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Iridium Flares

Iridium satellites are the communication satellites used by satellite phones. These satellites are quite plentiful (and each has its own number, eg Iridium 26), and have three big flat aerials at one end that reflect sunlight really well. When the aerials are at the right angle they reflect sunlight straight towards us, resulting in what looks like a flare of light that lasts for twenty or thirty seconds or so, brightening then dimming again as the satellite moves across the sky, rotating as it goes.

The flare makes a spot of light on the ground only a few kilometres across, so not everyone will be in the path of the reflected light, but the flares can be annoying for astronomers, as the light can be bright enough in the centre of the path to burn out sensitive equipment.

Thankfully they're quite predictable, and even for casual observers they can be quite impressive, getting as bright as magnitude -8 (occasionally -9.5), and as much as -6 quite regularly. (The lower the magnitude number the brighter it is, so -8 is brighter than -6.) Both of these factors makes them an attractive target for spotting.

Iridium satellites are not the only satellites that can flare, but they possibly the brightest. In appearance the flare can seem like the landing lights of an approaching aeroplane, but no plane ever appears. If not flaring the satellite can be seen at magnitude 6, which means a very dark sky away from city light pollution is required to see them.

Heavens Above has pages for predicting various types of satellite flares including Iridium flares for the next week (from the summit of Mt Eden).

On Friday 18 January 2008 I took a photo of Iridium 26 when it flared to magnitude -6.

The cropped photo gives an indication of how bright it got from how wide the light flared in the photograph.