Banks' Orbitals (A1-0)

Diagrammatic comparison in size between a Banks' Orbital and a Ringworld. A Banks' Orbital orbits the star like a planet and is tilted to provide a day night cycle; this type of megastructure is generally one or two million kilometres in radius. A ringworld is a megastructure that completely encircles a star, and would typically be more than one hundred fifty million kilometres in radius around a standard star.

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A Banks' Orbital is an extremely large hoop-shaped artificial habitat that rotates once per day to create artificial gravity along its inner surface. These large constructs can provide natural-seeming and self-maintaining environments over surface areas that are hundreds to thousands of times that of a typical rocky planet. The tremendous size of a Banks' Orbital and the stresses involved mean that exotic matter is required for its construction. A Banks' Orbital is not to be confused with circumstellar Ringworld megastructures. A typical Banks' Orbital consists of a ribbon of material arranged in a ring that has a radius measured in millions of kilometers. The spin and radius of the orbital are set so that the sunlight and surface acceleration along the inner surface simulate the day length and surface gravity of a planet.

Solids, liquids, and gases comparable to those of the desired planetary type are held against the inner surface by centrifugal effects and prevented from slipping off the edge into space by walls hundreds of kilometres high. The entire structure orbits a star within whatever constitutes the life zone for the inhabitants. From the point of view of an inhabitant, a completed Banks Orbital functions as a cylindrical planet, and in fact an unsophisticated inhabitant might not see any difference. Typically it has the equivalents of a lithosphere, hydrosphere, atmosphere, and a functioning biosphere and/or mechosphere. In the case of a Banks' orbital these elements are sometimes referred to as the Lithotorus, Hydrotorus, and so on, since they conform to the shape of the Orbital.

No form of ordinary matter will support the tensions of a Banks Orbital's spin, so exotic matter is required. Though some other kinds of exotic matter might theoretically serve the purpose, the supporting structure of a standard Banks' Orbital is ordinary matter reinforced with Magmatter.

Banks' Orbitals are also stable; once constructed their maintenance requirements are extremely low. Unlike smaller habs they do not require imports of materials and careful management to maintain the ecosystem and mechosystem in a stable state, and unlike more typical megastructures that use mass stream technology they do not need constant attention to prevent physical collapse. The estimated maximum period over which a Banks Orbital would maintain its environment without intervention varies according to the orbital and the estimator, but analyses suggest that a standard Banks Orbital might last for millions of years without maintenance before it became uninhabitable. However, since large quantities of magmatter must be created and manipulated to construct the support structure for Banks' Orbitals they are still relatively quite rare in the Galactyan Empire.

Design

Though the scale is orders of magnitude greater, the fundamental design of a Banks' Orbital is very much like that of a Bishop Ring. It is a cylindrical 'open-air' hab with a radius that greatly exceeds the width of the cylindrical band. The spin of the cylinder (or torus) is strong enough to simulate the gravity of a world along its inner surface. Like a Bishop Ring, a Banks Orbital is open to space along its inner surface, so that inhabitants see a sky that is very similar to that seen from a planet's surface. If, as is usual, an atmosphere is maintained on the interior surface of the Orbital, it is prevented from leaking away by high walls along the rim. One thing that makes a Banks Orbital different from a Bishop Ring is that the inner surface is illuminated directly, rather than receiving light from a central artificial source. To imitate the day-length and the surface acceleration of a typical gardenworld in this way the Banks Orbital must be millions of kilometres in radius.

In some cases, a Banks' Orbital will have spokes, constructed primarily of magmatter. These help to maintain a more perfectly circular shape against tidal forces or serve as support members for various additional structures. Such spokes are so extraordinarily thin in proportion to the rest of the Orbital that it may be difficult to distinguish a Banks Orbital with spokes from one that does not have them except by observing any structures they may be supporting. This may make travel between the Orbital's inner surfaces hazardous for those who are unaware of their location.

The required radius for a Banks Orbital is directly proportional to the desired 'gravity' on the inner surface. It is also proportional to the square of the desired day length. This means that each possible combination of diurnal period and surface acceleration leads to a different size of Banks' Orbital. It also means that creating long days is relatively expensive.

The rimwalls of a Banks Orbital may be constructed in a variety of ways according to the preferences of the designers. Some are truly vertical and wall-like. In such cases, there may be entire clades and their associated ecosystems specially adapted to living on and in these nearly sheer surfaces. In other cases the walls may rise more slowly and appear to be very high mountains as seen from the interior of the Orbital. In other cases yet again a range of radically different habitats may be produced by creating a step-like series of mountains and plateaus, so that the middle layers have thin atmospheres and the upper layers are in vacuum or near-vacuum. This is often the case where a number of clades with radically different requirements inhabit the same Orbital. Regardless of the particulars, the required height of the rim walls above the lowest surfaces of the Orbital depends on two factors. It is inversely proportional to the Orbital's surface gravity, and directly proportional to the desired atmospheric pressure on the rim surface. While most rimwalls are simply self-supporting foamed diamondoid, in some designs the rims are created by varying the underlying magmatter weave; the circular strands of magmatter underlying the rims simply have fractionally smaller radius than those of the Orbital's floor. In this way rimwalls of any height may be constructed.

Typically Banks' Orbitals have very little width in proportion to their radius, but even this provides a very large surface area. A Banks' Orbital is typically built with a band width of at least 3,000 kilometres, though aesthetic considerations or material shortages may lead to narrower structures.

A Banks' Orbital's axis of rotation is usually inclined to the ecliptic. Even a degree or two of inclination will prevent the sunward portion of the ring from shading the outer portion for most of the Orbital's year unless the Orbital is unusually wide. Often a Banks' Orbital designed to simulate Galactya's conditions will have an inclination of 25 or more degrees to provide some seasonal variation that will in turn drive local weather patterns and provide for adequate distribution of rainfall along the inner surface and to help prevent the depths of bodies of water from becoming stagnant. A Banks' Orbital's movement around its primary star may be nearly circular or it may be somewhat eccentric, again to provide some seasonal changes according to the preferences of the designers.

Structure

The tension in the ring of a Banks Orbital is tremendous. No bonds between ordinary atoms are sufficiently strong for this purpose. The foundation of any Banks orbital is magmatter in the form of magcarbon, the magmatter analogue of carbon. This magcarbon is formed into the magmatter equivalent of nanotubes. Though raw magmatter simply passes through ordinary matter, and would be useless as the foundation for a Banks Orbital, it can be treated so that it acts as a solid foundation by bonding a weave of magnanotubes to a ferromagnetic substance such as steel. The resulting composite is orders of magnitude stronger in tension than is required for even the largest known Banks Orbitals, and also strong enough in compression to support the weight of the ordinary matter that overlies it. Though it is quite massive, this composite layer is invisibly thick, even where it consists of several layers of magnanotube weave and intervening atoms of the steel. For safety's sake, it is enclosed in a layer of ordinary nickel-iron that is much thicker; at least a meter and in some cases dozens of meters thick, that helps prevent exposure of the magmatter filaments. The outer portion of the Banks' Orbital may have additional layers of foamed diamondoid and other materials, in part as an ablative layer in case of accidental impacts against the structure. However, most of the Banks' Orbital's mass is inward from the magmatter composite layer. Most commonly there is a layer of foamed diamondoid, some hundreds or thousands of metres thick, which forms the underlying contours of the terrain on the inner surface of the Banks' Orbital and helps provide rigidity. A similar layer makes up the Orbital's rimwalls in most designs. This is followed by a layer of corundumoid and/or silicate rock hundreds or even thousands of meters thick; this forms the Orbital's 'lithotorus' and serves the practical purpose of protecting the diamondoid from an oxygen-bearing atmosphere. Over these are liquids and gases forming the 'hydrotorus' and 'atmotorus'. A Banks' Orbital can support what inhabitants see as extreme differences in local topography, and often does so to provide varying environments. It is also not uncommon for a Banks' Orbital to support walls or mountain ranges equivalent to the rimwalls in height, providing complete separation between compartments along the Orbital's circumference that may have radically different environments. Also like the rimwalls, they are usually sheathed in foamed corundumoid or ordinary silicates.

Though a Banks' Orbital may be constructed without them, it is not uncommon for one to be built with spokes and a central hub. Such spokes must have a magmatter core, of course, to be able to support the tensions involved. They are useful as anchors for transportation structures (vac trains, Shuttles and Maglev Pods to and from the rimwalls, for instance, or launching and landing stations for interplanetary or interstellar vehicles). They may also, in some implementations, be anchors for any roofs or canopies that might be placed over portions of the Banks' Orbital habitat, where the designers find it useful to exclude light or other radiation, or to maintain an air pressure greater than that allowed by the rim walls.

Maintenance

A Banks Orbital requires relatively little maintenance compared to other kinds of habs. Lighting, temperature, and atmosphere are stable without interference, and the ecosystem is large enough to cycle on its own within the habitable range without active management. The structure itself does not require active repair except on a scale of centuries or even hundreds of millennia, and is not dependent, as are some other megastructures, on the huge quantities of energy, materials, and attention required to coordinate mass streams. Because their radius is much greater than their width, Banks' Orbitals are quite stable in their rotation, unlike tubular habs such as McKendree Cylinders. Also unlike fully enclosed habs, there is no envelope to maintain against the outflow of gases. Banks' Orbitals do not require countermeasures against precession, except on a scale of many thousands of years. Because of their size, Banks Orbitals do not require shielding against cosmic radiation, and because gases and liquids are held against the floor of the hab entirely by its spin, no attention is required to retain them. The chief vulnerability of a Banks' Orbital is the junction between its magmatter skeleton and the ordinary matter placed on top of it. Few natural forces are likely to disturb these. Conceivably if the diamondoid foam underlying the terrain were exposed, and later subjected to a hot natural fire, it would burn through with sufficient heat to melt iron and ultimately allow the atmosphere and hydrosphere to be ejected into space through the resulting hole. While this might happen after thousands of years if erosion created such a point of contact and a strong wildfire occurred in the region it would be far more likely as the result of a large comet or meteor strike on the Orbital's inner surface.

The ordinary matter portions of a Banks' Orbital require some long term maintenance, since natural erosion on the inner surface is not counteracted by any natural geological forces. This would ultimately lead to the chemical alteration of the original minerals, the leveling of mountains, and the filling and increasing salinity of oceans and lakes.

Few Banks Orbitals are created to be entirely inert. Most have repair and self-maintenance systems comparable in complexity to that of an extremely large plant or plantbot. This allows them to maintain their assigned orbital dynamics, and otherwise maintain physical homeostasis. It is not uncommon for them to have sentience, up to and including sophont-level capability or power into the transapient or near-Archailect range. In the modern day, most Banks Orbitals are dedicated ISOs, with ratings up to S3 on the toposophic scale.

The landscape on a Banks' Orbital can be engineered to closely resemble that found on a typical terraformed planet but they are millions of kilometers in circumference and the evidence of this can be seen in the sky.

Habitable Surfaces

Directions

From the point of view of an inhabitant on the inner surfaces of a Banks' Orbital the 'world' looks flat, just as it does to someone living on a planetary surface. Just as a planet has its north, south, east, and west, zenith, and nadir from a point on the surface, there are six fundamental directions. Zenith, or up, is towards the axis of the orbital. Nadir, or down, is away from the axis of the orbital. Unless the Orbital's spin is retrograde to its motion about its sun, east is in the direction of the Orbital's spin, and is the direction in which the sun appears to set (contrary to the situation on a typical planet), and west is away from the orbital's spin, and the direction in which the sun appears to rise (again unlike the situation on a typical planetary surface). Just as on a planet, northward is the direction on one's left if one is travelling east, and on the right if one is travelling west, whereas south is on the left if one travels east and on the right for an eastbound traveller. The extreme north and south of the ring do not have the connotations the polar directions would for dwellers in the northern or southern hemispheres of a planet. The regions near the rimwalls may be marginally cooler or warmer than the general floor of the Orbital at certain seasons due to shadows or to heat trapped by rimwalls, depending on the height of the rimwalls and the axial tilt of the Banks Orbital. This gives them distinctive climatic patterns, but they are not cooler on average than other parts of the Orbital even though their seasons are more extreme.

Topography

The archetypical Banks Orbital is usually designed to maximize shoreline space. Oceans have a large number of islands, and the land is dotted with lakes. Waterfront property is innately attractive to baseline humans, and therefore attractive not only to human nearbaseline clades but also to some human-designed clades. However there are other reasons for such designs. Maximizing transitional environments makes for a richer ecosystem, and placing land and water next to one another helps encourage daily and seasonal weather patterns that transport water inland. Large landmasses without lakes or seas may be created if dryland or desert environments are desired. The terrain typically includes mountains and other variations in terrain, in part to make for a more varied and interesting environment, and in part to allow for different climates within the orbital. Otherwise there would be little variation, since unlike a planet the strength of sunlight on a Banks Orbital does not vary from one part of the structure to another.

A Banks' Orbital also gives the option of extremely high mountains, on the scale of the Orbital's rimwalls. Like the rimwalls, these can be unique environments: steep slopes or vertical cliffs extending upwards to tremendous heights. Such extreme topography may also be used to separate different regions of the Orbital, so that travel between sections is impossible without technological aid. This may be simply to prevent wildlife from traveling between two similar sections, or it may be to allow for radically different environmental conditions on opposite sides of the megamountain range.

Sunlight, Ringlight, Starlight

For someone living on the inner ring, sunrise and sunset are more or less as they would be on a planet. The day begins in and ends in long shadows, and is brightest at noon when the sun is most directly overhead. Unlike the situation on a planet, one may be able to see dawn and dusk advancing towards one's location along the ring at a distance. However from the point of view of someone on the rim floor the sun still appears to rise and set. The more distant portions of the ring form a great arch in the sky that narrows and darkens as it rises towards the zenith. If the ring's axis of rotation is tilted relative to the ring's plane of revolution about the local star, there is some variation in the intensity of light (though no significant variation in day length) according to the time of year: two minima and two maxima per orbit. If the orbital is given an eccentric elliptical orbit there is some variation in the apparent size of sun and the brightness of the sunlight according to the time of year as well, with a brief maximum as the Orbital completes the faster leg of its orbit and a much longer dim period when it is further out from the primary.

Nights on a Banks Orbital are very well lit by ringlight. At night, the lit portion of the ring covers half the sky, and even the dark portion of the ring is faintly visible as it reflects back ringlight. This is because it covers so much of the sky and because an earthlike surface is more reflective than one like Old Earth's moon due to clouds. Wider rings provide even more light in proportion. Astronomers on the inner surface of a Banks Orbital have a difficult time observing the fainter stars and planets, just as observers under a bright full moon might from the surface of a planet.

An orbital with no axial tilt relative to its motion around the sun eclipses the sun all of the time, and one with a larger axial tilt eclipses the sun just twice per year. For the narrower orbitals this is a minor effect, possibly hard to see against the glare of the sun. For instance, a 1000 kilometer wide orbital going around a standard star has 2% the apparent width of the sun at the zenith and accordingly only blocks out a fraction of the sunlight. At the other extreme, a 35,000 kilometer wide orbital creates a total eclipse, and intermediate sizes may still have a significant effect. This is one reason why most orbitals have at least a small degree of axial tilt. For an orbital with a 90 degree axial tilt and a width of 1000 kilometres, circling a standard star at exactly 1 AU, the biannual eclipse lasts for 21.8 minutes at the height of one of the year's two summers. The length of this event is in direct proportion to the width of the Orbital, in inverse proportion to the length of Orbital's year around the local star, and in inverse proportion to the sine of its axial tilt. Such eclipses are barely noticeable on the narrower Banks Orbitals, but may be a dramatic event for others.

The regions of the orbital near the rim wall are in shadow for half of the year, and receive extra light reflected from the rimwalls for the other half of the year. Since on the inner surface of the orbital the rimwalls are typically experienced as mountains, this is somewhat like being on the south slope of a mountain during one half of the year and on the north slope during the other half of the year. For an Orbital with a 23 degree tilt, the top of a 100 kilometer high wall casts a shadow that is 42 kilometers long when it is at its greatest extent, once per year at midwinter during one of the biannual cool seasons. The transparent balloon wall casts a longer, much less noticable shadow.

Weather & Climate

Unlike planets, Banks' Orbitals do not have great differences in insolation between pole and equator to drive their weather systems. In fact they do not have any seasonal variation at all, other than that produced by an eccentric motion about the primary or by a rotation that is tilted to the ecliptic. Those that do have a tilt to the ecliptic have a pair of warm seasons once per orbit, each marked by a midsummer eclipse of the sun. There are also two cool seasons, and the middle of these is marked by the sun's lowest point in the sky. These differ only in that on one occasion the sun dips nearer the south rimwall and on the other occasion it makes its closest approach to the north rimwall. These seasons are not strongly marked unless the orbital's inclination is quite sharp. The seasons are only half as long as they would be on a planet, and although the angle of the sun in the sky changes the length of the days does not vary as it would on a planet; there are no long summer days or long winter nights.

Regions near the rim wall may be warmer than the rest of the torus for one half of the year, and significantly colder for the other half of the year. This means that the north and south rims have opposite seasons; during one ringwide cool season the north rim will be cooler than the rest of the ring and the south will be warmer, while the reverse will be true one half orbit later. However, this is important only if the orbital has a significant tilt, and such regions are in any case a narrow strip, less than 100 kilometers or so wide, on an orbital that is typically thousands of kilometres in breadth. Any mountains placed on the inner surface of the Orbital produce a similar, if smaller effect; the north and south slopes are unusually warm or cold in alternation.

The Coriolis Effect is not significant in the weather of a Banks' Orbital. What Coriolis effect there is acts vertically, deflecting falling air antispinward and rising air spinward, rather than horizontally as on the surface of a planet. However, over the mere 10 kilometer high region of active weather air currents and cloud shapes are hardly altered at all.

Without Coriolis effects and without major temperature gradients between different parts of the surface, there are no equivalents to the cyclonic storms known to most rotating planets, or to the steady strong winds found on planets with a captured rotation. There are in fact few large scale weather systems of any kind. There are cumulus clouds, and sometimes even thunderstorms, as the surface heats by day and cools by night. There are also onshore winds during the day and offshore winds at night as water and land heat and cool differently. Where the orbital's axis is tilted, there are some monsoon-like effects as water and land masses heat differently: cooler drier winds out of the interior of the continents during the two local yearly winters, and warmer wetter winds from the oceans during the local summers. Likewise an orbital with a somewhat eccentric orbit has a short warm season and a longer cool season, with similar effects. Overall, though, weather on a Banks Orbital is gentler and more local than weather on a planet with the same insolation, orbit, and axial tilt. Exceptions to this general rule may be created for all or part of an Orbital that has spokes by using structures mounted on them to increase or reduce the amount of sunlight falling on particular areas according to some pre-determined schedule. This option is common on Orbitals that are designed to produce a large number of radically different environments on the same surface.

Tides & Currents

Tidal effects on a Banks' Orbital are what they would be for a planet in the same position. While there are no lunar tides unless some body is in orbit around the hab, there are solar tides. As with planets, if a particular basin of water has a natural period resonant with the period of the tidal effect then tides may be quite large. If the basin does not have a resonant period then the tidal effect is small or negligible. A Banks Orbital alone in an orbit of 1 AU around a standard star will have tides one third the height of those on Earth. Those orbiting red dwarf stars at a comfortable distance can have much larger tides. Such extreme tidal effects make habitat maintenance difficult, and maintaining the orientation of the Orbital as it moves so close to its the star requires considerable energy and attention. As a result, constructs around the dimmest red dwarf or white dwarf stars are not usually Banks Orbitals but are miniature Ringworlds though built to the same scale as a Banks Orbital.

Since there is nothing comparable to the contrast between cold polar oceans and warm equatorial oceans, the seas on a Banks Orbital do not have any natural circulation between their depths and their surface. The result is that waters below the first two hundred metres are anoxic. If deeper oxygen-bearing waters are desired the hab's designers must take countermeasures. Strong seasonal variations, allow for a yearly overturn of waters if a hab is designed to have such extremes in its climate. Elsewhere, artificial sources of heat may be applied to the depths of a body of water to keep it in circulation, or subsurface waters may be simply pumped to the surface or onto nearby continental regions, or the underwater topography may be carefully designed so that mixing occurs due to tidal effects.

Rims, the Underside, and the Inner Worlds

Though Banks' Orbitals were first designed to produce environments very like those of a planet, they offer a number of other unique options. The ecosystems and associated sophont clades of the steeper ringwalls have already been mentioned. However, the outer surface of a Banks' Orbital is an entire world of its own. Originally such regions consisted only of the Orbital's transportation and maintenance mechosystem, but these regions have since been colonized by specially adapted bots and vecs or in some cases by vac-adapted bionts. Outer surfaces of the rimwalls were the first to become inhabited, but it was not long before entire mechologies and their associated clades capable of clinging to the Orbital's underside grew up. Many such are so thoroughly adapted to such a life that they cannot live successfully anywhere else. To a lesser degree, there are a variety of possible habitats within the voids and caverns of the foamed diamondoid layers of a Banks Orbital, especially underneath the taller geographic features of the inner surfaces, or within the rimwalls. Again, entire civilizations have grown up within these places, some consisting of clades and cultures that rarely if ever encounter the light of the outside world.

Transportation

The scale of a Banks' Orbital is such that the modes of transportation used on planets or smaller habs are not sufficient for travel between distant parts of the Orbital. Even the most rapid high speed vacuum tube maglev trains, with cruising speeds of over 10,000 kilometers per hour, take weeks to circumnavigate a Banks' Orbital. Rapid travel between distant portions of the Orbital requires the same technologies used for interplanetary travel. This varies according to local customs and technologies, but is most commonly a matter of spacecraft launched from the rimwalls or similar high points.