I grinned back at him. "I did some reading. A lot of the design choices for O’Neill cylinders will apply to topopoleis as well. Chances are there’s an outside shell that doesn’t rotate. It will be thick and designed to absorb micro-meteor impacts and high-energy radiation. You don’t want that much mass loading the rotating section, so the much thinner but structurally stronger inner shell will rotate, giving simulated gravity on its inner surface. There will be magnetic bearings or some such between the two shells."