allowing propellantless transfer between orbits. Around an airless world, such as the moon, the lower tip can actually touch the ground with zero horizontal velocity. As with any momentum exchange tether, orbital energy is gained or lost in the transfer. Fictional A number of structures have appeared in fiction which may be considered megastructures. Stellar scale The Dyson sphere has appeared in many works of fiction, including the Star Trek universe. Larry Niven's series of novels beginning with Ringworld centered on, and originated the concept of a ringworld, or Niven ring. A ringworld is an artificial ring with a radius roughly equal to the radius of the Earth's orbit (1 AU). A star is present in the center and the ring spins to create g-forces, with inner walls to hold in the atmosphere. The structure is unstable, and required the author to include workarounds in subsequent novels set on it. 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, and eventually expands to encompass a volume greater than the orbit of Jupiter. In White Light by William Barton and Michael Capobianco, a Topopolis is presented as taking over the entire universe. In the Heechee Saga series by Frederik Pohl, a 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 strings, spanning over 10 million light years. In Freelancer, The Dom'Kavosh's Dyson shell that is inhabited by a drone race created by the Dom'Kavosh, Nomads. This is reached via a hyper gate, created by the same creators as the Dyson sphere. The Saga of Cuckoo series novel Wall Around a Star mentions a proposal to build a super dyson sphere, completely enclosing the Galactic Center. The title of the novel Helix by Eric Brown directly references a stellar-scale helical megastructure. Different types of environments and habitats are interspersed along the structure, while their varying distance from the central star affects the climate. The player's central quest in computer game Dyson Sphere Program is to construct Dyson sphere. Gameplay focuses on constructing planetary scale factories as a means towards this end. The Quarg in the game Endless Sky are shown building a massive