keithl (01/26/83)
Before some of you flame about the imprecision of most satellite decay
predictions, you might consider the physics of the situation, and the
resultant difficulties.
First, the height of a satellite's orbit is influenced by gravitational
effects from the Moon and the Sun. It won't behave like "last time".
Second, the density of the atmosphere changes exponentially with height,
resulting in very small changes in orbital properties to start with, followed
by very large ones just before re-entry. As the height of the atmosphere is
dependent on seasons, solar activity, and even weather, this can be hard to
predict. Even the decay rate with an ideal atmosphere behaves exponentially
with time; the time constant can be measured in minutes or hours. You try
extrapolating an exponential like that with radar precision and a many-day
baseline!
Third, the ground track of a near-Earth, high inclination orbit changes with
every orbit. In a twelve hour period, a satellite comes within range of
most of the Earth's surface. The Earth turns!
With no aerodynamic data for a partially disassembled, non-streamlined,
out-of-control satellite, the best prediction that can be made is that it will
fall down somewhere in the range of latitudes the orbit crosses. A few orbits
before re-entry, of course, this can refined.
I think it's amazing that such things can be predicted to within days!
Narrowing it down to a fraction of one orbit would involve a degree of
omniscience it scares me to think about!
Keith Lofstrom
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