Iridium 11 flashes observed

Frank Reed (f-reed@metabien.com)
Sun, 19 Jul 1998 15:22:09 -0700

Hi Rob -

With a little help from my wife, I managed to wake up this morning in time
to see the Iridium 11 pass, and was rewarded by a 1x flashing display that
lasted nearly 4 minutes.  I posted the following report to the SeeSat
mailing list:

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I observed a protracted episode of flashing from Iridium 11 early this
morning.  At 1x I saw 17 flashes ranging from apparent negative magnitudes
to approximately magnitude 2, more or less evenly spaced over a time
interval of almost four minutes.  Most of the flashes were negative
magnitude.  Time intervals between flashes clustered around 5.5 seconds and
15 seconds with an occasional shorter interval.  Unfortunately, my stop
watch only has enough memory for 9 lap times.  Listed below are the clocked
times of the first eight flashes and the last flash.

The column labeled DT is the clocked time interval (lap time) 
from the previous observed flash, in seconds.

#           UTC            DT    Comment   
-- ---------------------- ----- -----------------------------
01 07/19/1998 11:00:09.50       Bright flash (approx mag. -2)
02 07/19/1998 11:00:25.20 15.70 Dim flash (approx mag. 2)
03 07/19/1998 11:00:30.80  5.60 Bright flash
04 07/19/1998 11:00:44.70 13.90 Bright flash
05 07/19/1998 11:00:49.98  5.28
06 07/19/1998 11:00:55.60  5.62
07 07/19/1998 11:01:01.14  5.54
08 07/19/1998 11:01:06.82  5.68
09 07/19/1998 11:01:09.90  3.08
      ...                       7 additional flashes observed    
17 07/19/1998 11:03:44.90  1.21 Dim flash (last flash observed)

This was an unprecedented sighting for me.  In the past, flashing episodes
I've seen of Iridium 20 and 11 lasted only a few seconds.

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My guess is that most of the flashes I saw were from one or more MMAs.  I'm
still thinking this satellite may be spinning around it's vertical axis and
I may be seeing flashes from more than one MMA.  If so, it's probably
wobbling somewhat (unlike Iridium 20 which seems to be in a "flat spin"),
which would cause the normal vector(s) from the reflecting surface(s) to
move in a pattern more complicated than a cone, and perhaps explains why
the data you've looked at is "all over the board".  Do you have any
esitmates of the primary moments of interia for the Iridium satellites?

I suppose that from the long duration of the episode, something can be
inferred about the geometry of the spin axis and mirror angles in relation
to the sun and observer, but it's not clear to me yet.

I'll certainly want to look at this satellite some more, but not every
morning.  Any suggestions on selecting which passes to watch?  I suppose I
should go for passes that are positioned differently in the sky from recent
observations.

- Frank


-- Frank Reed
Scottsdale, Arizona, USA  111.898W  33.484 N  1227 feet