Radiation shielding: how much regolith over the habitat

Question

How much soil does a habitat need overhead to cut the radiation dose?

Assumptions

• Surface dose0.64 ± 0.12 mSv per day, Curiosity RAD at Gale crater (Hassler 2014)
• Cruise dose1.84 ± 0.30 mSv per day inside the MSL spacecraft (Zeitlin 2013)
• LimitNASA career limit of 600 mSv effective dose (NASA-STD-3001, 2022)
• Depth modelRöstel 2020, Table 3, average solar modulation; equivalent dose in a 15 cm water sphere, which the authors call an upper estimate
• Loose regolith1.5 t per m³ (our assumption)
• Habitat6 × 12 m footprint, 2 m margin each side, flat cover, no side berms (our assumption)

The numbers

Unshielded surface:  0.64 mSv/d × 365 ≈ 234 mSv/yr → 600 mSv in ≈ 2.6 yr
Cruise (2 × 180 d):  1.84 × 360 ≈ 662 mSv  (Zeitlin 2013)

Areal density for 50 mSv/yr, dry rock (Röstel 2020):
  205 cm × 2.8 g/cm³ ≈ 574 g/cm²     265 cm × 2.2 g/cm³ ≈ 583 g/cm²

Depth in loose regolith at 1.5 g/cm³ (our approximation: areal-density scaling):
  574 / 1.5 ≈ 383 cm ≈ 3.8 m

Footprint (6 + 4) × (12 + 4) = 160 m²
Cover  160 m² × 5.74–5.83 t/m² ≈ 920–930 t

Hydrated cover:  50% water 164 cm × 1.4 ≈ 230 g/cm²;  10% water 137 cm × 2.4 ≈ 329 g/cm²
  → 40–60 % less mass than dry rock for the same target

Result

Modeled: keeping a buried habitat under 50 mSv a year needs about 575 g/cm² of dry cover: about 2.1 m of dense rock, or about 3.8 m of loose soil, roughly 920 t over a small habitat. Ice-rich ground cuts the mass by roughly half. The round trip alone, about 662 mSv (Zeitlin 2013), already exceeds NASA’s 600 mSv career limit with MSL-like shielding.

Trade-offs

Open problems

• Table 3 is a model (GEANT4/AtRIS) validated against RAD at the surface only; there is no measurement 2 m down.
• Scaling depth by areal density across materials is our approximation, not a published result.
• Doses are higher at solar minimum. Side shielding is not modeled.
• The rate and energy to move about 900 t robotically before the crew arrives are unknown.

What would change the answer

• Put long-occupancy rooms at the centre under the deepest cover.
• Use local ice or water in the overburden once extraction works.

Sources

1. Hassler et al., “Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover,” Science 343 (2014)
2. Zeitlin et al., “Measurements of Energetic Particle Radiation in Transit to Mars on the Mars Science Laboratory,” Science 340 (2013)
3. NASA-STD-3001 Vol. 1 Rev B (2022): career effective dose limit 600 mSv
4. Röstel, Guo, Banjac, Wimmer-Schweingruber, Heber, “Subsurface Radiation Environment of Mars and Its Implication for Shielding Protection of Future Habitats,” JGR Planets 125 (2020)
5. Zhang, Guo, Dobynde, Wang, Wimmer-Schweingruber, “From the Top of Martian Olympus to Deep Craters and Beneath: Mars Radiation Environment Under Different Atmospheric and Regolith Depths,” JGR Planets 127 (2022)