What Microgravity Quietly Does to the Astronaut Body Living on the Space Station

Within hours of reaching the ISS, fluid that gravity normally pools in the lower body migrates upward. Astronauts describe it as a permanent head cold, pressure behind the eyes, a stuffed sensation in the sinuses, a face that looks puffy in photographs while the legs below go thin and almost wasted. This is not a side effect. It is the body doing exactly what it is designed to do, redistributing itself for an environment where "down" no longer exists.
The cardiovascular system, built to pump blood uphill against gravity, suddenly has no hill. It keeps pumping at the same pressure. The result is that the upper body becomes fluid-heavy while the lower body, no longer needing to hold a blood column against gravitational pull, loses both fluid and the muscle that supported it. Astronauts on the ISS lose roughly one litre of blood plasma volume in the first week alone, according to data from NASA's Human Research Program.
Bone is not static tissue. It responds to mechanical load, the daily compression of walking, carrying weight, standing upright. Remove that load entirely and the cells that build bone (osteoblasts) slow down while the cells that break it down (osteoclasts) keep working. The net result: astronauts lose bone density at roughly 1 to 2 percent per month in weight-bearing regions like the hips and lower spine. A six-month mission can produce bone loss equivalent to a decade of normal ageing.
Muscle atrophy follows the same logic. Without resistance, the postural muscles of the back, the calves, and the thighs begin to waste within days. The ISS has a dedicated exercise protocol, two hours daily on resistance machines and a treadmill with a harness system that simulates body weight, specifically because without it, astronauts would return unable to walk. Scott Kelly, who spent 340 days aboard the ISS between 2015 and 2016, described the pain of relearning to bear his own weight on Earth as one of the more surprising parts of the mission.

One of the more troubling findings from long-duration spaceflight is a syndrome NASA calls SANS: Spaceflight Associated Neuro-ocular Syndrome. Fluid pressure in the skull, elevated by the same upward shift that causes the puffy face, pushes against the optic nerve and can flatten the back of the eyeball. Some astronauts return with permanently altered vision. Others recover. The pattern is not yet predictable.
In the NASA twin study, which compared Scott Kelly's physiology to his identical twin and former astronaut Mark Kelly over the course of the 340-day mission, researchers found changes in gene expression, cognitive function, and the microbiome in addition to the vision changes. The study, published in Science in 2019, was the first to use a genetically identical control subject for a long-duration spaceflight. Some changes reversed within months of return. Others, including certain epigenetic markers, had not fully reverted at the time of publication.

The ISS orbits at roughly 400 kilometres above Earth, inside the planet's magnetic field, which deflects a significant portion of cosmic radiation. Even so, astronauts on a six-month mission absorb radiation doses estimated at about 80 millisieverts, roughly 80 times the annual limit recommended for radiation workers in India under the Atomic Energy Regulatory Board's guidelines. Beyond the ISS, on a mission to the Moon or Mars, that shielding disappears entirely.
Radiation damages DNA directly and increases the long-term risk of cancer, cataracts, and central nervous system effects. It also appears to suppress immune function. Studies of ISS crew members have found reactivation of latent herpesviruses, including the Epstein-Barr virus and varicella-zoster, which causes chickenpox, suggesting the immune system is operating under chronic stress even in the absence of obvious illness. Exercise, nutrition, and sleep protocols address the musculoskeletal and cardiovascular effects of microgravity. Radiation has no equivalent countermeasure on current missions.ISRO's Gaganyaan programme is designed to carry Indian astronauts to low Earth orbit for missions of up to three days. At that duration, the acute effects of microgravity, the fluid shift, the early muscle response, will be present, but the cumulative bone loss and SANS changes that accumulate over months will not be the primary concern. The four astronaut-designates currently training include Group Captain Shubhanshu Shukla, who is also assigned to the Axiom Space mission to the ISS as part of a collaboration with NASA, a mission that will expose him to the longer-duration environment that Gaganyaan itself will not.

The physiological data collected from that mission will matter to ISRO's longer-term planning. India has announced ambitions for a space station by 2035 and a crewed lunar mission by 2040. At those timescales, the quiet biological toll that six months on the ISS produces stops being an American or Russian problem and becomes a problem that Indian mission planners will have to solve with Indian bodies, Indian data, and protocols built for missions that have not yet been designed.
Every system in the body that microgravity disrupts, skeletal, muscular, cardiovascular, visual, immune, was shaped by millions of years of life under gravity's constant pull. The ISS does not break the body. It reveals how thoroughly the body was built for one specific condition, and how quickly it begins revising its assumptions the moment that condition is removed.