Iridium Flares—A Bright Light in the Sky

Original article appears in GuideStar September, 2015.


Object: Iridium Satellites
Class: Satellites
Magnitude: Can be very bright
Location in the sky: Varies, but predictable

Why this object is interesting: Timing is everything. And a bit of planning helps too. The photo by Loyd Overcash is a good representation of what you’ll see in the night sky. This picture was taken over a few seconds. What you see is a satellite that brightens considerably over the few seconds that it’s visible to a sharp peak in brightness for perhaps a second.

What are these satellites, and why are they so bright? This ‘constellation’ of 66 satellites has been in low earth orbit (485 miles up) since 1998. The purpose of the satellites was to provide direct-to-satellite global communications capability. Many parts of the world do not have cell phone capabilities and the idea was to be able to communicate (i.e. make a phone call) from anywhere on the planet to anywhere else on the planet. As a business proposition, the service wasn’t successful. It was too expensive, and, as cell service became ubiquitous, less necessary. There remain places for which the Iridium network represents the only way to communicate — the South Pole, for example.

The large antennas on the satellites can reflect sunlight back to earthbound observers in sufficient quantities to create a ‘flare’ in the sky.

These events happen quite regularly, can be predicted, and can be easily observed even from the brightly lit city.

There are several ways you can find out when there will be an Iridium flare available for you to see. The availability and brightness of a flare is very dependent on your location on earth, so you need to have your latitude and longitude accurately identified. There are plenty of ways to do this. The compass feature on my iPhone does this, many GPS units do this, and you can get this information on your computer from Google Maps.

Once you have this information, one of the best web sites for predicting Iridium flares is You need to specify your location (click ‘Change your Observing Location’. You can type ’Houston’ and then move the map pointer around to get your location identified for the web site.  Also on the home page is a link to ’Iridium Flares’, where you’ll get a list of flares to see. You get altitude, azimuth, time (down to the second) and brightness in magnitude.

The ones in darker skies, higher from the horizon (altitude), and brighter are easier to see. The effect is much better under dark skies.

I use an iPhone app called GoSatWatch to predict Iridium flares. It’s $9.99 on the Apple app store. There’s an Android version of this app as well.

(For you amateur radio operators, you can use the apps to predict passes of amateur radio satellites as well. With a handheld radio capable of operating 2 meters and 440 Mhz and a $100 handheld antenna, you can communicate through these satellites.)

GoSatWatch also shows you a simple map of the sky showing where the flare will be and shows in real time the position of the satellite on its way to the flare point.

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