A megastructure is a very large manmade object, though the limits of precisely how large this is vary considerably. Some apply the term to any especially large or tall building.
Some sources define a megastructure as an enormous self-supporting artificial construct. Other criteria such as rigidity or contiguousness are sometimes also applied, so large clusters of associated smaller structures may or may not qualify. The products of megascale engineering or astroengineering are megastructures.
Some fictional megastructures in Space Opera
Larry Niven's series of novels beginning with Ringworld centered around, and originated the concept of a ringworld, or Niven ring. A ringworld is an artificial ring with a radius roughly equal to the radius of the Earth's orbit (1 AU). A star is present in the center and the ring spins to create g-forces, with inner walls to hold in the atmosphere. The structure is unstable, and required the author to include workarounds in subsequent novels set on it.
Several structures from the fictional Halo universe? are megastructures.
- The original twelve Halos, seen in Halo?: Cryptum?, were 30,000 kilometers in diameter; a separate array of six Halos are 10,000 kilometers in diameter, with one of the original twelve later being reduced to this size in Halo: Primordium?.
- The Ark? is a 127,530km diameter structure from which the Halo Array can be activated and capable of building 10,000km Halos. The "greater" Ark, seen in Cryptum and Primordium, is capable of producing 30,000km Halos.
- Onyx? is an artificial planet made entirely out of Forerunner? Sentinels. At its core is a "shield world", contained within slipstream space, that is approximately one astronomical unit in diameter. The much smaller Shield World 0459, (approximately 1,400 km in diameter), is the setting for the latter half of Halo Wars. A third shield world, Requiem?, is the primary setting for Halo 4?. Requiem is a normal planet but is encased in an artificial shell.
- High Charity?, the Covenant's? mobile planetoid station.
These are vast, brown dwarf-sized bubbles of atmosphere enclosed by force fields, and (presumably) set up by an ancient advanced race at least one and a half billion years ago. There is only minimal gravity within an airsphere. They are illuminated by moon-sized orbiting planetoids that emit enormous light beams.
Citizens of the Culture live there only very occasionally as guests, usually to study the complex ecosystem of the airspheres and the dominant life-forms: the "dirigible behemothaurs" and "gigalithine lenticular entities", which may be described as inscrutable, ancient intelligences looking similar to a cross between gigantic blimps and whales. The airspheres slowly migrate around the galaxy, taking anywhere from 50 to 100 million years to complete one circuit. In the novels no one knows who created the airspheres or why, but it is presumed that whoever did has long since sublimed but may maintain some obscure link with the behemothaurs and lenticular entities. Guests in the airspheres are not allowed to use any force-field technology, though no reason has been offered for this prohibition.
The airspheres resemble in some respects the orbit-sized ring of breathable atmosphere created by Larry Niven in The Integral Trees, but spherical not toroidal, require a force field to retain their integrity, and arose by artificial rather than natural processes.
One of the main types of habitats of the Culture, an orbital is a ring structure orbiting a star as would a planet. Unlike a Ringworld or a Dyson Sphere, an orbital does not enclose the star (being much too small). Like a ringworld, the orbital rotates to provide an analog of gravity on the inner surface. A Culture orbital rotates about once every 24 hours and has gravity-like effect about the same as the gravity of Earth, making the diameter of the ring about 3,000,000 km, and ensuring that the inhabitants experience night and day. Orbitals feature prominently in many Culture stories.
Ringworld-like megastructures exist in the Culture universe but are referred to simply as "Rings" with a capital R. These habitats are not described in detail but one is recorded as having been destroyed (along with 3 Spheres) in the Idiran-Culture war. In Matter, the Morthanveld people possesses ringworldlike structures made of innumerable various-sized tubes. Those structures encircle a star just like Niven's Ringworld and are about the same size.
These are asteroids and other non-planetary bodies hollowed out for habitation and usually spun for centrifugal artificial gravity. Rocks (with the exception of those used for secretive purposes) are described as having faster-than-light space drives, and thus can be considered a special form of spaceship. Like Orbitals, they are usually administered by one or more Minds.
Rocks do not play a large part in most of the Culture stories, though their use as storage for mothballed military ships (Pittance) and habitats (Phage Rock, one of the founding communities of the Culture) are both key plot points in Excession.
Shellworlds are introduced in Matter, and consist of multilayered levels of concentric spheres in four dimensions held up by innumerable titanic interior towers. Their extra dimensional characteristics render some products of Culture technology too dangerous to use and yet others ineffective, notably access to hyperspace. They were built millions of years ago as vast machines intended to cast a forcefield around the whole of the galaxy for unknown purposes. The species that developed this technology are now lost, and many of the remaining shellworlds have become inhabited, often by many different species throughout their varying levels. Many still hold deadly secret defence mechanisms, often leading to great danger for their new inhabitants, giving them one of their other nicknames: Slaughter Worlds.
Ships in the Culture are intelligent individuals, often of very large size, controlled by one or more Minds. The ship is considered the Mind's body. Some ships (e.g., GSVs) are tens or even hundreds of kilometers in length and may have millions or even billions of residents who live on them full-time, and together with Orbitals represent the main form of habitat for the Culture. Such large ships may temporarily contain smaller ships with their own populations, and/or manufacture such ships themselves.
Dyson spheres also exist in the Culture universe but are only mentioned in passing and are simply called "Spheres". Three spheres are recorded as having been destroyed in the Idiran-Culture war.
In Matter, the Morthanveld Nestworld of Syaung-un? is a "Sphere World" consisting of a complex, recursive arrangement of transparent tubes within tubes within tubes, all revolving around a small central star. The Nestworld is alleged to contain forty trillion Morthanveld, more intelligent beings than on all the Culture and associated worlds put together. There are also noted to be other Nestworlds, but none as big as Syaung-un.
Robots and Foundation
Trantor?, the capital of an interstellar empire in Isaac Asimov's Foundation series, is a planet entirely covered in one huge metal clad building, with only one small green space: the Emperor's palace grounds.
The Ori Supergate seen in a number of episodes of Stargate SG1? could be classed as a megastructure
In The Hitchhiker's Guide to the Galaxy? series, Earth, as well as other planets, were artificial megastructures. Earth was intended to function as a gigantic computer, and was built by a race of beings who made their living by manufacturing other planets.
Mata-Nui? in the Bionicle? franchise is classifiable as a megastructre. In the story he is a massive robot as tall as a planet, and inside his body, every inhabitant of the Bionicle Universe all live, unaware that they live inside a massive, space-traveling entity.
The Citadel? in the Mass Effect universe? is an enormous space station constructed by an ancient race of machines called the Reapers? millions of years before the games in the series. At the time of Mass Effect 2?, its population is 13.2 million.
Warhammer 40 K
In the Warhammer 40,000 series?, the Imperial Palace (site of the Golden Throne wherein the Emperor of Mankind is kept alive indefinitely) could be considered a megastructure. The palace is a complex of continent-wide structures with the Golden Throne being located in an area stretching across the whole of the Himalayan mountains.