Iridium 30 flare, and monster flare questions.

sbolton (sbolton@nbnet.nb.ca)
Fri, 2 Jan 1998 12:39:10 -0400 

Happy New Year to all.

Observed a "monster" flare of Iridium 30 at 22:40:51 UT on Jan 1.
Iridflar predicted mag -8.2. The event was spectacular to say the least.
I now will search for the "perfect flare". Criteria include:

1. Dark sky - no moon. Need I mention a clear sky? Sun at least -10 deg
elevation.
2. Minimum distance to Iridium. Needing a zeneth pass at perigee. Although
the orbits are 
   nearly circular, the 15 km should make a few tenths of magnitude difference.

Last night's Iridium 30 flare came close. There was a cresent moon low in
south-west, but the sun was -18 deg, and sky clear. The max flare was at
elevation of 49 deg, this put the distance at 995 km. This makes
consideration of perigee/apogee unimportant. I was predicted to have a flare
angle of only 0.02 deg. Centerline would have put me in dense forest!

My questions are:

1. What is the consensus opinion on trying to move to better location (ie
flare angle 0.00)
   My understanding is that attitude constraints of the sat are such that
anything less than 
   flare angle 0.05 is acceptable, and within the "error window" of actually
being perfect.

2. Can someone calculate the effect on magnitude of a 15 km difference in
distance? In fact
   examination of a list of Iridium elsets (Dec 31) indicates the following:
   1. Iridium 21 has the highest eccentricity at 0.0013. Perigee 628 km,
apogee 647 km.
   2. Almost all the so far launched Iridiums have perigees in the 770's
(km). There ARE a
      few exceptions. Specifically:
      Ir 21      628
      Ir 27      554
      Ir 45-49   665-669 range

    Now, this is far more variation than the typical 4-5 km per/apo range of
the sats with
    almost circular orbits. Therefore flares of Irid 21, 27 and the 45-49
group have the 
    greatest potential magnitude.

    Let us therefore calculate the difference of, say, Ir 27 vs a typical Ir
both at perigee.
    Brightness wil vary with inverse square of distance. I calculate a 1.97
difference!!

    If the absolutely typical Ir 30 can produce a -8.2 at 995 km range then
Ir 27 should 
    produce, for an optimal pass, the distance difference produces a
brightness difference of
    3.25. Given the brightness difference for a one magnitude change is 2.5,
then the flare 
    be in the -9.5 range.
    
    It would seem that observing the right satellite, rather than peri/ apo
considerations 
    is most important!

I wonder why those Iridiums have different orbital parameters than the rest
of the fleet?

Finally, there has been some discussison on how to accurately measure the
magnitude of the
monster flares. The eye fails, it has no standard of reference with which to
compare. Perhaps a CCD camera could do the job. I know that variable star
observers can get extremely accurate magnitudes using these devices.

In closing, and I apologize for the length of this post, my son's comments
on seeing the flare last night were comical. "Wow, what is that?- an
anti-vampire weapon?

Steve Bolton
Lat 45.432 N
Long 65.976 W