Returning to Earth after months in space is one of the most physically jarring experiences a human body can go through. Astronauts who spent six months on the International Space Station typically need several weeks to feel normal again, and some changes to their bones, eyes, and cells can linger for a year or more. The body adapts remarkably well to weightlessness, and that’s precisely the problem: it then has to re-adapt to gravity.
What Re-Entry Feels Like
Before astronauts even begin dealing with gravity, they have to survive the ride home. Astronaut Bob Behnken described descending in SpaceX’s Dragon capsule as the vehicle “coming alive” once it hit the atmosphere, with rumbling noise outside and shimmying motions felt through the body. Parachute deployment was more violent. Behnken compared each jolt to “getting hit in the back of the chair with a baseball bat.” Trunk separation was lighter, but the parachute firings delivered a significant impact.
Once the capsule touches down (either splashing into the ocean or landing on the steppe in Kazakhstan), astronauts are lifted out and placed in chairs. Many can’t walk unassisted. Their cardiovascular systems, accustomed to pumping blood without gravity’s pull, struggle to keep blood from pooling in their legs. Standing up can cause dizziness or fainting. NASA astronaut Jasmin Moghbeli, after 197 days in space, described feeling wobbly for the first two days on the ground.
Balance and the Rewired Brain
In microgravity, the inner ear’s balance sensors send signals that no longer match reality. There’s no “down.” Over weeks and months, the brain compensates by turning down the volume on inner-ear input and leaning more heavily on vision and touch to figure out body position. Research published in Cerebral Cortex confirmed this pattern: brain regions handling vision and touch showed increased activity after spaceflight, while the normal vestibular processing pathways were suppressed.
Back on Earth, this rewiring causes real problems. Astronauts experience significant declines in balance and locomotion within the first four days of landing. Walking feels unstable, turning corners is disorienting, and some report the sensation that the room is spinning. These balance difficulties typically require several weeks of readaptation. The brain changes observed on imaging scans persisted for over a month before beginning to fade and returned to baseline values by about three months post-landing.
Bone and Muscle Loss
Bones lose between 1% and 1.5% of their mineral density per month during a four-to-six-month mission, according to NASA. That rate is roughly ten times faster than age-related bone loss on Earth. Weight-bearing bones like the hip and spine take the biggest hit. Muscles atrophy too, particularly in the legs and back, since they no longer work against gravity.
Astronauts exercise vigorously on the ISS (about two hours a day on resistance machines and treadmills) to slow these losses, but they can’t fully prevent them. Recovery of bone density after landing is slow and sometimes incomplete. Some astronauts regain most of their bone mass within a year, but studies have shown that certain bone sites never fully return to pre-flight levels, especially after longer missions. Muscle recovery is faster, with most strength returning within a few months of consistent rehabilitation.
Vision Changes That Can Persist
One of the more concerning effects of spaceflight is a condition called Spaceflight Associated Neuro-ocular Syndrome, or SANS. In microgravity, fluid shifts toward the head and increases pressure around the brain and eyes. This can flatten the back of the eyeball, swell the optic nerve, and shift vision toward farsightedness. In one well-documented study of affected astronauts, five showed optic disc swelling, five had globe flattening, and six experienced a shift in their vision that made close-up objects harder to see.
Unlike balance issues, these eye changes don’t always resolve quickly after landing. Lumbar puncture measurements taken days to weeks after return showed borderline elevated pressure readings, suggesting the fluid dynamics behind SANS don’t snap back immediately. Some astronauts have reported lasting changes in their vision prescription.
Immune Shifts and Virus Reactivation
Spaceflight suppresses and dysregulates the immune system in ways that persist throughout a six-month mission. One notable consequence: latent viruses that most people carry harmlessly, like the chickenpox virus (varicella-zoster), can reactivate. Viral DNA has been detected in astronauts’ saliva during and immediately after spaceflight, even though none was present before launch. Most astronauts don’t develop symptoms, but the shedding of active virus after landing poses a theoretical risk to people around them, particularly newborns or anyone with a weakened immune system.
Changes at the Cellular Level
NASA’s landmark Twins Study, which compared astronaut Scott Kelly to his identical twin Mark during a year-long mission, revealed changes that go all the way down to the chromosome. Scott’s telomeres (the protective caps on the ends of chromosomes, often linked to aging) grew unexpectedly longer during spaceflight, then shortened rapidly after his return. Within six months on Earth, his average telomere length returned close to preflight levels, but he was left with an increased number of abnormally short telomeres, which is generally associated with accelerated aging and higher disease risk.
Gene expression shifted as well. Most changes returned to normal within six months, but a subset of genes involved in immune function, DNA repair, and bone formation remained disrupted beyond that window. The long-term health implications of these persistent genetic shifts are still being tracked across NASA’s astronaut corps.
The 45-Day Reconditioning Program
Rehabilitation starts the day astronauts land. NASA’s post-flight reconditioning program runs two hours a day, seven days a week, for 45 days. Early sessions focus on the basics: walking without falling, rebuilding cardiovascular endurance, and restoring the body’s sense of where its limbs are in space (a sense called proprioception that microgravity dulls). As the weeks progress, sessions advance to strength training and more complex movements, all tailored to each astronaut’s test results and medical condition.
Despite the structured timeline, individual recovery varies widely. Frank Rubio, who completed a record 371-day U.S. spaceflight, said he felt mostly recovered within a couple of weeks thanks to consistent workouts on the station. Loral O’Hara reported feeling almost completely normal just one week after returning from 204 days in space. Others take longer. The general pattern is that longer missions mean longer recovery, but fitness level going into the mission and exercise compliance during it make a significant difference.
Cognitive and Psychological Readjustment
Motor-related cognitive functions also take a hit. Dual-tasking (doing two things at once), motion perception, and manual dexterity all decline after six months on the ISS. The encouraging finding is that these cognitive deficits resolved within about four days of landing in the studies that tracked them. The brain bounces back from motor-cognitive effects much faster than from vestibular disruption.
The psychological transition can be subtler but equally real. Astronauts go from an extraordinary, purpose-driven environment where every moment is scheduled and meaningful to the mundane rhythms of life on Earth. Some describe difficulty reconnecting with daily routines. The contrast between the profound experience of seeing Earth from space and the ordinary demands of grocery shopping and paying bills can be disorienting in its own way. Readjusting to family life after months of separation adds another layer, as relationships have continued evolving in the astronaut’s absence.
Sensory experiences on the station can also leave a mark. Some astronauts have reported illusions or mild hallucinations during missions, likely related to prolonged isolation and limited sensory input. These typically stop after return, but they underscore how deeply the space environment reshapes perception.