Ah, yes, the Bernal Sphere. It’s too bad it’s incredibly impractical. Looks cool as hell. Prefer O’Neill Cylinders, anyway.
No more impractical than any other long-term space habitat. The combination requirements of radiation shielding, artificial gravity and atmosphere retention impose some design pressures that gravitate towards certain shapes. The Island Three-style cylinder is even somewhat worse than the Island One/Two sphere, in that the mirrors must support themselves under significant loading (or require a massive and mostly redundant and dangerously heat concentrating stationary and constantly corrected cone array) and the sun windows constitute a significantly larger proportion of the inner surface, making for structural problems and a general waste of space. An inner sun-tube is preferable, but comes with the issue of either light transport or power conversion (and thus heat dissipation).
Island-2 is largely impractical because of the very small inhabitable space it contains, compared to its entire mass. Only a small strip in the middle has enough artificial gravity for normal habitation, leaving the rest of the station only useful for other operations.
As far as the windows and mirrors in an O’Neill Cylinder go, at the scale of such a colony, the support structure necessary to hold them in place would be sufficient to reduce structural weakness to acceptable levels, and the thickness of the glass would be great enough to withstand sustained meteoroid impact. The mirror arrays also make for effective radiator mounts, as the sunward side reflects light into the colony and the anti-sunward side rarely, if ever, comes into direct exposure to sunlight, allowing it to constantly radiate heat away instead of absorbing it.
The greatest impracticality to any space colony is material. The scale of a colony prohibits using earthbound materials to construct, due to the cost of putting those materials in orbit. Materials would have to be mined, en masse, in space, and construction would take place over decades.
I redact my previous statement about the space inefficiency of the Bernal Sphere. Earth’s habitable surface accounts for roughly 20% of its surface.
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