Spending months aboard the International Space Station (ISS) offers a unique window into how the human body adapts to life beyond Earth. Orbiting roughly 250 miles above the planet, astronauts experience a microgravity environment that challenges their physical health in ways not seen on the ground. While the ISS provides a controlled setting with advanced equipment, the prolonged absence of gravity, combined with other factors like radiation exposure, takes a measurable toll on the body. This article examines the medical side effects of extended stays in orbit, breaking them down into key areas of impact, and includes a look at the recent return of two astronauts who faced an unexpectedly long mission.
Bone Density and Muscle Loss
One of the most noticeable effects of long-term time in space is the loss of bone density and muscle mass. On Earth, gravity acts as a constant force that keeps bones strong and muscles engaged. In microgravity, however, the body no longer needs to work against this force. As a result, bones begin to lose minerals, particularly calcium, at a rate of about 1 to 2 percent per month. This process mirrors osteoporosis, a condition common in older adults, but it happens much faster in space. The hips, spine, and legs—areas that typically bear weight—are hit hardest.
Muscles face a similar challenge. Without regular resistance, they weaken and shrink. Astronauts combat this by exercising for about two hours a day using specialized equipment like treadmills and resistance machines. Even with these efforts, some muscle loss still occurs, especially in the legs and back. When astronauts return to Earth, they often struggle with standing or walking until their strength rebuilds, a process that can take weeks or months depending on the length of their mission.
Fluid Shifts and Cardiovascular Changes
Another striking change happens to the body’s fluids. On Earth, gravity pulls blood and other fluids downward, pooling them in the lower half of the body. In space, this pull disappears, and fluids shift upward toward the head and chest. Astronauts often notice puffy faces and thinner legs as a result, a phenomenon sometimes called “moon face.” This shift puts extra pressure on the heart and blood vessels, forcing the cardiovascular system to adjust.
Over time, the heart doesn’t need to pump as hard in microgravity, which can lead to a slight reduction in its size and efficiency. Blood volume also drops because the body senses an excess of fluid in the upper regions and signals the kidneys to remove it. These adaptations aren’t necessarily harmful in space, but they pose challenges upon returning to Earth. Back in normal gravity, astronauts may experience dizziness or fainting as their bodies readjust to pumping blood against gravity once more.
Vision Impairment
A less obvious but significant issue involves the eyes. Some astronauts report blurry vision during or after long missions, a condition tied to changes in the skull and brain caused by fluid shifts. The increased pressure from fluids moving upward can flatten the back of the eyeball, alter the optic nerve, or even cause swelling in surrounding tissues. Researchers have labeled this Spaceflight Associated Neuro-ocular Syndrome, or SANS. Not every astronaut is affected, but for those who are, the changes can linger after they return, raising questions about the long-term health of their vision.
Radiation Exposure
Beyond Earth’s protective atmosphere and magnetic field, the ISS sits in a region exposed to higher levels of cosmic and solar radiation. This radiation, made up of charged particles from the sun and distant stars, can penetrate the body and damage cells. Over months, this exposure adds up, increasing the risk of developing cancer later in life. It can also harm the immune system, making astronauts more susceptible to illness during their mission. Shielding on the ISS helps reduce this exposure, but it can’t block it entirely. The longer someone stays in orbit, the greater the cumulative dose they receive.
Immune System and Sleep Disruption
The immune system doesn’t function as well in space, partly due to radiation and partly because of the stress of living in an enclosed, artificial environment. Studies show that certain immune cells become less active, leaving astronauts more vulnerable to infections. While the ISS is kept clean and sterile, even minor bugs—like a cold—could spread quickly among the crew. This weakened immunity often persists for a time after landing, complicating recovery.
Sleep also takes a hit. The ISS orbits Earth every 90 minutes, creating 16 sunrises and sunsets each day. This rapid cycle throws off the body’s natural clock, making it hard to maintain a consistent sleep schedule. Astronauts use strict routines and artificial lighting to cope, but many still report poor sleep quality. Lack of rest can worsen mood, slow reaction times, and strain overall health, compounding other physical challenges.
Digestive and Metabolic Adjustments
Eating and digestion don’t work quite the same in microgravity. Food floats, and so does waste, requiring careful handling. The stomach and intestines adapt, but some astronauts experience slower digestion or mild discomfort. Metabolism shifts too—without gravity to help regulate it, the body burns fewer calories at rest. Exercise helps offset this, but maintaining a healthy weight and nutrient balance remains a constant task. Deficiencies in vitamins like D, which the body makes less of without sunlight, can also creep in over time.
Psychological Factors
Though not a direct physical side effect, the mental strain of long missions influences the body. Isolation, confinement, and the constant hum of machinery can lead to stress or fatigue. These feelings sometimes trigger physical symptoms like headaches or a racing heart. The lack of fresh air and natural scenery adds another layer of difficulty. Crew members train extensively to handle these pressures, but the body still registers the toll, especially during missions lasting six months or more.
The Return of Two Stranded Astronauts
In a striking example of the challenges of long-term spaceflight, two NASA astronauts, Butch Wilmore and Suni Williams, recently completed an extended stay on the ISS that stretched far beyond their original plan. Launched in June 2024 aboard Boeing’s Starliner spacecraft for what was meant to be an eight-day test flight, the pair ended up spending over nine months in orbit—286 days—due to technical issues with their capsule. Problems with the Starliner’s thrusters and helium leaks made it unsafe for their return, prompting NASA to send the craft back to Earth empty in September 2024. Wilmore and Williams stayed on the ISS, joining the Crew-9 mission, until a SpaceX Dragon capsule could bring them home.
Their return journey began early on March 18, 2025, when the Dragon spacecraft undocked from the ISS at 1:05 a.m. EDT. Alongside fellow NASA astronaut Nick Hague and Roscosmos cosmonaut Aleksandr Gorbunov, they embarked on a 17-hour trip back to Earth, set to splash down off the coast of Florida around 5:57 p.m. EDT that same day, weather permitting. This long-awaited homecoming followed the arrival of the Crew-10 team, who took over their duties on the station. During their extended stay, Wilmore and Williams conducted research, performed maintenance, and even completed spacewalks—Williams breaking the record for the most spacewalking time by a woman, logging over 62 hours outside the station.
Their experience highlights the physical demands of prolonged microgravity exposure. After landing, they’ll face the task of readapting to Earth’s gravity, likely dealing with muscle weakness, fluid readjustments, and temporary balance issues. Medical teams will monitor them closely to assess how their bodies held up over the unexpectedly long mission, adding valuable data to our understanding of human endurance in space.
Summary
Extended time aboard the ISS reveals just how much the human body relies on Earth’s conditions to stay healthy. Bones and muscles weaken without gravity’s pull, fluids shift in ways that alter the heart and eyes, and radiation quietly raises long-term risks. The immune system falters, sleep becomes elusive, and even digestion adjusts to the strange environment. The recent return of Wilmore and Williams after more than nine months in orbit underscores these effects, showing both the resilience of astronauts and the challenges they face. While exercise, diet, and technology help manage these changes, the body doesn’t fully escape the strain. Each mission adds to our understanding of these impacts, shedding light on what it takes to live beyond our planet—and what it costs the people who do.
