A megastation is a free-floating space structure, usually manned, whose dimensions are so colossal that it approaches the scale of natural moons and planetary bodies. They are also sometimes called Megabases, Metal Planets, Mech Planets, Mech Worlds, Artificial Planets, and Artificial Worlds. Related structures of this scale, such as Balloon Worlds and Planetary Halos, are discussed in their own articles.
Many space opera settings have at least one Megastation as part of their background. The Death Star from the Star Wars films is probably the most famous example of such a structure, followed closely by Cybertron and Unicron from the Transformers universe. OíNeill Colonies from the Gundam anime series can also be considered megastations.
Construction of a megastation (or any megastructure) is usually a colossal undertaking by the building civilization, and so much manpower and resources wonít be wasted for trivial reasons. Megastations could serve a number of important purposes. In stories, megastations usually serve as living space, holding many environments and habitats in its interior. Because they may be big enough to have their own natural gravity field, one holding a breathable atmosphere on its surface could circle a star and sustain thriving cultures on its open surface. In other stories, they may serve as an ark or reserve to contain threatened species and races.
Megastations can be used as platforms for immense weapons and as military bases, as exemplified by the Death Star. They can also be used for industrial purposes. Large enough megastations could be used to process entire asteroids and perhaps even small moons in their interiors. Unicron Ďeatsí small astronomical bodies in a similar manner, but the raw materials extracted are used to maintain the megastation/robot itself.
Building such a structure is a bit more complicated than just laying down a core of trusses in orbit and building out from there. After a certain point, the stationís mass will start exerting a significant amount of gravity that will tug continually on its own internal structure. Though even for smaller megastations the gravitational forces of a few gigatons of metal might seem negligible, over a significant amount of time it will put significant stress on station.
Also of consideration will be the stationís orbit, if any. Tidal stress may put potential strains on the enormous structure that smaller stations could likely ignore, and will have to be taken into consideration as well during design and construction.
Like most other stations, provisions will have to be made to maneuver the structure for station keeping. The larger the stationís mass, the more energy and fuel such maneuverings will consume. And for something as immense and ponderous as a megastation, the energies required with even a simple orbital correction could be vast.
Overall shapes for megastations are usually depicted as spheres, rings, disks, or cylinders, in order for the station to be rotated to provide artificial gravity on its inner surfaces. However, almost any shape can be used if rotational artificial gravity is not an issue. In these cases, depending on actual design, inhabitants will either be in total freefall or be under the influence of the megastationís natural gravity. Very advanced stations may use gravitic technology to generate earth-normal gravity on all decks.
The megastationís general design will also depend on what the stationís major function will be. If its main purpose is living space, it will probably very closely resemble space colonies like a Bernal Sphere or OíNeill colony. A military station, like the Death Star, may be mostly power generators by volume in order to serve its many weapons. An industrial megastation may be a free-frame structure in order to take advantage of pervasive microgravity conditions. One type of proposed a megastation, a whole-asteroid processing node, would basically be an enormous hollow cylinder that would allow the disassembly of an entire asteroid herded into its interior.
Megastations can be made completely mobile, and in effect become mega spaceships, but are still usually called Ďstationsí as people will just assume that theyíre immobile because of their size. For example, both the Death Star and Unicron were never actually called ships despite their being able to travel easily between star systems.