is a built structure
typically at least 1,000 kilometers in length -- in other words, at least 1 megameter
, hence the name. The definition is often informal and varies from source to source.
By extension, some people apply the term to any especially large or tall building.
Most megastructure designs could not be constructed with today's level of industrial technology.
This makes their design examples of speculative (or exploratory) engineering.
Those that could be constructed easily qualify as megaprojects.
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.
Megastructures are also an architectural concept popularized in the 1960s where a city could be encased in a single building, or a relatively small number of buildings interconnected together.
Such arcology concepts are popular in science fiction.
Megastructures often play a part in the plot or setting of science fiction movies and books.
In 1968, Ralph Wilcoxon defined a megastructure as any structural framework into which rooms, houses, or other small buildings can later be installed, uninstalled, and replaced; and which is capable of "unlimited" extension.
Many architects have designed such megastructures.
Some of the more notable such architects and architectural groups include the Metabolist Movement, Archigram, Cedric Price, Frei Otto, Constant Nieuwenhuys, Yona Friedman, and Buckminster Fuller.
This type of framework allows the structure to adapt to the individual wishes of its residents, even as those wishes change with time.
Other sources define a megastructure as "any development in which residential densities are able to support services and facilities essential for the development to become a self-contained community".
There are structures on Earth that may be considered megastructures, such as
- The Great Wall of China is a human-built megastructure, a few meters wide and 3,947 mi (6,352 km) in length, about 4,975,318 square yards (4,160,000 m²)
- The Rice Terraces of the Philippine Cordilleras, a 10,360 square kilometer (4,000 square mile) sprawling agricultural landscape carved in the mountains by free tribesmen of Ifugao some 6,000 to 2,000 years ago.
- Skyscrapers represent our current state-of-the-art in large structure engineering. See also the list of largest buildings in the world.
Networks of roads or railways, and collections of buildings (cities and associated suburbs), are usually not considered megastructures, despite frequently qualifying based on size. However, an ecumenopolis might qualify.
Most stellar scale Megastructure proposals are designs to make use of the energy from a sun-like star while possibly still providing gravity or other attributes that would make it attractive for an advanced civilization.
- The Alderson disk is a theoretical structure in the shape of a disk, where the outer radius is equivalent to the orbit of Mars or Jupiter and the thickness is several thousand miles. A civilization could live on either side, held by the gravity of the disk and still receive sunlight from a star bobbing up and down in the middle of the disk.
- A Dyson sphere (also known as a Dyson Shell) refers to a structure or mass of orbiting objects that completely surrounds a star to make full use of its solar energy.
- Larry Niven's Ringworld is an artificial ring with a radius roughly equal to the radius of the Earth's orbit. A star is present in the center and the ring spins to provide artificial gravity.
- An Orbital is a space habitat similar to but much smaller than a Niven Ring. Instead of being centered around a star, it is orbiting a star, thus its diameter is typically on the order of magnitude of a star. By giving a tilt to its orbit, there's a convenient day and night experience on its surface.
- A Matrioshka brain is a collection of multiple Concentric Dyson Spheres which make use of different wavelengths of light.
- A Stellar engine either uses the temperature difference between a star and interstellar space to extract energy or serves as a Shkadov thruster.
- A Shkadov thruster accelerates an entire star through space by selectively reflecting or absorbing light on one side of it.
- Topopolis (also known as Cosmic Spaghetti) is a large tube that rotates to provide artificial gravity.
- Orbital ring is an enclosed loop slightly larger than the circumference of the Earth so that it can maintain low earth orbit.
- Globus Cassus is a hypothetical proposed project for the transformation of Planet Earth into a much bigger, hollow, artificial world with the ecosphere on its inner surface. This model serves as a tool to understand the World's real functioning processes.
- The Bernal sphere is a proposal for a space colony with a maximum diameter of 16 kilometers.
- The Stanford torus is a different design with a diameter just under 1.7 kilometers.
- The O'Neill cylinder is yet another space colony proposal.
- A Skyhook is a very tall structure that hangs down from orbit.
- A Space elevator is a skyhook that is fixed to the ground
- A Space fountain is held up by the momentum of masses which are shot up to the top at high speeds from the ground.
- A Lofstrom loop ("Launch loop") is a 2000km long iron loop that projects up in an arc to 80 km that is ridden by maglev cars while achieving orbital velocity.
- Rotovator proposals call for a large tether to transfer momentum between spacecraft in transit.
- The Dyson shell (including its variation, the Ringworld) has appeared in many works of fiction, including the Star Trek universe.
- Larry Niven's series of novels about his Ringworld centered around, and originated the concept of the Ringworld, or Niven ring.
- In the manga Blame! the Megastructure is a vast and chaotic complex of metal, concrete, stone, etc, that covers the Earth and assimilates the Moon.
- In White Light by William Barton and Michael Capobianco, a Topopolis is presented as taking over the entire universe.
- In the Heechee books by Frederik Pohl the race of pure energy beings called The Foe have constructed the Kugelblitz, a black hole made of energy and not matter.
- In the Xeelee series of books by Stephen Baxter, the eponymous alien race constructed the Ring, a megastructure made of cosmic string, spanning over 10 million light years.
- In the game Airforce Delta Strike a large Space Elevator called the Chiron Lift is used to send supplies out into outer space. Later in the game it is destroyed by an artificially enhanced human named Pierre who fights for the Orbital Citizens Community (OCC).
- The Ark from the Halo universe which served as the last place of safety for the Forerunners from the flood, with the ability to rebuild destroyed halos.
- Death Star from Star Wars
- Borg Transwarp Hub and Unicomplexes, Federation Starbases and Fleet Yards
- Buster Machine III from Gunbuster.
- Culture Orbital
- 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
- Coruscant, capital city in the Star Wars universe, entirely covers its host planet. It serves as capital of first the Republic and then later the First Galactic Empire.
- 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.
- The Star Forge from Knights of the Old Republic
- Despite being sentient, the Transformers universes Unicron's 'metal planet' form is comparable to a megastructure.
- Cybertron itself is a megastructure, comparable to trantor or coruscant on a smaller scale.
- The seven Halos from the Halo universe are 10,000 km wide rings.
- In the Robotech Sentinels novels, Haydon IV is an artificially constructed cyber-planet with android citizens.
- In the Invader Zim episode "Planet Jackers", two aliens surround the Earth with a fake sky in order to throw it into their sun.
- Nightmare's fortress from Kirby: Right Back at Ya! can be classified as a megastructure because of the fact that it's the size of a small planet.
- In several works, Arthur C. Clarke writes about a colossal hollow tube, first described in "Rendezvous with Rama" (1973), and inhabited by different races.
Structures that might not be classified as "Megastructures
" because they do not meet the requirements, but are indeed "Mega
" sized structures/constructions.