Tuesday, March 14, 2006

Playing hide & seek with Lacrosse 5

Got a nice bunch of data on Lacrosse 5 (2005-016A, #28646) last evening. I observed two passes (19:08 and 20:50 UTC) , under poor conditions (moonlight, veil of very thin high altitude clouds with more solid patches in it). Nevertheless obtained some good trails: 3 out of 4 photographs yielded useful points (6 points reported in total, 5 of which are consistent).

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Observations on March 11 & 12 had suggested to me that Lacrosse 5's "disappearance trick" might occur when it reached 64-65% illumination. If that would be correct it should do the trick again at about 19:09:30 UTC. In fact, it did at 19:09:41 +/- 3 s UTC, the satellite having reached 65% illumination at 19:09:39 UTC. This is very intrestinng: what is happening at 65% illumination thatt could cause this behavour?

At the next pass, I observed it being bright (+1.5, brighter than the previous pass when it was +2.0 to +2.5), at illuminatons between 68% and 70%. So wat ver happens it to 'dsappear"at 65% illumination, the effect is gone again by 68% illumination.

An attempt to also capture Lacrosse 2 failed as a thick patch of clouds occupied the part of the sky where it would pass.


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