Satgen363   Orbit Decay  by GM4IHJ                         9 March 96

Recent attention to the rapidly descending satellite Jianbing, and to
amateur radio satellite Ao13, has raised several questions as to what
factors affect a satellites orbit. The most frequent of these questions
is " At what perigee height or apogee height will a satellite fall out
of orbit " ?
The answers has to be that this depends on several factors :-
1.  Almost every orbiting satellite going below 10000 kms altitude in
any part of its orbit, and some satellites which go much higher , must
eventually come back to earth. At a satellite height of 1000 kms it can
take perhaps 200 years. At 600 kms altitude in a roughly circular orbit
it may take 3 to 5 years. But at 200 kms it will take only a few weeks.
However all these times are highly variable for the reasons which
follow.
2.  The earth atmosphere which produces the drag which gradually forces
the satellite to take up an increasing lower orbit, varies enormously
in density up at typical satellite heights. The chief variable is in
step with the Solar sunspot cycle. At Sunspot maximum the upper
atmosphere will de orbit a satellite many times faster than will the
much thinner atmosphere typical of sunspot minimum. Worse still for
would be orbit decay predictors, the atmospheric density differs from
night to day , and it can be grossly changed during solar storms.
Indeed these are the reasons why anyone expecting to plot a steadily
declining orbit decay curve , finds that the real curve proceeds by
jumps and bumps and is anything but regular.
3. Satellite volume is another factor varying the decay rate. A big
balloon like satellite decays much more rapidly than a simple cube or
sphere.
4. Satellite weight is also a factor , with heavier satellites staying
up longer than light weight satellites.
5. When the satellite gets into the denser part of the atmosphere
another feature emerges which complicates the orbit decay picture ,
whereby the satellite can in some instances begin to act like a glider.
So in the final stages of its descent it can behave more like an
aeroplane than a bomb. Indeed, in the past satellites which heat to the 
point where the atmosphere begins to tear them apart, have finished up, 
having their various pieces distributed over hundreds of
kilometres, because some pieces were better aerofoils than others.

For the above reasons, please be aware that prediction of satellite
orbit decay is a somewhat imprecise art. But not perhaps so imprecise
as one astronomy magazine presently predicting that Jianbing will decay
in May 96. Last week Jianbing was at apogee 480 kms with apogee
dropping 15 kilometres per day . So if it did not speed up at all
thereafter ( which it promptly did), it had to come down in 32 days or
less ie in March rather than May.