Whence Iridium 48?

Alan Pickup (alan@wingar.demon.co.uk)
Mon, 5 Apr 1999 08:54:30 +0100

The current NASA 60d decay forecast predicts the decay of Iridium 48 on
April 16. This was a happy member of the Iridium constellation until (at
least) March 27 when this elset was published:

Iridium 48       3.0  0.0  0.0  6.0 d            780 x 775 km
1 25107U 97082D   99086.69162521 -.00000018  00000-0 -13567-4 0  2489
2 25107  86.4037 156.9570 0003287  61.1570 298.9954 14.34216903 66335

The next elset appeared 3.9 days later, an unusually long interval for
this object, and assigns it a high drag and increasing mean motion:

Iridium 48       3.0  0.0  0.0  6.0 d            779 x 770 km
1 25107U 97082D   99090.18004916  .01791987  00000-0  58243+0 0  2498
2 25107  86.4089 155.5107 0006409 104.8280 255.7512 14.35243644 66834

Further elsets continue to show the orbit to be contracting quite
steadily, the latest being:

Iridium 48       3.0  0.0  0.0  6.0 d            713 x 710 km
1 25107U 97082D   99093.50475838  .03413279  00000-0  73667+0 0  2643
2 25107  86.3969 154.0938 0002395 106.6441 255.2083 14.54305980 67310

I conclude from all this that Iridium 48 is manoeuvring downwards for
some reason and I have reclassified this as "Iridium 48 man" in my
Iridium status file and iridiums.tle file, available via my WWW page.

However, an alternative possibility, suggested by Mike McCants (thanks,
Mike), is that Iridium 48 has released a piece of debris which SpaceCom
is now tracking instead of Iridium 48. In Mike's view, the drag has
remained too constant and the orbit too circular for this to be an
orbital contraction under ground control. The debris, if such it is,
must have an unusually large area to mass ratio to be decaying so
rapidly at this height.

So where is Iridium 48? Visual observations will tell us whether it is
in its original orbit (the first elset above) and whether it appears
normal or is tumbling after some untoward event. The final (high drag)
elset above is already too old to give reliable predictions and,
unfortunately, OIG's database has not been updated since late Saturday.
(The latter point also explains why I have been unable to post an update
on the decay of #25665, the Progress M-41 Soyuz rocket.)

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