1. source: https://gizmodo.com/nasa-considered-an-inflatable-donut-for-a-space-habitat-1737138377 Built from lightweight three-ply nylon cord and butyl elastomere, the sticky rubber-like material had an internal fabric bulkhead that could be independently pressurized during emergencies. The station was equipped with a solar collector for power, and yet-to-be-built space shuttles could dock at ports along the outside of the torus. The only problem? An inflatable space station is extremely vulnerable to rips from meteorites (or micrometerites), or even from overly-enthusiastic astronauts bounding around the interior of the donut. From ground tests on a 3 meter (10 foot) diameter prototype, engineers also found the station to be unstable enough that vigorous activity could be enough to induce a wobble in the entire structure.