When astronauts Sunita Williams and Butch Wilmore returned to Earth after spending nine months on the International Space Station due to a technical issue aboard the Boeing Starliner in March 2025, the pair took time to readjust to the Earth’s gravitational pull on their bodies, requiring extensive rehabilitation. If nine months sounds like a long time, consider this: two Russian astronauts — Oleg Kononenko and Nikolai Chub — broke the record for the longest stay on the ISS, spending a total of 374 days in orbit, before returning to Earth in September 2024. Of course, astronauts are rigorously trained to cope with the rigours of space travel. Even so, the prolonged stay in microgravity can significantly alter the human body. Here’s how it does.
In microgravity, without the tug of Earth’s gravity, the muscles and bone mass on our limbs begin to gradually diminish. The muscles in our back, neck, calves and quadriceps, which help us maintain our posture, are the most affected since they no longer need to work hard. This is where atrophy sets in — the shrinking of organs and tissues due to lack of physical activity. According to the Canadian Medical Association Journal, muscle mass is reduced by up to 20% after a two-week space flight. This will further diminish by 30% on longer missions. The bones, similarly, lose density and strength. It is said that astronauts can lose up to 1% to 2% of bone density per month in the hip and spine. This makes them prone to fractures and back pain once back on Earth.
Even when orbiting on ISS, an astronaut still experiences a gravitational pull 90% as strong as it is on Earth. This makes their physical weight drop to near zero as they are in a state of free fall around the Earth. However, their actual mass (amount of matter in the body) stays the same. Over a prolonged period of time, they can lose about 5% of their overall body mass and experience reduction in muscle volume and bone density because their bodies no longer work against gravity, while the blandness of dehydrated food can lead to consuming fewer calories.
About 70% of the astronauts have reported experiencing eyesight deterioration in space, a condition called Spaceflight-Associated Neuro-Ocular Syndrome (SANS). This occurs because microgravity causes blood to accumulate at the back of the head, some of which accumulates around the optic nerve in the back of the eye, leading to decreased sharpness and physical changes such as nerve swelling and hypermetropia (far-sightedness). This usually begins just after two weeks in space, but the risk increases over time. Vision gradually improves for many astronauts after returning to Earth, but some may experience permanent damage.
Astronauts often experience thinner and more sensitive skin during long-duration space missions. Studies have indicated that up to 20% of the epidermis — the outer layer of the skin — can become thinner after several months in space. In addition, the low-humidity environment inside spacecraft can also cause the skin to lose moisture, leading to itching and drying. In microgravity, cuts, scrapes and bruises can take longer to heal because changes in blood circulation, immune function and cell growth affect the body’s normal repair processes.
Astronauts may experience a weakened immune system, making them more susceptible to infections and the reactivation of dormant viruses in the body. White blood cells, especially T cells that help fight infections, do not respond as efficiently in microgravity. This means the body may be slower to recognise and destroy viruses and bacteria. Astronauts, once they return, undergo medical monitoring and, if needed, rehabilitation or treatment. Most immune changes resolve over time.
Studies have shown that the extremity of space can alter the gut microbiome. The unique conditions of outer space require astronauts to rely on food items that are not perishable. These items are usually processed to last longer and lack vitamins and fibre, which are crucial for maintaining healthy gut bacteria. Without these elements, good bacteria such as Lactobacillus and Bifidobacterium might struggle to survive. Further studies have reported that the gut microbiome can also change due to radiation exposure and chronic stress in space.
An astronaut training. | Photo Credit: Flickr
Published - July 20, 2026 10:39 am IST