Bubbleworld

A Bubbleworld or Shellworld or Ederworld is an orbiting hollow megastructure nearly half a million kilometres across, where gravity holds the structure together against atmospheric pressure just as a planet does. A core cell of gas halts its own expansion through self-gravitation. The gravity field of this core cell of gas would attract the shell/hull of the habitat to retain pressure, resulting in no net force on the shell.

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. The living space is 2,400 km deep with a surface area approximately 1,400 times that of Earth, but if the living space were divided into three and a half km high "floors" the useable volume would be about one million times that of Earth.

Its population limit is 36,000 trillion based on assumptions of energy use and black body radiation of waste heat. To control weather, "floors" are divided with vertical partitions, creating Earth sized cells 10,000 km across. There is also a steel cap which under the local acceleration of gravity of 0.02 m/s2 has to be 500 m thick to produce a force down equal to the atmospheric pressure up. Therefore, the Ederworld is a micro-gravity environment.

Eder himself 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. Nor did he consider the engineering that allows humans to live comfortably in a wider range of atmospheric pressures and gas mixtures. In addition his limits due to energy use and waste heat did not take into account nanotech or active cooling technologies. As a result, space habitats built using Eder's concept are much more diverse than he might have imagined.

Other types of shellworlds

 * A planet or a planetoid turned into series of concentric matryoshka doll-like layers supported by massive pillars. A shellworld of this type is in Ian M. Banks' novel Matter.
 * A megastructure consisting of multiple layers of shells suspended above each other by orbital rings supported by mass stream technology. This can be suspended above any type of stellar body, including planets, gas giants, stars, black holes, and even supermassive black holes at the center of galaxies.
 * Freespheres are large, transparent, double-walled open bubbles filled with air. These have a free-fall ecology and a layer of water between the inner and outer walls, and a magnetic screening which provides an effective radiation shield. With a hull completely made of buckycables a Freesphere can be made much larger than a rotating habitat made of the same material; as large as 5,000 km across even with full, baseline comfortable, atmospheric pressure. Freespheres may employ complex internal environmental systems, including airwalls, fusion or conversion reactor "suns", and various feedback-based systems to promote airflow, control temperature, and manage both general humidity and larger concentrations of moisture (ranging from small "pools" to "seas" several kilometers across).