Standard magnitude of Shuttle and ISS

From: Ted Molczan (seesat@rogers.com)
Date: Mon Aug 08 2005 - 19:33:56 EDT

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    I have analyzed Russell Eberst's observations of space shuttle and ISS to
    determine their standard visual magnitude.
    
    1. Shuttle
    
    During 1990-2001, Russell observed shuttles in free flight on nine missions. The
    following chart plots the relationship between visual magnitude normalized to
    1000 km range and phase angle:
    
    http://satobs.org/seesat_ref/Shuttle_and_ISS/Shuttle_stdmag.jpg
    
    The standard magnitude is 0.2 (1000 km range, 90 deg phase angle).
    
    As with most satellites, there is a statically weak correlation between
    brightness and phase angle, but the shuttle's variation is quite a bit greater,
    no doubt due to it's unique shape and pattern of reflectivity. When seen nose-on
    or tail-on, it presents a much smaller cross-section than when seen cargo bay-on
    or heat shield-on. And of course, the heat shielded side is far less reflective
    than the cargo bay and the top of the wings.
    
    2. ISS
    
    Russell cautioned me about using his ISS magnitude data:
    
    "There are very few comparison stars or planets that have magnitudes in the
    range around -2 or so, that I.S.S. frequently exhibits. Really only Jupiter (or
    rarely Mars) are in the right magnitude range. So this means that if these
    planets are not on view at the time of the transit, all that you have to go on
    is your "magnitude memory". As a result, I would expect my estimates to be quite
    unreliable, and I would not be surprised if any analysis based on them were to
    be described as 'flimsy'."
    
    "However, if you want to proceed in the absence of other estimates, then it
    might be worthwhile including a statement that the figures are 'indicative' and
    should not be regarded as quality data."
    
    I accept Russell's cautionary comments; however, I have no qualms in relying on
    his "magnitude memory", developed by making more than 200,000 precise
    observations over more than four decades. His accuracy and reliability have been
    confirmed as a result of standard magnitude evaluations across numerous examples
    of identical satellite buses and rocket bodies. Much of our hobby's ability to
    accurately predict satellite visual magnitudes derives directly from Russell's
    observations.
    
    One complication in the ISS analysis is that Russell roughly apportioned the
    total observed magnitude over the various docked components. I recombined them
    on the assumption of equal reflectivity among all of the components. I computed
    a pseudo reflecting surface area assuming the relationship, mag ~ 2.5
    log10(1/area), then applied this formula to the sum of those areas. Russell
    confirms that my results are within several tenths of a magnitude of his
    original observations.
    
    The following plot is of 47 observations during 2002 December to 2005 July, when
    ISS was in its present configuration, plus or minus a Progress supply ship:
    
    http://satobs.org/seesat_ref/Shuttle_and_ISS/ISS_stdmag.jpg
    
    The standard magnitude is -1.3 (1000 km range, 90 deg phase angle).
    
    Note the much lower variation in brightness for a given phase angle, compared
    with the shuttle.
    
    Based on the difference in their standard magnitude, most of the time ISS can be
    expected to be about 1.5 mag brighter than a shuttle; however, there is
    sufficient variation to frequently allow a shuttle to outshine ISS.
    
    Ted Molczan
    
    
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