ISS brightness & flash predictions

From: Matson, Robert (ROBERT.D.MATSON@saic.com)
Date: Thu Dec 14 2000 - 10:27:53 PST

  • Next message: Dale Ireland: "RE: ISS brightness & flash predictions"

    Hi Kevin,
    
    > Arles,France,43633N, 4.717E
    
    > Yes Bruno the Top Solar Panels Casted a shadow at 17:59:04
    > on the Zarya,And the Zvenda Modules.The mag of the ISS was
    > then reduced to a magnitude 6.2 From this Shadow.The
    > Shadow lasted 3-4 seconds it looks like on This screen.
    
    Respectfully, this is a mistake.  There is absolutely no
    sunlit configuration of ISS that will drop its magnitude
    by a factor of over 200 from its peak (but non-specular)
    brightness -- assuming you're not talking about trying
    to observe ISS at an elevation of 3 degrees above the
    horizon.  This is not a stealth space station, and
    the solar panels are not 99.5% absorptive.
    
    I appreciate that you've done a lot of work to model the
    3-D structure of satellites, but without continuous, precise
    pitch, yaw and roll orientation as a function of time, and
    the optical properties of all major external surfaces -- not
    just reflectivity, but bireflection distribution functions --
    you cannot predict the visual magnitude of a complex object
    like ISS to any better than about a factor of 2 (.75 visual
    magnitudes).
    
    As for spinning or tumbling satellites, except for the special
    circumstances of spin-stabilized nadir-pointing satellites, or
    satellites for which spin axes have been derived from multiple
    observations, you cannot accurately predict when they will
    glint.  You can forget the tumbling Iridium satellites -- they
    have far too many specular surfaces (I know of at least 10
    major ones, some of which can be involved in double or even
    triple bounce reflections) to deconvolve their tumble dynamics.
    Only through a tremendous amount of observational effort (years
    of data from dozens of observers) have the spin dynamics of
    Superbird A (#20040) been determined.  To a lesser extent,
    the axes of TDF-1 (#19621) and ETS-6 (#23230) have also been
    derived, though not their long term precessions.
    
    I want to reiterate that I applaud your efforts in working
    to uncover the rotational dynamics of specific satellites with
    the goal of providing glint predictions for our list members.
    I share your goal, as do several others on our list, and have
    been doing just that for many years, within the limits of
    what can be divined with eyes, binoculars, telescopes and
    stopwatches.
    
    Enthusiasm is ALWAYS welcome here!  I would only caution that
    your enthusiasm be tempered by an equal amount of skepticism
    until your model's predictive accuracy has been confirmed by
    multiple observations.  Overstating the model's predictive
    powers before it has been "put through its paces" only does
    a disservice to the people who sit out in the cold at 2 am
    to record the observations that make all this magic possible.
    
    Best wishes,
    Rob Matson
    
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