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I have always loved the idea of going to outer space. I grew up reading Isaac Asimov and Philip K. Dick. There is something unique and wonderful about the idea that humans could free ourselves from Earth’s gravity and take to the stars. Like many kids, I dreamed of one day traveling to another world.

Unfortunately, looking at the situation with an adult’s perspective, the reality is quite different. Elon Musk has recently stated that SpaceX will, at least right now, no longer be focusing on traveling to Mars—apparently the newest fantasy is cities on the moon—and it’s not hard to see why. There are innumerable challenges with human space travel, but one huge one that we have no good solutions for is that space is really, really bad for your health.

There are a few main issues that we know about when it comes to space and human well-being. The first is obvious: confinement and isolation. A return journey to Mars would be anywhere from two and a half to three years long, and any intrepid adventurers would have to spend that entire time in cramped, unpleasant spaces with a handful of other people. We have some solutions to this issue—space psychologists, for example, can help astronauts function as a team without losing their minds. There’s a really good podcast series called The Habitat on one series of experiments testing how humans could get through the group confinement of a Mars trip. But it’s still a persistent and worrisome problem.

The next issue is space radiation. The Earth’s atmosphere provides us with a great deal of protection from radiation of all kinds, but once you’re out in space, the risks of cancer and other organ damage start piling up quickly. Most people know that even taking a commercial flight exposes you to a dose of radiation—about one X-ray’s worth. That’s not a big deal, but if you get up higher, it is. The astronauts on the International Space Station receive a dose of around 240 to 480 X-rays on a six-month jaunt up in the exosphere. NASA estimated in 2017 that a human-crewed Mars mission lasting three years would result in those people receiving 3,600 X-rays’ worth of radiation over the course of their travel.

We could possibly correct for radiation. For example, there are potential plans to use underground cave systems on the moon as habitation, which would reduce the radiation danger substantially. But for the actual space travel—the time humans spend commuting across the vacuum—we currently have no real solution. Technically you can shield people from radiation with thick barriers of water, but getting the water or other protective substances into space and constructing the shields has thus far proved prohibitively expensive.

But those two problems are just the easier ones to solve. We may develop better radiation shields in the future. Virtual reality systems that allow astronauts to take breaks in other realistic-seeming environments could reduce the mental load of space travel. This is not impossible.

A much bigger problem is the impact of microgravity on the human body. There are immediate issues, such as blocked noses, which impact virtually all astronauts as they acclimate to space. (The “upward shift of fluid ultimately leads to symptoms of ‘puffiness’ and nasal congestion experienced by astronauts during their adaptation period,” write the authors of a recent paper on the issue.) But there are also much more pernicious problems. For example, the long-term damage to kidneys. Kidneys require gravity to function properly, and the longer people spend in space, the more their kidneys start to malfunction. A 2024 study in human and animal models suggested that even one month in microgravity can permanently alter kidney pathways and cause irreversible damage. In a similar vein, astronauts who have spent six months in space show damage to their arteries and endocrine system that is consistent with over a decade of aging.

Each of those problems has a potential solution. Blocked noses can be managed with over-the-counter medications, or ignored entirely. They’re irritating, but not life-threatening. Kidney function can be partially managed with the use of various medications, and exercise also appears to help reduce the risk of kidney stones (luckily, space treadmills are a thing). Arterial stiffness can potentially be reduced with further drugs, as can insulin resistance.

But then there are the problems with the musculoskeletal system, which, like other systems in the body, evolved to function with Earth’s gravity pulling down on it. Astronauts lose around 1 percent of their bone density in certain bones per month that they’re in space. Exercising for two hours a day can partially ameliorate this issue, but it doesn’t prevent the problem entirely. One 2019 study found that adding a bisphosphonate—a type of drug that slows down bone loss—can reduce the bone density loss even further, but it’s not clear that even this entirely fixes the issue.

In addition, spending time in microgravity causes muscles to waste away. This, too, is something we can partially correct with exercise, but there’s no real long-term fix. Every paper I can find on the topic discusses keeping astronauts fit for the duration of a mission, not keeping humans well indefinitely in moon cities or building a colony on Mars.

Any solution you can find to the woes of space is essentially a stopgap measure designed to keep astronauts relatively healthy for six-month stretches. Every study I could find on the topic unanimously agrees that the ultimate solution to microgravity-induced disease, for example, is for astronauts to come back to Earth. We don’t even know exactly what the impact of a 12- or 24-month stay in space would be, except to say that it would probably be very, very bad.

And all of these issues are just the things that we already know about. We’re looking at a very small sample of astronauts who have spent a maximum of six months each in low Earth orbit, as well as various studies on nonhuman animals. There are undoubtedly a whole range of dangers to space that we will only discover once we get people out beyond the atmosphere for a year or more.

Realistically, we were never going to Mars, at least in my lifetime. We are probably not going to meaningfully live on the moon. We could put people on the moon for six-month stretches, but any longer than that and they’d be seriously risking their lives with every passing day. From a health perspective, I doubt that any humans will spend more than 12 months living in space—or on the moon—during my lifetime without suffering serious, long-term health consequences. Space may be fascinating, wonderful, and exciting, but most of all, it is incredibly dangerous. As far as human space travel goes, it’s probably best that it stays in the realm of science fiction, at least for the foreseeable future.