Spotted an Iridium Flare

Well Sydney had it’s first night in a long time where we got to do some good observing. The clouds did eventually roll in, but there were a few good hours of observation before the first cloud appeared.

At 8:30pm, the First Quarter Moon was up around 55 degrees above the horizon so there was a little bit of light pollution coming from the Moon. Jupiter was nearby and a few stars were appearing in the night sky.

As I looked up, I suddenly noticed one of the stars was moving across the sky towards the northeast. As I watched it, the star suddenly brightened by several magnitude for a few seconds before growing fainter and then completely disappaearing.

Iridium Flare (Click to enlarge)

This of course was one of the countless thousands of satellites orbiting the earth. To be exact, this was an Iridium satellite called Iridium 30+ which at 8:32pm should have had a magnitude of +7.34 which meant it should not have been visible with the naked eye. However, i think the sun reflecting off it was slowly increasing it’s brightness so that I could eventually see it.  This made it bright enough to easily see with the naked eye but was not as bright as Jupiter which has an apparent magnitude of -2.28. When the satellite suddenly flared, it was slightly brighter than Jupiter so I estimate it’s magnitude briefly went to a magnitude of  -2.5 before going back to mag +7.34 where it disappeared from sight.

Iridium communication satellites have 3 highly polished mirrors, one of them facing in the direction the satellite is taking. At times the Iridium satellite will reflect light from the Sun directly down onto the Earth where an observer looking at the satellite would see a bright flare in the sky lasting only a few seconds.

This was what I saw at 8:32 tonight. It was the first Iridium flare I have ever witnessed and it was pretty impressive to see.

These iridium flares can be as bright as a magnitude of -8 which is where they can be seen during the daytime. As they can get so bright,  these satellites can be an annotance to astronomers in that they can ruin a critical observation by flaring at the wrong moment or they can even damage light sensitive equipment.

I have tried to recreate my experience tonight in the picture above. From left to right, the satellite is travelling across the sky like any satellite in the first picture. In the second picture, there is a flare from the satellite making it brighter and easier to see. In the final picture, the satellite became very faint after a few seconds and then completed faded from sight.

~ by sydneystargazers on November 25, 2009.

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