What are Iridium flares or flashes?
One specific set of satellites is of special interest to viewers. The Iridium satellites are communications satellites with shiny dish antennas which can give flashes of reflected sunlight. If you’re in the right place, the flash of light–lasting just a few seconds–will be brighter than Venus. Predicting what you can see is very dependent on your location–just a few kilometers makes a noticable difference. Web sites such as Heavens Above give predictions for viewing Iridium flashes; for these, it is best to specify your latitude and longitude to within 0.01 degrees (some web sites can help you find this for the part of town where you are observing from). Also, you should calibrate your watch with a time standard (an atomic clock standard, or on line via Heavens Above) since the flashes last only a few seconds. These predictions depend not just on the satellite’s orbit but also the satellites orientation: if the direction the shiny antenna is pointing in changes just slightly, the pr
The Iridium satellites are relatively small telecommunications satellites in a low Earth orbit. They were a part of a world-wide system for mobile communications operated by the now defunct Iridium LLC Corporation. As can be seen from the diagram below, each satellite has three main mission antennas (MMAs), which are flat, highly reflective surfaces, that can reflect the Sun’s rays to an observer on the ground when the Sun-Satellite-Observer geometry is correct. The satellite’s attitude is controlled so that the long axis remains vertical, with one MMA always pointing forwards. Given knowledge of the attitude, together with the orbital position of the satellite and the Sun and observer’s location, it is possible to calculate the angle between the satellite, observer and the line of a perfect reflection of the Sun. This is the so-called “mirror angle” and determines the brightness (magnitude) of the flare. This flare, or glint, usually lasts from five to 20 seconds. Some keen-eyed obser
