Iridium is a global network of communications satellites. The system was originally a product of Motorola. Iridium’s 66 satellites provide wireless mobile communications as they move in polar orbits at altitude of 485 mi (781 km). They are able to provide global coverage from pole to pole.
This brief video illustrates the ability of the constellation of satellites to receive a ground signal, pass it to other satellites, then deliver it to the ground in another part of the world.
The original Iridium satellites carry three highly reflective antennae as shown in the image below. Because of their mirror-like surfaces being positioned much of the time in intense bright sunlight, they sometimes reflect a bright spot of light to the ground. The ground track of the reflections is known precisely. If you happen to see it in the sky above, it grows in brightness over a few seconds and can be many times more intense that Venus and then it fades away. They are most easily seen just after sunset and before sunrise. They can also be seen in bright daylight if you know where to look.
I received an email earlier in the week telling me such a reflection, or Iridium flare, was to pass right over my house going south. In fact, another Iridium satellite in nearly the same orbit was to also flare me only 24 seconds after the first. The sky was clear the night of 13 May as I set up for a time exposure photograph.
About 45 seconds before 9:22:14 pm, I started the exposure using NightCap Pro on my iPad 2. The flare maximum occurred as predicted right on time. Coming right behind it was the next one. It passed and peaked at 9:22:38 pm. After 90 seconds I ended the time exposure.
How does one know when and where to look? I subscribe to a service called CalSky. It emails me when significant events like this are to occur. It offers many options and services but is not the easiest to use. There are others which are simpler. The best in my opinion for the general user is Heavens Above. In these services, you need to input your location. Heavens Above makes it easy to do by clicking on a map for your location.
Using the CalSky interface, I produced the sky chart of where the Iridium flares were going to pass. It gave this chart making it easy to know where to point my camera.