Torus habitats can best be described as resembling wheels spinning out in space. They are intended to provide artificial gravity for the entire population through centripetal force without requiring as many resources as an O’Neill cylinder or similar megaproject. A free-spinning torus will often have a circular crosssection to maximize radiation deflection and best carry the pressure loads from the internal atmosphere. Hybrid habitats where the torus is part of a larger overall structure tend to have a rectangular crosssection for ease of manufacturing and increased overall volumetric efficiency. Even then, the edges where the faces of the torus meet tend to be rounded to prevent stress fractures from the internal pressure.
Docking with the rotating ring of a torus presents a difficult orbital mechanics problem, to put it lightly, so most torus habs locate their docking modules or spaceports at the center of rotation. Spoke arms connect this core or hub out to the ring. If the core is not de-spun from the ring, any visiting vehicle must match the station’s rate of rotation about the approach axis to dock. This is not as much of an issue for large torus habitats with a slow rate of rotation.
The radius of any spin habitat is proportionally related to the decimal fraction of Earth-normal gravity desired and the inverse-square of the radial velocity. This means that the most significant limitation is the rate of rotation the inhabitants can withstand without Coriolis effects on the neurovestibular system causing discomfort and nausea. In baseline humans, the maximum safe speed is about two revolutions per minute. Beyond that, an increasing proportion of the population experiences nausea, some of whom never recover.
The smallest 1 g torus habitats are, thus, nearly 500 meters in diameter across the axis of rotation, while a 0.1 g torus can be as small as 50 meters. Many torus habs associated with the Planetary Consortium approximate the Martian standard of 0.38 g to reduce the minimum size of the station while still providing an environment amenable to many, if not most, biomorphs.
“Double-decker” torus habitats actually consist of two counter-rotating torus sections around a de-spun core section. This allows the two sections to cancel out each other’s momentum on the core without the complicated machinery required of single section habs. However, moving between each section does require temporary exposure to microgravity during the portion of the trip moving through the core.
The majority of torus habs have a common environment through the entire structure. Industrial and commercial outposts tend towards a more functional layout that takes up the entire interior like decks on a ship, while colonies have an open air volume and exterior windows along the core-facing side and an artificial biosphere resembling a valley that wraps back around on itself. In the classic Stanford torus layout, a system of reconfigurable mirrors provides a day-night cycle through the windows. However, some stations accommodate multiple environments by partitioning the ring or adopting the double- (or even quadruple-) decker configuration.

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