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: http://satobs.org/seesat_ref/Lacrosse_5_stdmag/first_28_days.jpg http://satobs.org/seesat_ref/Lacrosse_5_stdmag/apparent_final_configuration.jpg 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: http://satobs.org/seesat_ref/Lacrosse_5_stdmag/first_9_days_1.jpg 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: http://satobs.org/seesat_ref/Lacrosse_5_stdmag/first_9_days_2.jpg 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 launch: http://satobs.org/seesat_ref/Lacrosse_5_stdmag/apparent_final_config_2.jpg 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 phase: 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 ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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