Orbital (The Culture)

In Iain M. Banks' fictional Culture universe, an Orbital (sometimes also simply called an O or a small ring) is a purpose-built space habitat forming a ring typically around 3 million km (1.9 million miles) in diameter. The rotation of the ring simulates both gravity and a day-night cycle comparable to a planetary body orbiting a star.

Its inhabitants, often numbering many billions, live on the inside of the ring, where continent-sized "plates" have been shaped to provide all sorts of natural environments and climates, often with the aim of producing especially spectacular results.

Banks has described Orbitals as looking like "a god's bracelet" hanging in outer space. Orbitals are ribbon-like hoops of a super-strong material reinforced and joined with force fields. Each Orbital possesses a "hub", a station suspended at its rotational centre which houses the Orbital's governing Mind.

An Orbital is similar to a ringworld but is much smaller and does not enclose its primary star within itself, instead orbiting the star in a more conventional manner, making it much more intrinsically stable than a ringworld. Many different civilizations are known to use Orbitals sized according to the preferences of the builders; the Culture's Orbitals are approximately 10 million kilometres in circumference, which, together with their rotational speed, creates gravity and day-night cycles to normal Culture standard. To put this another way, with a diameter of 3 million kilometres the orbital completes a full rotation once per standard Culture day to simulate normal Culture gravity via centrifugal force, and as the orbital is itself orbiting a star this in turn gives the day-night cycle. They have widths varying between one thousand and six thousand kilometres, giving them a surface area of between 20 and 120 times that of the Earth (but comprising significantly less mass).

Orbitals are regarded as highly matter-efficient, providing vastly more usable living space for their constituent mass than primitive arrangements like planets. The Culture therefore prefers to leave planets unterraformed, treating them like wilderness areas.

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