It’s been over half a century since humanity last set foot on the Moon. Those early “baby steps” towards the stars remain some of our most defining achievements, yet the promise of a “giant leap” to another planet still hangs unfulfilled. Interplanetary travel remains a dream, though visionaries like Elon Musk insist it’s only a matter of time. “Each launch,” he says, “is about learning what’s needed to make life multiplanetary.”
But while SpaceX and others are tackling the technical side of rockets and propulsion, the far greater challenge may not lie in the machinery at all. Imagine reaching Mars only to collapse from exhaustion, or facing an invisible killer that warps your DNA. This isn’t science fiction. It’s the biological reality that awaits the first humans who dare to venture into deep space.
In this first of a two-part exploration, we spoke to NASA scientists and physicians to understand the devastating effects of long-term space travel on the human body, and what we’re doing to survive it.
The human body vs. the void
A trip to Mars isn’t measured in miles but in months. Six to nine of them, traveling through a vacuum that’s actively hostile to life. Unlike the relatively short missions to the Moon or the manageable environment of the International Space Station (ISS), the journey to Mars represents an unmatched test of endurance.
Space doesn’t just remove gravity. It removes the very conditions under which the human body evolved. Muscles weaken, bones dissolve, fluids redistribute, and the heart itself begins to shrink. Even the brain and immune system start to behave in unpredictable ways. In space, adaptation isn’t optional. It’s a desperate, ongoing battle for survival.
Microgravity: The enemy within
As soon as astronauts enter microgravity, every organ system begins to change. On Earth, gravity provides constant feedback that keeps the body balanced, muscles active, and bones strong. Take it away, and everything begins to unravel.
The first thing astronauts feel is disorientation, which NASA calls “space adaptation syndrome.” Without gravity, the brain’s balance system is thrown into chaos. Vision says one thing, while muscle and joint sensors say another. The result? Nausea, dizziness, headaches, and fatigue are feelings our expert describes as “being a grumpy drunk.”
The brain eventually adjusts, but the body continues to degrade. In zero gravity, bones lose density at an alarming rate, roughly 1% per month, compared to about 1% per year for an elderly person with osteoporosis on Earth. Muscles, especially in the legs and spine, begin to atrophy since they no longer need support or movement.
Even the spine itself stretches, causing astronauts to “grow” a few centimeters taller in orbit, but at the cost of back pain. By the time astronauts return to Earth, their muscles are weaker, their bones more fragile, and their balance systems need retraining.
The body fluid revolution
On Earth, gravity pulls fluids toward the lower half of the body. In space, that distribution evens out. Blood and other fluids shift toward the head, creating the space travelers’ signature “puffy face” and “chicken legs” look.
This shift increases pressure in the skull and behind the eyes, sometimes leading to permanent vision problems. It also triggers a chain reaction in the cardiovascular system. The body senses too much fluid near the heart and brain and responds by eliminating what it perceives as excess. Astronauts start to urinate more, losing about 10–15% of their blood plasma within days.
The heart, now working under less strain, begins to shrink. Red blood cells are destroyed to maintain balance, and blood pressure regulation becomes more erratic. The combined effects make returning to gravity or standing upright a painful, dizzying experience.
DNA, immunity, and the cellular cost of space
Space doesn’t just challenge muscles and bones. It reaches into the body’s most fundamental processes. Studies comparing astronaut Scott Kelly, who spent a year aboard the ISS, to his twin brother Mark on Earth revealed startling differences. Scott’s genes showed changes in expression related to DNA repair and immune response. His telomeres, protective caps on chromosomes, lengthened in space, only to shorten rapidly upon return.
Deprived of Earth’s microbial environment and under constant radiation exposure, the immune system becomes unpredictable. Astronauts experience changes in white blood cell behavior, inflammation, and even allergic responses. Prolonged exposure could increase risks of cancer or autoimmune disorders, dangers that will grow exponentially on longer journeys.
Fighting back: Survival through exercise and engineering
NASA has learned one essential truth. Exercise isn’t optional; it’s medicine. Astronauts aboard the ISS dedicate more than two hours daily to workouts designed to mimic gravity’s effects. They run on treadmills while strapped down by bungee cords, lift vacuum-based weights for resistance, and cycle for cardiovascular health. Despite this grueling regimen, astronauts still return home weaker than when they left. The problem is time. For more than 21 hours each day, their bodies are still weightless, continuing to waste away.
Researchers are now exploring advanced countermeasures: nutritional plans rich in protein and essential minerals, electrical muscle stimulation, and even pharmaceuticals that could slow down muscle and bone loss.
But the ultimate fix may even come from physics and not biology. Engineers are experimenting with artificial gravity, such as spinning spacecraft to generate centrifugal force. Early centrifuge tests proved that even short periods of artificial gravity can help maintain muscle tone and bone density. The challenge lies in scaling the technology for long missions without adding excessive weight or energy demands.
Radiation: The invisible threat
Outside Earth’s protective magnetic field, astronauts are exposed to cosmic radiation, a constant shower of high-energy particles that can damage DNA and tissue. While the ISS orbits within Earth’s magnetosphere, deep-space missions will not have that protection.
Solar storms are the most immediate danger. These massive bursts of charged particles from the Sun can deliver lethal doses of radiation in hours. During the Apollo program, a massive solar storm in 1972 narrowly missed astronauts; had it occurred during a lunar mission, it could have been fatal.
NASA’s current shielding, layers of metal, plastic, or even water, offers limited protection. On the Moon or Mars, astronauts must always be within an hour’s reach of a heavily shielded habitat. Even a short delay during a solar event could mean radiation sickness, internal damage, or death.
On Mars, long-term exposure to galactic cosmic rays poses another challenge. Unlike solar protons, these high-energy particles are nearly impossible to block completely and may significantly increase cancer risks over the years.
The isolation factor
Beyond the physical dangers lies one that’s harder to measure—the human mind. Living for months in a confined, silent void millions of miles from Earth will test psychological endurance in ways never before experienced.
Astronauts on the ISS already deal with confinement and stress, but can still communicate with Earth in real time. On Mars missions, communication delays could stretch up to 40 minutes round-trip. A medical emergency or technical failure could unfold faster than Earth can respond.
To survive, crews will need complete medical and psychological autonomy, an “Earth-independent healthcare model,” as one of our experts said. This means astronauts will have to diagnose, treat, and manage crises without ground support, even if a crew member becomes incapacitated. The burden will be immense, both physically and emotionally.
Mars: The ultimate test
The sheer scale of a Mars mission makes every problem exponentially harder. The ISS orbits just 400 kilometers above Earth, a six-hour journey. The Moon is 384,000 kilometers away, a three-day trip. Mars, at its closest, is 225 million kilometers from Earth.
The round-trip could take three years. In that time, astronauts must endure radiation, bone loss, muscle wasting, isolation, and limited food supplies. Even with perfect engineering, the human body itself may be the greatest limiting factor.
Current food technology can’t sustain nutrient-rich diets for that long without spoilage. Radiation shielding for multi-year exposure remains inadequate. And the psychological impact of long-term isolation in an alien environment is still poorly understood.
Despite these immense challenges, progress continues. Space agencies and private companies are working to bridge the gap between imagination and possibility, developing new life-support systems, medical technologies, and spacecraft capable of sustaining humans beyond Earth’s orbit.
The dream of walking on Mars might seem distant, but so did the Moon once. Each mission to the ISS, each rocket launch, each experiment on human adaptation brings us closer to understanding how to survive in space. The journey to Mars will not just be humanity’s next ig step. It will also be its greatest test.