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Comment by idlewords | Hacker News Reader

The Martian atmosphere does not provide meaningful shielding against heavy ion radiation unless you're way down in the Hellas basin (where no one particularly wants to land). You reduce the flux by half compared to deep space just by having a big rock blocking half of the sky, but you'll still get all the cancer sooner than later.

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A little napkin math says the columnar mass of the Martian atmosphere is 18 g/cm^2, versus 10g / cm^2 of aluminum shielding discussed in the paper. So being on the Martian surface would probably give you something like 1/10 (atmosphere + planetary mass + environment suit) the heavy ion radiation exposure of deep space. You'd need to be pretty deep underground most of the time (no windows, at least) to be safe for the duration of a reasonable visit. I'll pass!

The napkin math falls short a bit because the heavy ions that absorb in the atmosphere will create a shower of secondary radiation that reaches the surface. Similarly, heavy ions that hit the surface will create secondary radiation from rock. This factor is why partial shielding in a spacecraft can give the crew a bigger absorbed dose than having no shielding at all.

Nobody is arguing radiation isn’t a problem on Mars. Just that heavy-ion bombardment per se isn’t something to be concerned with.

That is not true; the heavy ion component of galactic cosmic radiation in particular is the single biggest risk factor in going to Mars (other than the spacecraft breaking).

> galactic cosmic radiation in particular is the single biggest risk factor in going to Mars

Sorry, I was unclear. It’s absolutely an issue going to Mars. It isn’t front and centre on Mars.

It is, though. It's about half as bad on Mars (since the planet physically blocks half the sky) but it remains a showstopper. And trying to shield against it runs you into the problem of getting heavy construction equipment to Mars, plus you run afoul of rules against contamination that we're bound by international treaty to follow, including a ban on digging.

Lol - let’s have a Mars colony without digging.

> rules against contamination that we're bound by international treaty to follow, including a ban on digging

Is this OST related? Or Mars specific?

It's the Outer Space Treaty as interpreted by COSPAR, which has introduced concepts (like Special Regions) that are unique to Mars. One Special Region is defined as anything more than 5m underground.

Actually, there is : https://caseyhandmer.wordpress.com/2019/10/20/omg-space-is-f...

It is time we move away from the LNT (Linear No Threshold) radiation risk modeling. See https://en.wikipedia.org/wiki/Ramsar,_Iran#Radioactivity

You're right, actually, that means even more exposure than I calculated since even the relativistic ions that are absorbed will still have potential biological effects via the secondary particles. Do you know how to calculate the flux from those?

This paper looks like a good starting point: https://www.sciencedirect.com/science/article/abs/pii/S22145...

The Martian atmosphere may not but the surface of the planet does, cutting half the yearly dose versus being in deep space. And I'm not sure I buy the argument that the atmosphere provides little protection. There's a lot of atmosphere there still, enough you can push down on it and rise into the sky if you have some carbon fiber blades moving at very high speeds. I'd think a iron nuclei would hit a lot of atoms on the way down.

> you'll still get all the cancer sooner than later.

Cancer risk from radiation is probabilistic. If you spend all day outdoors then you increase your lifetime risk of cancer by a significant degree, but it's not like you're definitely going to get cancer. Additionally, most habitation modules, at least early on, are planned to be buried in martian regolith.

Also the moon is worse than Mars in all respects with regard to this so you're effectively arguing that humans should never leave the orbit of Earth. I'm not a fan of that future.

(BTW, we deal with much higher radiation levels in hot cells in reactors here on Earth, and those even have windows into them. I'm sure we can work out something.)

You don't have to buy the argument; you can look up the scientific papers. Heavy ion radiation is hard to shield against and creates high energy secondary radiation when it hits air, metal, or rock. Moreover, there is strong evidence cancer risk from heavy ions is considerably greater than predicted by the absorbed dose model we use for other kinds of ionizing radiation. Search on "non targeted effects heavy ions" to read up on this; it's a fascinating topic. The upper error bar right now for a 1000 day Mars mission is upwards of 20% risk of radiation-induced death; most of this is from the heavy ion component of cosmic rays.

Rather than speculation it'd be useful if there was more hard science on the subject, you know by actually doing long term experiments on life in space being exposed to these heavy ions.

You can find scientific papers on a lot of subjects where it's hard to get at objective fact arguing one way or another and it's hard to tell how factual they really are.

Also why would you quote the "upper error bar"? I'd like to know the size of that error bar and what the median is.

> The upper error bar right now for a 1000 day Mars mission is upwards of 20% risk of radiation-induced death; most of this is from the heavy ion component of cosmic rays.

This is just an argument that you need to bury things in regolith, at least early on, until we get better shielding designed for heavy ions.

Here's the specific citation:

Assuming 940 day mission, percent risk of radiation-induced death for a 40 year old woman is:

* mean: 8.8; 95% confidence [2.78, 21.0]

For a 40 year old man:

* mean: 6.49; 95% confidence [2.58, 13.6]

Source is "Cancer and circulatory disease risks for a human mission to Mars: Private mission considerations", Acta Astronautica, 2018.

Narrowing this uncertainty range requires long-duration human or animal experiments outside the magnetosphere.

Thanks for the raw data. That's a very wide range.

It’s also dependent on the effectiveness of various cancer treatments as well as the specific location etc.

These figures aren't limited to cancer, they include the risk of (also poorly understood) heavy ion radiation induced cardiovascular disease.

I'm not very knowledgeable, but couldn't we wrap the space suits in tinfoil or something?

This works for lower energy particles (like protons from the sun), but for heavy ions partial shielding makes the problem worse than not having shielding at all, the small number that hit the metal instead of passing through create a shower of damaging secondary radiation.

Effectively shielding people from cosmic rays requires either a thick atmosphere, a couple meters of soil, or really thick slabs of metal. The best solution on Mars might be to build a shelter inside a cave or lava tube, but then you run up afoul of international treaty obligations to not contaminate these sensitive environments.

The short answer to your question is "yes, but the foil has to be three feet thick."

I mostly agree with that but you don’t need to hand wave as much.

The atmosphere of mars is roughly equivalent too 2 inches of soil if the source is directly overhead and 3 inches at a 45 degree angle, but far more than that at low angles. Anything below the horizon is blocked by the surface of Mars, so you end up with well over 50% shielding from cosmic rays vs the trip to Mars. Being further from the sun is also beneficial.

You definitely want to live and work underground on Mars or at minimum below a large water tank, but even just landing in a valley can noticeably reduce radiation exposure as the mountains block portions of the sky so too do they block a portion of cosmic rays.

I think you have this backwards; GCR flux increases with distance from the sun.

For GCR, but not total radiation exposure. Cosmic rays are a combination of solar energetic particles and GCR.

Though this is dependent on how active the sun is: “If the instrument had been taking measurements on Mars during the comparable period of the solar-activity cycle about 11 years ago, it would probably have seen eight events in 300 sols, contributing an unknown but certainly significantly higher amount to the overall dose.” And of course if Mars was closer to the sun these numbers would be significantly higher. https://www.nasa.gov/jpl/msl/mars-rover-curiosity-pia17600.h...

I've been talking since the top of the thread about the heavy ion component of galactic cosmic radiation, not space radiation in general.

I realize. That line was simply pointing out that while being further from the sun is a net negative in terms of GCR it’s a net positive in terms of total radiation exposure.

> in the Hellas basin (where no one particularly wants to land)

Why don't people want to land down there? Is it too rough a surface, more so than at higher elevation? Would it be feasible to land outside the basin and make your way down?

There's a great StackExchange answer on this. Basically, both the terrain and weather are bad:

"Sites of areologic interest are far apart, its seasonally induced micro-climate conditions are rough and pronounced due to somewhat larger surface atmospheric pressure, winds are stronger, dust storms longer lasting, it experiences higher seasonal and diurnal surface temperature variations, shorter days and exposure to sunlight during Southern hemisphere winters, and as a deep impact basin, terrain is fairly rugged, inclined, and often covered with deep pockets of sand.”

https://space.stackexchange.com/questions/12158/why-has-no-l...

The thing is, if you intend to do something like establishing a completely self-sustaining population on Mars (one that could live indefinitely of the earth disappeared), then you need to have a presence in many many places on Mars, because you need access to numerous raw materials.

For a simple visitation mission, you can pick an easy place, sure (the other reply points out why this basin is not in fact that easy). But this does nothing to prove that long-term habitation of Mars is in any way possible.

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