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BDO of the Day: Bernal Sphere

A Bernal Sphere is the space habitat cousin of the most famous BDO of all: the Dyson sphere. First proposed by Bernal in 1929 with a 10 mile diameter, it reappeared in smaller form in the 70s as two of O'Neill’s space colony proposals: Island One (1600 foot diameter), and Island Two (a mile in diameter). The total inner surface of such spheres is about 300 square miles (Bernal), a quarter of a square mile (Island One), and 3 square miles (Island Two), respectively.

Bernal sphere exterior (public domain image from NASA)

A Bernal sphere rotates to provide gravity, and as a result only an equatorial band has adequate gravity to be inhabited, with the remainder of the sphere devoted to windows, trees, and other purposes. This reduces the usable area, making it a relatively impractical layout compared to cylinders and other shapes (even though it photographs well).

Bernal sphere interior (public domain image from NASA)

Note that a real Dyson sphere is far too large for the curvature of the sphere to be visible.