From: utzoo!henry Newsgroups: net.space Title: Lagrange point satellite Article-I.D.: utzoo.2163 Posted: Sat Jun 12 23:22:14 1982 Received: Sat Jun 12 23:22:14 1982 It turns out that, although the straight-line Lagrange points are unstable, there are *nearly* stable orbits around them. These orbits are called "halo orbits" because the satellite's orbit looks like a halo above the nearer of the two major bodies. Halo orbits are not quite stable, but the fuel requirement to correct perturbations and keep a satellite in such an orbit is quite modest. Such orbits were first investigated for putting a relay satellite around the far Lagrange point (don't remember which Lx number it is) of the moon, as a radio link between Earth and probes on the Lunar farside. As it turns out, the first application was a halo orbit around the middle Earth-Sun Lagrange point. I believe it was one of the ISEE (International Sun- Earth Explorer) satellites that was placed into that orbit a few years ago. A very useful spot, because it sees things like disturbances in the solar wind well ahead of Earth-orbiting sensors; the halo orbit is something like a million miles sunward.