Megastructures are large artificial constructs, typically seen in science fiction, but also exist in reality such as the Great Wall of China. Examples include the Death Star, Orbitals, and the Ringworld. A space elevator made of carbon nanotubes could be the first true megastructure, with plans to build one by 2031.
A megastructure is an artificial construct of enormous size. The term is informal but megastructures generally have a size of at least a few thousand kilometers along one dimension. Megastructures appear more frequently in science fiction, even though the Great Wall of China is probably a megastructure. The Great Wall can be seen from low Earth orbit, but also highways, farmlands and large buildings. Low Earth orbit is only 200 km (125 mi.) so this makes sense. Megastructures are more frequent in science fiction and it seems only a matter of time before our civilization starts building them. A megastructure is meant to be a contiguous and self-contained entity, i.e. not composed of an aggregate of smaller constructs.
The most familiar fictional megastructure would be the Death Star. It has been suggested that the Death Star was similar in size to the Moon. If so, it would be approximately 3,500 km (2,000 mi.) in size. Its core would be under enormous pressure and would probably need to be solid rather than containing structural elements, like in the movies.
Globus Cassus, an abstract design project, proposed to “unfold” the earth’s matter in a series of rotating rings whose inner surface we would then inhabit. A similar concept appears in the novels of Ian M. Banks Culture, where huge ring-shaped megastructures dozens of Earth diameters called “Orbitals” rotate silently, their internal surfaces covered by hundreds of continents and long oceans. In Larry Niven’s Ringworld series, there is an even bigger ring: this ring goes around the Sun with a surface covered with earth. It is made of a fictional material called scrith with atomic bonds as strong as the bonds between protons and neutrons in the atomic nucleus. Scientists calculating the necessary strength of this megastructure found that it should actually be that strong.
Back closer to reality, a space elevator, megastructure that could be built out of a material we can already make, albeit expensive and in small quantities – carbon nanotubes – would be around 50,000 km long, stretching all the way to geosynchronous orbit. Such an elevator could be relatively thin, with a nanotube core only 1 to 10 cm wide. The taller parts of the elevator would need to be thicker to withstand impacts with space debris. There is already one group, Liftport, that plans to build a space elevator by 2031. If they are successful, it could be the first true megastructure and herald a revolution in space travel.
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