A review by Eric He, published 2 July 2026 in the Journal of Radiological Protection (Volume 46, Article 031001), synthesised lunar and orbital measurements, radiation-transport models, beam studies, and neutron-transmission experiments. It finds that the first tens of grams per square centimetre of regolith reduce dose equivalent by fragmenting high-LET ions and lowering average quality factors, while recommending approximately 30–40 g/cm² of regolith combined with a hydrogen-rich inner liner.
For habitat design, 30–40 g/cm² corresponds to roughly 0.17–0.23 m of compacted regolith at an assumed bulk density of 1.7–1.8 g/cm³, although actual thickness must be determined from measured emplacement density. More regolith is not automatically better: increasing thickness can produce secondary neutrons, creating diminishing radiation-protection returns. A hydrogen-rich interior layer is therefore superior to regolith alone, particularly for galactic cosmic rays, solar energetic particles, and albedo-generated neutron and gamma fields.
Adopt 30–40 g/cm² as the preliminary minimum areal-density requirement for permanent Ark modules, vaults, sleeping areas, and storm shelters, with an internal hydrogen-rich liner specified from the outset. Require Monte Carlo transport models to be benchmarked against beam and neutron experiments; measure local regolith density, hydrogen content, and composition before finalising thickness; and maintain separate protection analyses for chronic galactic cosmic rays and acute solar-particle events. Evaluate lava-tube or subsurface siting as a mass-saving option, but do not treat natural overburden as validated until its geometry and radiation field are characterised.