Re: Spectacular USA 86 blue flare

From: Bjoern Gimle (
Date: Tue Mar 07 2000 - 23:17:55 PST

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    >I was just curious, but what causes an Iridium flare to occur on one
    >pass, and not another?  Is it the satellite's orientation in space, or
    >the sun-satellite-earth angle, or something else?
    It is always these two causes. More or less flat surfaces on the orbiting
    object beam a reflection in one direction. If the surface is not flat and
    mirror-like (specular reflection) the flash can be seen over a wide area.
    Non-operational satellites are often tumbling or rotating, so the flashes
    repeat frequently, and can reach a large area. If there is one, or a few,
    flat surfaces, the rotation axis and the orientation of the surfaces can be
    computed from locations of bright flashes, and they can then be predicted.
    Operational satellites like most Iridiums and USA 86... are maintained in a
    controlled attitude. If the only reflecting surface is solar panels, the
    reflection is usually back towards the Sun. Iridiums have three large, flat
    microwave antennas oriented 40 degrees down from the spacecraft body, which
    is kept pointing at local zenith, one pointing forward, the other two 6o
    degrees from the backward direction. (There are three more reflecting
    surfaces, possibly predictable soon).
    So, during a visible pass, an operational Iridium is essentially stable in
    space, and the three reflected beams trace three bands, approximately
    parallell to the S-N-S orbit track (if they reach ground). Because of the
    panels' flatness and mirror-like surfaces, the beams are about 1 degree wide
    where they can cause negative-magnitude flares, or about 15-20 km at
    reasonable distances. The interval between two operation Iridiums is about 9
    minutes, and the Earth rotates up to 27 km E / minute, depending on
    latitude. So the chance that one pass will cause a bright flare is quite
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