RE: Lacrosse 5: analysis of brightness

From: Ted Molczan (
Date: Sun Oct 23 2005 - 01:58:20 EDT

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    Tonight it occurred to me that Lacrosse 5's unusual photometrics might not have
    been present during all of its first 28 days in orbit.
    Reviewing the two relevant comparison plots I posted Friday, the most obvious
    difference during the first 28 days is that the 1000 km normalized magnitude
    frequently exceeded 4, and afterward it seldom did:

    Closer inspection of the data reveals that all but one of the faintest points
    were observed during the first nine days, resulting in this plot:
    This produced a notable improvement in the correlation between magnitude and
    phase angle. It also revealed a near-absence of 1000 km normalized magnitudes
    brighter than 2. 
    Closer inspection of the data reveals that the three brightest points near phase
    angle 1.5 rad were all observed by Russell Eberst on the very first rev after
    launch, less than 15 min after spacecraft separation. Most likely, the
    spacecraft had not completed its initial deployment of communications antennae
    and solar arrays, which depending on how they were stowed, and the their
    orientation with respect to the observer, could have altered the spacecraft's
    brightness - even made it brighter. So, on the assumption that those three
    points were not representative of the later configuration of the spacecraft, I
    removed them, resulting in this plot:
    Not surprisingly, the correlation improved further.
    Next, I plotted all of the observations made after the first nine days to-date.
    There was a gap in the observations, so the  data begins on the 12th day after
    This result is not much different than the one at the second URL above, which
    started early in the 3rd month after launch.
    Visually comparing these final two plots reveals an even more distinct
    difference in photometrics than the earlier versions. During the first 9 days,
    many points were fainter than mag 4, and none were brighter than mag 2;
    virtually the reverse was true after the first 9 days. 
    The numerical comparison is equally stark, both in brightness and coefficient of
                   std mag   phase coeff
    1st nine days    4.1        1.86
    all later obs    2.9        0.46
    I continue to speculate that this difference may have been due to the SAR
    (synthetic aperture radar) antenna not having been deployed during the first 9
    days, as I explained in my original post.
    Finally, in my original post, I included a comparison graph of Lacrosses 2 and
    3, during their first 28 days after launch, which did not resemble Lacrosse 5
    during its comparable period. In light of the discovery that Lacrosse 5's
    unusual photometrics occurred during its first nine days, I re-visited the
    Lacrosse 2 and 3 data, and found that there had been no observations during that
    comparable period. I have quite a few points from Lacrosse 4, but they are from
    a single observer, who mostly reported the same magnitude, apparently a roughly
    indicative value, which I do not regard as reliable for this kind of analysis.
    So, there is no observational basis to believe that Lacrosse 5's early
    photometrics were unique.
    Lacrosse 5's std mag of 2.9 in its final configuration, does represent a
    significant increase in apparent brightness - about 0.7 magnitudes - relative
    its predecessors. So far, Ed Cannon has responded to my question about colour,
    agreeing with my observation that it is less red. Also, I note Thierry Marais'
    argument in support of the idea that the difference colour may account for much
    of the perceived difference in brightness.
    Ted Molczan
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