When most people think of worlds, they think of big balls of rock or gas orbiting a star. Variations usually tend toward strange atmospheres, odd compositions, or exotic native life. One world might have a crimson sky and a dozen moons; another with a vibrant ring system may be locked in a perpetual ice age.
But these are only worlds as nature might make them. Just as mankind has created new, artificial chemical elements, so too is it possible for the human race to eventually create entirely new classes of worlds. Worlds that may stretch around planets or trap the energy output of entire stars. Worlds that may challenge the scale of the universe.
These new worlds are immense artifacts collectively known as megastructures.
This article is an overview of the many types of megastructures that have been featured in science fiction stories over the years. Megastructures are easily one of the most awe-inducing concepts in speculative fiction, and the musings of various creators have given birth to a startling variety of them.
Basically, a megastructure is defined as any single artifact that challenges human preconceptions of size. A modern city may not be considered a megastructure because it is composed of many smaller buildings. The Great Wall of China, however, could be considered one, because it is a single, unified structure.
Megastructures require vast amounts of material, capital, manpower, energy, and logistic planning to construct. Some may take decades, centuries, or even thousands of years to construct. So why bother building one at all?
For some, form follows function. Some feats can't be performed by anything except the very large. Space elevators, for example, need to be tens of thousands of kilometers in length in order to propel cargoes into orbit. In order to trap all of the sun's radiative energy, a structure as large as a Dyson sphere is a necessity. The enormous expenditure of building such structures may be mitigated by the valuable services and resources they can provide.
Another reason to construct one may be as simple as the need for living space. Interstellar travel may turn out to be impractical because of one circumstance or another, meaning we may be forever trapped within our own star system. Unless draconian measures of control are used, burgeoning growth will eventually outstrip the capacity of terrestrial worlds to support their populations. However, if a civilization were to convert all of the material in the system to the job of supporting life, by creating the vast habitable surface area of, say, a Ringworld or Dyson sphere, the problem could be circumvented.
In some cases, as in Niven's Ringworld and Malibu Comics's Godwheel, larger megastructures may be used as enormous life and cultural laboratories. Specimens from many thousands of worlds can be held and studied in detail on the vast surface of the artifact. Sociological and biological experiments can be conducted on a grand scale without endangering the source stock on distant homeworlds.
A megastructure can also act as a means of preserving or protecting a culture from disasters or outside influence. In the novels Farthest Star and Wall Around a Star by Frederick Pohl and Jack Williamson, a Dyson sphere is used as an intergalactic lifeboat to transport survivors away from an exploding galaxy. On a smaller scale, space-borne O'Neill Colonies could be used as refuges for cultures that wish to retain their ideological "purity" away from outside contact yet still remain self-sufficient.
But perhaps the most significant reason to build a megastructure is to simply prove that one can, to create some lasting and unforgettable monument to the power and ingenuity of its builders.
A number of megastructures, such as space elevators, rotovators, and O'Neill colonies, have already been discussed in detail in previous Strange Horizons articles. Links are provided at the end of this article.
The term "arcology" is a fusion of the words "architecture" and "ecology," first coined by Paolo Soleri in his landmark 1969 book, Arcology: City in the Image of Man. Arcologies have also been incorporated into many science fiction works since, such as the movie Blade Runner and novels like Neuromancer by William Gibson and Oath of Fealty by Larry Niven and Jerry Pournelle.
Arcologies are envisioned as optimal human urban environments; large, self-enclosed mega-skyscrapers with volumes measured in the cubic kilometers that contain everything a human population could ever need. They would be miniature cities onto themselves, engineered for maximum comfort and efficiency. The structure would not only supply living space but would have its own commercial, industrial, and business districts. A resident would not only live within the structure, but also commute and work and shop all within its enclosed walls. The interior would also be configured with a great many windows, parks, and mirrors to give the illusion of much greater space in individual hallways and chambers.
Because space within the structure is used much more efficiently, an arcology would require only about two percent as much land as a modern city of equivalent population. Actual living space per person would be about the same, but the support structure can be built on a much more efficient three-dimensional scale, and the need for road infrastructure (which can consume as much as sixty percent of the space of modern urban sprawls) would be eliminated altogether. Arcologies of many different configurations have been proposed, many able to hold up to half a million residents or more.
Though arcologies today are seen primarily as an urban development, they can be built for nearly any environment. Cliffside arcologies, farmland arcologies, and even dam arcologies have all been proposed. Many see the development and perfection of Earth-bound arcologies as a necessary first step for engineering livable colonies in the oceans and in space.
Megaships are an outgrowth of ever-larger ship designs, exemplified by super-carriers like the USS Enterprise and luxury liners like the Queen Elizabeth II, each designed to hold many thousands of passengers and crew. A new class of megaship, built on a scale dwarfing even those vessels, would become the sea-borne equivalent of arcologies.
The Freedom Project, the most well-publicized civilian endeavor along those lines, envisions a massive vessel 4,500 feet along its beam, making it nearly a mile long and four times the size of the Queen Mary. It is designed to hold no less than 100,000 people at any one time, including 40,000 full time residents and 20,000 permanent crew, with the rest as passengers and visitors. The builders plan on having, among other facilities, first-class hotel accommodations, a casino, a fully-equipped hospital, a school system, and the world's largest duty-free shopping mall.
One would think a ship that size would crack in two just from its sheer bulk. However, the Freedom ship would not really be one solid unbroken hull, but over a dozen separate aircraft-sized modules. Each module floats independently, much like a ten-story-tall barge, joined to the others at many decks via seamless gimbaled joints but maintaining enough flexibility along its dimensions to absorb and dampen wave effects.
The ship, the groundwork for construction of which is already being put into place, would slowly circumnavigate the globe every three years, stopping not only at major ports but also at many islands accessible only by sea. The US military is also looking into developing a similar-sized ship, but one whose individual modules would sail separately and then assemble into a mega-sized ocean-going air base.
An orbital rosette is three or more objects of comparable mass spaced equidistantly along the same circular orbit. The objects will circle about the system's center of gravity, even if no object is occupying the center of the system, bound together and balanced by each other's gravity. This arrangement of bodies was first discussed by W. B. Klemperer in The Astronomical Journal in 1962, and is sometimes known more formally as a Klemperer rosette.
An orbital industrial infrastructure would require a great deal of natural resources, and lifting so much material off Earth or even the Moon would involve great cost and effort. An alternative would be to move asteroids into geosynchronous Earth orbit, arranged in a rosette for stability. With megatons of raw ore readily available, huge construction projects in space could become much more cost effective and feasible.
Rosettes of asteroids would just be the beginning. Centuries from now, when humanity begins exploiting and colonizing the moons of the gas giants, those small wordlets can be moved into rosettes to allow for more efficient mining of their resources, as well as facilitating travel between colonies set up on them. It is generally faster and less fuel-consumptive to move between objects in the same orbit than changing orbits.
If technology ever becomes fantastically advanced enough to move planets, it would be possible to create rosettes of major worlds. In the novel Ringworld by Larry Niven, an alien race called the Puppeteers created a rosette of five Earth-sized worlds to function as a colossal generation ship to migrate interstellar distances.
However, a rosette is prone to progressional instability. Progression is the tendency of rotating objects to drift along the plane of their spin. It's the phenomenon that allows one to easily balance on a moving bicycle, among other things. If left to their own devices for long enough, the objects in a rosette would eventually begin drifting out of alignment. Rosettes could be constructed with enough precision that such drifts may take centuries or millennia before they become apparent, but some kind of active, long-term maintenance would still be needed to keep a rosette in place.
These structures are called haloes here to better distinguish them from naturally-occurring planetary rings. As the name implies, these are circular structures that completely encompass a planetary body.
The first subclass of this megastructure would be more properly called a halo swarm, made up of thousands if not millions of objects independently orbiting in a tight circle about the planet, giving the impression from a distance of a solid ring. Because of the relative closeness of the objects, all their orbits would have to be careful choreographed and interwoven to avoid collisions.
A halo swarm may not be a planned structure, but may evolve over time. Geosynchronous orbit is prime real estate in space, already relatively crowded with communication satellites. In centuries to come, geosynchronous orbit may become the orbital equivalent of urban sprawl, choked with thousands of satellites, habitats, and other structures. But a halo swarm may also be planned, as civilization realizes the value of certain orbits and partitions space along them carefully to thousands of interested parties.
A solid halo is possible for very advanced cultures, and may be a natural outgrowth of things like space elevators, asteroid rosettes, or a halo swarm, all of which may be necessary to provide the preliminary framework and resources needed to construct it. A solid halo with an orbital altitude of 22,300 miles and a cross section of one square kilometer would have an internal volume of some 70,000 cubic kilometers. If spinning at the same rate as the planet below (a very advantageous arrangement, as it would allow easy access back and forth to the surface via space elevators) it would have a very modest gravity along its outermost surface, about 1/20th of a g.
Like a rosette, one of the main problems with engineering a solid halo (beyond its immense scale, of course) is progressional instability. Attitude jets or other means of maintaining the immense artificial ring's position in the plane of its orbit would be necessary. Solid halos were seen in novels such as The Fountains of Paradise and 3001, both by Arthur C. Clarke.
This unusual kind of deep space structure is basically an enormous hollow shell held rigid by gas pressure within. Using advanced materials, such a balloon megastructure could be built up to about the size of Jupiter and still remain structurally stable. However, smaller such balloon worlds several thousand kilometers across may prove much easier to both build and maintain.
Unlike other megastructures which could serve multiple purposes, perhaps the only real reason for constructing a balloon world is to function as a habitat for life, provided it is filled with a breathable atmosphere. Light from its parent star could be let in through large transparent sections or through a series of gigantic gimbaled mirrors near its rotational poles.
The best example of such a construct can be found in Karl Schroeder's Sun Of Suns, a novella serialized in the magazine Analog from November 2005 through March 2006. The hollow world in his story was what he described as basically a "fullerene balloon" some 5000 kilometers in diameter, pressurized with breathable air and stocked with enough volatiles and organic materials within to equal the mass of Pluto. However, this mass is widely dispersed within the sphere, giving birth to a life-supporting but completely gravity-free environment. The makers of that world wanted to separate themselves from the rest of humanity, so they placed it far away from any star. In order to keep things comfortably warm for its human inhabitants and ecologies, a great many radiative fusion plants, the "suns" of the title, were dispersed throughout.
Take a band of unbelievably strong material a million miles wide and stretch it around a star at a distance comparable to Earth's distance from the sun. Spin it to simulate gravity on the inner surface. Put a wall a thousand miles high along the inner edges to keep in the atmosphere. Now sculpt the interior surface to resemble an Earth-like world, complete with soil, water, oceans, forests, and so on. Further in, orbiting the sun, place over a dozen enormous squares, casting shadows on the inner surface and simulating the day-night cycle of natural worlds. What you end up with is a vast world with a habitable surface area equal to approximately three million Earths. Totally self-supporting, it could easily hold tens of trillions of inhabitants.
The structure described above was first proposed by Larry Niven in his seminal novel Ringworld. The scale of the Ringworld and other star-centric megastructures can be difficult to grasp intuitively. For example, on Niven's Ringworld, two great Oceans exist on either side of the Ring, each stretching from one rim wall to the next, making them nearly a million miles wide. That's large enough to swallow the planet Saturn if the oceans were made deep enough. Entire civilizations can rise and fall on the artifact over millennia, exploring many planet-lengths in all directions, but never knowing more than a tiny portion of their world.
Ringworlds would require the disassembly on an entire solar system for building materials, as well as fantastically advanced technology, such as large-scale matter conversion and the production of near-indestructible materials to handle the immense structural stresses the artifact would constantly endure. They would also station-keeping thrusters and attitude jets to keep them stable.
Iain M. Banks's Culture novels introduce a much scaled-down version of Niven's monumental Ringworld called Orbitals. Like their much larger cousin, Orbitals are spun up for gravity, and their interiors are sculpted and terraformed to provide a suitable environment for habitation. However, they are "merely" several million miles across and several thousand kilometers wide. They are canted at an angle in the plane of their orbits about their primary star, so the shadow cast by the forward part of the ring provides the "night" of the habitat's day/night cycle.
An Orbital-like structure is also featured in the Halo science fiction action game series.
Instead of being made of uber-metal and rock, a Ringworld can also be constructed of micron-thin solar sail material and a sparse framework. Its main purpose would be energy gathering, trapping a significant percentage of a star's energy. As the ring would not be required to sustain life, it can be built with a much smaller radius and would require much less material to construct, requiring perhaps the disassembly of only a single large moon.
Type I Dyson Sphere
The modern concept of a Dyson sphere is generally attributed to famed physicist Freeman Dyson, though Dyson-sphere-like objects can be found here and there in Golden Age science fiction. There are a number of different variants on this idea. The original, as proposed by Dyson in 1959, is generally referred to as a type I Dyson sphere, or sometimes as a Dyson swarm.
A Dyson sphere is a physical shell completely encasing a star. Its main purpose is energy gathering, as it would trap all of the sun's radiative energy for use by its building civilization. The type I variant of this idea assumes that this shell is constructed entirely of loose objects, each with a different and independent orbit, arranged and layered in such a way that they intercept all of the star's light. These objects could be trillions of small satellites, space stations, O'Neill colonies, natural planets, Ringworlds, or some combination of all of the above.
Though such a sphere would trap all outgoing light, it would still radiate waste heat. Dyson proposed searching for the low-infrared signatures of Dyson spheres as a means of locating advanced extraterrestrial civilizations. None have yet been found.
Type II Dyson Sphere
A far more ambitious undertaking than the type I variant, a type II Dyson sphere is a single, unified object that completely englobes a star. Such a sphere constructed with a radius of Earth's distance from the sun would have a potential habitable surface area of about one billion Earth-sized planets. If the entire surface were terraformed, it could hold over one hundred trillion inhabitants. Like a Ringworld, a Dyson sphere has such a mind-boggling amount of habitable room that it could support millions of diverse ecologies and thousands of independent civilizations with no strain whatsoever.
However, this type of Dyson sphere has a number problems that would require very exotic technologies to overcome. It would have to be made out of unbelievably strong material, far more resilient than anything now known, in order to withstand the enormous tidal stresses it would have to endure both during construction and afterward. It, like a Ringworld, would require station-keeping thrusters and attitude jets to keep its orbit and position stable. Even if set spinning, some means of artificial gravity generators will be needed to keep objects not on the interior equator from flying off toward the sun. And there is the question of whether there would be enough silicates even in the entire solar system to form soil for the entire inner surface, or enough nitrogen and oxygen to provide enough breathable atmosphere. Chances are technologies such as matter conversion would have to be employed or material would have to be imported from dozens of nearby star systems.
Other Dyson Sphere Variants
A variation on the Dyson sphere is the gossamer sphere. Like the similar Ringworld equivalent, it would be composed mostly of a sparse framework and micron-thin solar cell material. Its main purpose would be energy gathering, and it can be constructed with much smaller dimensions than an inhabited sphere.
A compromise can be built between the standard and gossamer sphere, and avoid some of the logistic problems of the former. The equator of the sphere can be a habitable Ringworld, as described above. The rest of the sphere, however, can be made from gossamer material for energy gathering, and supply power to the trillions of inhabitants of the ring.
Still another variation is the solid Dyson sphere. It has the same dimensions as a standard type II sphere, but is solid all the way through to nearly the surface of its center star. Even with matter conversion, it would take the material resources of dozens of star systems to construct such a mammoth artifact. It could hold perhaps a quadrillion inhabitants with millions of continent-sized habitats throughout its volume. The aforementioned novels Farthest Star and Wall Around A Star featured this type of Dyson sphere.
This disk-like megastructure has not one star at its center, but two. The central stars orbit closely about each other about a common center of gravity. A disk built around them, perpendicular to their mutual orbit, would take advantage of their circling to simulate a day-night cycle over its alternate sides. The suns would rise and set successively over each surface during their short orbital period. If the disk is built large enough, its potentially habitable surface can cover the entire life zone around a star, theoretically from about the orbit of Venus to the orbit of Mars, on both sides of the disk. Gravity would have to be generated purely by artificial means.
The godwheel was created by Larry Niven for Malibu Comics's Ultraverse line, and was a central feature in this short-lived but intriguing super-hero universe.
Perhaps one of the most mind-boggling megastructures yet proposed, this was the central feature in Stephen Baxter's novel Ring. Whereas previous structures were built on the scale of planets and stars, the Xeelee Ring was built on a scale dwarfing galaxies. This is ironic, as the "material" that made up the Ring is far too small to be seen by the naked eye: a cosmic string.
A cosmic string is essentially a singularity—the pinpoint of super-collapsed matter at the heart of a black hole—stretched into a line light years long. Thought to be remnants of the Big Bang, the gravitational influence of cosmic strings is theorized to be one of the primary factors responsible for the formation of the first galaxies.
In the novel, the alien Xeelee are losing a universe-wide war against dark matter beings for control of the cosmos. So they retreat through an "escape hatch" from the universe—created by ripping open a spinning naked singularity with a 10 million light year wide loop of cosmic string. This wormhole-like "escape hatch" has a mass of several galactic clusters and measures some 300 light years across. The 10 million light-year-wide Ring's purpose is to rip away the singularity's event horizon to allow easy access in and out of the portal. The Xeelee Ring is so huge that the Xeelee's enemies throw entire galaxies at it to try and disrupt it.
It's never clear if the Xeelee simply gathered the cosmic string from around the universe or found some way of manufacturing it through some insanely advanced process. Either way, however, this represents resources and technology of a mind-boggling level.
From the Pyramids at Giza to the Great Wall of China, to modern projects such as the Hoover Dam and the artificial Palm Islands off Dubai, mankind has a long history of dreaming big and acting upon such notions. As our technology becomes more sophisticated and more powerful, so too will our ability to create grand artifacts that can dwarf their creators. The age of the megastructure is just beginning.
Arcology: City in the Image of Man by Paolo Soleri
Ringworld by Larry Niven
Ring by Stephen Baxter
On The Web:
Space Elevators [strangehorizons.com]
O'Neill Colonies [strangehorizons.com]
The Freedom Ship homepage [freedomship.com]
Article on the Freedom Ship [dickran.net]
Dyson Spheres [nada.kth.se]