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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 (see also unobtainium) 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 ten million kilometres in circumference, which, together with their rotational speed, creates gravity and day-night cycles to normal Culture standard.:275 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.
The inside of the hoop can be formed to any type of planetary environment, from desert to ocean to jungle to glacier. The structure is usually divided into individual 'plates', similar to continents. Though there need be no directly visible indication for the transition from one plate to another, on some Orbitals the border between neighboring plates is marked by an artificially high string of mountains known as a 'bulkhead range'. These also serve to contain the atmosphere in those areas where the Orbital may not yet be complete, with those gaps in the plate ring being bridged only by forcefields.:331
Some plates mimic natural environments very closely, others are wild exaggerations possible only by advanced matter forming and intricate (but usually hidden) machinery—such as a gigantic river circumnavigating the whole Orbital, which in some regions travels on immense, kilometre-high bridge- or mountain-range-like constructions, and in other regions might act as an immense 'waterslide' for a floating event stadium.
Orbitals spin to mimic the effects of gravity and are sized so that the rate of rotation necessary to produce a comfortable gravity level is approximately equal to one day. In the case of the standard Culture day and gravity, this diameter is around three million kilometres. For such an orbital to reproduce the equivalent to the Earth's gravity (9.8 m/s2 at sea level), whilst maintaining Earth's 24 hour period of rotation, it would need to have a diameter of approximately 3.71 million kilometres. By tilting the axis of the Orbital relative to its orbit around a star a convenient day-night cycle can be experienced by the inhabitants. Since the edges of the Orbital are built as high walls, the rotation prevents the atmosphere from escaping, thus protecting the inhabitants from radiation. The walls are typically hundreds or thousands of kilometres high, made of a 'monocrystal' material.
Responsibility for day-to-day administration of one of the Culture's Orbitals and the management of its complex systems mainly rests with a Mind, which is situated in a structure in space at the centre of the Orbital, known as the Hub. The Mind is generally referred to simply as "Hub" by the inhabitants of the Orbitals, who never tend to be more than a millisecond away from the personalized contact and care it provides (via a contact terminal, usually worn as a piece of jewelry).
As the Hub Mind is extremely advanced, it could simultaneously hold conversations with every one of the billions of citizens a fully settled Orbital has, and constantly controls millions of avatars, (usually) humanoid representatives of itself, throughout the world, though it can also interact in myriad other ways. It will also provide near-instant aid or material comforts, usually via service drones or matter displacement - being a near-omnipresent, omniscient as well as generally all-benevolent presence in the life of an Orbital citizen. As an insurance policy against unscrupulous Hub Minds, and to represent the community to visitors from more traditionally hierarchical societies, each Orbital also elects a body called a General Board from among its human and drone population. As a further check on the power of the Hub Mind, matters of public concern are decided by referendum.
Other civilizations also build Orbitals, although it is not known if all of them are similarly managed.
The culture within the Orbitals is typical for the philosophic-hedonistic slant of all the Culture. They are also prime examples of the Culture's post-scarcity society, for within some physical limits, all material wishes can be fulfilled (or will be fulfilled by the Hub on request).
Orbital culture is thus heavy on enjoyment, arts and crafting, creative endeavours of all kinds, learning, as well as sports and games. The mere building of an Orbital is an adventure itself, in which the Hub Mind involves its inhabitants; beginning with two plates orbiting its future Hub, with more plates added to them at regular intervals. The final joining plates may not be fully formed and 'landscaped' until after a very long time - at least as measured from the viewpoint of a biological member of the Culture.
While the number of people living on an Orbital tends to be in the many billions, the sheer size of the habitat, as well as the casual lifestyle of the Culture, ensure that it almost never feels crowded. Citizens can choose to withdraw into large areas of primal (if ultimately manufactured) nature or into their own spacious homesteads, and tend not to live in cities unless they prefer the increased activity and the proximity of friends.
Orbitals also serve as residences for 'Ambassadors' of other societies to the Culture - though as shown in some of the books, the Culture understands this term differently: the alien is fully intended to eventually consider the Culture superior to his own society and become an ambassador for the Culture.
In the video game trilogy Halo, players visit structures from which the series takes its name, and which are similar to an Orbital. Halo's developer Bungie Studios has mentioned Banks' Culture as an inspiration for Halo.
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