Re: Decay watch: Iridium Long March rocket

Alan Pickup (alan@wingar.demon.co.uk)
Tue, 20 Jan 1998 22:51:45 +0000

The latest elset I have is
Iridm CZ-2C r   10.0  3.0  0.0  3.0 d            145 x 122 km
1 25079U 97077C   98020.44963137  .15626146 -14890-5  11418-3 0  1404
2 25079  86.2668  98.5370 0017762 348.1915  12.7438 16.52293750  6806
but even this may not be the last orbit since my final (so far)
evolution yields yet another equator crossing at 12:14 UTC...
Iridm CZ-2C r   10.0  3.0  0.0  3.0 d            130 x 114 km
1 25079U 97077C   98020.50995796  .47151752  30314+1  21067-3 0 91388
2 25079  86.2706  98.5023 0012154 350.3347   9.6419 16.56639467  6816

My present estimate is that decay came in the vicinity of this latter
equator crossing which was at 155 deg E longitude.

In the end, SatEvo didn't do well on this and I'm still trying to figure
out why. It seems as though the net drag fell by ~25% during the final
day, making it run later than I expected and prolonging its life more
than the 15% "uncertainty" estimate I usually advertise. One possibility
is that the rocket adopted a more end-on attitude, at least near
perigee, so that it flew like an arrow through the denser parts of the
atmosphere. Another factor, perhaps not unrelated, may be a transition
in the atmospheric flow regime experience by the rocket from "free-
molecule flow" to a lower-resistance "slip flow" as the perigee fell
below 150 km. If you'd like to know more about this effect, see section
2.3.5 of King-Hele's book "Satellite Orbits in an Atmosphere: Theory and
Applications" (Blackie, 1987).

Alan
-- 
 Alan Pickup | COSPAR site 2707:   55d53m48.7s N   3d11m51.2s W    156m asl
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