Microgravity is the condition of near-weightlessness experienced by objects and people in orbit around Earth or during other phases of spaceflight. It is not zero gravity in the strictest sense; aboard the International Space Station (ISS), for instance, the pull of Earth’s gravity is only about 10 percent weaker than at the surface. What creates the floating sensation is that the station and everyone inside it are in continuous free fall around the planet, so the net gravitational force they feel is vanishingly small. That seemingly simple physical state turns out to have profound consequences for the human body, for microorganisms, for plant growth, and even for how drugs work, and the picture researchers have assembled over decades of spaceflight is both fascinating and sobering.
How Fluid Shifts Reshape the Body Within Hours
On Earth, gravity pulls a significant volume of your blood and other fluids into your lower body. The moment that pull disappears, the distribution changes fast. Roughly two liters of fluid migrate from the legs toward the head and chest, giving astronauts characteristically puffy faces, congested sinuses, and noticeably thinner legs, a combination sometimes described as “puffy face, chicken legs.”1Precision Clinical Medicine. Long-term spaceflight and the cardiovascular system This shift is not just cosmetic. The heart, suddenly receiving more blood volume in the chest than it is used to, begins to remodel. In one early study, the left ventricle lost an average of about 12 percent of its mass in four male astronauts after only ten days in space.2Precision Clinical Medicine. Long-term spaceflight and the cardiovascular system The heart essentially shrinks because it no longer has to pump against gravity to get blood to your brain. That adaptation is efficient in orbit but creates problems on return, when the heart must suddenly resume its full workload.
Why Astronauts Lose Bone and Muscle
Your skeleton is a living tissue that constantly builds itself up and breaks itself down in response to mechanical loading. When you walk, run, or simply stand, the impact and compression tell bone-forming cells to keep working. Remove that loading, and the balance tips. In microgravity, weight-bearing bones experience almost no compressive force, so the cells responsible for building new bone slow down while the cells that break bone down ramp up.3Npj Microgravity. The effects of microgravity on bone structure and function The result is a net loss of bone mineral density that can reach about 1 to 2 percent per month in the hip and spine, a rate roughly ten times faster than typical age-related bone loss on Earth.4PubMed Central. Osteoclasts and Microgravity
Skeletal muscle faces a similar problem. Without gravity pulling on them, the postural muscles that hold you upright on Earth have almost nothing to do. The soleus muscle in the calf, one of the hardest-working anti-gravity muscles, shows measurable changes in its protein composition during spaceflight. Researchers comparing muscle biopsies from two astronauts found small but significant shifts in the types of contractile proteins expressed in the soleus at different phases of a mission, reflecting the changing demands placed on the muscle as loading disappeared.5npj Microgravity. Spaceflight on the ISS changed the skeletal muscle proteome of two astronauts Without countermeasures, astronauts can lose a substantial fraction of their leg muscle mass over a six-month stay.
Eyes Under Pressure
One of the more surprising discoveries in spaceflight medicine is that microgravity can change astronauts’ vision, sometimes permanently. The condition is now called spaceflight-associated neuro-ocular syndrome, or SANS. It appears to stem at least partly from the same headward fluid shift that puffs up astronauts’ faces. Increased pressure around the brain and optic nerve leads to a constellation of eye problems including swelling of the optic disc, flattening of the back of the eyeball, folds in the retina and choroid layers, and shifts toward farsightedness.6JAMA Ophthalmology. Space Flight–Associated Neuro-ocular Syndrome
These changes can affect one or both eyes and vary widely between individuals, which has made SANS difficult to predict and prevent. Ground-based experiments that simulate headward fluid shifts using head-down tilt bed rest have shown that intermittently returning subjects to an upright posture can reduce retinal thickening, suggesting that periodic relief from the fluid shift may help.7PubMed Central. Optic Nerve Vasculature and Countermeasure Assessment in a Bedrest Analogue of Spaceflight-Associated Neuro-Ocular Syndrome Whether artificial gravity devices aboard a spacecraft could provide that kind of relief is one of the open questions in the field.
A Confused Inner Ear
Your sense of balance depends on gravity. The otolith organs in your inner ear are literally small crystals sitting on sensory hairs, and they work by sensing which direction gravity is pulling them. In microgravity, those crystals float rather than settle, and the brain loses a major source of orientation information. During the first few days of a mission, many astronauts experience space motion sickness, with nausea, disorientation, and difficulty coordinating movement.
The brain adapts by leaning more heavily on vision. After returning to Earth, astronauts tend to show an increased reliance on visual cues for balance, compensating for the fact that their inner-ear and body-position signals are temporarily unreliable.8PubMed. Space flight and neurovestibular adaptation This re-adaptation period is why returning astronauts sometimes stumble or have difficulty walking in a straight line. For short missions the effect is temporary, but the long-term consequences of repeated adaptation cycles remain unclear.
Immune Suppression and Dormant Viruses Waking Up
If you have ever had chickenpox or a cold sore, the viruses responsible never truly left your body. They sit dormant in your cells, kept in check by your immune system. Spaceflight appears to weaken that immune surveillance enough to let those viruses reactivate. Around 60 percent of astronauts on long-duration ISS missions shed at least one type of herpes virus in their saliva or urine. Varicella zoster virus, which causes chickenpox and shingles, was detected in the saliva of roughly 96 percent of all astronauts tested. Epstein-Barr virus showed up in about 65 percent, and cytomegalovirus appeared in the urine of approximately 61 percent. Both the frequency and magnitude of viral shedding increased with longer missions.9npj Microgravity. Challenges for the human immune system after leaving Earth
These reactivations rarely cause clinical illness in healthy astronauts, at least on current mission durations, but they signal that the immune system is under real stress. For a hypothetical multi-year Mars mission, where crew members would be far from medical facilities, immunological weakening of this sort becomes a serious planning concern.
Space Anemia
Astronauts have long been known to come home with lower red blood cell counts than they left with, a phenomenon called space anemia. For years, the leading explanation was that the body simply destroyed excess red blood cells during the initial fluid shift and then stopped producing as many. More recent research tells a different story. A study of 14 astronauts throughout their six-month ISS missions found persistently elevated markers of red blood cell destruction, including elevated carbon monoxide in exhaled breath and higher iron levels in the blood, throughout the entire mission, not just at the start.10PubMed Central. Hemolysis contributes to anemia during long-duration space flight In other words, the body keeps destroying red blood cells at an accelerated rate for as long as it is in microgravity. Even a year after landing, some of these effects persisted, including higher-than-normal rates of both red blood cell destruction and production.11PubMed Central. Hemolysis contributes to anemia during long-duration space flight
Animal studies complement this picture. Rodent red blood cells exposed to spaceflight conditions showed altered shape and membrane composition, along with signs of oxidative stress, suggesting that microgravity and radiation together damage the cells directly.12PLoS ONE. Effects of Long-Term Space Flight on Erythrocytes and Oxidative Stress of Rodents Space anemia is now considered a hemolytic condition, one that should factor into the medical screening and monitoring of anyone who flies in space, including the growing number of private space tourists.
What Happens to Cells at the Microscopic Level
At the cellular level, gravity plays a larger role than most people would guess. Cells sense mechanical forces through their internal scaffolding, a network of protein filaments called the cytoskeleton. In microgravity, this scaffolding reorganizes. Decades of spaceflight experiments have documented that cells in orbit undergo changes in shape, stiffness, and the signals they send and receive, all stemming from that disrupted internal structure.13PubMed. Cells respond to space microgravity through cytoskeleton reorganization
Bone-forming cells are a clear example. In microgravity, the key structural protein actin disassembles more readily inside these cells, and a protein called YAP that helps regulate bone formation drops in activity. Laboratory experiments have shown that applying gentle compression can restore YAP activity even in microgravity, hinting that mechanical countermeasures at the cellular level are possible.14npj Microgravity. Cellular mechanotransduction of human osteoblasts in microgravity This kind of finding helps explain why resistive exercise in orbit partly protects bones and muscles: it manually reintroduces some of the mechanical signals that gravity normally provides.
Microbes Get Tougher
Humans are not the only organisms that respond to microgravity. Bacteria and fungi do, too, and their responses are less than ideal from a crew health standpoint. Microgravity tends to enhance biofilm formation, the sticky colonies of microbes that adhere to surfaces and are notoriously difficult to clean. It also appears to boost pathogenicity in certain bacteria and increase antibiotic resistance.15Life Sciences in Space Research. Microbial resilience in space: Biofilms, risks and strategies for space exploration Bacteria grown under simulated microgravity become better at surviving secondary stressors like temperature changes, pH swings, and antimicrobial treatments.16PubMed Central. Spaceflight and modeled microgravity effects on microbial growth and virulence
On the ISS, multi-drug-resistant biofilm-forming pathogens have been isolated from surfaces.17PubMed. Multi-resistant biofilm-forming pathogens on the International Space Station In a closed environment where the crew’s immune systems are already under stress, hardier and more resistant microbes are a real concern. Spacecraft environmental control systems were designed with this in mind, but the problem scales with mission duration: the longer the flight, the more opportunity microbes have to evolve and colonize surfaces.
Your Gut Microbiome Shifts Too
The trillions of bacteria living in your digestive tract are sensitive to environmental changes, and spaceflight delivers plenty of them. Cosmic radiation, microgravity, confinement, altered diet, and disrupted sleep cycles all conspire to change the composition of the gut microbiome. Research has shown a decrease in the diversity of beneficial gut microbes during spaceflight, along with shifts in the metabolites they produce.18PubMed Central. Gut microbiota shifts in spaceflight: a case study evidence and countermeasures for microbial homeostasis These changes can affect immune function, metabolism, and even mood, forming yet another thread in the web of health challenges that microgravity introduces.
When Microgravity Meets Radiation
On Earth, our atmosphere and magnetic field shield us from much of the radiation that permeates space. Astronauts in low Earth orbit receive radiation doses many times higher than what people experience on the ground, and outside the protection of Earth’s magnetic field, as on a trip to Mars, the exposure would be substantially greater. What makes this relevant to microgravity specifically is that the two hazards may amplify each other. Microgravity appears to interfere with the cellular machinery that repairs DNA damage, potentially allowing radiation-induced lesions like double-strand breaks and chromosome abnormalities to accumulate rather than being fixed.19PubMed Central. Interplay of space radiation and microgravity in DNA damage and DNA damage response The evidence here is mixed, with some studies finding clear synergies and others not, but the possibility that microgravity worsens the cancer risk from space radiation is taken seriously in mission planning.
How Drugs Behave Differently in Space
If microgravity changes your cardiovascular system, your fluid distribution, your gut microbiome, and the way your cells function, it stands to reason that medications will not work the same way either. Drug absorption depends partly on how quickly your stomach empties and how much blood flow your gut lining receives, both of which change in microgravity. Drug distribution changes because fluid volumes in different body compartments shift. On top of that, the drugs themselves may degrade faster: the radiation environment, temperature swings, humidity, and vibration that medications experience during launch and in-flight storage can reduce their shelf life and effectiveness.20PubMed. A review of the physiological effects of microgravity and innovative formulation for space travelers This is a practical problem right now on the ISS, where astronauts keep a pharmacy of common medications, and it becomes an even larger issue for long missions where resupply is impossible.
Plants and the Question of Growing Food in Orbit
Gravity guides plant growth on Earth. Roots grow downward, shoots grow upward, and the distribution of growth hormones through the plant depends partly on gravitational pull. In microgravity, those hormone patterns change. Experiments with pea seedlings grown aboard the ISS found that the interplay between two major plant hormones, auxin and cytokinins, was significantly altered. Reduced polar transport of auxin led to increased cytokinin levels, which in turn changed growth direction and inhibited overall elongation of the shoots.21Life Sciences in Space Research. Comprehensive analyses of plant hormones in etiolated pea and maize seedlings grown under microgravity conditions in space Plants can still grow in orbit, and astronauts on the ISS have successfully cultivated lettuce and other crops, but yields and growth patterns differ from what you would see in a greenhouse on Earth. For long-duration missions where fresh food is important for nutrition and morale, understanding how to optimize plant growth in microgravity is a genuine engineering challenge.
Countermeasures That Help and Gaps That Remain
The ISS is essentially a floating laboratory for figuring out how to keep people healthy without gravity. The most important countermeasure is exercise. Astronauts spend roughly two hours a day working out on a combination of a treadmill, a cycle ergometer, and a device called the Advanced Resistive Exercise Device, or ARED, which uses vacuum cylinders to simulate weightlifting. Studies have shown that ARED provides significant protection against bone loss compared to older, less capable exercise devices, although some loss in the hip and femur neck still occurs.22PubMed. Bisphosphonates as a supplement to exercise to protect bone during long-duration spaceflight Current countermeasures have reduced but not eliminated musculoskeletal losses for missions of several months, and whether they will be sufficient for multi-year exploration missions remains an open question.23PubMed Central. Towards human exploration of space: the THESEUS review series on muscle and bone research priorities
Artificial gravity is a newer area of investigation. Researchers have tested protocols that combine cycling exercise with centrifugal force generated by a short-arm centrifuge, essentially spinning the astronaut while they pedal. Early results suggest this combination significantly boosts cardiovascular stress beyond what exercise alone provides, which could help maintain the reflexes that keep blood pressure stable when you stand up in a gravitational field.24npj Microgravity. A combined individualized cycling exercise and artificial gravity training protocol as a spaceflight deconditioning countermeasure Whether continuous rotation of an entire spacecraft habitat, the science-fiction approach to artificial gravity, will ever be practical is a question of engineering economics as much as biology.
Studying Microgravity Without Going to Space
Sending experiments and subjects to orbit is expensive and logistically constrained, so much of microgravity research relies on ground-based analogs. The most widely used is head-down tilt bed rest, in which volunteers lie in bed with their feet elevated six degrees above their heads for days or even months at a time.25PubMed. Analogs of microgravity: head-down tilt and water immersion This position shifts fluid toward the head, unloads the legs, and prevents most weight-bearing activity, replicating several key features of spaceflight. Other methods include water immersion, where a subject floats at the surface with their head above water, and dry immersion, which wraps the subject in a waterproof sheet so they float without getting wet.
These analogs duplicate many of the body’s responses to weightlessness reasonably well, but they are imperfect. A recent study comparing brain activity in head-down tilt volunteers to data from astronauts in actual microgravity found that while bed rest reproduced some central nervous system changes, it did not capture all of them, particularly those related to body-position sensing and motor control.26PubMed Central. Brain power comparison between microgravity and head-down tilt bed rest: an electroencephalography approach The ground-based models are invaluable for studying things like fluid shifts and cardiovascular deconditioning, but they cannot fully replace actual spaceflight research for understanding the complete picture.
Crystal Growth and Materials Science
Not all microgravity research is about health risks. One of the longer-running commercial applications of the microgravity environment is protein crystal growth. When crystals form in a solution on Earth, gravity-driven convection currents disturb the process, producing smaller and less orderly crystals. In microgravity, those currents are suppressed, and crystals can grow larger and with fewer defects. This has been investigated and used for both academic and industrial purposes since the mid-1950s, and the benefit has been supported by individual studies as well as meta-analyses of crystallization experiments.27Springer Nature (Current Stem Cell Reports). Protein Crystallization in Microgravity: Commercialization and the Next Chapter Better crystals mean sharper structural data, which can accelerate drug design. Several pharmaceutical companies have sent experiments to the ISS specifically for this reason, and as launch costs decrease, the field is entering a new phase of commercialization where growing crystals in orbit may become economically routine rather than a rare research opportunity.