The Invisible Problem of Sending People to Mars

Sending people to Mars won’t be easy. There are the obvious challenges of getting people and supplies into space and landing them safely on another planet. And once they get there, they’ll need a safe place to live, with air to breathe, water to drink, and food to eat. But the biggest obstacle to manned Mars exploration may be something entirely invisible and often overlooked: space radiation, which 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 may 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 leaving not just its gravity, but its protective bubble. And we’re only just beginning to uncover 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 dying stars, and that radiation is part of the vacuum of space when you travel,” explained radiobiologist and radiation expert Eleanor Blakely.

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

The particular challenge of space exploration is that it involves prolonged exposure to low levels of radiation, very different from most radiation exposures that occur here on Earth.

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

This kind of space radiation is not like the low-dose exposure from a chest X-ray. Instead, imagine a charged particle traveling at nearly the speed of light, shooting straight through your brain, disrupting 10,000 cells in a row, all in a microsecond. It doesn’t necessarily damage those cells, but it activates them in a very unusual way. And we don’t yet know what it does.

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

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

Another concern is that astronauts aren’t just exposed to radiation. During space travel, they also have to deal with microgravity, which is well known to cause health problems.

There are the more obvious effects, like the loss of muscle tissue because the muscles aren’t working against gravity. But there’s also evidence of other effects like brain remodeling. “That means the tissues are being activated in a different way than usual,” Blakely explained, like changes in 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 beginning to study how the effects of microgravity and radiation exposure might add together.

“There’s 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 unclear whether the effects add to each other or if they produce an even worse outcome when combined. Nelson pointed to evidence of changes in bone health, the blood-brain barrier of the central nervous system, and specific features of the eye as areas of open research.

A combination of radiation exposure and sleep deprivation could also lead to further cognitive impairments, according to recent research in rodents. That’s without even considering the additional effects of isolation on long-duration space missions and the psychological toll of confinement.

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

NASA calculations show that longer missions to Mars could expose astronauts to radiation exposures exceeding 1 sievert, which is above the agency’s acceptable limit for lifetime exposure. However, when sending people to Mars, the greatest radiation risk occurs during the time they are on the ground. On the surface of Mars, there is some protection from being on the surface of the planet, so the real concern is the 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 get into the range where, at least in rodent studies, you can see some changes,” Nelson said. “And how that extends to humans, we don’t know with great certainty yet.”

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

The time spent in space could be reduced by using technologies such as nuclear propulsion, which NASA is working on, but this comes with risks, especially if something were to go wrong during launch, as an explosion could disperse radioactive material into the Earth’s atmosphere.

There are ways to protect astronauts from radiation, such as shielding. But this is also not a simple proposition.

“Intuitively, we’ve all come to think, ‘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 the radiation comes from one direction. But with charged particles, which come from all directions, that’s not the case.

“With charged particles, one of the things that happens is they break into pieces,” Nelson said. “And the smaller pieces have the ability to penetrate deeper than the larger ones. So sometimes more shielding actually makes the problem worse.”

There is a “sweet spot” for radiation shielding that protects against some of the larger pieces without creating too many secondary pieces. Some of the most effective shielding is actually materials like polyethylene rather than metal, because they have more hydrogen atoms and are less likely to create small pieces.

In certain circumstances, it is possible to create layers of material to act as protection, such as having astronauts sleep in more shielded areas, but eventually astronauts will have to venture out and explore.

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

NASA has strict limits on the amount of radiation an astronaut can be exposed to during his or her career, equivalent to a 3 to 4 percent excess risk of death from all causes. These limits were recently changed, somewhat controversially, because it is difficult to determine how much radiation exposure is safe. Different types of radiation affect people differently, based on factors such as the parts of the body exposed, as well as the person’s age, gender, and general health.

“We need to provide an informed risk assessment to the crew members,” Nelson said. “Here’s your risk if you go into space: as far as we know, this is your excess risk in any category. And then the person has to decide. Are they willing to accept that in exchange for a benefit, for themselves, for NASA, for the general public? Is your family okay with that? Is your lawyer okay with that?”

When it comes to health risks, astronauts are often quite willing to take risks for their own safety. After all, space exploration is dangerous for a variety of reasons, including the very real danger of a potential spacecraft or launch vehicle failure that could cause death. Next to that, the risk of developing cataracts or an increased risk of cancer may seem like a minor concern.

But agencies like NASA also need to consider the perspectives of family members and others in the lives of astronauts. “There are family stakeholders here who really have a stake in what happens and want to weigh in on those decisions,” Blakely said. “And when that’s included, it brings a new perspective to what you come up with as the constraints. [for radiation exposure].”

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

But considerations of individual harm must be balanced with the potential for discovery that comes from exploration, including all the things we might learn about the human body.

“Exploration is thought to be important to our country for a lot of reasons, and we’ve learned a lot about health from it. It’s incredible,” Blakely said.

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