These are megastructures that can be built without the presence of a gravity well.

O'Neil Colonies

Proposed several decades ago, these are basically large, hollow cylinders, from several hundred meters to several hundred kilometers long, that are spun about their long axes to simulate gravity on their inner surfaces. The interior is pressurized and the inner surface is sculpted into cities, parks, and even waterways to make as earth-like an environment as possible. They are usually powered by massive solar arrays.

An O'Neil colony's primary purpose is living space. A colony 5000 meters long and 1000 meters wide would have an interior surface area of about 15 square kilometers, and could support about one hundred thousand inhabitants if completely self-sufficient, more if food were imported.

O'Neil colonies are small worlds unto themselves, and will usually have all the features of other worlds in microcosm. There will be rich sections and poor sections, crime, political intrigue, and so on, making them rife with adventure potential. Some O'Neil colonies may have been built and settled exclusively by one political or religious faction or another, who use the colonies to further their isolation and ideological "purity". One colony may be set up by a group of Utopians, for instance, who try to create a "perfect" society away from contaminating influences. Another may set up a colony to act as a trading post to further its financial standing.

Babylon 5 of the TV series of the same name is an example of an O'Neil colony. An O'Neil colony is also the setting for Dark Horse's comic-book mini-series, Dirty Pair: A Plague of Angels, by Adam Warren.

Hollowed asteroids are a variation on this theme. As the name implies, these are O'Neil colonies built in the interior of large asteroids. All the features of a normal colony applies, except that the exterior shell of rock is many kilometers thick, providing protection from natural radiation hazards and potential attacks.

Rotovator

An intriguing concept proposed only in the last few years, a rotovator is an orbital transfer device that is essentially a spinning length of orbiting cable thousands of kilometers long. Rotating perpendicular to the surface of the planet, the end of the cable would "dip" into the upper atmosphere of the planet, scoop up transfer vehicles that have flown up to meet it, and then use its tremendous rotational acceleration to "fling" the transfer vehicle into a higher orbit at the apex of its spin, when the end of the cable is furthest away from the planet.

Returning a space ship to the planet could be accomplished by following the procedure in reverse. A spinning rotovator 8000 or so miles long orbiting Earth (which has a circumference of about 25,000 miles) would be able "dip" into the upper atmosphere about three times per orbit, when one end of the cable would reach the lowest point in its spin.

For structural integrity reasons, the cable would have to be much thicker in the middle than at its ends, by about ten to one. Thus, if the ends of the Rotovator were three meters thick, the center would need to be thirty meters wide. The cable material would also have to be incredibly strong, to resist the tremendous centrifugal force its own weight and constant spin would put on it.

The center of a Rotovator could also hold an inhabited station of some sort, perhaps to function as a command and control center for the structure. Also, if built wide enough, a rail system could be constructed along the rotovator's length so that supplies and passengers can be transferred easily to and from the central station or the opposite end.

Rotovators are discussed at length in the book Indistinguishable from Magic and featured in the novel Timemaster, both by Robert L. Forward.

Mech Planet

This is an artificial construct that approaches the scale of planets. It's basically a space station so massive that it has its own gravity well and could conceivably have its own free-standing atmosphere.

Again, the main purpose of a mech planet would probably be living space. A mech planet sphere with a diameter of 1000 kin would have an interior volume of over 500 million cubic kilometers. This is more than enough space to hold many billions of people. It would be a full-fledged world onto itself, with both its surface and interior volume used to its full capacity for food production, manufacturing, and living quarters. Like an Orbitville (see below), a Mech Planet could be used as an industrial or defensive center.

However, unlike an Orbitville, a Mech Planet could be made mobile. To a race with no means of FTL travel, a Mech Planet would be the ultimate generation ship, housing many millions of people in a selfenclosed environment for the decades or centuries it would take to travel between the stars. An industrialized, mobile Mech Planet could move through an asteroid belt and consume entire planetoids to synthesize materials for a burgeoning civilization. And a heavily-armed Mech Planet would be a fearsome weapon indeed.

The most famous example of a Mech Planet is the Death Star from the Star Wars movies.

A variation on the Mech Planet is the Converted Planet, where a planet-sized mass is gutted with machinery and attached to massive engines, converting it into a mobile platform. This is actually much more complicated than a standard mobile Mech Planet, as the internal structure of the planet might not be able to withstand the processes of gutting or the stresses of being moved. The engines needed to move it would have to be massive undertaking in and of themselves, requiring vast energies (perhaps tapped from the planet's core itself) to move such an enormous mass delicately.

Warworld, from the Superman comics, is an example of a Converted Planet.

MegaStructures Sci-fi Settings That Challenge the Scale of the Universe

Introduction
Free-Floating Structures
Planet-Centric Structures
Star-Centric Structures
Other Genres