The human body is a masterpiece of engineering, refined by four billion years of evolution to thrive under very specific conditions: one gravity, a nitrogen-oxygen atmosphere, a magnetic field that deflects cosmic radiation, and a 24-hour light-dark cycle driven by a star 93 million miles away. Remove any of these conditions, and things start to go wrong.
This is the central challenge of human spaceflight. We can build rockets powerful enough to escape Earth's gravity. We can design habitats that provide air, water, and warmth. But we cannot redesign the human body, and so we must understand -- in meticulous, sometimes uncomfortable detail -- exactly what happens to flesh and bone when we send it into space.
The science of space medicine has been advancing for more than sixty years, and what it has revealed is both alarming and strangely hopeful. Space breaks you in ways you would not expect. But the body fights back, and the countermeasures we have developed are getting better every year.
Bones: The Slow Dissolution
On Earth, your skeleton is in a constant state of renovation. Specialized cells called osteoclasts break down old bone tissue while osteoblasts build new bone to replace it. This process is regulated, in part, by mechanical loading -- the stress of gravity pulling on your skeleton and the impacts of walking, running, and simply standing upright.
In microgravity, the loading disappears, and the balance shifts. Osteoclasts keep working, but osteoblasts slow down. The result is a net loss of bone density at a rate of roughly 1-2% per month, concentrated in the weight-bearing bones of the hips, spine, and legs. Over a six-month ISS mission, an astronaut can lose as much bone density as a postmenopausal woman loses in a year.
This is not just a number on a scan. It means weaker bones, increased fracture risk, and elevated calcium in the blood (which can cause kidney stones -- a medical emergency that has nearly occurred in space). Upon return to Earth, bone density does recover, but the process takes years, and some studies suggest it may never fully return to pre-flight levels after very long missions.
The primary countermeasure is resistance exercise. The Advanced Resistive Exercise Device (ARED) aboard the ISS allows astronauts to perform squats, deadlifts, and other loaded exercises that simulate the mechanical stress of gravity. It has been remarkably effective -- astronauts who exercise diligently on ARED lose significantly less bone than those on earlier stations without it. But for Mars missions lasting two to three years, exercise alone may not be enough, and researchers are investigating pharmaceutical interventions, including bisphosphonates (drugs commonly used to treat osteoporosis on Earth).
Muscles: Use It or Lose It, Faster Than You Think
Muscle atrophy in space follows a similar logic. Without gravity to work against, the muscles that normally keep you upright -- the soleus in the calf, the erector spinae along the back, the quadriceps in the thighs -- begin to waste. Studies have shown that astronauts can lose up to 20% of their muscle mass during missions of just five to eleven days on early shuttle flights, before rigorous exercise countermeasures were implemented.
The ISS exercise program, which mandates roughly two hours of combined resistance and aerobic exercise per day, has dramatically reduced muscle loss. But it has not eliminated it. Even with diligent exercise, some degree of atrophy occurs, particularly in the fine motor control and coordination of the postural muscles. Astronauts returning from six-month missions often struggle with balance and coordination in the first days back on Earth, sometimes requiring assistance to walk.
For Mars missions, the challenge intensifies. After six to nine months of transit in microgravity, crews will need to be physically capable of working on the Martian surface (which has about 38% of Earth's gravity) almost immediately upon arrival. There will be no medical team waiting with wheelchairs. The crew will need to land, egress, and begin surface operations largely on their own. Maintaining sufficient muscle function during transit is a critical unsolved problem.
The Fluid Shift: Your Head in Space
One of the most visually obvious effects of microgravity is the fluid shift. On Earth, gravity pulls blood and other fluids toward your feet. Your body compensates with complex vascular mechanisms -- valves in your veins, reflexive changes in heart rate and blood vessel tone -- that keep blood flowing to your brain.
In microgravity, gravity's pull disappears, and roughly two liters of fluid shift from the legs toward the head and upper body. Astronauts' faces become puffy and round -- they call it "moon face" or "bird legs" because their legs simultaneously become thinner. This fluid shift happens within hours of reaching orbit and persists throughout the mission.
The consequences go beyond appearance. The elevated intracranial pressure associated with the fluid shift is believed to be a primary driver of one of the most concerning spaceflight health effects discovered in recent years: Spaceflight-Associated Neuro-ocular Syndrome, or SANS.
SANS: The Vision Problem No One Expected
SANS was identified when multiple astronauts returned from long-duration ISS missions reporting significant changes in their vision. Ophthalmological exams revealed flattening of the eyeball, swelling of the optic nerve, and folds in the choroid layer of the eye. Some astronauts experienced lasting vision changes that required new corrective lens prescriptions.
The condition appears to be caused by the chronic elevation of intracranial pressure due to the fluid shift. On Earth, standing up and lying down creates pressure variations throughout the day. In microgravity, the pressure in the head is constantly elevated, with no relief.
SANS affects roughly 70% of astronauts on long-duration missions to some degree, making it one of the most prevalent spaceflight health concerns. It disproportionately affects male astronauts, possibly due to anatomical differences in cerebrospinal fluid dynamics, though the reasons are not fully understood. Research into SANS is a top priority for NASA's Human Research Program, because untreated vision degradation on a Mars mission -- far from any eye doctor -- could be mission-threatening.
Current countermeasures being investigated include lower body negative pressure devices (which pull fluid back toward the legs), specialized sleeping positions, and pharmaceutical interventions. None has been validated as fully effective yet.
Radiation: The Invisible Threat
On Earth's surface, you are protected from the majority of cosmic radiation by two shields: the planet's magnetic field and its atmosphere. In space, both shields are diminished or absent.
Astronauts aboard the ISS, which orbits within the partial protection of Earth's magnetosphere, receive radiation doses roughly ten times higher than the average person on Earth. Crews on lunar missions, outside the magnetosphere, would receive significantly more. And crews in transit to Mars, spending months in deep space with minimal shielding, would accumulate radiation doses that push against or exceed current career exposure limits.
The health effects of chronic, low-level radiation exposure in space include increased lifetime cancer risk, potential damage to the central nervous system (including possible cognitive effects), cardiovascular damage, and cataracts. The uncertainty in these risk estimates is itself a major concern -- we simply do not have enough data from humans exposed to deep-space radiation to make precise predictions.
Shielding helps, but mass is expensive to launch. Water, polyethylene, and other hydrogen-rich materials are effective shields, and some Mars mission concepts envision surrounding the crew habitat with water tanks or waste storage. Pharmaceutical radioprotectants are under investigation. But the radiation problem remains one of the most significant barriers to long-duration human exploration beyond Earth orbit.
The Twin Study: Scott and Mark Kelly
In 2015-2016, NASA conducted one of the most ambitious human spaceflight experiments ever attempted. Astronaut Scott Kelly spent 340 days aboard the ISS while his identical twin brother Mark -- also an astronaut, but retired -- remained on Earth as a genetic control subject.
The results, published in a landmark 2019 paper in Science, were detailed and sometimes surprising. Scott's telomeres (the protective caps on chromosomes that typically shorten with age) actually lengthened during spaceflight, though they shortened rapidly upon return to Earth. His gene expression changed significantly, with alterations in immune function, DNA repair mechanisms, bone formation genes, and cardiovascular markers. Most changes reverted to normal within six months of landing, but roughly 7% of gene expression changes persisted.
The study also documented cognitive performance changes, microbiome shifts, and epigenetic modifications. It was a treasure trove of data, but also a humbling reminder of how much we still do not understand about the long-term effects of spaceflight on human biology.
Preparing for Mars
Everything we have learned from six decades of human spaceflight points to one conclusion: sending humans to Mars will be the most demanding medical challenge in the history of exploration. A round-trip Mars mission would expose the crew to 2-3 years of microgravity (minus time on the Martian surface), chronic radiation exposure far exceeding ISS levels, extreme isolation and confinement, communication delays of up to 22 minutes each way, and the knowledge that rescue is impossible.
The countermeasures being developed are wide-ranging: better exercise equipment, pharmaceutical interventions for bone and muscle loss, artificial gravity concepts (rotating spacecraft sections), improved radiation shielding, advanced medical autonomy systems (because the crew will need to handle medical emergencies without real-time ground support), and extensive psychological preparation.
None of these solutions is complete. Each represents an active frontier of research. But the progress is real, built on the hard-won knowledge of every astronaut who has lived in space and every researcher who has analyzed the data they brought home.
The human body was not designed for space. But human ingenuity is gradually learning how to bring it there -- and bring it home healthy. That is the quiet, persistent triumph of space medicine, and it is the foundation upon which every future mission beyond Earth will stand.
