Satgen 536  Satellite Collisions ? by GM4IHJ  (BID SGEN536) 1999-07-03

There has been a recent surge of interest in the possibility that the
increase in satellite traffic density at an altitude around 780 kms,
could lead to satellite collisions, followed by a rapid sequence of
further collisions . Whereby the debris from the first collision
spreads like shrapnel through the whole satellite fleet in a chaotic
chain reaction of destruction.

Nature Vol399 pages 743/744 discusses collision possibilities based on
the Iridium system of 80+ satellites ( not 66 as stated ).
Unfortunately this thesis makes misleading use of an illustration
which suggests that all the satellite orbits meet at a point directly
above the pole. In fact the satellite orbits pass 3.6 degrees of
latitude below the pole ( ie about 400 kms ), with each orbit plane
reaching the pole at different longitudes around the pole. A factor
which does not make collisions impossible ,but does spread potential
collision points out widely, some distance from and around the poles.

Less clear however , in respect of this problem , is why it is
suggested that this is a new problem. LEO satellite constellations
have been around for nearly 30 years. Although being military or
government sponsored they have not had the publicity that has
accompanied Iridium and the other new commercial communications
fleets. Doppler Navigation satellites have been around in considerable
numbers for 30 years. Any dark clear night over Scotland you can see
one every ten minutes or so passing east of the pole north going,
coming west of the pole south going,if Russian , or in the equally
numerous American fleet passing west of the pole north going, east
south going . Just as numerous are the NOAA and US Defence Department
Weather satellites, again going west or coming east of the pole ,
whilst their Russiam Meteor sat equivalents take the opposite route.
Add to this 26 or so Amsats. Then there are other groups in lower
inclination orbits , like the Orbcom fleet , this amounts to a lot of
LEO sats in one sky. But even this is totally eclipsed by the Russian
military "Follow My Leader" comsats. Launched in groups of six or
eight , several times per year, for a number of years. Many of these 
satellites may have been silent for years, but this in no way reduces
their collision potential and they are far more numerous than Iridium.

Of course not all these satellites orbit at around 780 kms altitude,
but as their orbits gradually decay they often constitute a relatively
long term ( 3 to 10 year) problem at that height. Equally true, not
all the satellites are at the same orbital inclination. Although most
of them group at certain inclinations ( 52 to 60 degs, 82 to 86 degs,
and 92 to 98 degs), and all these orbits cross somewhere. Though
interestingly enough, the most likely collision points seems to be
near but not at the poles , and, near the equator. Concentration
features which may add to the likelyhood of collisions. But given sat
concentrations in the past "Why have'nt we seen plenty of collisions
already". There have been one or two isolated incidents of satellites
breaking up or exploding. But most opinion to date has blamed
accidental explosion of residual fuel. Time will tell whether this
problem is serious and which orbit parameters are most accident prone