ISS/HST/Iridium flares

Matson, Robert (ROBERT.D.MATSON@cpmx.saic.com)
Thu, 22 Apr 1999 17:44:34 -0700 

Hi Mal,

> I guess some of you remember my post from a couple of days ago.
> I introduced myself to the list and described the first dramatic
> flare I ever saw, and the first time I had ever seen HST flare
> at all.

Meant to mention to you that bright flare-ups from HST have been
seen fairly frequently by Seesat observers.  Theory is that they are
specular reflections off the telescope aperture door.

Regarding ISS, you wrote:

> I know the flare is attributed to a greater surface
> area being exposed to the sun (i.e., solar panels,
> longitudinal axis of the satellite body, etc.) of a
> given satellite in respect to the observer at a given
> time and location as the satellite maneuvers locally
> on axis or changes its position relative to this ground
> based observer.  Is my understanding of this in the
> ballpark?

Not quite.  When you see a satellite increase its brightness from, say,
magnitude +1 to magnitude -4, you're seeing a change of 5 visual
magnitudes -- which is a factor of 100 in brightness!  Changes in
projected surface area and phase cannot explain such a large
increase.  (Rarely does a satellite change its projected surface area
and phase factor by more than a factor of 4 -- 1.5 visual magnitudes).
No, what you're seeing is a specular effect off a flat, polished surface.

In the case of HST, it would be the back of the telescope or the
aperture door as I mentioned above.  Curved surfaces (cylinders,
spheres) cannot produce bright specular reflections.  Of course,
solar panels are flat and good reflectors, but since they generally
track the sun, the specular reflection doesn't intersect the earth's
surface.

Now for ISS, the only thing I can imagine that could produce a
-4 glint would, in fact, be a solar panel.  Since the station is not
yet manned, it wouldn't surprise me if the spacecraft controllers
opted to put the station into a slow spin so as not to "cook"
one side of the station and freeze the other.  We call this
"barbecue" mode in the aerospace biz -- a way of reducing
thermal stress.  If so, the solar panel pointing might not be
optimum, allowing glints to occasionally be directed toward
the ground -- particularly if the satellite was lower in the east-
southeast in the evening, or west-southwest in the morning.

> I have tried several times to witness an Iridium
> flare, but with no luck.

Have you tried using the IRIDFLAR program or the GSOC site?  If
so, you may have a problem with one of the input parameters.  Knowing
your location accurately (at least 2 decimal places) is crucial for
accurate flare magnitude calculation.  I notice in your signature
you give your coordinates to 6 1/2 decimal places, so you must
be fairly confident of your location (that's an accuracy of 2 inches,
by the way!  ;-)

Iridium flares are extremely reliable -- so much so that you can
set your watch by them (assuming 5-second accuracy is good
enough for you).  For example, I show a magnitude -1.5 flare for
your location on Friday night, April 23rd at 9:20:50pm EDT.  It
will occur 20 degrees above your northern horizon (about 5
degrees west of due north).

Cheers,
Rob