[net.space] Lagrange point satellite

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.