colonization of five equilibrium points in the orbit of planets or moons
A diagram showing the five Lagrange points in a two-body system, with one body far more massive than the other (e.g. Earth and Moon). In this system L3–L5 will appear to share the secondary's orbit, although they are situated slightly outside it.
The Lagrange points L4 and L5 are stable if the mass of the larger body is at least 25 times the mass of the secondary body. Thus, the points L4 and L5 in the Earth–Moon system have been proposed as possible sites for space colonies. The L5 Society was founded to promote settlement by building space stations at these points.
Gerard K. O'Neill suggested in 1974 that the Earth–Moon L5 point, in particular, could fit several thousands of floating colonies, and would allow easy travel to and from the colonies due to the shallow effective potential at this point. A contemporary NASA team estimated that a 500,000-tonne colony would cost US$5.1 billion (equivalent to US$27 billion in 2020) to build.