Observing Iridium Flares

On the evening of Wednesday, October 7th, my girlfriend, Christine and I had the pleasure of experiencing a bright Iridium flare at 6:30 pm EDT. Earlier that afternoon, my Insight Observatory Associate, Harry Hammond called me to inform me of a -3 in magnitude Iridium flare that could be seen roughly 62 degrees high and directly due north. Harry's source for the sighting was the website Heaven-Above. Heavens-Above tabulates the altitude and azimuth of each iridium flare event, but you can also use the site to plot a particular flyover against the stars as follows: once you have the list of flares generated by the steps given above, click on a particular spacecraft ("Iridium 53," say), and then click on "Passes." This will give you a list of that spacecraft's passes over your location for the next several days. Click on one of those passes by date, and you can get a star chart showing the spacecraft's quickly changing position among the stars.

Iridium Satellite 54 - Photo by Christoph Lohuis.

My first visual experience of an iridium flare occurred when I was attending the 2006 amateur telescope-making convention known as Stellafane which is held annually during mid-summer in Springfield, Vermont. I was relaxing in my lawn chair staring up at the summer Milky Way overhead roughly around midnight. As I was enjoying the clear night sky, I overheard two gentlemen discussing iridium flares. Not knowing much about them myself, I couldn't help but start to listen in on their conversation. As they continued to discuss the topic, one interrupted the other and muttered, "Look right below Sagitta (the constellation of the arrow) In 5, 4, 3, 2, 1..." Immediately after his countdown, a bright burst of light that resembled a star suddenly brightening, then fading, appeared right below the constellation Sagitta as promised.

Of the roughly 3,000 spacecraft in Earth's orbit, just about 100 stand apart: the Iridium communications spacecraft, which skim the uppermost, most rarefied region of the atmosphere (the exosphere) at altitudes around 800 kilometers in six steeply inclined orbital planes (orbits that nearly pass overhead at the North and South Poles). Known as an iridium flare, the glare from these satellites is well-known to many astronomers.

How we see Iridium Flares

What causes Iridium Flares? Iridium satellites are unique because their flat, shiny, door-sized antenna arrays periodically reflect sunlight toward the ground, causing brief, but brilliant flares that can momentarily reach an apparent magnitude of –8, brighter than the planet Venus. Also, these flares are predictable and their orbital elements are public information. Thanks to websites such as Heavens-Above, satellite-watching enthusiasts are able to witness these brilliant occurrences. The illustration to the left demonstrates how Iridium Flares are seen from the Earth.