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Given an O'Neill "Island Three" style cylindrical habitat located at Mercury–Sun L5 orbit, how much radiation shielding would be required to protect the occupants, and how would it need to be structured?

The Island Three O’Neill Cylinder design has two counter-rotating cylinders aligned with the axes pointed towards the sun, with a sun shade/solar power facility at the sunward end, and mirrors to reflect light into the the habitats. A sun-shade and radiator seemed to be enough for the recent MESSENGER mission.

I haven't found a source for how much radiation to expect in Mercury orbit, which comes within 46 million kilometers of the Sun.

In Orbital Space Settlement Radiation Shielding (Globus 2016), Globus recommends a limit of 20 mSv/year exposure for the general population, and 6.6 mGy/year exposure for pregnant women.

Globus recommends water as the most effective shielding material, but his calculations are for the LEO environment, where Earth's radiation belts are more of a concern than GCR.

Ignoring the cost, assuming all the materials needed, and the technology to construct them, what radiation shielding would be required to limit the risk of life-long occupants, especially pregnant women, of such a habitat, and how would the shielding need to be configured?

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  • $\begingroup$ I couldn't post more than one link in the body of the question, but here's a quick description of the Island 3 habitat design: nss.org/settlement/space/oneillcylinder.htm $\endgroup$ – David Jul 24 '17 at 14:54
  • $\begingroup$ It's more of an engineeringSE question, imo and possibly a list type question, sorry. You could think of moving it there if you don't get answers here. $\endgroup$ – Countto10 Jul 24 '17 at 15:04
  • $\begingroup$ I was originally going to post this stuff as an answer but it really is not an answer to your question as asked. However it's certainly something to think about! $\endgroup$ – uhoh Jul 26 '17 at 3:23
  • $\begingroup$ Not sure why you need to be at Mercury's L5. If you want a stable lagrange point, might as well pick Earth's, better balance of solar input for what humans need. Also that close you will need far more shielding, but if you want/need to be close to the Sun and/or Mercury, might as well go with a Sun facing polar orbit of Mercury to take advantage of it's small magnetic field and access to resources. Also you can use anything for shielding, it just takes more of some stuff than other stuff. The solar system has a lot of water in the asteroid belt to be put to use. $\endgroup$ – Brooks Nelson Jul 26 '17 at 17:51
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    $\begingroup$ Why? To play with an interesting sf setting! :D The motivations of the setting's habitat architects are more complicated of course. I'm definitely seeing that either a Mercurial orbit or a purely heliocentric orbit are probably better choices—thanks to @uhoh for that insight—but whichever way it's still going to require a lot of shielding. $\endgroup$ – David Jul 26 '17 at 23:18
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You have to worry about the solar wind - the charged particles that are emitted by the Sun, and which are quite effectively shielded by the Earth's magnetic field. The magnetic field of Mercury is very weak (150x weaker than Earth's), and probably wouldn't do a very good job deflecting the solar wind. See details here. It also has negligible atmosphere (low gravity and strong solar wind stripped away any semblance of atmosphere as we would recognize it).

Fortunately, charged particles are stopped quite efficiently by water. But you also need to worry about neutrons (byproducts of the fusion process) and of course the intense heat and gamma radiation.

It's not a bad start to look at the radiation dose that astronauts receive in the ISS - see for example this summary that tells us that they might get 160 mSv of radiation over a 6 month period. We need to scale that number because Mercury is closer... with an eccentricity of 0.21, it is sometimes "much closer" to the Sun than at other times (from 4.6E10 to 7.0E10 m, compared to Earth at 1.5E11 m).

When it is closest to the sun, its orbit is 3x closer, and the radiation intensity would be 9x greater. To get back to the levels of the ISS, you need shielding that cuts the radiation dose by about 10x. But that ignores the fact that the ISS is operating "within Earth's magnetic field", and thus is still benefiting somewhat of the shielding that that provides.

A lot of detail can be found in this article - although it is specifically talking about the issues for a Mars mission, the principles are the same; it's just that the radiation levels on Mercury will be much greater, as the orbital radius is about 1/4 of that of Mars... note also that this means that when there is a significant solar flare, you don't have much time to hide...

A "lot of water" should do the trick for most of the radiation.

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    $\begingroup$ Just to clarify, I'm looking at Mercury–Sun L5 (or is it preferable to say Sun–Mercury L5?), the trailing Trojan orbit (unless I have that backwards and it's leading). I'm guessing there's no protection from Mercury's magnetic field there. $\endgroup$ – David Jul 25 '17 at 14:59
  • $\begingroup$ Thanks for the clarification. You're right there is no magnetic protection from Mercury at the L5 point. The first paragraph was talking about "on Mercury" - noting that the usual mechanisms for protection on Earth (atmosphere, magnetic field) don't do much for you there. At the L5 point you are at the same distance from the Sun as Mercury (IIRC - it's an equilateral triangle of the 3 bodies) so the rest of the math holds. $\endgroup$ – Floris Jul 25 '17 at 15:26

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