Re: Iridium Glints to Mag -9

Jim Varney (
Mon, 13 Oct 1997 14:44:48 -0700 (PDT)

---Leigh Palmer <> wrote:
> >The glint occurred on Oct 12, 1997 at 0206 UT in twilight.  Skysat
> >listed a sun-mirror divergence of only 0.05 degrees.  Range was only
> >979 km.  51 deg above the horizon.  I assume that the max
> >brightness occurs when the divergence is 0 degrees and the range is
> >small.
> If one takes the model of a perfectly reflecting antenna 188 cm x 86
> reflecting the Sun, then any pointing angle within a quarter of a
> should yield the maximum brilliance flash. There should be no
maximum at
> zero degrees. The brightest the mirror can seem is the brightness of
> surface of the Sun.

The Sun's brightness is not constant across the disk due to the limb
darkening effect.  If an Iridium MMA "samples" the limb vs. the
center, there should be an appreciable difference in brightness.

> >The glint was much brighter than the two space shuttle reentries I
> >have seen, which tend to be estimated around mag -6 or -7.  From
> >I put Iridium 13 at mag -9.
> Using the above model, for an observer-satellite distance of 1000
km, I
> reckon that about -8 m is the brightest an Iridium flash can get.

Again due to the limb darkening effect, if you distribute the Sun's
magnitude of -27 uniformly across the disk I assume the limb's
brightness will be overstated while the center is understated.  If my
assumption is correct, then it is not out of the question for a
central disk reflection to hit -9.  That's only one more magnitude
than your -8 estimate.

> What is the altitude of these satellites?

The nominal height is about 775 km.
Jim Varney   121.398W  38.458N 8m   Sacramento, CA,

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