Iridium double flares -- 3 nights running

Ed Cannon (ecannon@mail.utexas.edu)
Wed, 25 Nov 1998 02:56:41 -0500

It seems to be time for these Iridium double flares to be 
explained.  Other folks have again been reporting Iridiums 
doing double flares (as also happened quite a few months ago 
I believe), and this message is to report three double-flare 
events -- three different satellites on three consecutive 
nights.

Monday 23 Nov. UTC (Sunday evening local), Iridium 38 (25043,
97-69E).  Iridflar predicted -4 at 00:42:00.  Mike McCants 
timed its peak at 00:42:01.2 -- and it was really -6 or 
brighter, a real wowser!  It faded a lot but then brightened 
again to about +1.5, and Mike timed that peak at 00:42:46.5.  
Location was 30.314N, 97.866W, 270m.

Tuesday 24 Nov. UTC (Monday evening local), Iridium 18 (24872,
97-34D).  Iridflar predicted -2 at 00:35:51.  It faded some 
and I didn't have my clock, but wondering if it might double,
I started counting.  At about 50-count it flared up again to 
at least -1 and was easy one-power through 70-count.  Location 
was 30.286N, 97.739W, 150m (UT Austin campus).

Wednesday 25 Nov. UTC (Tuesday evening local), Iridium 39
(25042, 97-69D).  Iridflar predicted +1 at 00:29:45.  I had 
my clock this time, and in spite of some scattered thin 
cumulo-stratus type high clouds, I was able to observe again 
two flares.  In this case I'm almost certain that the second 
one, which peaked 60-70 seconds later than the first, was the 
brighter of the two, perhaps -1.  Location was UT Austin.

In each case, the predicted flare was from the left MMA.  The 
first night the predicted flare was much brighter.  The second 
night the predicted flare was just somewhat brighter at my
location than the second flare.  The third night the second 
flare was brighter than the predicted one.  Can anyone explain 
this phenomenon?  I remember some months ago someone from 
Motorola said that a couple of other components (bus and 
battery module?) are as reflective as the MMAs, but I don't 
know how big they are or where they are relative to "front", 
"left", and "right" MMAs. The solar panels are bigger, but 
would they be positioned correctly?

Also, I'm sure others have noticed that from one night to the
next, for predicted flares in any given plane, the flare 
maximum time is just a few seconds more than six minutes 
earlier than the previous night.  I had kind of thought that
time would vary more with the seasons, but -- just going on
memory -- it seems that it's been just over six minutes 
earlier each night through two or three seasons now -- since
there were enough of them in planes to have consistent 
consecutive-night passes.

Iridiums 20 and 44 have been flashing nightly here; 20 does
*spectacular* groupings of several-to-many flashes (and, I'm
embarrassed to admit, it just registered with me Tuesday 
evening that Iridflar predicts 20's flash events; so much 
time went by that I forgot about that feature!).  Ir 44 seems 
to exhibit fewer but some very bright flashes.

Recently I've been thinking about the Iridium future.  Imagine 
what the night skies may hold years from now, if/when the first 
generation of Iridiums is replaced -- 150 or more of them up 
there, about half in-plane and flaring, the other half drifting,
and quite a few tumbling!  And perhaps added to them will be a 
two or three hundred Teledesics (but maybe those will be like 
Orbcomms and Globalstars -- not very bright).  And of course 
ISS, second-generation Space Telescope, etc.  The catalog will
be up to 30,000; IDs will includes things like 08-175Q (or 
2008-175Q --not a piece of debris, but a vehicle that launched 
16 payloads)!  On the other hand, the sky may be so bright in
many places that about all that can be seen are ISS, HST2, 
Iridiums, some descending Zenits and Titans, and maybe for the
low-latitude folks, good ole 00694, 45 or 50 years old by then, 
perigee at 320 km (200 miles), magnitude +0 or brighter when 
the phase angle's right....

Ed Cannon - ecannon@mail.utexas.edu - Austin, Texas, USA