Wednesday, 24 January 2007

Odds of a mag. -7.5 Iridium flare, and twice Lacrosse 5 its tricks

Another clear (and cold) evening, although clouds began to appear at 19h UTC. Positions were obtained on Lacrosse 5 (05-016A, #28646) and Lacrosse 4 (00-047A, #26473), as well as Iridium 03 (98-048A, #25431).

Highlight of the evening was a splendid magnitude -7.5 flare by Iridium 03 at an altitude of 44 degrees and azimuth 28 degrees (north-northeast) at 18:22:50.8 UTC.

(click image to enlarge)

The image above was shot from the street in front of the almshouse complex (it was just too low in the sky to be visible from the courtyard of the complex) with a 10.7s exposure between 18:22:47.1-18:22:57.8 UTC. The flare had a predicted brightness of mag. -7.5, and indeed must have been in that order. The center of the flare path was 4.8 km West of me (while the Iridium satellite was over Denmark, at 785 km altitude and a range of 1069 km north-northwest of me).

Chance had it that the people from Grimbergen observatory in Belgium (Philippe Mollet et al.) located 135.9 km south of my location, observed the same flare from the same Iridium. While I had the center of the flarepath at 4.8 km West of me at 18:22:50.8 UTC, they had the center of the flare path at 4.6 km East of them at 18:22:28.9 UTC. So the geometries were virtually the same (with a predicted brightness of mag. -7.7 for Grimbergen, versus -7.5 for me), they having the flare maximum 20.9s earlier in time than me. What a coincidence!

I observed one pass of Lacrosse 4 (two good images, yielding 4 positions), and two of Lacrosse 5 (3 images, half of the points dropped because of too faint trail ends, hence 3 positions).

Lacrosse 5 (05-016A, #28646) did its "disappearance trick"again at both passes. During the first pass it did it at 17:30:44 UTC, while the camera was open for its second exposure, resulting in the image below. The true end of the trail at the end of the exposure should be at the right of the star near the trail end: it is at the left in the image, as the satellite dropped below the imaging treshold in brightness before the end of the exposure. The drop was rather fast.

(click image to enlarge)

A few tens of seconds after it, I could see the satellite as a very faint object (mag. +4 to +4.5) with the naked eye (before the drop in brightness it had been +2). A third image exposed between 17:31:46.1-17:31:56.8 UTC shows a very faint trail after some image manipulation (see below)

(click image to enlarge)

Lacrosse 5 did the same trick again during a second pass. This time it disappeared some 5 seconds before my camera opened, at 19:13:15 UTC.

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