Bernal spheres are artificially constructed hollow spheres, built to hold atmosphere for inhabitants living on the inner surface, Bernal spheres are usually spun to provide residents with gravity along an equatorial strip, with gravity lessening higher up and reaching zero at the axis of rotation. A ring of mirrors direct sunlight to focal points that illuminate the interior via day and night cycles through large windows near the axis points. Bernal spheres are regarded as less efficient than other habitat types since only a portion of the interior surface provides full gravity. Though designs have been produced for Bernal spheres up to 16 kilometers in diameter, most are much smaller, ranging from 0.5 to 1.5 kilometers in diameter and providing living space for populations ranging from 10,000 to 200,000. A few smaller spheres are also kept at zero gravity, providing ample internal air space for ﬂoating structures and ﬂight-capable morphs.
Bernal spheres are often coupled with other station types, particularly cluster habs and toruses. Clusters are more common, housing the agricultural, industrial, and docking modules that the habitat needs.
From security and inﬁltration standpoints, Bernal spheres present several challenges. The axial access points provide a chokepoint for entering and leaving (though some models have escape pods with exterior access ports built into the structure for emergencies). Inside, most Bernal spheres are small enough that you can reach anywhere with a short 20-minute drive or train ride at most. Almost all areas of the sphere’s interior are visible to other areas, making overhead surveillance, tracking,and targeting quite easy, even at range, except when large structures and terrain provide limited cover.