LAGRANGE POINT

   In 1772 the French mathematician Joseph Louis Lagrange (1736-1813) calculated that in the orbit of Jupiter around the Sun there would be two stable positions, one 60deg ahead of the planet, the other 60deg behind, where a comparatively tiny mass would remain in stable orbit around the Sun rather than being swept up Jupiter's gravitational field. (More than acentury later two groups of ASTEROIDS, the Trojans, were found at these positions in Jupiter's orbit.) This is a general principle, part of what is sometimes called the three-body problem, although usually more than 3 bodies must be considered; for example, if planning to site a SPACE HABITAT at one of the Lagrange Points (or Lagrangian Points) of theEarth-Moon system, one must take into account also the gravitational presence of the Sun (the mass of the habitat itself can be discounted as trivially small). There are 5 Lagrange Points in the Earth-Moon system; they are not absolutely fixed in relation to the Earth and Moon but, because of the Sun's influence, slowly circle "Lagrange Regions". They are numbered L1 to L5.The Princeton physicist Gerard K. O'Neill (1927-1992), an important propagandist for space colonies, argued in The High Frontier (1977) that good sites for such colonies would be L4 and L5, 60deg aheadof and behind the Moon in its orbit. He particularly liked L5, and this region soon became something of an sf CLICHE as the site for fictional space cities consisting of clusters of habitats.

   PN