Astronaut chilling on the Mars surface, getting his daily dose of Galactic Cosmic Rays.
Radiation risk to Mars exploration crews – the journey there and the orbit
To quantify the risk level associated with radiation on Mars to astronauts and potential Martians, we must first measure it.
The first measurements of radiation close to Mars were conducted by The Mars Radiation Environment Experiment (MARIE). This instrument was on board of the 2001 Mars Odyssey orbiter. MARIE measured radiation on its way to Mars and, after orbit insertion, in the Mars orbit. This experiment showed what we expected: because Mars lacks a strong magnetic field, astronauts on Mars orbit would be exposed to about 2.5x higher Sun particle and Galactic Cosmic Rays particle radiation than astronauts in ISS on the orbit around Earth. This is not great, but not terrible.
If we assume that the stochastic risk of developing cancer linearly increases with the cumulative radiation dose, we can make a comparison of the risk of cancer to astronauts orbiting Mars (~1.2 mSv/day) to astronauts on the orbit around Earth (~0.5 mSv/day).
So far we know that astronauts, many of which has spent more than 2 years in the Earth orbit, have developed less cases of cancer than would be expected for the general population. This information might be surprising to you, as we all know that risk of cancer in astronauts due to radiation is increased. However the risk is calculated based on models using studies with enormous error bars: “NASA’s radiation limits set a 3% cancer fatality probability as the upper bound of acceptable risk and considers uncertainties in risk predictions using the upper 95% confidence level (CL) of the assessment.”
Because of the large error bars due to epidemiological uncertainties, NASA highly overestimate the risk to astronauts, and the predicted risk is therefore much less than 1% of the increase chance of fatality before the astronaut reach 80 years of age. And so far this predicted risk has not translated into increased cases of cancer in astronauts.
To put that into perspective, 1 in 6 people worldwide prematurely dies of cancer. Hence NASA allows the astronauts to increase risk of dying from cancer from the “natural” 16% up to about 17%. For comparison, if you decided to pick up smoking, your change of dying from lung cancer caused by the smoking would be 7.7% if you smoke up to 5 cigarettes a day, and 26.4% if you smoke a pack a day. These percentages are however just associated with the lung cancer. The other causes of premature death caused by smoking, such as all other types of cancers, strokes, and heart attacks almost guarantee that you will prematurely die for your passion if you pick up smoking: A privilege that NASA astronauts expanding our horizons are not allowed to have.
Radiation risk to Mars exploration crews – the surface
So far we’ve discussed orbits of Mars and Earth. But nobody wants to live in the Mars orbit. So will you be prematurely killed by the radiation living on the Mars surface?
For that we will have to check data from The Radiation Assessment Detector (RAD). This instrument is on board of Curiosity rover and measures radiation levels on the Mars surface to prepare for future human exploration. RAD measures and identifies all high-energy radiation, such as protons, energetic ions of various elements, neutrons, and gamma rays. That includes not only direct radiation from the sun and space, but also secondary radiation produced by the interaction of radiation with the Martian atmosphere and surface rocks and soils.
The results provided by RAD were analyzed and assessed for the risk to the future astronauts by international teams of researchers (here and here) and the effect of the Mars radiation have been summarized in the COPSAR publication (here).
To build up an intuition about the dose equivalent, the average American receives 6.2 millisieverts (mSv) yearly from all radiation sources. If you undergo a cardiac CT scan, you will receive 20 mSv in a single dose. In some high-background radiation environments on Earth you can receive up to hundreds of mSv a year. One such place is a city Ramsar in Iran where people have lived for centuries receiving dose upwards of 300 mSv/year from the uranium and thorium in the bedrock, without any documented adverse effect on their health.
In comparison, unshielded on the Mars surface you would be on average exposed to only between 100 and 200 mSv/year, depending on the Solar cycle and altitude. This mean that unshielded Mars explorers would be exposed to about twice smaller radiation dose than families occupying high radiation environments on Earth for many generations.
And so if you sleep in your cave on Mars, and don’t explore Martian surface during solar storms, radiation is unlikely to have any statistically significant adverse effects on your health. Compare that to living the “modern” life without sufficient exercise and without healthy diet that is know increase your risk of dying much more significantly: Spending your days on a couch in front of the TV exposes you to more health risks than radiation on Martian surface.
How is that possible? Isn’t radiation the silent killer?
My fellow Czech researchers showed that humans when exposed to a prolonged low-level radiation respond by increasing their DNA repair efficiency. In other words: you will get used to Mars-level radiation on a molecular level. Your body can combat the adverse effects due to radiation thanks to training and hormesis.
NASA and COSPAR are extremely cautious when it comes to radiation exposure of the astronauts and presume a linear dose-response model. Hence their “risk” assessment is exaggerated. When NASA talks about radiation “danger”, always remember that they talk about the less than 1% increase (from ~16 to 17%) of a chance of dying from cancer before 80 years of age. The variation within population is much higher than the 1% increase. Yes, there is still a lot of unknowns, but I would be willing to bet my life that the radiation on Mars is not an issue we should worry about.
The question is, should we allow NASA astronauts to undergo a risk of a premature death caused by radiation that is an order of magnitude lower than a risk arising from many bad lifestyle choices people make every day?
Radiation risk to the hypothetical microbial Martians: they are fine (except for UV).
To understand why the cosmic radiation is much less of a problem for a single cell life form than for humans, consider that to develop cancer, just a single cell of the ~30 trillion cells making up your body must undergo genetic changes to become a “mutant precursor cell” that will grow into tumor that might eventually kill you. In contrast if one in one million bacterial cells mutates in Martian soil … nothing changes.
To kill E. Coli bacteria population, you would have to expose it about 10 million higher dose of radiation than is currently on Mars (LD50 is 700 Gy/day).
The only radiation related issue for Martians is the UV radiation, that can however be easily shielded by few millimeters of soil.
As long as there are more cells produced than destroyed, single cell life would not be much affected by the radiation. The levels of particle radiation on Mars is negligible from the bacterial point of view.
Martians have these two options:
- In the case life found a way to replicate under the current Mars surface conditions, the particle radiation (galactic cosmic rays and solar energetic particles) is not an issue and life can be either hiding under a thin layer of Mars dust, or if protected by pigment from the UV, it can be directly on the surface.
- In the case life did not find a way to replicate in the currently arid and cold environment, the dormant cells would be eventually destroyed by the cosmic rays down to several meters underground. Living cells can be however waiting for the high obliquity periods few meters down, for occasions when the surface of Mars becomes transiently wet.
The second option is a more conservative interpretation that I am inclined to believe in, but we do not have data refuting the first option.
A microbe hiding 20 ft. underground in the Lasagna ice on Mars would be as safe from radiation as ancient bacteria deep in Earth glaciers, which we know can survive imbedded in ice for millions of years. From there it can be ejected via meteoric impact onto the Mars surface. At the times of high obliquity, this ejected microbe is likely to land in a wet spot, metabolizing the accumulated high energy molecules: perchlorates and oxygen as electron acceptors and carbon monoxide or meteorite-delivered organics as electron donors and fixing CO2. During the high obliquity period the exhumed microbes would replicate fast and re-populate the Mars surface.
The radiation would not be an issue if life can reproduce faster than the radiation very slowly damage it.
To find the Mars life, we must analyze the subsurface ice as ALFA Mars suggests.
Conclusion
When someone tells you that something is dangerous, ask them to quantify the danger ideally in micromorts or microlives. Chronic exposure to Mars surface radiation puts you in much less danger of premature death (if any) than being chronically overweight.
A friend of mine told me that pilots have a higher chance of getting cancer because they are daily exposed to high levels of cosmic radiation. This issue has been reviewed in a recent article. This study shows that although the pilots and air crew are exposed to about 50% higher levels of cosmic radiation than general population, there is still no evidence of radiation dose-cancer relationship. Authors of the study speculate that pilots and air crew might have higher risk of cancer than general population because of the stress related to their disruption of circadian rhythm. Note that millions of people live in regions with background radiation above 10 mSv/year, at radiation dose higher than the air crews without any adverse effect on their health.
