Gigantic inflated, self-gravitating megastructures

Edersphere - diagram
Image from Steve Bowers, after AlanSE
Ederworlds are also variously known as Ederbubbles, Edercolonies, Ederspheres, Ederhabitats, Bubbleworlds or simply Eders. Legend has it that an Information Age bright, Dani Eder, conceived the idea for these habs during a conversation at a dinner party. Someone asked, "What is the largest orbiting habitat that could be built?" The conventional answer for a habitat suitable to humans was well known to zar Eder's contemporaries. It was an O'Neill cylinder about 50 kilometres across and 250 kilometres long. This limit was a result of the pressure vessel's "radius of curvature" and the tensile strength of the materials available. In later centuries, new materials and technologies would allow the construction of megastructures much larger than anything O'Neill described (Banks Orbitals, Bishop Rings, Dysons, Shellworlds, Smoke Rings, and Supramundane Planets, to name some prominent examples). However, zar Eder began to devised a different approach to the problem, one that would have allowed the creation of a megastructure nearly half a million kilometres across even if using only the materials and technologies of the day.

Zar Eder's insight was that all mass, even the gas mixture inside a pressurized vessel, generates a gravitational effect. If one can bypass the size limits inherent in more conventional habitats during the design stage one can find a new equilibrium: a habitat in which gravity holds the structure together against atmospheric pressure just as a planet does. However, instead of having a rocky core holding on to an atmospheric envelope one would have a core cell of gas that is massive enough to halt its own expansion through self-gravitation. The gravity field of this core cell of gas would attract the shell/hull of the habitat and in doing so the shell would weigh down the gas so that its pressure would not simply taper off to zero. Furthermore if the mass of the gas and shell were just right, pressure and gravity would be balanced so that the shell would not need much in the way of structural strength. It could, in fact, simply float upon the atmosphere while it was in turn weighing down and containing the gas. A variety of habitat sizes are possible with this design, since different gases have different densities. If one wished to increase the habitat's size one would have to use a lighter gas mixture, as otherwise simply adding more gas causes it to contract faster* under it's own gravity. As an example, a Freesphere with an earth standard atmospheric mix and temperature would be at Eder's calculated equilibrium if it were approximately 68,000 kilometres across. However the water envelope would have to be several kilometres thick, making it impossible to view the outside from inside (and this view is a prime reason Freespheres would be built) and the atmospheric pressure would increase to fatal levels (for baseline humans) as one approached the centre. Such a Freesphere would also have a micro-gravity field, and a mass almost four percent the mass of the planet Earth.

Dani Eder of course was trying to answer the question "what is the largest orbiting habitat that could be built for humans?" so he chose the lightest gas for his core cell: hydrogen. Zar Eder's hab, as he described it, would be "built like a soap bubble." Hence the more popular names "Bubbleworld" and "Ederbubble." Its living space is a shell of breathable gases surrounding and held up by a core cell of hydrogen and "capped" by an outer solid shell. As the whole habitat is held together by a balance of forces its size is set by the initial assumptions about internal conditions. Zar Eder assumed his living space would be separated from the core cell by a thin membrane so its temperature and pressure at that barrier would have to be the same as the living space. As zar Eder was designing it for baseline humans that meant a temperature of 300 Kelvin and a one bar pressure at the membrane, so the radius of zar Eder's non-rotating sphere was limited to 240,000 km. Zar Eder further assumed the living space would have an average density of 10 kilograms per cubic metre, the majority of this being the mass of its population, artefacts of their civilization and the supporting biosphere. Zar Eder also limited the range of atmospheric pressure from sea level to the equivalent of 3,000 metres on earth, which under the weak gravity of his hab meant the living space was 2,400 kilometres deep. Zar Eder's colony would have had a surface area approximately 1,400 times that of Earth, but if the living space were divided into three and a half kilometre high "floors" the useable volume would be about one million times that of Old Earth. Finally Zar Eder set a population limit of 36,000 trillion based on assumptions of energy use and black body radiation of waste heat. All of this, he calculated, would use a mass only three times that of Earth's. To control weather zar Eder suggested subdividing the "floors" with vertical partitions, creating Earth sized cells 10,000 kilometres across. Finally there was the "cap." Dani Eder suggested steel, which under the local acceleration of gravity of 0.02 metres per second squared (generated by both the mass of the core cell and the living space) has to be 500 metres thick to produce a force down equal of the atmospheric pressure up. As a whole an Ederworld is a micro-gravity environment but it is possible to hang smaller, spinning habitats from the cap.

Dani Eder himself did not believe anything as large as his bubble world would ever be built. He claimed that it would be "putting too many eggs in one basket." In this he was far too conservative. Not only have larger megastructures like Banks Orbitals, Dysons, Smoke Rings, Supramundane Planets and others been built, but his own design was quickly refined and enlarged. Zar Eder did not consider the use of a thermal break between the core cell and the living space which would allow the hydrogen to expand from heating (a sandwich of aerogel and metallized mylar is still effective even today). Nor did he consider the gengineering that allows even the common human nearbaseline of today to live comfortably in a wider range of atmospheric pressures and gas mixtures than the baselines of zar Eder's day. In addition his limits due to energy use and waste heat did not take into account nanotech or active cooling technologies. As a result modern habs built using Eder's concept are much more diverse than he might have imagined.

MPA and Negentropy Alliance Ederworlds

Saarinen, an MPA Ederworld
Image from Steve Bowers
Saarinen Ederworld includes several habitats located inside the 'bubble' volume. To reduce the risk of fire, this edensphere is filled only with helium; this makes the whole structure somewhat smaller and denser than other edenbubbles. The habitats are connected by a network of gravity -propelled vactrains which orbit within the sphere.
In the current era the MPA has built several worlds to the designs inspired by Eder, and the Negentropy Alliance has also done so as it appreciates how a large population can be housed with so little mass. Elsewhere Ederbubbles are rare, built either for their novelty or to house clades that favour micro-gravity environments. The following is a short list of notable examples.

The Black Giant
Black Giant, an Ederworld
Image from Steve Bowers
The Black Giant is a dark structure in deep space, very difficult to detect (except when it eclipses the local white dwarf star)
Built by the Deeper Covenant, this Eder is 457,900 km in diameter and orbits a white dwarf 4,579 kilometres in diameter. This system is an exercise in contrast. The white dwarf is a super dense, super hot stellar object while the Eder's large size hides its low mass. The Black Giant uses Hider technology to remain cold, dark and generally "silent" along all electromagnetic wavelengths, even under the ultraviolet radiation it receives from its star.

Forest- an Ederworld in the Red Star M'Pire
Image from Steve Bowers
Unlike most Eders, Forest has a biosphere located above the iron shell, and rotates slowly to ensure each hemisphere receives enough illumination
This Red Star 'M'pire Eder is 470,000 km in diameter and one of the very few with an exterior day/night cycle. Normally giving a megastructure 'soap bubble' any kind of rotation would turn it into an oblate sphere but the lums of the Red Star 'M'pire countered that effect by adding an extra layer of cometary material to the outside surface of the computronium cap. The regolith has been piled on thickest at the equator and thinnest at the poles. This Eder was named Forest because the lums found the combination of micro-gravity and comet regolith to be perfect for the planting of dyson trees on the outside surface, almost 785.4 billion square kilometres of Dyson trees.

Madison Avenue
Eder (Madison Avenue)
Image from Steve Bowers
The vast Madison Avenue Edersphere, Centaurus sector, NoCoZo, completely covered in advertisement displays. Each pixel of these vast displays is itself a giant advertisement made up of pixels that are themselves advertisements.
Deep in the Non-Coercive Zone, this Eder's surface is a fractal array of imagers. It acts as a billboard, displaying an ever changing tapestry of adverts. This is effective enough if seen from a distance but as one gets closer one finds that each pixel in each image is itself an advert whose own pixels are adverts. This 'ad in ad' system continues down the size scale to the multi-micrometer level.

Neptuna Eder Sphere
Image from Steve Bowers
Neptuna has an oceanic environment completely filling its habitable shell
This smaller Utopia Sphere Eder has a core of helium and its cap is a layer of water four kilometres thick sandwiched between layers of diamondoid and braced by vertical walls of utility liquid. It is the home of a clade of provolved, transapient Bluefins as well as other Acties, some Cymbium sapiens, Iniliak, Streyas and a few unique clades of Europans. There are even various anomalocaroid species, including a derivative of the huge Cygnus Belt Leviathans which serve as bioship transports to and from Neptuna. The walls of utility liquid recycle all waste and supply all oxygen and food. There is no atmosphere/water interface that would allow air breathing clades nor any "air filled" living space as the cap is the living space. Some of the Acties are currently involved in a "race" of sorts. They are competing to autoevolve the perfect form for high speed, long distance underwater travel. The race is open to all, bioborgs and cyborgs too.

Other Eders Also of note are two Eders known only by their location designations. In the first case this is for administrative reasons, given a diversity of local names, and in the second case the name used by the inhabitants cannot readily be translated:

One in the Solar Dominion is in a halo orbit between the two G type stars of a triple star system (the other star is a red dwarf which orbits both). The surface of this Eder is a near perfect reflector of nanite prepared and nanite maintained gold foil. Active cooling keeps the internal temperatures comfortable.

The second was built by freed Sagittarius Sphere autowars. They have made a new home for themselves in the dark bands of the Lagoon Nebula and lead a relatively peaceful life of cloud harvesting. Their Eder was built to aid in the harvest. Its low gravitational field acts as an attractor for the gas and dust but still allows for easy landings and launchings for the autowars. To build it the autowars pooled their utility fog into one sphere just 2,000 km in diameter. As this intercepted the gas and dust it passed the gas to its interior and the sphere grew into a shell. The gas is pre-sorted by molecular weight with hydrogen molecules in the core followed by a layer of helium3, a mixed layer of helium4 and deuterium and the rest follow in turn. Each layer is separated from the others by a layer of the utility fog forming an airwall. This Eder has no cap as such, only an extra thick airwall layer saturated with dust and water. The whole arrangement allows the autowars to pass through the Eder picking up the material they need for reproduction, refueling, repairs, etc. In the centuries since the Version War these autowars have developed good trade relations with the MPA and STC for the excess gas and dust they've collected and they plan to build additional Eders to increase their collection rate.

  • An Edersphere is not stable. It will contract slowly over the course of millions of years as its gases cool, just as a gas giant planet does. However as long as it is inhabited this contraction can be halted if it is internally heated, just as a star's contraction is halted by the energy released by fusion at its core.
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Development Notes
Text by AI Vin
Additional material by Ryan B and AlanSE
Initially published on 10 September 2005.

Additional Information
An analysis of the Edersphere concept by AlanSE
Ederworld Analyzed (Concentric Gravity Balloons to Maximize Volume)