A concept to build giant stations in space far from Earth and the Moon, L-5 colonies would be cities in space, located in a gravitational node in the Earth-Moon system. These colonies would be home to tens of thousands of people each, and serve as bases for building solar power satellites to generate electricity for Earth. L-5 colonies were extensively studied in the 1970s, but the high costs of building them prohibited their construction and they have been largely ignored since.

L-5 is the designation given to one of five Lagrangian points that existin the Earth-Moon system. These points, also known as libration points, exist where the gravity of Earth and the Moon partially cancel each other out. The first three points, L-1, L-2, and L-3, exist on a line connecting Earth and the Moon. These three libration points are considered unstable: An object placed near...