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Iridium Satellite Flares

There are bright points of light in the night sky. They are not Venus, Jupiter, or the Space Station but something that can be just as bright. It is sun glint reflecting from one of the three main mission antennas (MMA), or occasionally a solar panel, of one of the nearly 100 Iridium satellites in orbit that form a worldwide satellite telephone communication system.

Seen in the night sky near dusk or dawn, the Iridium satellites remain illuminated by sunlight. They are barely visible under ordinary conditions. However, each antenna, acting as a mirror, reflects sunlight forming an intense beam that when projected onto Earth is only about 10 kilometers in diameter.

If you happen to be standing in this spot an intense flash of light will be seen for perhaps 1 to 4 seconds. This flash can be as bright as -8 magnitude, looking as bright as the Space Station and brighter than either Jupiter or Venus.

At first glance, it looks like an airplane dropped a magnesium flare. These so-called “Iridium flares” can originate from any one of the satellites in the constellation. The real feat in this sun-satellite-earth dynamics is to know when and where to be standing so that they can be observed when everything comes into alignment.

Rob Matson, an amateur astronomer from Southern California wrote a program that makes such calculations, the algorithm for which is also employed by the Iridium flare predictor on the website Heavens-Above. (Editor's note: Iridium flare predictions are listed under Satellites on the left side of the page).

Given your GPS location, Rob’s program does a remarkable job of predicting the flares down to the very second. An impressive parlor trick is to take your friends outside to observe a bright flare in the night sky at the end of your countdown.