Space changes nearly every system in the human body, from bones and muscles to eyesight, immune function, and even the bacteria living in your gut. Most of these changes begin within days of reaching orbit and intensify the longer an astronaut stays. Some reverse quickly after landing, while others linger for months or years. Here’s what actually happens, system by system.
Bone and Muscle Loss
On Earth, gravity constantly loads your skeleton and muscles just by keeping you upright. Remove that force, and the body starts shedding tissue it no longer thinks it needs. Bones lose between 1% and 1.5% of their density each month during a typical four-to-six-month mission, with weight-bearing bones like the spine and hips hit hardest. For comparison, a postmenopausal woman on Earth loses roughly 1% to 2% per year. In space, that same loss happens in a single month.
Muscles shrink even faster. After six months on the International Space Station, astronauts’ calf muscles shrank by about 13% on average, with the deeper soleus muscle losing around 15% of its volume. This happened despite a demanding exercise schedule: roughly five hours per week of aerobic work and resistance training three to six days a week. Without that exercise, losses would be far worse, but current routines still can’t fully prevent atrophy. Rebuilding bone density after landing can take years, and some astronauts never fully recover the mineral content they lost.
Heart and Circulation
The moment you enter microgravity, blood and other fluids that normally pool in your legs shift upward toward your chest and head. Astronauts often notice this immediately: their faces puff up, their legs thin out, and their sinuses feel congested. The heart, no longer pumping against gravity to push blood upward, begins to change shape. It rounds out from its usual elongated oval into something closer to a sphere, more like an air-filled balloon than a water-filled one.
Blood pressure also drops during flight. Average 24-hour systolic readings fall from around 120 mmHg on the ground to about 106 mmHg in space. The good news is that current countermeasures, including exercise during the mission and fluid loading before landing, work well enough that astronauts in recent studies did not experience fainting or dangerous drops in blood pressure during normal activities in the first 24 hours back on Earth. Blood pressure typically returns to preflight levels right after landing. Still, the long-term consequences of months or years of altered circulation remain an open question for future Mars-length missions.
Vision Changes
One of the more surprising discoveries in space medicine is how profoundly microgravity affects the eyes. About 70% of astronauts on the ISS experience some degree of swelling at the back of the eye. This condition, known as spaceflight-associated neuro-ocular syndrome (SANS), can flatten the eyeball, shift the optic nerve, and change an astronaut’s vision prescription. The upward fluid shift that puffs up the face also increases pressure around the brain and optic nerve, and that sustained pressure appears to be the primary driver. Some of these eye changes persist after astronauts return to Earth, making SANS one of NASA’s top concerns for long-duration missions.
Radiation Exposure
Earth’s atmosphere and magnetic field block most cosmic radiation. In low Earth orbit, astronauts lose much of that protection. During a six-month ISS stay, a crew member absorbs roughly 80 to 160 millisieverts of radiation, depending on the sun’s activity cycle. To put that in perspective, the average person on Earth receives about 2 to 3 millisieverts per year from natural background radiation. Six months in space delivers decades’ worth of ground-level exposure.
A round trip to Mars would be far more extreme. Outside the protection of Earth’s magnetic field, astronauts would be exposed to continuous radiation for approximately three years. Current models estimate that this exposure would give astronauts more than a 3% risk of eventually dying from radiation-induced cancer. That number may sound modest, but it represents a significant added risk layered on top of all the other hazards of a Mars mission, and it drives ongoing research into better shielding and faster transit times.
Immune Suppression and Virus Reactivation
Spaceflight weakens immune function in ways that have real, measurable consequences. Key immune cells called T-lymphocytes become less effective in orbit, and this suppression allows viruses that have been dormant in the body for years to wake up. Most adults carry several latent herpesviruses picked up during childhood, and spaceflight reliably triggers their reactivation.
Epstein-Barr virus shedding increases roughly tenfold during flight. Cytomegalovirus and varicella-zoster virus (the virus behind chickenpox and shingles), which healthy adults on Earth rarely shed at all, show up in astronauts’ saliva and urine samples during missions. Herpes simplex virus can reactivate with visible cold sores. Most of this viral shedding happens without symptoms, but it signals that the immune system is struggling to keep old infections in check, a concern that grows more serious the longer a mission lasts.
Gut Bacteria Shifts
The trillions of bacteria in your gut play a major role in digestion, immunity, and inflammation. Spaceflight disrupts this ecosystem significantly. Overall microbial diversity drops, likely due to a combination of stress, dietary changes (astronaut food is low in live microbes), altered physical activity, and the space environment itself.
Specific changes paint a concerning picture. Beneficial bacteria like Lactobacillus and Bifidobacterium decline, and in a year-long study of Soviet astronauts aboard the Mir station, both genera disappeared entirely. Meanwhile, bacteria associated with inflammation tend to increase. The gut microbiomes of astronauts on the same mission also start to look more alike over time, converging as shared environmental pressures override individual differences. These shifts can contribute to nutrient deficiencies, metabolic imbalances, and further immune dysregulation. On top of that, certain harmful microbes grow more aggressively: the common foodborne pathogen Salmonella, for example, shows enhanced growth under simulated microgravity and becomes lethal to mice at lower doses than it would on Earth.
Changes at the Genetic Level
NASA’s famous Twins Study, which compared astronaut Scott Kelly to his identical twin Mark during Scott’s year in space, revealed changes that go all the way down to DNA. One of the most striking findings involved telomeres, the protective caps on the ends of chromosomes that normally shorten as you age. Scott’s telomeres unexpectedly lengthened during flight, then rapidly shortened again after landing. The meaning of this is still being studied, but it suggests spaceflight triggers unusual dynamics in how cells age and divide.
Beyond telomeres, the study found shifts in gene expression, with hundreds of genes becoming more or less active during the mission. Most of these returned to normal after Scott came home, but a small subset of changes persisted at the end of the study period, along with DNA damage, thickening of the carotid artery walls, and some cognitive shifts.
Sleep and Circadian Disruption
Your internal clock relies heavily on the natural cycle of daylight and darkness. On the ISS, that cycle is obliterated. The station orbits Earth every 90 minutes, meaning astronauts see 16 sunrises and 16 sunsets every 24 hours. This rapid flickering between light and dark throws off circadian rhythm, the biological timer that controls when you feel alert and when you feel sleepy.
To counteract this, NASA has been replacing the station’s old fluorescent lighting with tunable LED panels that let crew members adjust both brightness and color spectrum throughout the day. Bluer, brighter light promotes alertness during work hours, while warmer, dimmer light in the evening helps prepare the brain for sleep. Even with these tools, sleep quality in space remains a persistent challenge, compounded by noise from station equipment, the disorientation of sleeping while floating, and the psychological weight of living in a confined, high-stakes environment.
What Recovers and What Doesn’t
Many of the body’s changes reverse after landing, but the timeline varies dramatically. Blood pressure normalizes almost immediately. Cardiovascular fitness and fluid distribution return to baseline within weeks. Muscle mass can be rebuilt over several months with targeted rehabilitation, though regaining full strength and coordination takes dedicated effort.
Bone density is the slowest to recover. Some astronauts need one to three years to rebuild what they lost, and studies suggest that not all of them regain their preflight bone mineral content. Vision changes from SANS can persist indefinitely. The small subset of genetic and epigenetic alterations identified in the Twins Study also remained changed months after return, raising questions about whether some effects of spaceflight leave a permanent mark on the body.