As is the way of news cycles, in recent days we're back to hearing about plans for setting humans up on Mars. A few years ago this idea was in the spotlight because of now-defunct efforts like Mars One, which somehow got 200,000 people to express interest in what would have been a lifelong trip to the red planet. We've also seen Elon Musk's vision of how SpaceX would eventually provide a human "backup plan" by permanently settling Mars.

This past week Musk brought the idea up again, in typically provocative fashion, by talking about sending 1 million people to Mars by 2050, using no less than three Starship launches per day (with a stash of 1,000 of these massive spacecraft on call). He also raised the possibility of giving wannabe martian settlers loans to enable them to pay for the opportunity. Naturally, for many observers this also provoked discussion of indentured servitude for those "seeking a new life in the off-world colonies", to paraphrase a famous line from the 1982 movie Blade Runner.

But whatever you think about Musk's pronouncements, or his businesses, there are some very serious scientific hurdles to setting humans up on Mars (and in full disclosure, I own a few Tesla shares and I greatly admire his vision and drive for terrestrial change as well as the space-launch business, but I'm also somewhat wary of people being taken seriously just because they have amassed a lot of cash).

One of those hurdles is radiation. For reasons unclear to me, this tends to get pushed aside compared to other questions to do with Mars's atmosphere (akin to sitting 30km above Earth with no oxygen), temperatures, natural resources (water), nasty surface chemistry (perchlorates), and lower surface gravitational acceleration (1/3rd that on Earth).

But we actually have rather good data on the radiation situation on Mars (and in transit to Mars) from the Radiation Assessment Detector (RAD) that has been riding along with the Curiosity rover since its launch from Earth.

The bottom line is that the extremely thin atmosphere on Mars, and the absence of a strong global magnetic field, result in a complex and potent particle radiation environment. There are lower energy solar wind particles (like protons and helium nuclei) and much higher energy cosmic ray particles crashing into Mars all the time. The cosmic rays, for example, also generate substantial secondary radiation - crunching into martian regolith to a depth of several meters before hitting an atomic nucleus in the soil and producing gamma-rays and neutron radation.

An analysis by Hassler and colleagues, published in 2014 in Science, noted that a human expedition with 360 days total in interplanetary space, plus 500 days on Mars itself, would expose astronauts to just over 1 sievert of radiation. Now statistically that's not too awful. It would increase the odds of you getting fatal cancer by some 5% over your lifetime.

However, if we consider just the dose on Mars, the rate of exposure averaged over one Earth year is just over 20 times that of the maximum allowed for a Department of Energy radiation worker in the US (based off of annual exposure).

And that's for a one-off trip. Now imagine you're a settler, perhaps in your 20s and you're planning on living on Mars for at least (you'd hope) another 50 Earth years. Total lifetime exposure on Mars? Could be pushing 18 sieverts.

Now that's kind of into uncharted territory. If you got 8 sieverts all at once, for example, you will die. But getting those 8 sieverts spread out over a couple of decades could be perfectly survivable, or not. The RAD measurements on Mars also coincided with a low level of solar particle activity, and vary quite a bit as the atmospheric pressure varies (which it does on an annual basis on Mars). 

Of course you need not spend all your time above surface on Mars. But you'd need to put a few meters of regolith above you, or live in some deep caves and lava tubes to dodge the worst of the radiation. And then there are risks not to do with cancer that we're only just beginning to learn about. Specifically, there is evidence that neurological function is particularly sensitive to radiation exposure, and there is the question of our essential microbiome and how it copes with long-term, persistent radiation damage. Finally, as Hassler et al. discuss, the "flavor" (for want of a better word) of the radiation environment on Mars is simply unlike that on Earth, not just measured by extremes but by its make up, comprising different components than on Earth's surface.

To put all of this another way: in the worst case scenario (which may or may not be a realistic extrapolation) there's a chance you'd end up dead or stupid on Mars. Or both.

There is also a real difference between a small group of astronauts being constantly monitored, advised, and trained to optimize their time on Mars (whether brief or long term), and a million settlers eager to be pioneers. The old trope of "what could possibly go wrong?" springs to mind.

Obviously no one, not even an emboldened SpaceX, is going to plop humans down on Mars en masse without worrying about all of this. But I think it's an open question as to just how big a challenge the radiation hurdle turns out to be, along with all the other hurdles.