Why did the study favor the Bernal sphere for 10,000 people?
Under the 1977 study’s relaxed design assumptions, simulated gravity of 0.85 ± 0.15 g and a rotation rate of 1.9 rpm, the rotation radius fell to 236 m. At that scale, the study judged that a cylinder supplied far too much space for 10,000 people, while a sphere supplied about the right amount. That made the proposed Bernal sphere a closer fit to the target population, not an all-purpose superior design[1].
The mass trade-off was substantial. The study estimated that the Bernal sphere needed 300,000 tonnes less radiation shielding than its nearest competitor, the banded torus; that figure was not a direct comparison with the cylinder. Against the other possible forms, however, it estimated 3 to 4 times as much atmospheric mass and 2 to 4 times as much structural mass. The study therefore left the preferred geometry dependent in part on the relative costs of shielding, atmosphere, and structure. These were historical design estimates, not measured performance of built habitats[2][3].
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