Space is hard, especially on our bones and muscles, our hearts, our eyes, and almost all human organs. However, we cannot stay away from it. About 700 people have flown so far, and that number is set to grow as private jets begin to take off. But when it comes to long-term endurance in space, how much can the human body really take?
Earlier this year, two Russian cosmonauts broke the record for the longest stay on the International Space Station (ISS), spending 374 consecutive days in microgravity. Astronauts aboard the ISS are helping scientists study the effects of spaceflight on the human body. Spoiler alert: they’re not good.
A weightless environment causes loss of bone density, muscle wasting, reduced blood volume, reduced heart muscle activity, blurred vision, and confusion. NASA and other space agencies hope to learn more about these effects to help reduce the risks to astronauts on long space missions.
A human mission to Mars will take about three years, according to NASA. But what would that journey—and even longer distances in space—do to the human body? In this Giz Quiz, we spoke to experts to understand the challenges of living in a weightless environment for long periods of time. How long can a human survive during deep space travel? And in the worst case scenario, what would happen if someone were permanently trapped 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.
Simple answer: it depends. Several professional government astronauts have spent at least one continuous year in space, with little or no ill effects. We know that this can be done, at least for those who are in very good health to begin with and stick to a strict regimen of moderation (especially exercise). How long can this be extended? It depends on what is expected of the people in space, what fighting methods they have, and whether they will return to Earth or not.
If their only job is to stay alive, regardless of their ability to do any meaningful work, then it is a matter of survival. In this case, people can live in space for a very long time. Without controversial measures such as physical exercise, their time can be relaxed and enjoyable. Their only goal is to enjoy the experience, which can be a lot of fun. For a while. Ultimately, a lack of even a little physical exertion (which we get on Earth just by working against gravity to stay upright) can cause serious deterioration of bones, muscles, and the heart. These changes may not be bad if these people live in zero gravity, but this displacement of life would likely prevent their ability to return to Earth’s gravitational field.
Even if these physical changes are not fatal or fatal, there are other stressors that can worsen over time. The psychological challenges of living in a small space with a small population can be significant—especially without the broader goal of making the hardship worthwhile. Without the relative safety of low-Earth orbit, deep-space radiation can have far-reaching consequences. Some of these effects can be long-lasting: an increased risk of cancer with increasing time in space. Other factors may depend on rare events such as solar flares where protection may not be sufficient, and which can produce dangerous results very quickly.
Along with these problems is the mysterious effect of weightlessness on the distribution of fluids in the body. Without gravity, these fluids (blood, cerebrospinal fluid, lymphatic fluid, etc.) disperse evenly, rather than being drawn into the legs. It is thought that some of the effects of this fluid shift—which has already been in space for several months—are changes in the shape of the eye, the brain’s elevation in the skull, and minor changes in brain function. These may be signs of actual nerve damage from long time in space. It’s possible that people can stay in space for very long periods of time but start to gradually lose sensory function—things like vision and motor control. If others on board are available to help, these people may live longer. But for what purpose? These are among the biggest dangers we know. There will likely be more to come as humans spend more time in space. It’s these unknowns that could be the limiting factors, but we don’t know what they are.
I would hazard a guess at five years, perhaps more, to live in space under the conditions just described. But these people would die in space, with little benefit other than to set biological limits on being able to survive in such a harsh environment. Contradictory measures can help reduce some of the medical problems, where the time may reach maybe ten years, and maybe even allow a return to Earth if the exercise is strong enough.
Once hikers start working out, the chances of injury increase, but so does the need to maintain a high level of physical fitness. This is a challenge. When they leave with a mission to do and a purpose to return to Earth, the answer changes. In this case, mere survival is not enough: the ability to perform meaningful work and maintain bones, muscles, and heart condition is required. Even the best current exercise and countermeasures in nutrition, radiation and isolation will do harm. With little supporting evidence, I would put this limit at about four years. With artificial gravity it may be much longer. In this case the limitations may be primarily due to psychology and radiation. If artificial gravity is used properly, with radiation shielding and attention to psychological concerns, there may actually be no limit to the amount of time that can be spent in space.
Not only does the final answer depend on the factors just described, it also depends on the individual – his genetics, lifestyle, and ability to cope with stress. The numbers here carry a large amount of uncertainty, but they provide a starting point, show factors to consider, and show how different machine conditions contribute.
Professor in the department of health physics and diagnostic sciences within the School of Integrative Health Sciences at the University of Nevada, Las Vegas.
ISS found ia [radiation] average volume is about three times lower than deep space due to the Earth’s shadow blocking about one third and the Earth’s magnetic field another third. The surface of Mars is about one third of deep space due to the body and atmosphere of Mars.
Shielding the ISS is sufficient to reduce doses from even large solar particle events so there are no significant risks of severe radiation sickness. Therefore, the main risk is called late effects (cancer, heart disease, cataract) and the possible risk of changes in understanding and memory, which are seen in mice and rats but not strictly confirmed in humans.
So another way to answer is to ask how much risk a person is willing to accept? If unlimited risk is accepted the answer is related to the probability of various diseases.
Radiation causes DNA damage and creates radicals due to ionizations in tissues that lead to increased oxidative stress. This can lead to genetic mutations, chromosomal changes, tissue changes such as immune disorders and abnormal biochemical reactions. These are the changes that precede various health diseases.
With protection like the ISS, a person can survive but has a higher chance of developing fatal diseases or illness than a 10% chance after a few years in deep space.
I think the main point I have to ask is whether the effort to spend a few years in space is important enough to deal with the risks, and if the space agencies are making huge investments to reduce the risks. Later results take time to be seen depending on the type. Short periods after exposure include visual impairment (a little more than five years), leukemia (two years), solid cancer (about five years), heart disease (about 10 years), cognitive changes are not well known. So maybe another question would be, how long can a person stay in the space if the treatment [to those diseases] it is impossible.
A physicist at Stanford University who has 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 astronaut Valeri Polyakov, who spent 437 days and 18 hours on the Mir space station from January 1994 to March 1995. This shows that a person can stay in space for more than 1.2 years . Can a person last long? Definitely. However, the health risks are getting worse.
Let’s consider a 1,000-day trip to Mars, which would be the expected time with our current technology. In microgravity, muscles and bones weaken due to the lack of normal weight-bearing activity.
In the research we did, a collaboration between NASA and Stanford University, we developed a predictive mathematical model. This model shows that on the Mars mission, 100% of the astronauts are likely to develop osteopenia. [when bone density is lower than normal]33 percent risk of osteoporosis, depending on factors such as age, gender, and race. The most concerning is exposure to radiation. With deep space missions such as Mars missions, the risk of cancer increases significantly due to high exposure to galactic cosmic rays (GCRs) and solar radiation. Mars missions may expose astronauts to 0.7 to 1 sievert (Sv) of radiation, with 1 Sv increasing the risk of cancer by about 5%. This is much higher than the average radiation dose on the International Space Station (ISS), which is about 0.3 Sv for a six-month stay.
In addition, space travelers face other serious health challenges: Spaceflight-Associated Neuro-Ocular Syndrome (SANS), heart disease, and possible damage to the nervous system. Vision problems caused by fluid shifts in microgravity may persist even after returning to Earth. Mental health is also a concern, as prolonged isolation, confinement, and distance from Earth can lead to depression, anxiety, depression, and mental retardation. Altered immune responses during prolonged exercise also raise concerns about fighting infections or managing medical emergencies.
In my opinion, a three-year mission to Mars is possible, although the astronauts would return with significant health problems, some of which could be serious. A mission longer than this would push the limits of human endurance.
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