RE: SeaSat 1 unexpectedly bright.

From: Ted Molczan (
Date: Sun Sep 12 2004 - 14:48:51 EDT

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    Paul Thomspon wrote:
    <<I was curious however, about the brightness of SeaSat 1 ... was due for a pass
    ... [at predicted] 3.9 mag. This data is all from Heavens-Above. The satellite
    was easily visable, and not as dim as expected, appearing as bright as perhaps
    3.2-3.3 mag.>>
    <<... I wondered if its orientation is variable, and with it, its brightness
    Your speculation is correct. Seasat 1 is very irregular in shape and surface
    reflectivity, resulting in a large variance in magnitude for a given set of
    circumstances (range and solar illumination).
    Heavens-Above's magnitude estimates are based upon standard magnitude (aka
    intrinsic brightness) data derived by hobbyists from their observations.
    Seasat 1's standard magnitude is 3.9, when observed at a range of 1000 km, and a
    phase angle of 90 deg. But that value is an average, derived statistically from
    a large number of observations, as in the following graph of more than 500 of
    Russell Eberst's observations:
    The linear regression line on the graph confirms that the std mag is 3.9, but
    notice that the observed values range +/- 3 magnitudes from the line. As a
    result, predicted magnitudes are uncertain by +/- 3 magnitudes.
    Almost all satellites can flare several magnitudes on occasion, but most of them
    stay within about +/- 1 magnitude of the regression line, most of the time, for
    example, 93026B:
    Comparing the two graphs, it is clear that Seasat 1's magnitude is far less
    predictable than 93026B's.
    Perhaps, we should consider including statistical confidence limits with the
    standard magnitude. A simpler solution would for prediction software and
    services to prominently state that magnitude estimates are uncertain by at least
    2 magnitudes.
    Ted Molczan
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