Astronauts lose 70% of their eyesight in space:What is SANS syndrome, and can it cause permanent eye damage?

Did you know that space travel could change an astronaut’s eyesight? For astronauts spending months in space, vision changes are a real health concern. NASA says around 70% of astronauts aboard the International Space Station experience swelling at the back of their eyes, a condition linked to long-duration spaceflight called Spaceflight-Associated Neuro-Ocular Syndrome (SANS). That is why NASA and other space agencies are studying SANS closely before humans begin spending longer periods around the Moon and eventually travel to Mars. What exactly is SANS? Spaceflight-Associated Neuro-Ocular Syndrome, or SANS, is not simply a case of astronauts needing stronger glasses. It is a collection of changes involving the eyes, optic nerves and brain that can occur after spending a long time in microgravity.
NASA says the condition can include swelling of the optic nerve, folds in the retina, flattening of the back of the eyeball and other structural changes.
The exact reason why SANS develops is still not completely understood. However, scientists believe that the movement of body fluids in microgravity plays an important role. On Earth, gravity constantly pulls blood and other body fluids towards the lower parts of the body. In space, that gravitational pull is largely absent. As a result, fluids that would normally remain lower in the body can shift towards the head and upper body. This can increase pressure around the brain and eyes and may contribute to changes in the optic nerve and the shape of the eyeball. Cases where astronauts experienced SANS
NASA has documented astronauts experiencing changes in their vision during long stays aboard the ISS.
In one case, astronaut Michael Barratt and Canadian astronaut Robert Thirsk developed difficulty reading procedures during a six-month mission and needed stronger glasses. Their symptoms led researchers to investigate the eye changes associated with long-duration spaceflight. In another notable example is NASA astronaut John Phillips, who began his 2005 mission with normal 20/20 vision. After spending about six months aboard the ISS, his vision had dropped to 20/100, showing how significantly long-duration spaceflight can affect eyesight. Some astronauts therefore develop what scientists describe as a farsighted shift, making close-up tasks such as reading more difficult. ESA astronaut Sophie Adenot has also described regular vision checks in orbit because microgravity can cause changes in visual sharpness and farsightedness.

Also read: NASA invites ISRO to join moon base mission: Both India and US to explore greater scientific data sharing under Artemis Accords

Does SANS mean astronauts go blind? This is an important distinction. The statement that astronauts “lose 70% of their eyesight” is not what NASA says. The 70% figure refers to astronauts experiencing swelling at the back of their eyes, not a 70% loss of vision. The effects also vary from astronaut to astronaut. Some experience relatively mild changes, while others can develop more significant changes in their vision and eye structure. NASA says the long-term health consequences are still being investigated. However, researchers are concerned that certain structural changes may persist, particularly after longer missions.
What about Indian astronauts? India’s human-spaceflight programme makes this question particularly relevant. Indian astronaut Shubhanshu Shukla became the first Indian to visit the International Space Station during the Axiom-4 mission in 2025. NASA identified him as an ISRO astronaut and pilot of the mission. However, there is currently no public evidence from ISRO or NASA showing that Shukla developed SANS or suffered permanent vision damage during that mission. This is an important distinction: astronauts can experience normal physiological changes in microgravity without necessarily developing clinically significant SANS.
Why are Mars missions a bigger concern? A mission to Mars is far more challenging than a stay on the ISS because astronauts will spend much longer away from Earth. NASA says the risk of SANS increases with longer space missions, raising concerns about astronauts’ vision during deep-space travel.
With communication delays also increasing, astronauts may need to monitor their eye health themselves rather than rely on doctors on Earth. NASA already checks astronauts’ eyes in space Astronauts on the ISS regularly undergo eye checks using Optical Coherence Tomography (OCT), which captures detailed images of the back of the eye to detect changes. However, current OCT systems are bulky and often require support from specialists on Earth, making them less practical for future deep-space missions. Siloton: An eye scanner small enough for space This is where British health-tech company Siloton comes in. The company is developing a much smaller OCT-based eye-scanning system using photonic-chip technology. Traditional OCT machines use a large collection of optical components such as lenses, mirrors and fibre optics. Siloton says its technology can compress much of this system onto a photonic chip smaller than a coin. The company has already used its technology to capture images of human retinas, and its portable system is being developed for use outside traditional hospitals and specialist eye clinics. Could SANS cause permanent eye damage? This is one of the biggest unanswered questions. NASA is still studying the long-term effects of SANS. While some eye changes may improve after astronauts return to Earth, scientists are concerned about longer missions. Early detection through regular eye scans could help identify changes before they significantly affect an astronaut’s vision or ability to work. Also read: AI traffic may outpace humans by 1,000 times, predicts Musk: Non-human traffic overtook human-generated traffic in May 2026, reports Cloudflare

Does every astronaut get SANS? NASA says astronauts can experience different levels of eye changes. Some have mild effects, while others experience clinically significant changes. Scientists are still trying to understand why.
Factors such as the length of the mission, individual differences, fluid movement, pressure changes, and other physiological factors may all play a role. This is also why collecting more data is important.
Why eye health matters for future space missions Astronauts are no longer being prepared only for short trips into orbit. Space agencies are planning missions around the Moon and are studying what it would take to send humans to Mars. But getting astronauts there safely means solving problems that may not appear during shorter missions. SANS is one of them. An astronaut who develops weaker vision on Earth can simply visit an eye doctor and get new glasses. An astronaut travelling to Mars does not have that luxuryhat is why a tiny eye scanner could become more than just another piece of medical equipment. As space missions become longer, protecting astronauts’ eyesight may become just as important as protecting their bones, muscles and brains.