Why Radiation Could Add Dozens of Tons

In the Lewis study, crew safety had a substantial mass cost. Its proposed spacecraft had a heavily shielded central vault where the crew would shelter during large solar flares, passage through radiation belts and nuclear-rocket operation; they would also sleep there to reduce exposure to cosmic rays^[1].

For a 420-day voyage, the study estimated 23.5 tons of shielding if major flares could be avoided and the permitted total dose was 100 REM. Allowing for one major flare raised the estimate to 82 tons at that dose limit; lowering the limit to 50 REM while allowing one flare raised it to 140 tons. REM (Roentgen Equivalent Man) is a measure of radiation dose^[2].

That is the engineering trade: a more protective dose limit or a more severe assumed flare meant much more shielding to carry. These were study estimates, not universal Mars-mission requirements. The Lewis researchers stressed that radiation hazards in space were not yet well understood, so the mass figures depended on assumptions they could not firmly verify^[3].