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 them will quickly drift away. The other two points, L-4 and L-5, are in the orbit of the Moon, 60 degrees ahead and behind the Moon.Unlike the other three Lagrangian points, L-4 and L-5 are relatively stable: An object in orbit around either point will remain there.
The first person to propose L-5 colonies was Princeton University physicist Gerard K. O'Neill. Concerned in the early 1970s about both the effects of industrialization on Earth's environment and the energy crisis, he proposed developing giant space stations capable of hosting up to 10,000 people. These space colonies, as O'Neill called them, would be used to support the construction and operation of large solar power satellites that would convert sunlight into microwave energy to be beamed to Earth and converted into electricity.
Many concepts for space colonies* were developed in the 1970s, but most shared key characteristics. They had spherical, cylindrical, or wheel shapes, a kilometer (0.6 miles) or more in diameter, and rotated to generate artificial gravity. The interiors were designed to resemble small towns, complete with houses, parks, and farms. O'Neill estimated that one basic design, called Island One, would cost about $100 billion in 1978 dollars (about $275 billion today.)
Placing colonies at L-5, rather than on the Moon or in a closer Earth orbit, had a number of advantages. At L-5 the colonies would have continuous sunlight and would be free of the gravity of both Earth and the Moon. The L-5 location would also make it easy to transport building materials from the Moon. At L-5, colonies could be built to support whatever level of gravity was desired, from normal Earth gravity to weightlessness.
The concept of L-5 colonies attracted the attention of the National Aeronautics and Space Administration (NASA), which funded several studies of them and solar power satellites in the 1970s. Interest in such colonies among the general public also led to the creation of the L-5 Society, a precursor to the present-day National Space Society. However, by 1980, NASA's interest in space colonies and space solar power waned and it stopped funding additional studies. Also around this time, supporters discovered that the shuttle would not offer the low launch costs needed to make colonies feasible. There has been only sporadic interest in L-5 colonies since then.
see also Domed Cities (volume 4); Dyson Spheres (volume 4); O'Neill Colonies (volume 4); O'Neill, Gerard (volume 4); Settlements (volume 4).
O'Neill, Gerard K. "Colonization at Lagrangia."Nature (August 23, 1974).
——. The High Frontier: Human Colonies in Space, 3rd ed. Burlington, ON: Apogee Books, 2000.
Combs, Mike. "The Space Settlement FAQ."<http://members.aol.com/oscarcombs/spacsetl.htm>.
*In the mid-1970s, the U.S. State Department prohibited the use of the term "space colony" because colonialism is a system that denies human equality.