Re: Decay watch: Iridium 69 rocket

Alan Pickup (alan@wingar.demon.co.uk)
Sun, 7 Jun 1998 21:48:20 +0100

Thanks for all the (unfortunately negative) reports. This one kept going
and going... (but I think it is down now)

The final elsets from OIG were:
Iridm 69 CZ r   10.0  3.0  0.0  4.5 d            174 x 127 km
1 25321U 98026C   98158.32757099  .16197737 -96548-6  20721-3 0  1336
2 25321  86.3224 275.6778 0036363  22.8842 337.4378 16.45843550  5673
Iridm 69 CZ r   10.0  3.0  0.0  4.5 d            163 x 124 km
1 25321U 98026C   98158.38833279  .14674012 -12136-5  15131-3 0  1341
2 25321  86.3249 275.6465 0029808  42.3417 318.3720 16.48326710  5681
Iridm 69 CZ r   10.0  3.0  0.0  4.5 d            128 x 117 km
1 25321U 98026C   98158.50957176 -.48118245 -14423-5 -22672-3 0  1369
2 25321  86.3218 275.5713 0008200  33.8551 327.0040 16.56420592  5706
Iridm 69 CZ r   10.0  3.0  0.0  4.5 d            127 x 110 km
1 25321U 98026C   98158.56997804  .24558294 -16985-5  94005-4 0  1377
2 25321  86.3198 275.5366 0012854   5.5493 355.1966 16.57905241  5718

The last two are certainly inconsistent - one has a strongly negative
drag (a bit unlikely), they have too-similar mean motions and the drag
on the final one is probably too low. Unless the final elset is a
prediction, the rocket was still in orbit at the northbound equator
crossing at 13:40 UTC. My best guess is that decay was on the latter
orbit, possibly near the southbound equator crossing at 14:24 UTC near
16 deg W longitude.

At least this has given me some useful data to improve SatEvo. My hunch
is that, not for the first time, the lifetime was extended at the death
by the rocket experiencing a changed atmospheric flow regime near
perigee; a shift from so-called free-molecule flow to "slip" flow. For
something as long as a Long March rocket, the effect would be to
approximately halve that drag coefficient (and double its remaining
lifetime) as its perigee fell below ~130 km - just about what was
observed.

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