The donuts, as we’d taken to calling them, looked like fat bicycle wheels. Carbon-fiber cables ran from the hub to the rim, providing most of the structural support. Three thicker spokes provided elevator access from rim to hub. The donut was oriented perpendicular to the sun, and mirrors between the rim and hub reflected sunlight into the interior through the transparent roof of the rim. Everything was designed as simply as possible, to minimize construction time and material requirements.