Iridium Flares

By Malcolm Gibb

Have you seen one recently? Anyone who has seen a magnitude -8 flare, which is about fifty times brighter than Venus, can only be impressed. It gradually increases in brightness to maximum then fades until it disappears, taking between 15 and 30 seconds to do so and at the same time moving across the sky. If you didn't know what you had seen, you could be forgiven for thinking you had seen a UFO, in fact a number of reported UFO sightings are probably just that!

But what is an Iridium Flare? It is a small telecommunications satellite in a low Earth orbit. There will eventually be a total of 66 in 6 orbital planes, approximately 780 km above the Earth. The flare or more properly, the reflection, comes from the main antenna on the satellite. There are three antennae, 120 degrees apart, 188 cm wide 86 cm long and angled 40 degrees away from the axis of the body of the satellite. They are highly reflective aluminium flat plates treated with silver coated Teflon. This plate provides a direct reflection of the Sun's disk, which is only tens of kilometres wide at the Earth's surface, and to observe them, one has to be inside this relatively small area.

When can you see one? Almost any day of the year. An operational Iridium satellite maintains its axial and longitudinal position within very strict tolerances. A computer programme has been developed which can calculate the time, position and magnitude of the projected specular reflection of the Sun from the antenna to an observer at a given position on Earth. Believe it or not, it can predict the time to within a few seconds up to seven days ahead. If you have access to the Internet, then the site to go to is . This site, the German Space Operations Centre, not only gives predictions for Iridium satellites, but for a whole host of others, including the ISS and MIR, also star charts, planetary information and links to other interesting sites.

Is this really observing? Watching satellites may not be astronomical observing in the purest sense and I have heard some complain about it causing light pollution and spoiling astrophotography, but remember, without satellites such as the Hubble Space Telescope and others, our knowledge of the Galaxy and the Universe would be very much poorer. Do try and see an Iridium flare, I'm sure you will give it a high 'Wow' factor, as Heather would say!