>I was just curious, but what causes an Iridium flare to occur on one >pass, and not another? Is it the satellite's orientation in space, or >the sun-satellite-earth angle, or something else? >---------------------- It is always these two causes. More or less flat surfaces on the orbiting object beam a reflection in one direction. If the surface is not flat and mirror-like (specular reflection) the flash can be seen over a wide area. Non-operational satellites are often tumbling or rotating, so the flashes repeat frequently, and can reach a large area. If there is one, or a few, flat surfaces, the rotation axis and the orientation of the surfaces can be computed from locations of bright flashes, and they can then be predicted. Operational satellites like most Iridiums and USA 86... are maintained in a controlled attitude. If the only reflecting surface is solar panels, the reflection is usually back towards the Sun. Iridiums have three large, flat microwave antennas oriented 40 degrees down from the spacecraft body, which is kept pointing at local zenith, one pointing forward, the other two 6o degrees from the backward direction. (There are three more reflecting surfaces, possibly predictable soon). So, during a visible pass, an operational Iridium is essentially stable in space, and the three reflected beams trace three bands, approximately parallell to the S-N-S orbit track (if they reach ground). Because of the panels' flatness and mirror-like surfaces, the beams are about 1 degree wide where they can cause negative-magnitude flares, or about 15-20 km at reasonable distances. The interval between two operation Iridiums is about 9 minutes, and the Earth rotates up to 27 km E / minute, depending on latitude. So the chance that one pass will cause a bright flare is quite small. -- bjorn.gimle@tietotech.se (office) -- -- b_gimle@algonet.se (home) http://www.algonet.se/~b_gimle -- -- COSPAR 5919, MALMA, 59.2615 N, 18.6206 E, 33 m -- -- COSPAR 5918, HAMMARBY, 59.2985 N, 18.1045 E, 44 m -- -- SeeSat-L / Visual Satellite Observer Home Page found at -- -- http://www2.satellite.eu.org/satintro.html -- ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
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