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What are Iridium ‘flares‘ or ‘flashes‘?

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 observers have even spotted flaring Iridium satellites in daytime skies.

Luckily, you don’t have to do all of the calculations!  All you have to know is your latitude and longitude (the more accurately the better), and the date. A visit to the German Space Operations Centre (GSOC) website, will let you predict exactly when the next Iridium Flare will be.  It will also let you investigate the details of the particular Iridium satellite that will cause the flare. 

What will you see when you see a flare?  Over the course of 5 – 10 seconds, you’ll see a spot in the sky brighten rapidly to a brief flash, moving with the motion of a satellite, followed by a steady dimming that can last as long as 30 seconds.  Good Luck!!!