Space is hard, especially on our bones and muscles, our hearts, our eyes, and basically nearly every human organ. And yet, we can’t stay away from it. Nearly 700 people have been to space so far, and that number is only going to increase as private spaceflight begins to take off. But when it comes to enduring long duration stays in space, how much can the human body really take?
Earlier this year, two Russian cosmonauts broke the record for the longest duration stay on board the International Space Station (ISS), spending 374 consecutive days in the microgravity environment. Astronauts on board the ISS help scientists learn about the effects spaceflight has on the human body. Spoiler alert: they’re not great.
The weightless environment causes loss of bone density, muscular atrophy, reduced volume of blood, decreased heart muscle functions, blurred vision, and disorientation. NASA and other space agencies are hoping to learn more about these effects in order to help mitigate the risks on astronauts traveling on long-duration journeys through space.
A human mission to Mars would take roughly three years, according to NASA. But what would that kind of journey—and even longer durations in space—do to the human body? For this Giz Asks, we spoke with experts to understand the challenges of surviving in a weightless environment for extended periods. How long could a person endure life during a deep-space voyage? And in the worst-case scenario, what would happen if someone were stranded indefinitely on the ISS? Here’s what they had to say.
Professor at Johns Hopkins University School of Medicine, vice president of the Human Research Program for Civilians in Space and Chief Scientist at the NASA Human Research Program from 2013 to 2016.
The simple answer: it depends. Several professional government astronauts have spent at least one continuous year in space, with little to no serious adverse effects. We know that this can be done, at least for those who are in excellent health to start with and who adhere to rigorous countermeasure protocols (mostly exercise). How long can this be extended? It depends on what is expected of the people in space, what countermeasures are available to them, and whether or not they will return to Earth.
If their only job is to stay alive, regardless of their ability to perform any meaningful work, then it is just a matter of survival. In this case, people could survive in space for quite a long time. Without countermeasures such as exercise, their time would be leisurely and enjoyable. Their only goal would be to enjoy the experience, which could be quite pleasant. For a time. Eventually, the lack of even minimal physical exertion (which we get on Earth just by working against gravity to stay upright) would cause severe degradation of the bones, muscles, and heart. These changes might not be bad if these people stay in the benign weightlessness of space, but this physiological de-conditioning would very likely preclude their ability to return to the Earth’s gravity environment.
Even if these physiological changes are not debilitating or fatal, there are other stressors that could take a toll over time. The psychological challenges of living in a small space with a small number of people can be significant—especially without an overarching goal to make the difficulty worthwhile. If outside the relative safety of low-Earth orbit, deep-space radiation might have substantial effects. Some of these effects would be cumulative: an increasing risk of cancer with increasing time in space. Other aspects would depend on sporadic events like solar flares for which protection might be inadequate, and which could produce acute effects very quickly.
In parallel with these issues is the little-understood impact of weightlessness on the distribution of fluids in the body. Without gravity, these fluids (blood, cerebrospinal fluid, lymphatic fluid, and others) disperse more evenly, rather than being drawn to the legs. It is thought that some effects of this fluid shift—seen already in spaceflights of several months—are changes in the structure of the eye, an upward shift of the brain in the skull, and slight changes in brain function. These might be harbingers of actual neural damage from extended time in space. It’s possible that people could stay in space for very long times yet undergo gradual deterioration of neural function—things such as cognition and motor control. If others onboard are available to assist, these people might survive for a long time. But to what end? These are among the major risks of which we are aware. There will likely be others that arise as people spend longer times in space. It is these unknown unknowns that could be the limiting factors, but of course we don’t know what they are.
I would hazard a guess at five years, maybe more, for survival in space under the conditions just described. But these people would die in space, having done little of value except establish biological boundaries on the ability to survive in such a harsh environment. Countermeasures would help to mitigate some of the medical issues, in which case the viable duration might extend to perhaps ten years, and maybe even allow return to Earth if exercise is sufficiently vigorous.
Once the space-farers start doing work, the chance of injury increases, but so does the need to maintain a higher level of physical fitness. This is a challenge. If they go with work to be done and the intention of returning to Earth, the answer changes. In this case, mere survival is not enough: the ability to perform meaningful work and to retain bone, muscle, and cardiovascular condition is necessary. Even with the current best exercise and nutrition countermeasures, radiation and isolation will take their toll. With little supporting evidence, I would place this limit at about four years. With artificial gravity it may be much longer. In this case the limitations may be predominantly due to psychology and radiation. If artificial gravity is properly implemented, with radiation shielding and attention to psychological concerns, there might in fact be no limit to the time that can be spent in space.
Not only does the final answer depend on the factors just described, it also depends on the specific individual – his or her genetic predisposition, lifestyle, and ability to cope with stress. The numbers here carry a huge amount of uncertainty, but they provide a starting point, indicate the factors to be considered, and show how different mission scenarios have an impact.
Professor for the department of health physics and diagnostic sciences within the School of Integrated Health Sciences at the University of Nevada, Las Vegas.
ISS has received a [radiation] dose-rate about three times lower than deep space because of the Earth’s shadow blocking about one-third and Earth’s magnetic field an additional one-third. The surface of Mars is about one-third of deep space due to Mars’ body and atmosphere.
The shielding of the ISS is sufficient to reduce the doses from even large solar particle events so there are no significant risks of acute radiation sickness. Therefore the main risk is so-called late effects (cancer, heart disease, cataracts) and a potential risk of changes to cognition and memory, which is observed in mice and rats but not firmly established in humans.
So one way to answer is to ask how much risk a person is willing to accept? If unlimited risk is acceptable then the answer has to do with probabilities of occurrence of the various diseases.
Radiation causes DNA damage and creates radicals due to ionizations in tissue leading to increased oxidative stress. This can lead to gene mutations, chromosomal aberrations, tissue environment change such as perturbation of the immune system and aberrant biochemical signaling. These are precursor changes to various health diseases.
With shielding such as on the ISS, a person can survive but has a high probability of fatal diseases or morbidity exceeding 10% probability after a few years in deep space.
I think the main point to ask is if the endeavor to spend a few years in space is valuable enough to take the risks, and should space agencies make large investments to reduce risks. Late effects take some time to appear dependent on which type. Minimum times after exposure include vision impairing cataracts (a little over five years), leukemia (two years), solid cancers (about five years), heart disease (about 10 years), changes to cognition is less known. So perhaps another question would be, how long can a person stay in space if treatment [to those diseases] is not possible.
A physicist at Stanford University who’s worked with NASA on developing biomaterials to prevent and treat bone loss in astronauts during spaceflight.
As of 2024, the record for the longest continuous stay in space is held by Russian cosmonaut Valeri Polyakov, who spent 437 days and 18 hours aboard the Mir space station from January 1994 to March 1995. This shows that a person can remain in space for over 1.2 years. Could someone stay longer? Absolutely. However, the health risks become increasingly severe.
Let’s consider a 1,000-day mission to Mars, which would be the expected duration with our current technology. In microgravity, muscles and bones weaken due to the lack of regular weight-bearing activity.
In a study we conducted, a collaboration between NASA and Stanford University, we developed a predictive mathematical model. This model shows that on a Mars mission, 100% of astronauts are likely to develop osteopenia [when bone density is lower than normal], with 33% at risk for osteoporosis, depending on factors like age, gender, and ethnicity. Even more concerning is the radiation exposure. For deep space missions like a Mars journey, the cancer risk rises significantly due to higher exposure to galactic cosmic rays (GCRs) and solar radiation. A Mars mission could expose astronauts to 0.7 to 1 sievert (Sv) of radiation, with 1 Sv increasing cancer risk by about 5%. This is much higher than the typical radiation dose on the International Space Station (ISS), which is around 0.3 Sv for a six-month stay.
In addition, space travelers face other serious health challenges: Spaceflight-Associated Neuro-Ocular Syndrome (SANS), cardiovascular disease, and potential nervous system damage. Vision problems caused by fluid shifts in microgravity may persist even after returning to Earth. Mental health is also a concern, as extended isolation, confinement, and the distance from Earth can lead to stress, anxiety, depression, and cognitive decline. The altered immune system response during long-duration missions also raises concerns about fighting off infections or handling medical emergencies.
In my opinion, a three-year mission to Mars is feasible, though astronauts would likely return with significant health issues, some of which could be severe. Missions longer than this would push the limits of human endurance.
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