truchon (06/10/82)
#N:uiucdcs:12700001:000:305 uiucdcs!truchon Jun 10 13:47:00 1982 In an issue of Astronomy magazine from a couple of years back, there was a small article which told of a satellite which had been put into orbit about a lagrange point between the Earth and Sun. I do not remember which issue it was but I am pretty sure that it was from 1979 or 1980. Lee Truchon
henry (06/13/82)
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.