Hi Pat,
A satellite will spiral in if it is in a more or less circular orbit, but
an eccentric orbit like this ones, with small perigee and large apogee means
that it encounters atmospheric drag which slows its speed down when it is
near perigee only. Slowing an object at perigee lowers its apogee halfway
around its orbit, so most of the decay is only in the apogee (but caused when
at perigee...) until the orbit is nearly circular when it decays much more
rapidly. If the perigee drops due to luni-solar perturbations or other
affects, then if the perigee drops too low, it can re-enter at that point as
well. If I remember right, the height at which it is unlikely to survive
another orbit is something like 60 kilometers. I'm sure someone can be more
exact and it would depend on the drag coefficient and on the orbit
particulars.
Jim.
On Wed, 12 Oct 2005, iridium43@att.net wrote:
>
> Last July 25600's apogee was 14,000 km
> its perigee was 125 km
>
> Now its apogee is 3056 km
> its perigee is 111 km.
>
> 11000 km off the top but a mere 15 off the bottom.
>
> Why don't satellites spiral in?
>
> Pat McNally
> Seattle
>
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Jim Scotti
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA http://www.lpl.arizona.edu/~jscotti/
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