Megastructure

A megastructure is an enormous device used for many purposes. A megastructure is defined as any artificial structure greater than 1,000 kilometers in length.
==Types of megastructures==
Many kinds of megastructure have been proposed. Most are designed to maximise living area or the area for collection of solar power. Civilisations capable of building these are likely to be high Kardashev class II or Kardashev class III.
===Dyson sphere===
A structure that surrounds a sun completely, with the inner surface being used for living space and solar power arrays. The idea was proposed (though not created) by Freeman Dyson in the late fifties as a possible means for a race to achieve Kardashev class II- using the total power of a sun. Most are designed to have a radius of one astronomical unit or thereabouts to keep them habitable.
A solid-shell Dyson Sphere with a hundred-million km radius, a wall thickness of 2km, and the density of iron would have a mass of approximately a solar mass- 2E30kg. To gather sufficient resources to build such a structure, thousands of planets or a few stars would have to be deconstructed. However, simply gathering the resources is not good enough- most will be hydrogen and helium, which must be fused or somehow changed into usable structural materials. Ores must be processed and refined, and everything must be moved into the right place without disturbing the star in the process.
When constructed, the shell will possess enormous gravity due to its mass. It can hold an atmosphere at about one atmospheric pressure to its surface (an artificial magnetosphere will also be needed), though if it rotates then all the materials will gather in the equator and the majority of the sphere's internal area will be wasted. Because this stress is acting over such a large area, it adds up to produce stress over ten thousand times the yield strength of structural steel- no conventional materials could withstand this. Increasing the shell thickness will not help solve this as it also increases the sphere's mass and so the load.
The gravity on the surface will not be sufficient to hold onto objects, so antigravity technology is required. They also tend to heat up, reaching equilibrium at about 120 degrees C unless serious cooling is used.
===Dyson Swarm===
To counter the prohibitive stress problems of a solid Dyson Sphere, Dyson suggested that the structure be made from trillions of artificial planetoids closer to the Sun than Earth is. Each would be equipped with a very large, thin collector mirror, and mass about 1E13kg, making the shell's total mass slightly less than Jupiter's. Each would be internally pressurised and act as a habitat.
A Dyson bubble is like a swarm, but the constructs are not orbiting the star. Instead, they act as solar sails, maintaining their position using radiation pressure. Such a structure at 1 AU would mass around 2.2E20kg. Materials science could not yet produce sails of sufficient lightness that are opaque, though carbon nanotubes are a possible candidate.
The simplest form of Dyson bubble is a Dyson ring, with only enough statites to surround the star's equator.
===Ringworld===
This was an idea proposed by the science fiction author Larry Niven, which he explored thoroughly in his novel Ringworld and its sequels. It is far more plausible and reasonable than the solid Dyson sphere, while still offering vast amounts of surface area.
Niven's Ringworld has a radius of one AU, a mass of 2.5E27kg (about one and a half Jupiters), a wall height of 2 million metres and a floor thickness of a kilometre. The structure can rotate to provide artificial gravity without worrying about deformation, so allowing an atmosphere and oceans on the inner surface. Walls rising above the atmosphere prevent these from slipping around the edge. An inner ring of shadow squares linked by very strong wire rotates at a different speed, producing shadows on the Ringworld to simulate day and night (these are also the sites of power generation).
The interior surface area is equivalent to that of a million Earths. Several Ringworlds of differing radii could be set up around a star. A disadvantage is that small peturbations will be amplified over time, causing a very small disturbance to be a possible serious threat.
A similar idea is the 6E21kg topopolis, which is a hollow ring and has the biosphere set up inside. If severed and stretched out as a cylinder, it would be about a trillion metres long and about a kilometre in cross-section. It would have a habitable area of about 20 Earths.
===Orbital===
Popularised by Iain M. Banks in his Culture books, an orbital is shaped like a Ringworld, but far smaller. It orbits a star without encircling it, rotating to provide its gravity. As it rotates, a day/night cycle is produced because one side is in darkness while the other is in light. It still requires superstrong materials, and has a radius of about 2 million km to produce a 24 hour day/night cycle.
An orbital with a width (of the surface) of 1000km masses around 8E22kg and has an area of 22.9 Earths.
===Alderson disk===
Far larger than any of the above, an Alderson Disk is shaped like a CD or record with a star at the centre of the hole. The mass of the structure will give uniform gravity perpendicular to the disk everywhere except at the structure's edges, providing enormous quantities of living area, and both sides can be inhabited.
The proposed design has an outer radius of 600 million km, which if in our solar system would be between Jupiter and the asteroid belt. The inner radius is 50 million km, just about the orbit of Mercury. The structure is 5 million km thick and has a uniform gravitation field of 0.16g. Naturally, it is enormously massive- about 6E33kg. It is the least feasible of these structures.
===Artificial planet===
==Comparison of megastructures==
Name 	Mass 	Living/Collector Area	Minimum energy required to build	Notes
Solid Dyson sphere 	2E30kg 	~600 million Earths 	1E43J 	
Dyson swarm 	1E27kg 	~600 million Earths 	1E35J? 	Structures can be self-replicating if given resources
Dyson bubble (whole)	2.2E20kg	~600 million Earths 	1E25J? 	As above
Dyson ring 	4E17kg 	~1 million Earths 	1E22J? 	As above
Ringworld 	2.5E27kg	~1 million Earths 	1E36J 	
Topopolis 	6E21kg 	~20 Earths 	1E27J 	
Orbital 	8E22kg 	~23 Earths 	1E28J? 	
Alderson Disk 	6E33kg 	~4 billion Earths 	5E44J 	
 	 	 	 	
==See also==
*Biochemistry
Category:Science