Sending people to Mars won’t be easy. There are the obvious challenges like getting people and supplies into space and landing them safely on another planet. And once they arrive, they’ll need somewhere safe to live with air to breathe, water to drink, and food to eat. But the biggest obstacle to crewed exploration of Mars might be something that’s totally invisible and often overlooked: the space radiation that can wreak havoc on the human body.

While Elon Musk is busy drawing up plans for a Martian city, experts working on human space exploration are more cautious. Getting to Mars might not even be the hardest part if we want people to explore safely.

We know from decades of research on the International Space Station that microgravity has a range of effects on the body, from vision problems to muscle loss. But leaving Earth means not only leaving its gravity but also leaving its protective bubble. And we’re only just beginning to learn about the many ways that exposure to space radiation can impact human health.

Space radiation comes from two main sources: solar activity in the form of solar flares, and energetic particles called galactic cosmic rays. “Galactic cosmic rays come from stars that are dying, and that radiation is part of the void of space when you travel,” explained radiobiologist and radiation expert Eleanor Blakely.

The health risks from space radiation are many, but poorly understood. It is thought to raise cancer risk, affect the central nervous system, increase degenerative effects like heart disease and cataracts, and change the immune system. Finding a way to mitigate these effects will determine whether astronauts can ever visit Mars safely or whether the health detriments make it too dangerous for people to ever set foot there.

The particular challenge of space exploration is that it involves long-term exposure to low levels of radiation, which is quite different from what most radiation exposure looks like here on Earth.

Most of the data we have looks at the health effects of radiation like gamma rays and X-rays, which cause damage across the body in a “uniform, spray-bottle kind of pattern,” explained radiation biologist Greg Nelson, who advises NASA on radiation health research. But galactic cosmic rays move through the body in a straight line, like a track. “So you concentrate damage on a microscopic scale, and that damage, because it’s so concentrated, is much more difficult for the body to repair,” Nelson said.

This type of space radiation isn’t like the low-dose exposure of a chest X-ray. Instead, imagine a charged particle traveling at nearly the speed of light, firing straight through your brain, perturbing 10,000 cells all in a row, all within a microsecond. It’s not necessarily damaging those cells, but it is activating them in a highly unusual way. And we don’t yet know what that does.

“It’s that feature, that we would call track structure, that lends itself to the possibility of new and different effects occurring,” Nelson said.

While most radiation on Earth can cause cancer by breaking apart DNA, the latest research suggests these charged particles could be damaging the brain in an entirely different way, such as by disrupting the connections between neurons or the mitochondria within neurons.

Another concern is that astronauts aren’t only exposed to radiation. On a space journey, they are also dealing with microgravity, which is well known to cause health issues. 

There are the more obvious effects, like loss of muscle tissue because muscles aren’t working against gravity. But there is also evidence of other effects such as brain remodeling. “That means the tissues are activated in a different way than they normally are,” Blakely explained, such as changes of the amount of gray matter versus white matter. But as for the effects of that: “What are the psychological or physiological consequences? We don’t know.”

Researchers are starting to look at how the effects of microgravity and radiation exposure can compound.

“There is some evidence that they interact,” Nelson said. “No one knows if it’s additive, or if it’s a synergistic effect at this point.” In other words, it’s not clear whether the effects stack on each other or if they produce an even worse outcome when combined. Nelson pointed to evidence of changes to bone health, to the blood-brain barrier of the central nervous system, and to particular features in the eye as areas of open research. 

A combination of radiation exposure and sleep deprivation could also add up to more cognitive defects, according to recent research in rodents. This isn’t even considering further effects of the isolation of long-duration space missions and the psychological toll of confinement. 

The health risks of traveling in space are many, and we don’t yet have enough information to know how they interact. 

NASA’s own calculations show that longer missions to Mars could take astronauts above 1 sievert of radiation exposure, which is above the agency’s acceptable limit for lifetime exposure. However, when sending people to Mars, the biggest risk from radiation is during the period they are traveling. On the Mars surface, there is some protection from being on the planet’s surface, so the real concern is time spent in space. 

For periods of up to a month, the effects are unlikely to be severe. But when you start looking at periods of six months to a year in space, “Now you’re getting into the range where, at least in rodent studies, you can pick up some changes,” Nelson said. “And how that extrapolates to humans we still don’t know with great certainty.”

You can choose when to travel to mitigate the radiation risk. The sun goes through a roughly 11-year cycle of activity, and if you travel when the sun is most active at solar maximum, there is more material coming from the sun that pushes away cosmic rays. But that coincides with more solar particle events, so you have more radiation from the sun to worry about.

You could lessen the amount of time spent in space by using technology such as nuclear propulsion, which NASA is researching, but that carries its own risks — especially if something were to go wrong during a launch, as an explosion could scatter radioactive material into Earth’s atmosphere. 

There are ways to protect astronauts from radiation, such as shielding. But that’s not a simple proposition either.

“Intuitively, we’ve all come to think, that ‘Oh, just put enough lead around me, make sure my underwear is lead, and I’ll be fine.’ That’s probably true for things like X-rays and gamma rays,” Nelson said, particularly when radiation is coming from one direction. But with charged particles, which come from all directions, that isn’t the case. 

“With regards to charged particles, one of the things that happens is they break up into pieces,” Nelson said. “And the smaller pieces have the ability to penetrate to larger depths than the big pieces did. So sometimes more shielding actually adds to the problem.”

There’s a “sweet spot” for radiation shielding that protects from some of the large pieces without creating too many secondary pieces. Some of the most effective shielding is actually material like polyethylene rather than metal as it has more hydrogen atoms and is less likely to create small pieces. 

You can build up layers of material to act as protection in certain circumstances — such as having astronauts sleep in more heavily shielded areas — but sooner or later, astronauts are going to need to venture out and explore.

“Shielding is effective, but we simply have to live with the fact that there will be unshieldable quantities of radiation that we have to deal with,” Nelson said.

NASA has strict limits on how much radiation an astronaut may be exposed to over their career, equivalent to a 3–4 percent excess mortality risk from all causes. These limits were recently changed, somewhat controversially, because it’s hard to come up with an amount of radiation exposure that is safe. Different types of radiation affect people differently, based on factors like which parts of the body were exposed, plus the age, gender, and general health of the person. 

“We have to provide an informed risk estimate to the crew members,” Nelson said. “Here’s the risk to you if you go to space — to the best of our knowledge, this is your excess risk in whichever category. And then the person has to decide. Are they willing to accept that against some benefit — to themselves, to NASA, to the public at large? Does your family agree with that? Does your lawyer agree with that?”

When discussing health risks, astronauts are often quite willing to accept risks to their own safety. After all, space exploration is dangerous for a whole host of reasons, including the real danger of potential failure of a spacecraft or launch vehicle that can result in death. Next to that, the risk of developing cataracts or an increased risk of cancer can seem like a lesser concern. 

But agencies like NASA also have to consider the views of family members and other people in astronauts’ lives. “There are family stakeholders here, who really do have a stake in what happens, and who want to weigh in on those decisions,” Blakely said. “And when that is folded in, it gives a new perspective to what you come up with as a limit [for radiation exposure].”

Considering the long-term health risks to astronauts, especially those who are younger, from the perspective of their families carries a different emotional weight than thinking purely about oneself. “I’m not sure if I was the mother of those people, that I’d want that,” Blakely said. 

But the considerations of individual harm have to be balanced against the potential of discovery from exploration — including all the things that we could learn about the human body. 

“Exploration is thought to be important to our country for many reasons, and we’ve learned so much about health from it. It’s amazing,” Blakely said. 

Whether it’s the glittering Martian cities envisioned by Musk or, more realistically, a small group of explorers heading to Mars for periods of a few months to a few years before returning to Earth, the payoffs of sending people to another planet could be profound — we just need to be clear about the costs.