Heaven’s River consisted of a stationary outer shell made of regolith interwoven with structural members and some kind of 3D carbon-fiber mesh, and a rotating inner shell made of a combination of metal and ceramics forming some kind of metamaterial, reinforced with the same carbon mesh. What I hadn’t realized, or maybe hadn’t paid attention to before, was the fact that the space between the two shells was only about ten yards. Since the structure was fifty-six miles in radius, the curve of either shell wasn’t discernible from my vantage point. But I could tell which way was which because the inner shell was rotating.