Re: Two Iridium flares right next to each other

From: Rod Sladen (
Date: Thu Jan 10 2002 - 18:59:22 EST

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    Kevin Mangis ( wrote:
    > Last night (9 Jan 02) while watching Iridium 42 flare to > -2 as predicted by Heavens-above (19:02:29 local time or 
    > 00:02:29 UTC), I saw a much brighter flare right next to 
    > it in the sky which appeared to be from a nearby Iridium > satellite that was not predicted by Heavens-above. 
    This must have been a very pleasant surprise.
    > I didn't think that two operational Iridium satellites 
    > would be so close together -- aren't there 11 satellites 
    > spread out evenly in one orbital plane?
    Indeed, the operational satellites are evenly spaced around each plane, and the next would not be along for around another 9 minutes.
    > Could the second have been a spare?  
    Correct.  Iridium 82, in fact, with a flare predicted to peak at mag. -4.1 within a second or so of Iridium 42 flaring to mag. -1.8.  Heavens-Above unfortunately omits predictions for some of the spare satellites.
    > My understanding was that the spare Iridiums are not 
    > always nadir-oriented like the operational satellites, 
    > and it seems to me to be too much of a coincidence that 
    > the spare happened to be oriented just like the 
    > operational satellite.
    The spares are maintained in correct orientation, though possibly not with quite the same accuracy as the operational satellites.
    But most of the spares are in orbits around 68km. below the operational sateliites, and thus "lap" the operational satellites roughly every 11 days.  So what you witnessed was a lucky coincidence.
    For making your own predictions, I recommend Rob Matson's IridFlar, which you can download from Mike MCants' site at
    My own Iridium Constellation Status page is at
    Rod Sladen
    Beeston, Nottinghamshire, UK
    52.923N, 1.219W
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