Iridium flare observation theory, and a few obs.
Ed Cannon (ecannon@mail.utexas.edu)
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
ecannon@mail.utexas.edu
Univ. of Texas at Austin, USA, 30.29N, 97.74W, 150m