Saturday, September 11, 2010

Spectacular Keyhole appearances

The KH-12 Keyholes USA 129 (96-072A) and USA 186 (05-042A) delivered some spectacular views in the early part of September.

USA 129 was making passes over Europe near its perigee, at no more than 300-310 km altitude. That is lower than the ISS! It resulted in a zipping fast speed, especially during zenith transits. It was very bright too, attaining magnitude +0 easily.

Below are two images of two such spectacular passes. One (3 Sept 2010) shows it in twilight low in the eastern sky, grazing the roof of my neighbours, at a distance to the observer of 800 km and an orbital altitude of only 305 km. The other image (5 sept 2010) shows it during a zenith pass, when it was at only 300 km orbital altitude at a distance to the observer of no more than 305 km! As can be seen, the FOV of my EF 2.5/50mm (about 24 x 18 degrees) was no longer adequate at that time! (the image shows the full frame: movement is from bottom to top).

click images to enlarge

Below diagram shows the geometry in question for the second image (based on elset 10248.82329557). The photograph was taken 30 seconds after perigee: during perigee itself, it could not be photographed as it coincided in time with emergence from the Earth's shadow.

click diagram to enlarge

A few nights later, USA 186 flared brightly to mag. +0 on September 9th at 21:15:10.7 UTC, yielding this nice picture plus brightness diagram (movement is from left to right):

click image to enlarge

click diagram to enlarge

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