Re: Iridium double-flare

From: Rod Sladen (rodsladen@crosswinds.net)
Date: Wed Mar 07 2001 - 12:34:26 PST

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    Last night's predicted mag. -8.1 flare from Iridium 6 was a no-show for me because
    of heavy cloud, even though the Moon was spookily visible through the mist in
    the same area.
    
    Tonight's predicted mag. -1.2 from Iridium 4 (24796) did not manifest itself
    as a double flare, but as an extended duration flare of more less steady magnitude
    lasting from perhaps 19:18:15 to around 19:18:45, asymmetric about the predicted
    centre time of 19:18:18 UT.
    
    (and then, perhaps my last view of Mir, rising to only 22 deg. and passing below
    Orion's feet.  Tomorrow only 17 deg, Friday only 13 deg, then ... nothing. 
    I shall miss Mir.)
    
    A few thoughts:
    
    1. Double flares may actually be fairly common, but are missed because the flares
    overlap and appear as a single extended flare, as the above, or as a single
    flare with more than one peak (see earlier report).  Only if checking timings
    or positions carefully is is evident that the flare is not symmetric about the
    predicted flare maximum time.
    
    2.  Extra flares prior to the predicted flare may easily be missed.  If concentrating
    on the expected flare location, a flare 20 or so away is likely to be missed
    unless very bright (but see below).
    
    3.  Extra flares after the predicted flare may also be missed.  Once the satellite
    has faded to invisibility, there is a tendency to look away to check notes or
    to look at something else.
    
    A correction to my report of 04 Mar 2001: the second flare from Iridium 61 peaked
    at something 19:37:05 rather than 19:40:05, i.e something like 40 seconds after
    the first flare.
    
    And a question : how far apart can the flare peaks be?  We already know that
    it can be at least a minute or so - *if* the source is a multiple reflection
    or indeed another surface (other than, presumably, the solar panels), will it
    perhaps be masked by the MMA responsible for the primary flare if the time separation
    is greater?
    
    And, finally, a report from sci.astro.satellites.visual-observe of what seems
    to be an extra flare *before* the predicted one:
    
    >Subject: Iridium Flare In Wrong Location?
    >Date: 07 Mar 2001 17:07:45 GMT
    >From: bbarksdl@aol.com (BBardskl)
    >
    >I was looking for Iridium 37 last night (03/06/01) from 
    >Marietta, GA:
    >Time: 20:08:01
    >Magnitude: -8
    >Altitude: 46 Deg.
    >Azimuth: 142 Deg.
    >I was facing South. The moon was shining brightly to my 
    >left, just out of my peripheral vision.  All at once it 
    >seemed the moon had flared up. I remember thinking there 
    >must have been a thin cloud blocking some of the moonlight, 
    >that suddenly moved away.  I'm not sure if I turned my 
    >head slightly, or if the Iridium flare just came into my 
    >field of vision, but it was at least as bright as Venus, 
    >moving from East to Southeast as it dimmed to invisibilty.
    >Anyway, the azimuth seemed more like 90 Deg. and the 
    >altitude seemed greater than 46 Deg.  The other elements 
    >seemed accurate, and at least it was an interesting 
    >experience.
    
    Comment: Az. of 90 deg. and the reference to the moon would put this flare at
    around 20:07:05 local time, say 56 seconds or so before the predicted flare.
    
    
    Rob, Bjoern:  keep up the good work of analysing these reports!
    Other SeeSaters: start watching early, keep watching after the predicted flare,
    keep reporting observations!
    
    
    -- 
    Rod Sladen, Beeston, Nottinghamshire, UK, 52.923N, 1.219W
    
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