# Planet and star glints for Iridium

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Wed Oct 25 2000 - 12:03:40 PDT

• Next message: Paolo Cosetti: "SATBUSTER - Software"

```Hi Allen,

> Secondary question: has anyone tried to see any except solar and lunar
> Iridum flares/blinks?  Venus, Jupiter, Sirus, I-flares for example?

About six months ago I wrote a modified version of IRIDFLAR
to search for glints from Venus, Jupiter, Mars, Sirius, Vega,
and a few other bright stars, without really thinking through
whether they would be visible.  Later, after working out the
math/optics it became clear that the glints would be of such
short duration (and the pointing tolerances so high) that
they probably would not be detectable.  (Consider how dim a
lunar flare is, even from a full moon.)

While the planets have the next brightest visual magnitudes
after the sun and moon, their ~radiances~ rather than their
~irradiances~ is what matters for this type of optical setup.
While Venus can be magnitude -4.5, its angular size (solid angle)
is hundreds to a thousand + times larger than that of an
Iridium MMA.  Thus, the MMA specular reflection only samples
a small fraction of Venus as it sweeps the planet.

Call it a factor of 1000, which is 7.5 visual magnitudes.  If
so, a Venus glint would be about magnitude +3.  If the planet
subtends ~15 arcseconds, then the MMA will sweep across the
planet's disk in about 1/30th of a second.  Therein lies the
problem.

Stars, while dimmer, have much higher radiances.  Since their
angular sizes are less than that of the MMA, you see a complete
reflection of the star.  Assuming the MMA has 100% reflectivity
(it's a little less than that, but close enough), the star
glint will be a mirror-image of the star -- i.e. the same
magnitude.  So a glint from Sirius would still be magnitude
-1.4.  But the duration of the event would be less than a
millisecond due to the satellite's motion.  --Rob

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