Iridium flare observation theory, and a few obs.

Ed Cannon (
Sat, 01 Nov 1997 00:51:45 -0600

Using fairly loose parameters, Rob Matson's program gives me 
the following (abridged) Iridium flare prediction:

Latitude: 30.30860, Longitude: -97.72790, Altitude: 150.0 m
Time Zone: UTC  -6.0 h; Date: 97-11-05; Iridium 13

  Time    Azm El  Rang I  Sun FlrAng Mag FlareLat FlareLong
17:55:20  266 06  2711 L -3.4  2.82  4.7 Does not intersect
17:55:44  262 05  2757 L -3.5  1.50  3.7 Does not intersect
17:56:10  258 04  2819 L -3.6  2.95  5.7 Does not intersect

("Does not intersect" means that the predicted centerline 
does not touch the Earth's surface.  "L" is "Lit".)

Given the 1.4-degree variability in the attitude of the 
satellites, I believe this prediction has at least a remote 
possibility of really having a "FlrAng" of 0.10, which is 
very good.  However, this one is 5 degrees above the horizon 
at a range of 2757 km., in bright twilight and not far from 
the Sun.  (The solar azimuth is 254.)  I believe that the 
geometry is such that I'm seeing the reflecting antenna at a 
very oblique angle, so it's almost "edge-on" and presents 
only a fraction of its area to me.  (I know there's a term 
for that view of the antenna, but I can't think of what it 
is.  The satellite is at a very poor phase angle.  What does
that make the reflecting antenna?)

Now my real question:  Given that I have a site with a low-
enough horizon, and assuming for discussion that the real 
antenna angle will be 0.10, what might I see?  It's about 
triple the range of the best ones, so right off it's only 
1/9 as bright.  But there's also the oblique orientation of 
the mirror, atmospheric extinction, etc.  So I guess my 
question really is, what it the likelihood of seeing a 
bright flash or flare or glint visible at one-power from 
fringe predictions such as this?  Or, how bright would such 
a flare be, and how long would it last?  

There was a predicted *real* Iridium "monster" (i.e., not a 
fringe prediction) at the site of a semi-annual star party
Friday evening, and I'm pretty sure that the sky in the area 
was very nearly perfect, so I imagine that a fair number of 
astronomers saw it.  Sue Worden was supposed to be out 
there, and I can hardly wait to hear/see her report of the 
event.  (I just was not able to get away from my office in 
time to go.)  So anyway, I consoled myself by going outside 
the building and even with a late start seeing 5 satellites 
at one-power within 26 minutes, from a poor location!  (Like 
I said, a perfect night!)  Cosmos 1346 Rk (82027B, #13121), 
Cosmos 1980 Rk (88102B, #19650), Cosmos 1697 Rk (85097B, 
#16182), Cosmos 2297 Rk (94077B, #23405), and HST (90037B, 
#20580).  Cosmos 2297 Rk made a nice pass through the zenith 
-- except that it was at a minimum at that point!  In about 
two or three minutes it tumbled once -- a long maximum 
fairly high in the NNW, a long minimum up around culmination, 
and another long maximum fairly high in the SSE. 

Ed Cannon
Univ. of Texas at Austin, USA, 30.29N, 97.74W, 150m