Decay watch: January 20 (Iridium 11A rocket decayed)

Alan Pickup (alan@wingar.demon.co.uk)
Wed, 20 Jan 1999 22:25:14 +0000

SpaceCom have just released the final decay notice for the Iridium 11A
rocket. the current notices are:

Object                            Notice issued    Predicted decay
                                       UTC              UTC

#25579 = Iridium 11A CZ-2C r      Jan 20 02:35     Jan 20 14:49 +-9h
#25579 = Iridium 11A CZ-2C r      Jan 20 10:20     Jan 20 16:43 +-4h
#25579 = Iridium 11A CZ-2C r      Jan 20 18:51     Jan 20 20:56 +-1h
#25579 = Iridium 11A CZ-2C r      Jan 20 21:42     Jan 20 21:21 +-30m

#25065 = ETS-7 H2 r               Jan 18 21:49     Jan 27 17:27 +-5d

               -------------------------------

The Iridium 11A rocket clearly survived well beyond the decay prediction
I posted yesterday. It also appears that SpaceCom got it wrong too, with
their predicted decay drifting 6.5 hours later over a period of 19
hours. This is close the limit of their declared "decay window".

SpaceCom have been busy issuing elsets for this, with 16 issued since 0h
UTC today, including five for the nodal crossing at 99020.699. There may
well be another elset yet to be released. The most recently issued
(highest numbered) elsets for the last few revs are:

Irid 11A CZ r   10.0  3.0  0.0  3.5 d            171 x 121 km
1 25579U 98074C   99020.57793235  .14667948 -11337-5  14361-3 0  1274
2 25579  86.3285 181.3412 0038355  49.7256 310.7361 16.47501277  5061
Irid 11A CZ r   10.0  3.0  0.0  3.5 d            165 x 118 km
1 25579U 98074C   99020.63863872  .14037275 -11779-5  11386-3 0  1308
2 25579  86.3295 181.3087 0036230  49.5439 310.9135 16.49264935  5072
Irid 11A CZ r   10.0  3.0  0.0  3.5 d            154 x 117 km
1 25579U 98074C   99020.69925184  .15999812 -12782-5  11254-3 0  1358
2 25579  86.3328 181.2702 0028229  51.2629 309.1181 16.51441291  5089
Irid 11A CZ r   10.0  3.0  0.0  3.5 d            146 x 113 km
1 25579U 98074C   99020.75981281  .14724353 -13422-5  82637-4 0  1367
2 25579  86.3319 181.2344 0025166  49.9027 310.5132 16.53629399  5096

Note that the drag term (0.14667948 in the 1st elset) shows little or no
tendency to increase as the orbit contacted. The atmospheric density
should have risen by a factor of at least 2.5 during this time, but the
near-stable drag implies to me that we are seeing the air-flow regime
changing from free-molecule flow to transition and slip flow, which have
a lower "drag coefficient". For a 10-m long rocket, transition flow
should occur at about 130 km and slip flow around 110 km. It is easy to
be wise after the event, but this appears to be the strongest example of
this effect that I can remember. Unfortunately, SatEvo does not (yet)
cope with it.  Attempts to force SatEvo evolutions through the above
elsets give a decay at ~20:30 UTC, but this could be as unreliable as
the one I posted yesterday.


Alan
-- 
 Alan Pickup | COSPAR 2707:   55d53m48.7s N   3d11m51.2s W   156m asl
 Edinburgh   | Home:   alan@wingar.demon.co.uk    +44 (0)131 477 9144
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