RE: Superbird A (89-041A, 20040)

From: Matson, Robert (
Date: Fri Dec 16 2005 - 13:11:24 EST

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    Hi Leo, Ed, Björn & other Superbird A trackers,
    A couple comments on the flashing behavior of this dead satellite.
    For years I think we've all been assuming that Superbird's large
    solar wings are the source of the bright (up to ~2nd magnitude),
    predictable glints that occur over a 5-8 minute window each night.
    (After all, these solar wings are much larger than the spacecraft
    bus itself, and offer significant surface area with which to direct
    solar glints down to the ground.)
    But the well-known phase shift that occurs during the flash window
    each night raises a small dilemma:  clearly at least two somewhat
    specular surfaces on the satellite are involved that must point in
    opposite (or nearly opposite) directions.  If surface #1 causes
    the glints at the beginning of the flash window, and surface #2 is
    the source of those at the end of the flash window, then the pattern
    of flashes is something like this:
    1 1 1 1 1 121212121212 2 2 2 2 2
    Currently ~22 seconds separates each pair of 1's and each pair
    of 2's, with flashes from surfaces 1 and 2 alternating every ~11
    seconds during the middle of the flash window.  So what's the
    dilemma, you may ask?  The problem is that the flashes from surface
    1 and surface 2 are the same brightness (or very close to the same
    brightness), which means that either the two surfaces share the same
    size and reflectivity (or much less likely, that the smaller size
    of one surface is exactly compensated by a higher reflectivity).
    Why is this a problem?  The satellite has two solar wings, after
    all.  The problem is that there is a significant difference in
    surface reflectivity between the front side and back side of each
    solar wing.  Thus if the two solar wings both faced in the same
    direction, then the flashes I've labeled "1" and "2" could not
    be of the same brightness.  They probably wouldn't even be of
    the same color.
    When I presented my talk on Superbird A at Eurosom 3 in Edinburgh,
    my proposed solution to this problem was simple:  the two solar
    wings must be pointed in opposite directions!  (I even theorized
    at the time that perhaps this was an emergency failure mode
    configuration guaranteed to always have sunlight falling on one
    solar panel. This asymmetric arrangement would also handily
    provide a mechanism for the observed spin-up of the satellite
    that continues to this day -- the Yarkovsky effect).
    The problem is that I don't think spacecraft designers ever actually
    do this (deliberately point the solar arrays in opposite directions)
    for the very reason that the reflected energy imbalance ~will~ spin
    up their satellites.  Which brings us back to square one:  how can
    we get equal magnitude glints from two surfaces on Superbird A
    pointed in opposite directions?  The only solution I can come up
    with is that the glints come from the body of the satellite itself.
    And while the solar panels must also generate solar specular
    reflections, they may not ever intersect the earth -- or if they
    do, they occur at a different time from the satellite body flashes.
    (There ~have~ been observations of secondary flashes from Superbird A
    at a different time then the main flashes -- perhaps these are from
    the solar panels...)  I'll have to check my archives, but my guess
    is that these secondary flashes only occur at the 22+ second period,
    not 11+ seconds.
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