RE: Consider the moon

Matson, Robert (ROBERT.D.MATSON@cpmx.saic.com)
Tue, 13 Jul 1999 19:08:10 -0700

Hi Karsten,

> I've got two "problems", I cannot solve:

> 1.) Every satellite visible prediction software calculates the sun as
> the light source of reflecting satellites.  The sun has a magnitude of
> -27. An iridium flare can reach a magnitude of -9.  So, it is easy to
> understand, that the sun provides the light for the flare.  But, what
> about the moon. The moon can reach a magnitude of -12.  Why doesn't
> any prediction software use the moon as a light source for satellite
> reflections?

Actually, I wrote a sister program to IRIDFLAR (the "other" Iridium
prediction program) that calculates lunar Iridium flares.  Several
Seesat members have attempted to see these flares, and one so far
has been successful.  Bright lunar flares are much less common
because of the phase requirement on the moon (3/4 full or better
is necessary for a reasonably bright reflection).  Even when the
moon is full, the brightest reflections never exceed about mag
+6.5 -- i.e. binoculars are required).  The satellite also has to
be in darkness; even a diffuse solar reflection from Iridium will
swamp a specular lunar reflection.

> Or what about an iridium flare, that is reflected to another iridium
> satellite and then it is visible on a special earth location?

Geometrically possible, but after a double-bounce it will be far
too dim.  The second satellite receives only a miniscule fraction
of the first satellite's specular beam.  The average separation of
satellites in the same plane is about 4000 km.  Even if the
satellites were only 100 km apart, and the beam was only 1/2-degree
wide, the spot size at the second satellite would be almost 900
meters in diameter.  A main mission antenna would only be able
to capture .00025% of this beam -- that's a reduction of 14 visual
magnitudes.  (At 4000 km range, the decrease is 18 visual mags).
Not much to see, I'm afraid.  Still, some insightful questions,
Karsten.

Cheers,
Rob