Lacrosse 5: analysis of brightness

From: Ted Molczan (seesat@rogers.com)
Date: Fri Oct 21 2005 - 15:26:20 EDT

  • Next message: Thierry Marais: "Re: Lacrosse 5: analysis of brightness"

    Brightness observations of Lacrosse 5 reveal that it is 0.6 to 0.7 magnitudes
    brighter than its predecessors. They also reveal a striking change in its
    coefficient of phase and brightness, between its first month in orbit, and the
    period from its third month to the present.
    
    1. First 28 days in orbit
    
    The following plot of 78 observations during 2005 May, with magnitude normalized
    to 1000 km, vs. phase angle, reveals a very large co-efficient of phase, of
    about 2 mag/rad:
    
    http://satobs.org/seesat_ref/Lacrosse_5_stdmag/first_28_days.jpg
    
    There are relatively few reliable observations of earlier Lacrosses during their
    first 28 days in orbit. The total of 36 observations of Lacrosses 2 and 3 during
    that period, reveal a small, insignificant correlation between magnitude and
    phase angle, similar to the predecessor Lacrosse life-time averages, between 0.1
    and 0.4 mag/rad:
    
    http://satobs.org/seesat_ref/Lacrosse_5_stdmag/Lac_2_&_3_first_28_days.jpg
    
    I speculate that Lacrosse 5's coefficient of phase was larger than usual,
    because its large SAR (synthetic aperture radar) antenna may not yet have been
    deployed. 
    
    GlobalSecurity.org has Charles Vick's conceptual drawing of earlier Lacrosses,
    showing essentially a long spacecraft bus, with a large dish-shaped SAR antenna:
    
    http://www.globalsecurity.org/space/systems/lacrosse_onyx-schem.htm
    
    A large co-efficient of phase is associated with long, narrow objects, e.g. a
    Lacrosse without its SAR antenna. Once deployed, the SAR antenna would tend to
    make the object appear less elongated on average, which would reduce its
    coefficient of phase. It would also increase its average reflective surface,
    causing an increase in brightness.
    
    That is exactly what was observed, beginning several weeks later into the
    mission, as discussed below.
    
    2. 2005 July to present
    
    During 2005 June, Lacrosse 5's orbit was not well placed for observation. Since
    early July, 100 brightness observations have been made, which reveal a much
    smaller coefficient of phase: about 0.4 mag/rad, similar to the predecessor
    Lacrosse life-time averages, between 0.1 and 0.4 mag/rad:
    
    http://satobs.org/seesat_ref/Lacrosse_5_stdmag/apparent_final_configuration.jpg
    
    Brightness also increased, by 0.7 mag relative the first month in orbit. Both
    changes are consistent with the object having increased its reflective surface,
    and become less elongated overall.
    
    An additional finding is that Lacrosse 5 is 0.6 to 0.7 mag brighter than its
    predecessors, suggesting that it has some combination of a larger, more
    reflective surface area.
    
    It has been my impression that Lacrosse 5 is not quite as red as earlier ones -
    perhaps more light yellow in colour. I would be interested in other observers'
    comments on the colour.
    
    In the event of a consensus that Lacrosse 5 is less red, could that explain its
    greater apparent brightness? Could that be tested by means of photometry?
    
    Ted Molczan
    
    
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