astronauts’ Eyesight

Spaceflight Associated Neuro-ocular Syndrome (SANS)

The environment of outer space presents unique challenges to human physiology, including effects on vision. The absence of Earth’s gravitational pull in a microgravity setting has been shown to lead to a phenomenon known as Spaceflight Associated Neuro-ocular Syndrome (SANS). This condition has been a subject of concern among researchers, clinicians, and space agencies due to its potential to adversely affect an astronaut’s ability to perform tasks that require keen eyesight.

Mechanisms Behind SANS

SANS primarily occurs due to the redistribution of bodily fluids towards the head in a microgravity environment. On Earth, gravity helps to distribute fluids downward, but in space, fluids tend to move towards the upper body. This shift can result in increased intracranial pressure, which in turn can affect the optic nerve and other ocular structures.

Symptoms and Changes

The symptoms of SANS can manifest in several ways:

  • Optic Disc Edema: Swelling of the optic disc occurs due to increased fluid pressure. This can affect the transmission of visual signals from the eye to the brain.
  • Globe Flattening: The back of the eye can become slightly flattened, altering the shape of the retina and potentially affecting vision.
  • Choroidal Folds: These are wrinkles in the vascular layer of the eye, which can distort vision.
  • Cotton Wool Spots: These are fluffy white lesions on the retina, and while they generally don’t affect vision, they are indicative of nerve fiber layer infarcts.
  • Refractive Error Changes: Changes in the refractive error, or the eye’s ability to focus light properly, have also been noted.

Long-term Consequences

The long-term implications of these changes are not yet fully understood. While some astronauts experience a reversal of symptoms upon return to Earth, others continue to show signs of ocular changes. This raises questions about the long-term viability of extended missions, such as those planned for Mars, where astronauts will be exposed to microgravity for extended periods.

Countermeasure Research

Efforts to counteract or mitigate the effects of SANS are ongoing. Space agencies are researching exercise regimens, specialized suits, and even medications that might help alleviate the increase in intracranial pressure.

Instances of Visual Impairment on ISS Missions

SANS has been increasingly reported in astronauts who have spent extended periods on the International Space Station (ISS). While specific medical information about astronauts is generally confidential, some instances have been publicly discussed to highlight the importance of understanding this phenomenon.

The Case of John Phillips

Astronaut John Phillips’ case is often cited as one of the earlier instances where the phenomenon was noticed. Phillips flew a six-month mission on the ISS in 2005. After returning to Earth, it was noted that his vision had deteriorated from 20/20 to 20/100. Although the specifics of his condition have not been publicly disclosed in detail, his case is frequently cited as an early example that drew attention to the issue.

Scott Kelly and the One-Year Mission

Perhaps one of the most publicized instances was the one-year mission involving astronaut Scott Kelly. While comprehensive data about his eye health have not been publicly released, the one-year mission served as an extended case study for various health impacts of long-duration spaceflight, including SANS. This mission has been instrumental in studying the long-term physiological effects of space travel.

Routine Ophthalmic Examinations

It is now standard practice for astronauts to undergo pre- and post-flight ophthalmic examinations, as well as in-flight assessments, whenever possible. These evaluations have indicated that even during a trip as short as two weeks, vision changes occur for about 1/3rd of American astronauts. When the Mission is longer, such as 4 to 6 months, that figure may double.

Monitoring and Diagnostic Equipment

Given the prevalence of SANS, the ISS is equipped with diagnostic tools to monitor eye health, such as the fundoscope for retina imaging and tonometers for measuring intraocular pressure. These tools allow for in-situ evaluation of astronauts’ eyes, facilitating immediate action if significant changes are observed.


The impact of space on astronauts’ eyesight is an important area of study, particularly as humanity looks towards longer missions beyond Earth’s orbit. SANS, characterized by symptoms such as optic disc edema, globe flattening, and choroidal folds, is primarily thought to be caused by the redistribution of bodily fluids in microgravity. Additionally, instances of visual impairment related to SANS have been reported from ISS missions, leading to more stringent monitoring and research into the phenomenon. Cases like those of Scott Kelly and John Phillips have been particularly illustrative, highlighting the need for further investigation. Current protocols now include routine ophthalmic examinations to understand and ideally mitigate the impacts of long-duration space travel on ocular health.

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