NASA’s lunar radiation guidance states that calculations using 3D-HZETRN and Monte Carlo transport codes found dose equivalent can increase when non-hydrogenous shielding is added beyond 20 g/cm², and therefore recommends surrounding crew with about 20 g/cm² of shielding in occupied vehicle and habitat spaces. The document says this target is not helped by adding more mass unless the concept uses meters of shielding, such as a surface habitat buried in regolith, and it also notes that lunar habitats still need dedicated solar particle event protection through a storm shelter and/or wearables.
The technical meaning is direct: more shielding is not automatically safer on the Moon. For long-duration habitation, the highest-value design is hydrogen-rich shielding placed where crew actually spend time, with the heaviest gains coming from the first 20 g/cm² rather than from thicker non-hydrogenous layers; this shapes layout, material choice, and whether regolith burial or local mass is used as structural shielding. The guidance aligns with NASA’s broader radiation-protection recommendations that vehicle shielding for missions longer than 6 months should be at least 20 g/cm².
For the Lunar Ark team, the action is to treat 20 g/cm² as a minimum design reference for occupied zones, then test whether the habitat can achieve it with existing structural mass, polyethylene-rich materials, water walls, or regolith burial. Monitor follow-on NASA and peer-reviewed validation of dose transport at moderate depths, and integrate storm-shelter architecture as a separate layer of protection so that archive integrity and crew survivability are not coupled to a single shielding strategy.