Re: Iridium 921 tum

From: Bjorn Gimle (
Date: Sun Dec 16 2001 - 07:11:31 EST

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    It is very reasonable that you saw Iridium 921 (tum)(bling) - and that goes
    for my friend Bengt also, who saw it about four minutes before the ISS+STS
    pass at 16:56 UTC on Dec.15, just above Mars.
    Incidentally, Bengt was one of the first to report an Iridium flare, in
    August 1997, before the phenomenon was identified. I think I noted that an
    Iridium was close, but it was only late autumn-winter, when IridFlar was
    available, that I could verify the flare.
    It is very difficult to find a flashing rate for a tumbling Iridium, since
    it has at least eight good flat surfaces (3 MMA, three body sides, flat
    bottom, (parallell) solar panels). Unless the rotation is very fast, a flash
    will be much fainter the next time, because the satellite has moved several
    degrees, and the reflection is no longer specular.
    I have collected quite a few negative-magnitude flash observations from 921
    (and other Iridiums) in the hope of determining a rotation axis, but because
    of the many surfaces, and (probably) a rapid precession of the axis, a large
    number of observations are needed in a short period (a month or so).
    If you wish to find notes about a possible pattern, search on SeeSat-L using
    Google, from page (but my
    search gave no true hit - I'm sure that's wrong)
    > At 06:38 UT (give or take a minute either way) this morning as I let my
    dogs out
    > into the backyard I happened to glance towards Polaris and saw a mag -1.0
    > Iridium type flare from an unknown. (55 Elev.  5 Azm.)
    > Using FindSat the nearest sats to this position were Cosmos 2322 and
    Iridium 921
    > tum. As the flare was travelling towards the north and Cosmos 2322 would
    > been travelling towards the SE, the obvious choice would be Iridium 921
    tum. The
    > only other time I have seen 921 was on the 13/06/01 at 23:38 UT when I had
    > nice -2.0 flare at (35 Elev. 95 Azm).
    > Does anybody have any information on this Sat i.e.:tumbling rate etc: to
    > my conclusions.?
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