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
Scotland | SatEvo page: http://www.wingar.demon.co.uk/satevo/