Two threats, not one
Galactic cosmic rays (GCR) and solar particle events (SPE) are usually discussed together and should not be. They differ in energy, in timescale, and in what can be done about them.
Galactic cosmic rays are a continuous, low intensity flux of very high energy nuclei from outside the solar system. They are not stoppable in any practical sense: a few centimetres of shielding makes things worse, because the primary shatters into a shower of secondaries. Only mass in bulk helps, and even then the return diminishes.
Solar particle events are sudden, intense bursts of mostly protons from the Sun. They are far lower energy and therefore genuinely stoppable, but they arrive with very little warning and can deliver a career dose in hours.
| Property | Galactic cosmic rays | Solar particle events |
|---|---|---|
| Source | Outside the solar system | The Sun, flares and coronal mass ejections |
| Energy | Up to 10¹⁰ MeV, GeV typical | 10 to 500 MeV typical |
| Timescale | Continuous, always present | Hours, a few times a solar cycle |
| Annual dose, unshielded | about 380 mSv | 0 to 1,000+ mSv in a single event |
| Shieldable | Poorly, and secondaries make it worse | Yes, effectively |
| Countermeasure | Bulk regolith, and time limits | A shelter, and warning |
The two metre target
Shielding effectiveness is measured in areal density, grams per square centimetre, not in thickness. Regolith is about 1.5 g/cm³ loose near the surface and closer to 1.8 at depth or compacted, so two metres of it is roughly 300 g/cm² or a little more. That is a great deal of shielding by any standard.
The unshielded figure needs a word, because it is quoted three ways. The one measurement is Chang'e 4's Lunar Lander Neutron and Dosimetry instrument, which recorded 1,369 microsieverts a day on the far side in 2019, at solar minimum when galactic cosmic rays are strongest: about 500 mSv a year as dose equivalent in a small detector. A body shields itself, so the effective dose to a person is lower; Matthiä and Berger's 2024 transport calculation puts it near 306 mSv a year for the same conditions, and Reitz, Berger and Matthiä estimated about 380 mSv a year at solar minimum and 110 at solar maximum in 2012. This document uses 380, the solar minimum estimate, as its design value, and it is the figure every table below starts from.
The curve below is the reason for that number and not a larger one. GCR dose falls steeply at first, then flattens, and past about 250 g/cm² the secondaries produced in the shield itself start to offset further gains. Two metres sits just past the knee.
The transport calculations published since put the knee in a less flattering place, and the chart should be read with them in mind. Matthiä and Berger find that the first 20 g/cm² of material cuts the effective dose rate by about a quarter, that secondaries then hold it nearly flat until well past 100 g/cm², and that 180 g/cm² of regolith gives about 190 mSv a year against 306 unshielded, with neutrons by then about half of the dose equivalent. Horst and colleagues' 2022 Monte Carlo study gets below about 100 mSv a year only with 400 g/cm² and a polyethylene liner inside it. The 126 mSv the chart shows at two metres is therefore a design estimate at the optimistic end of the published range, and the number to carry until a transport calculation is run for this hull and this cover lies somewhere between 126 and 190.
None of it is in place yet. The base is shown at its early increment: each module stands on a compacted regolith skirt to about a fifth of its height, which anchors the hull and protects its foot but is not radiation shielding. So the modules sit at the left edge of the chart below, at the unshielded figure, and bulk regolith over the vessels is the increment two job in section 08.
The storm shelter
Two metres over the habitat will handle the routine case once it is placed. Bulk regolith does not handle a large solar particle event either, because during one the dose rate inside a normally shielded module can still be significant, and because the crew may be outside when it starts. In the built increment the shelter is the only shielding on the base, so it matters more, not less.
The shelter is a volume under the habitation core deck, surrounded by the water and waste tanks the base has to carry anyway. Water is an excellent proton shield per unit mass, and putting the tanks around the shelter costs nothing that was not already in the mass budget. The whole crew fits, uncomfortably, for the duration of an event.
The first protons of a large event can reach the surface within about thirty minutes of the flare, and that is the warning the shelter is sized to; the forecast services being built for Artemis aim to give hours to a day of notice from the state of the Sun, which is what EVA planning uses. Thirty minutes sounds like plenty until you are 800 metres out at the excavation face in a suit. The rule is that no crew member operates further from an airlock than they can return from in twenty minutes, leaving ten for ingress and for the cupola shutter to close.
Dose budget
The table is the design case, with two metres of regolith in place. In the built increment the modules are unshielded except for the storm shelter, so the inside rate is the open surface rate, the 0.043 mSv an hour of the EVA row, and the routine dose is about 380 mSv a year: 380 divided by 365 is about 1.04 mSv a day, so a 180 day rotation collects about 190 mSv before the transit dose. With the transit dose that is about 200 mSv a rotation, inside the 250 mSv per rotation budget this design uses, but a third of NASA's 600 mSv career limit spent in one stay. The budget is chosen against that standard, NASA STD 3001, which sets the career limit at 600 mSv effective dose and also caps the dose to blood forming organs at 250 milligray equivalent in any 30 days and 500 in a year, the short term case the shelter exists for. The hull's own structure, around 20 g/cm², is not credited, which is conservative: the 2024 transport calculation finds the first 20 g/cm² of material cuts the effective dose rate by about a quarter before secondaries flatten the curve. Until the cover exists the dose is managed by the shelter, by the 180 day rotation and by the EVA hours below, which are a budget and not an estimate.
| Exposure | Dose rate | Hours per year | Annual dose | Note |
|---|---|---|---|---|
| Inside, shieldeddesign case, under 2 m regolith | 0.0144 mSv/h | 6,570 | 94.6 mSv | 75 percent of the crew's time |
| Inside the cupolaunshielded aperture | 0.036 mSv/h | 365 | 13.1 mSv | One hour a day, and worth it |
| EVA, suitedextravehicular activity, no bulk shielding | 0.043 mSv/h | 912 | 39.2 mSv | 2.5 hours a day of surface work |
| In the shelterduring events | 0.002 mSv/h | 48 | 0.1 mSv | Two events a year, 24 hours each |
| Transit, Earth to Mooneach way | 1.2 mSv/day | 192 | 9.6 mSv | Four days each way, twice |
| Total per 180 day rotation | 78.3 mSv | Against the 250 mSv per rotation budget |
The EVA contribution is the one that constrains operations. Surface work is 12 percent of the crew's time and 42 percent of their dose, which is the argument for autonomy doing more of the construction: every hour a machine works outside is an hour a person does not.
The cupola, and whether it is worth it
Once the cover is placed the cupola will be the only unshielded volume on the base, and it costs 13 mSv a year, about 17 percent of the crew's exposure, for an hour a day. In the built increment every module shares that exposure, so the trade here is about the design case. Purely on a dose budget it is indefensible.
It is defended anyway, on evidence. Every long duration isolation study, from Antarctic winter over to Mir to the station, reports the same thing: a window is among the most valued features of the environment and its absence is repeatedly cited in crew debriefs as a stressor. The glazing is cerium doped to resist browning, four panes deep, and behind a shutter that closes in ninety seconds.
The trade is stated here rather than buried because it is a real one and somebody will eventually ask why the safest option was not taken.
Materials and the secondary problem
Not all shielding is equal per unit mass. Hydrogen rich materials are better, because a proton scattering off a hydrogen nucleus of similar mass transfers energy efficiently, while a proton hitting a heavy nucleus produces a shower of secondary neutrons and gamma rays that can be worse than the primary.
| Material | Relative effectiveness per kg | Available here | Used for |
|---|---|---|---|
| Liquid hydrogenthe theoretical best | 1.00 | No | Not carried |
| WaterH₂O | 0.73 | Yes, already carried | The storm shelter jacket |
| PolyethyleneCH₂ | 0.71 | Shipped only | Sleep station liners |
| Regolithsilicates | 0.42 | Unlimited, on site | Bulk shielding over every module, a later increment |
| Aluminiumstructure | 0.38 | Structural only | Not used as shielding |
Regolith is not the best shielding material. It is the one that is already there in unlimited quantity, and 300 g/cm² of a mediocre shield beats 20 g/cm² of an excellent one that had to be launched.
What is not yet known
In the built increment the modules are unshielded except for the storm shelter, so the routine dose is the open surface figure, about 380 mSv a year and about 200 mSv per 180 day rotation with the transit: inside the 250 mSv rotation budget, but a third of the 600 mSv career limit in one stay, with no margin for a large event outside the shelter. The open surface figure itself is an estimate; the one measurement, Chang'e 4's, reads higher as dose equivalent and lower as effective dose once the body's self shielding is counted, and the solar cycle moves it by a factor of three. What two metres of regolith buys is the largest uncertainty in this document: the chart's 126 mSv a year is at the optimistic end of the transport calculations, which give 190 mSv at 180 g/cm² and reach 100 only with 400 g/cm² and a hydrogen rich liner, because secondary neutrons from the shield itself offset more of the gain than the simple curve allows. No transport calculation has been run for this hull, this cover and this ground. The cupola rate in the dose table, 0.036 mSv an hour, and the storm shelter's 12 g/cm² jacket are design allocations, not calculations for the glazing, shutter and tank geometry as drawn. A large solar particle event can still deliver a significant dose inside a normally shielded module, which is why the shelter exists. The cupola costs 13 mSv a year in the design case and is defended on crew evidence rather than on dose.
Increment plan, shelter first
Increment 1, the shelter and the skirt
The modules land with their own structure only, around 20 g/cm², and a compacted regolith skirt is built against the foot of each hull. The storm shelter arrives inside the core, so its water jacket is plumbed and provisioned in this increment; until it is, a large solar particle event means an emergency return. The routine dose is the open surface figure, about 190 mSv per 180 day rotation, so EVA hours are held to the budget in section 04, and at about 200 mSv a stay a crew member's career under NASA's 600 mSv limit is three rotations. This is a real constraint on the first rotations and is stated as such. It is the increment the site shows.
Increment 2, two metres over everything
The excavator places bulk regolith over every pressure vessel. This is the step that takes the routine dose to 126 mSv a year and gives the 180 day rotation its margin.
Increment 3, dosimetry and prediction
Personal dosimeters logged continuously, and a tie into the feed from the solar wind monitors at the Sun Earth L1 point so warnings arrive automatically rather than by voice call from Earth.
Sources and further reading
Every figure in this document traces to one of these. Agency documents and peer reviewed work first, reporting only where it is the primary record.