Satgen311    Satellite Hindsight No9      by GM4IHJ           11 March 95
Msg BID is SGEN311  Please use this BID if you retransmit via packet radio

Continuing to review those situations where satellites have met unexpected
problems, the scene moves to the Molniya high altitude elliptical orbits
and the geostationary and geosynchronous orbits. The important point here
being that the satellite gets out to at least 36000 kms from the Earth for
at least part of its orbit.

Quite often Solar wind pressure on the Sunward side of the earth ,
compresses the earths magnetic field pushing it below 36000 kms from the
earth. No one worried too much about this until the 1980s, when several
geosats failed completely or suffered serious problems soon after
increases in solar activity. What happened was caused by the solar wind
pushing the protective magnetic field below the satellites altitude
leaving it exposed to the solar charged particle bombardment which caused
a tremendous build up of static electricity on the satellites outer
surfaces. So intense was this charge build up, that eventually , flash
over occured across the face of the satellite. Thereby killing it, as
effectively as if it had been a convicted murderer in the electric chair.
Fortunately special protective outer coatings which dissipate the charge,
have now been developed. So far Amsats have been lucky. But all the Phase
3 sats can get into this situation at apogee, now that their orbits put
apogee at low latitude. So perhaps it is only a matter of time before one
of them gets a nasty shock.

The second high altitude problem has already affected Oscar 13 ( and
several Russian Molniyas ). The danger is caused by the gravitational
attraction of the Sun and the Moon. With geosats the Sun pulls them north
in the northern hemisphere summmer and south in winter. The pull being
strong enough to require regular use of thrusters to keep them on station
in a zero degree inclination orbit. Cosmos 2054 at 15W 10830 MHz is a
(good/bad) example of what happens when there is no more thruster fuel, as
it wanders about the equator on a permanently inclined orbit which makes
its relays of Mir TV very difficult copy in UK.

Oscar 13 does not have any thrusters to overcome solar gravity effects, 
so its apogee high point has increased markedly as it is pulled towards
the Sun. Unfortunately this means that its corresponding orbit perigee low
point is pulled nearer to the Earth. At present the solar pull on AO13 is
reduced but some predictions suggest that this is only a brief stay of
execution before the downwards pull on perigee resumes and AO13 enters the
Earths atmosphere and burns up. Russian Molniya-1 60 , object 1984 029A re
entered and burnt up last November , so AO13 is not the only satellite to
suffer from this effect.

What has to be said with 20-20 hindsight however is that none of these
disasters were inevitable. The use of protective " charge dispersing "
outer coverings on satellites prevents death by electrocution, and, by
careful choice of satellite launch orbit in the case of Phase 3 sats and
the Molniyas, it is possible to keep the satellite away from the point
where its orbit major axis points at the Sun. Thereby greatly decreasing
the solar gravity perturbation.
73 de GM4IHJ @ GB7SAN  or  gm4ihj@delphi.com