single flashes

From: Ed Cannon via Seesat-l <>
Date: Tue, 14 Apr 2015 06:46:36 +0000 (UTC)
I've seen these.  In general they are from slowly rotating 
satellites that have a specular surface.  During the pass 
the phase angle changes such that the specular surface is 
in the "correct" orientation relative to the observer such 
that only one flash is possible.

Consider a slowly rotating Iridium payload.  The specular
surfaces are only about two meters by one meter.  Usually
an Iridium is only about magnitude +6 on an ordinary pass, 
but they can flare up to -8, a change of 14 magnitudes. 
Well, if you have one that is no longer in service and is 
in fact rotating about once per minute, it's conceivable 
that during a given pass it could rotate such that the 
specular surface is just right for a -8 flare, but because 
it's rotating it's a quick flash instead instead of a slow
flare.  And during the one minute that it takes to rotate 
around again, it has moved ten or fifteen degrees across 
the sky such that it just can't flash anywhere near that
bright again on that pass.

A couple of other ones that can do this that come to mind 
are Topex (22076, 92-52A) and IRAS (13777, 83-04A), but 
really any one with a good specular surface can do it, if 
its rotation and travel are just so on a given pass.  Some 
old NOAA and DMSP payloads are rotating slowly.  I think 
I've seen Globalstar 33 (25909, 99-49C) flash just once on 
rare occasions.  Some "flashing geosynchs" are rotating so
slowly that they may only flash once during a session.
Molniyas and even GPS and Glonass payloads can do this as
well, in theory (or my hypothesis) at least.  USA 186 and
its cohorts can do this.  (Slewing is controlled rotation.)

I think that the brightest flare (or flash?) I have ever 
seen was from ETS 7 (25064, 97-74B) --

What an observer should do is note the position of the 
flash and the time as exactly as possible, and using those 
and the observing site coordinates and a complete set of 
orbital data and a program like Findsat try to identify 
the object.  It's often possible with good enough data.

Footnote.  I think a possibly useful distinction between 
"flash" and "flare" is that flashes result from the 
rotation of a spacecraft on its axis while flares are the 
result of the changing solar phase angle as the satellite 
travels along its orbital path.

My two cents...

Ed Cannon - Austin, Texas, USA
Seesat-l mailing list
Received on Tue Apr 14 2015 - 01:47:07 UTC

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