Part of my series on common misconceptions in space journalism.

“With no magnetic field, Mars has no defense against harsh solar radiation. If I were exposed to it, I’d get so much cancer, the cancer would have cancer.”

― Andy Weir, The Martian

“The Martian” is one of the hardest science fiction novels ever written. In a previous post, I gave its technical accuracy an A+. Andy is a smart person who takes the trouble to do the research and get things right.

That said, there is a common misconception that humans traveling in space will have their faces melted off by radiation like something out of Indiana Jones.

Radioactivity remains a relatively esoteric corner of chemistry and physics research. Fears of radiation have seriously harmed the nuclear energy industry, while technology restrictions on proliferation have ensured that radioactivity expertise remains relatively rare. There are few topics outside of religion that are so polarizing!

In large doses, or rapid doses, radiation can cause serious harm and death. This chart by XKCD gives an excellent summary of the various levels of radiation that humans can be exposed to.

It turns out that the surface of the Earth is a radioactive place, and always has been. We are constantly gently bombarded with “background radiation,” which includes galactic cosmic rays and decay products of both natural and artificial radioactive elements in our environment.

As one climbs through the atmosphere, the amount of radiation gradually increases. Pilots and mountain dwellers get a greater dose. Astronauts in orbit get more still. Astronauts who leave Earth’s orbit to go to the moon get even more, particularly while traversing the van Allen belts.

Out in space, away from radioactive rocks, there are many sources of radiation that can potentially cause harm to life. These include ultraviolet sunlight, solar wind, and cosmic rays. There are also a few places that have extreme levels of radiation, such as within Jupiter’s magnetosphere, but currently it is relatively easy to avoid visiting such places in person!

The Mars Rover Curiosity carries a radiation instrument specifically designed to measure the radiation (RAD) absorbed by a human flying to, and living on the surface, of Mars. Since 2012, this instrument has taken out a lot of the guess work. We now know what sort of doses astronauts flying to Mars would take.

There are two main phases of the mission to consider separately. The first is flying to Mars, which takes about 6 months and is in deep space, far from any planet. The second is living on the surface of Mars.

In deep space, it turns out that the radiation dose hovers around 500 mS/year. If absorbed all in one go (over minutes to a few days), 500 mS would cause symptoms of radiation poisoning, but with a very low chance of death. Fortunately, this dose is more like 250 mS over the six month flight. Like many poisons, the dose rate is important. Consuming 6 months worth of alcohol in a single sitting would also be very ill-advised.

This 500 mS/year background is caused by cosmic rays, high energy nuclear particles that can easily penetrate a few meters of material, so it’s not practical to shield them on a spaceship, which is typically made of a few millimeters of metal. It’s also worth noting that the Q-factor, or multiplicative damage factor, is relatively low for cosmic rays, compared to the showers of particles that incomplete shielding can generate.

Occasionally, on the trip to Mars, the radiation level will increase by one or two orders of magnitude, as shown in this chart from RAD.

These spikes are caused by passing solar flares, or coronal mass ejections. They are typically protons, neutrons, and helium nuclei that move at a substantial fraction of the speed of light, and with energies about 1000x lower than the cosmic rays. This means that they are pretty bad news as far as radiation goes – in fact they’d most likely kill any astronaut they hit. Fortunately, their lower energy means they can be shielded with a just a few inches of light elements, such as plastic or water. For spaceships taking humans to Mars, there will be a small shielded room in the middle of the structure where the crew can take refuge for a few hours while the solar flare passes.

Once on the surface of Mars, the radiation level drops quite substantially. This is due to both the planet blocking space radiation from below, and the miserably thin atmosphere blocking most of the solar wind from above. As a result, the unshielded dose on the surface hovers around 200 mS/year, with occasional spikes up to 250-350 mS/year (equivalent rate) during particularly energetic solar particle events.

Of course, any astronaut living on Mars for a long time would have a house with some shielding on the roof, so would only get the 200 mS/year equivalent dose when traveling outside in a spacesuit or light rover. Still, what sort of effect would taking that much radiation for the rest of your life cause?

There are a few ways of approaching this question.

First, this dose will very quickly exceed US Federal Radiation Worker limits in non-emergency scenarios. Many first responders in Chernobyl or Fukushima took much more, and over just a few hours, with no lasting ill effects. All the confirmed fatalities from Chernobyl were due to acute exposure over just a few minutes, rather than extended slow cooking. The occupants of the Chernobyl reactor control room took something like 10,000,000 times the rate on Mars, over a few minutes, and they all died.

There are a few places in the Chernobyl exclusion zone with radiation levels as high as 200 mS/hour, such as the cemetery. This is not morbid – the soil there was contaminated by fallout and couldn’t be remediated without disturbing the graves. Still, millions of animals live in the exclusion zone having a great time, now that their single greatest cause of premature death (humans) have been taken away. It is worth noting to prospective visitors of Chernobyl that the greatest radiation-related risk is the inhalation or ingestion of some tiny fragment of highly radioactive fallout, which would then barbecue some unlucky organ from the inside. Just hanging out in the cemetery is unlikely to cause harm, compared to stirring up dust and eating dirt.

When I began researching this post, I hoped that it would be possible to estimate the dose taken by early radiation workers in the 1920s. Deaths due to exposure to radium are quite famous, particularly in the case of the radium girls who painted luminous paint on clocks, dials, and sometimes themselves. On the other hand, while Marie Curie and some family members died in their 50s and 60s due to radiation-related illnesses, they survived decades of absurdly high levels of exposure before that point. Even today, Marie Curie’s personal effects such as her cookbook are considered too hot to handle. Curie’s youngest daughter Eve, gestated and raised in a bath of radiation during the isolation of radium, lived to the age of 103! In short, a few data points are not much better than anecdote. Can we do better?

One of the amazing things about the Earth’s geological processes is that over time, all the mixing, churning, erosion, and biology often concentrate specific minerals, which are sometimes mined. It is much easier to extract, say, iron, from a rich vein of ore than from generic dirt. It turns out that some geological processes concentrate radioactive elements, and so some places in the world have much higher levels of background radiation. The highest known inhabited place is a community built around a hot spring in Ramsar, Iran. The spring’s waters are thought to promote healing, and are loaded with radon, a radioactive gas.

One house has a total background level of 200 mS/year, equivalent to the unshielded exposure on Mars. So people in Ramsar should be keeling over left and right from radiation? To quote Andy Weir, surely their cancer has cancer? In short, not only do people in Ramsar live apparently long and normal lives, there is no local increased rate of cancer attributable to radiation exposure!

It can be challenging to conduct long term longitudinal studies on people and to extract meaningful information. In particular, Ramsar is not a huge place, so there aren’t that many people to begin with. Additionally, many of them are already of advanced age, so a greater incidence of cancers and other health problems is expected. But most importantly, nearly everyone there smokes and it’s hard to separate cancers caused from inhaling smoke compared to cancers caused by inhaling radon.

And here we’ve touched on an important point. There have been hundreds of studies conducted on incidents of accidental radiation exposure, which span anything from people grabbing highly radioactive cobalt-60 sources, to people living in apartment buildings in Taipei, contaminated with radioactivity. None of these studies has found conclusive evidence either for or against great harm caused by extended doses of elevated background radiation. It’s equally consistent with the data that small doses of radiation actually reduce risk of cancer, or have no effect, or have a disproportionately increased risk. It’s very controversial.

Yet if we do a longitudinal study of people exposed to cigarette smoke, there is a very clear signal. Heavy smokers double their lifetime risk of fatal cancer from 20% to 40%. Heavy drinkers have substantially elevated risks of cancer. People who live near freeways or endure exposure to car exhaust or diesel fumes also run a substantially increased risk of cancer and other health problems. People who get sunburned often develop fatal melanoma, particularly in my native Australia. One bad sunburn doubles the risk of cancer. Obesity carries a very clear signal of harm to health and early death.

The key point is that while I have no doubts that extended exposure to high levels of radiation isn’t great, it needs to be kept in context to understand its contribution to overall risk of premature death. On the one hand, we know that partly shielded astronauts living on Mars may be exposed to ~100 mS/year, which some studies have suggested causes a few percent increase in the risk of cancer. On the other hand, one would hope, they won’t be smoking, getting sunburned, or inhaling diesel fumes, all of which we know can increase risk of cancer by 5-50%.

Further, all astronauts have a substantially increased risk of premature death due to non-cancer causes. In fact, of the ~600 people who have ever been to space, not one has ever died or been injured by a radiation-related issue. On the other hand, ~20 astronauts have been killed in flight, another ~20 in training-related accidents, and many more than this have narrowly survived near-fatal accidents or mishaps. As far as professions go, being an astronaut is dangerous as well as cool, with a 7% career-length fatality rate.

What about an individual mission? The NASA verbiage for catastrophe is Loss of Vehicle/Loss of Crew, and for a long time experts in failure mode effects analysis have been attempting to estimate the risk for any given mission.

Recent estimates for risk during the Apollo lunar landings are one in six. That is to say, at the moment of launch, the astronauts had only 84% chance of coming back alive. Given the magnitude of the challenge involved this is actually pretty impressive. It also accords quite well with our flight experience, which was six successful landings and one narrowly averted catastrophe in Apollo 13.

The Shuttle was much safer, right? Not so much. Best estimates of catastrophe place early flights at one in nine, and the later flights as low as one in ninety. In 135 missions we lost two orbiters, which is actually pretty lucky considering the intrinsic risk. Most estimates place the chance of so few losses at about 5%.

An early reference standard mission to Mars, taking around 900 days, would realistically be lucky to be safer than one in ten odds, though NASA officially targets one in 100. On this mission, the astronaut might absorb 1000 mS of radiation over three years, increasing their lifetime risk of cancer by, at most, a few percent. That there are millions of people who’d take that risk is immaterial – the odds of dying of something other than cancer, such as launch or re-entry mishaps, are much, much greater.

Extended habitation on the surface has a different risk profile than Shuttle flights, where the highest time density of risk accumulated during launch and landing. Instead, we can compare the risk to other dangerous occupations in hostile environments, such as scientific outposts in Antarctica or professional rock climbing. In both cases, accidents are both relatively rare and often catastrophic, so may be modeled with a Poisson distribution. Like BASE jumping or riding motorcycles, making a habit of it tends to guarantee the outcome. The only question is how long until one’s luck runs out!

The implication for long-term cities on Mars is that humans old enough to have professional qualifications fly one way and, having survived the journey, set out to build a new life until they either fly back to Earth or die for some reason, including old age. While most humans on Mars will live almost entirely within climate controlled, pressurized habitable areas, they will be living in and working in an industrial environment that is vulnerable to fires, depressurization, system failure, design flaws, and anything else that can hurt people in confined spaces. People who work outside in space suits will face the risk of leaks and malfunctions every time they go to work, which more than offset their increased exposure to background radiation.

In the background is the higher-than-usual accumulation of radiation due to unshielded cosmic rays. The key point of this whole exercise is that if all other confounding sources of early death are low enough that, unlike in Ramsar Iran, there’s a clear signal for radiation-induced cancer, the Mars base will be one of the safest places in the solar system and a roaring success.

In summary, of all the things that I worry about killing people on Mars, radiation barely makes the list.