Double flash from Iridium 65

From: Willie Koorts (wpk@saao.ac.za)
Date: Fri Jun 14 2002 - 06:17:26 EDT


Hi All

Good to see SeeSat-L back on the air and sorry to hear about your big loss
Bart.

While you were away, Sue Wheayley and myself had more discussion on double
Iridium flares after a recent observation I made.  This is what was said:

Hi Sue

Regarding double Iridium flashes, I too since saw a double Iridium flash
which I cannot explain.  I gave a satellite talk to about 60 people last
Saturday night from a very dark site and noticed that we had a -2 mag
flare from Iridium 65, scheduled about 20 minutes into the talk.  When the
weather turned out 100% clear, we decided on plan B, ie, start the talk by
explaining about Iridiums first and then go out to observe the flash which
was very easy to find since the flash missed the "pointers" of Crux by 8
degrees only.

All worked out perfectly and we even managed to pick up the satellite long
before it flared (did I say it was a good site?) and followed it to the
predicted flare point where it did its thing.  Just as the applause and
the satellite started fading, it almost immediately flared up again as it
missed Alpha Cen by 2 degrees (where was my camera?).  Since I was trying
to keep my pose through all the cries of "why", I did not make a mental
note of the brightness of the 2nd flare but it came close to the first,
perhaps a little fainter.

In the heat of the moment, I explained it as perhaps a spare as with my
previous daytime double, as explained before but it worried me that we did
not see the 2nd satellite.  I later realised that this was untrue since I
used the full IRIDIUM.TLE to do the prediction and it only showed one
satellite.

So, it seemed like we DID observe a double flash!  Any explanations?

Regards
Willie

ps.  See  http://da.saao.ac.za/~wpk/satpasses/Iridium65Flash.gif  (13kB)
which shows a screenshot of the event with the (yellow) satellite name in
the original (first) flare position.  The LH-bottom corner gives the site
coordinates, date and time, etc. if anyone wanted to do any simulations.

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Sue then replied:

Glad your observing session turned out so well. It must be a spectacular
observing site. Your 13K file came through beautifully.  I wish Houston,
Texas, was as clear.

When Iridium 75 flashed, I thought the second flash was absolutely in line
with the first, so I thought they were the same...not a spare.

Regards
Sue and Willie

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