An Ohnyl Colony was a giant, cylindrical void station several kilometres long that lacked artificial gravity generation technology. Instead, it relied upon rotation around its axis to generate centrifugal force on its inner surface that was equal to or sometimes less than the gravity of Terra. These Ohnyl Colonies were first developed during the Age of Terra as a form of orbital habitat and could serve as a home in the void for hundreds of thousands or even millions of people.
During the Stellar Exodus of the late Age of Terra and the Age of Technology as Mankind first began to settle worlds beyond the Sol System, it is likely that these "Ohnyl Cylinders", as they were also known, were outfitted with Plasma Drives and served as the core design for the conventional human sub-light spacecraft used to colonise nearby star systems. Generations of people would live and die aboard these starships while they crossed the gulf between the stars. When the Warp-Drive and the Geller Field generator were invented in the 18th Millennium, such vessels were quickly rendered obsolete.
"Ohnyl" is a corruption of "O'Neill", named for Gerard K. O'Neill, who first proposed the design for the O'Neill space colony or "O'Neill Cylinder" in his 1976 book The High Frontier: Human Colonies in Space. O'Neill proposed the colonisation of space starting with low Earth orbit in the 21st century, using materials extracted from the Moon and later from asteroids.
As originally conceived by O'Neill, an O'Neill Cylinder would consist of two counter-rotating cylinders. The cylinders would rotate in opposite directions in order to cancel out any gyroscopic effects that would otherwise make it difficult to keep them aimed toward the Sun. Each would be 5 miles (8.0 kilometres) in diameter and 20 miles (32 kilometres) long, connected at each end by a rod via a bearing system.
- The Flight of the Eisenstein (Novel) by James Swallow, pg. 45