Topopolis

A topopolis is a tube-like space habitat, rotating to produce artificial gravity via centrifugal force on the inner surface, which is extended into a loop around the local star. Topopoles can be looped several times around the local star, in a geometric figure known as a torus knot.

The concept of Topopolis was first proposed by Dr. David A. Hardy in the 1970s. He envisioned it as a potential solution to the problem of overpopulation on Earth. He conceptualized a torus-shaped structure, similar to a giant donut, that spins to create artificial gravity for the inhabitants. The structure would be divided into compartments for living quarters, laboratories, and other facilities, and it would be powered by solar panels. The torus would be surrounded by a shell that would protect the inhabitants from radiation and other space hazards.

A topopolis can be compared to an O'Neill cylinder extended in length so that it encircles a star. A normal topopolis would be hundreds of millions of kilometers long and at least several kilometers in diameter. A topopolis with a big enough diameter can theoretically have multiple levels of concentric cylinders.