In science fiction, floating cities are settlements that strictly use buoyancy to remain in the atmosphere of a planet. However the term generally refers to any city that is flying, hovering, or otherwise suspended in the air via any means technological or magical.
In Jonathan Swift’s satirical novel Gulliver’s Travels, Swift envisioned Laputa, an island city that floated in the sky. The island was suggested to levitate above the Earth by use of the force of magnetism. In the 1920s, Hugo Gernsback speculated about floating cities of the future, suggesting that 10,000 years hence “the city the size of New York will float several miles above the surface of the earth, where the air is cleaner and purer and free from disease carrying bacteria.” To stay in the air, “four gigantic generators will shoot earthward electric rays which by reaction with the earth produce the force to keep the city aloft.”
Although Swift’s proposal was intended as satire, Buckminster Fuller proposed the concept more seriously in the form of the Cloud nine (Tensegrity sphere) megastructure, in which he envisioned structural spheres that float freely in the sky, allowing passengers a migratory lifestyle and a solution to the depletion of Earth’s resources. He proposed a 1-mile-diameter (1.6 km) geodesic sphere that would be heated by sunlight, functioning as a thermal airship.
A design similar to Fuller’s Cloud Nine might permit habitation in the upper atmosphere of Venus, where at ground level the temperature is too high and the atmospheric pressure too great. As proposed byGeoffrey A. Landis, the easiest planet (other than Earth) to place floating cities at this point would appear to be Venus. Because the thick carbon dioxide atmosphere is 50% denser than air, breathable air (21:79 Oxygen-Nitrogen mixture) is a lifting gas in the dense Venusian atmosphere, with over 60% of the lifting power that helium has on Earth. In effect, a balloon full of human-breathable air would sustain itself and extra weight (such as a colony) in midair. This means that any large structure filled with air would float on the carbon dioxide, with the air’s natural buoyancy counteracting the weight of the structure itself.
At an altitude of 50 km above the Venusian surface, the environment is the “most Earthlike in the solar system”, with a pressure of approximately 1 bar and temperatures in the 0°C-50°C range. Because there is not a significant pressure difference between the inside and the outside of the breathable-air balloon, any rips or tears would cause gases to diffuse at normal atmospheric mixing rates, giving time to repair any such damage. In addition, humans would not require pressurized suits when outside, merely air to breathe and a protection from the acidic rain.
Since such colonies would be viable in current Venusian conditions, this allows a dynamic approach to colonization instead of requiring extensive terraforming measures in advance. The main challenge would be using a substance resistant to sulfuric acid to serve as the structure’s outer layer; ceramics or metal sulfates could possibly serve in this role. (The sulfuric acid itself may prove to be the main motivation for creating the structure in the first place, as the acid has proven to be extremely useful for many different purposes.)
In addition to Venus, floating cities have been proposed in science fiction on several other planets. For example, floating cities might also permit settlement of the outer three gas giants, as the gas giants lack solid surfaces. Jupiter is not promising for habitation due to its high gravity, escape velocity and radiation, but the solar system’s other gas giants (Saturn, Uranus, and Neptune) may be more practical. In 1978, the British Interplanetary Society’s Project Daedalus envisioned floating factories in the atmospheres of Jupiter refining Helium-3 to produce fuel for an interstellar probe. Michael McCollum notes that the “surface” gravity of Saturn (that is, at the visible cloud layer, where the atmospheric pressure is about the same as Earth’s) is very close to that of Earth, and in his novel The Clouds of Saturn, he envisioned cities floating in the Saturnian atmosphere, where the buoyancy is provided by envelopes of hydrogen heated by fusion reactors. Uranus and Neptune also have “surface” gravities comparable to Earth’s, and even lower escape velocities than Saturn. Cecelia Holland populated Jupiter, Saturn and Uranus with mutant humans, the Styth, in floating cities in her only SF novel, Floating Worlds (1975).