April 6, 2020

AstroEvent of the Week: Spotting Iridium Flares.

Looking up at the dawn or dusk skies, it’s not uncommon to see a satellite brighten, flare up, and the abruptly disappear from view. What you’ve just seen is an Iridium flare, a glint of sunlight off of a refrigerator sized satellite panel. Motorola launched this series of 66 communications satellites in 1997 through 1998 and they are currently owned and operated by Iridium Communications, Inc.

Their primary purpose is to provide worldwide coverage for satellite phones and data services, although the proliferation of cheaper cell tower technology worldwide assured that the sat phone concept never quite took off as envisioned. Catching an Iridium flare is quite a sight, as they can approach magnitude -8, or about 40 times brighter than Venus. Such luminosity can cast a shadow and easily be visible in the daylight. I’ve seen Iridium flares by chance before (they’re that prevalent!) but the best way to catch and photograph one successfully is to plan ahead. As we approach the autumnal equinox, the sun angle is such that Iridium flares in the dawn or dusk skies will be common worldwide. Regardless of your location, several flares may occur overhead on a daily basis. By far, the most reliable sight to track flares is Heavens Above. Simply load in your locale and it’ll give you local predictions for the next 48 hours, complete with a ground tracking map. The closer you can get to the track, the brighter the flare. Do watch a few minutes on either side of the prediction, as satellite orbits do evolve over time. Want to photograph a satellite flare? Simply tripod mount your camera, aim it in the direction the flare is supposed to occur, lock the shutter open (it has to be of the SLR variety, with a Bulb setting) and wait for the flare to occur. A few pretest shots to gauge sky brightness is handy, as twilight can change limiting exposure times dramatically…remember, digital film is cheap!

The Astroword for this week is: Satellite flare. These brilliant flashes are a result of a solar or communications panel catching the Sun high overhead at just the right angle. The Iridium constellation of satellites are the most famous for this phenomenon, but spy satellites such as the X-37B and even the International Space Station are known to flare occasionally. Discarded boosters are known to flash rhythmically as well; this is a result of tumbling motion prior to reentry. If you are diligent, all kinds of strange and wonderful sights will grace a local sky near you!


  1. [...] to do it. Will us Earth-based amateurs one day watch the brilliant explosions visually similar to Iridium flares of Orion spacecraft departing on brave new voyages? One can only [...]

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