Thursday, May 12, 2005

History of Iridium Flares

I have been a member of the internet SeeSat-L newsgroup for many years, which led to my hobby of observing low-orbit Earth satellites. Several years ago an investment group began launching dozens of communications satellites that would enable telephone communications from anywhere in the world. They planned to orbit a total of 77 birds in all which, coincidentally, is the Periodic Table number of the rare metal Iridium, which turned out to be the name they chose for their system, purely for want of a better one.

Shortly after the launches began, keen-eyed SeeSat-L members began noticing that the Iridium satellites passing overhead would sometimes become very bright -- many times brighter than the brightest star -- only to fade away in a few seconds. The group members decided that the two door-sized main mission antennas on the birds were catching the sun's light and reflecting back to the observer on the ground, like a signal mirror.

I was following this thread with great interest since I had purchased stock in the fledgling Iridium company (and sold it at a very nice profit). One of the group members, a rocket scientist named Rob Matson was following it too, asking questions about the observations and the construction and in-orbit guidance of the satellites. Within a couple of weeks Matson had worked out a computer algorithm that was able to predict when and where on Earth the birds would suddenly flare up. He called them "flares", although his peers preferred the term "glints", and the name stuck. All of today's Iridium prediction software use his algorithm.

A few months later I learned that the astronomy club I belonged to had been invited to bring our telescopes and present a star party during a cookout hosted by the local Mensa Society, many of whom were engineers and scientists at NASA Langley. I wondered what I could possibly find in the sky to impress these folks, and went to my computer.

The cookout date coincided with a nice pass of the Russian space station Mir and I began to feel that I had something of interest to the Mensans. Then I cranked up Matson's SkyMap. Voila! A bright MINUS SIX Iridium Flare! I could hardly contain myself as I wrote down the predictions, and smirked.

On the evening of the cookout I looked around the yard and counted over fifty Mensa geniuses and maybe fifteen of us mortal astronomer guests. None of my friends there had ever heard of Iridiums. I looked at my watch which was set to the exact second and thought "This-had-better-work!"

The Mir pass was right on time and lasted for a couple of minutes, giving the Mensans time to come over and see what we were looking at over our heads. I took the opportunity to explain that in a few minutes another satellite, invisible at first, would suddenly brightly appear for a few seconds, and I pointed out a good location in the side yard where I'd meet them.

With two or three minutes to go I started calling out for everyone to join me -- and they all did. Every last person in the yard was soon standing in a gaggle as I pointed to a spot in the sky where our flare would appear, hopefully. With ten seconds to go I nervously looked away from my watch, took a big gulp, and pointed to the sky as I counted down.

When I reached "two" the sky where I was pointing suddenly lighted up like a flashbulb for about three seconds and an awesome "Ahhhh" arose from the Mensans as the entire group broke into appreciative applause.

It was one of those magic moments.

-- Dalton Hammond

More of My Astronomy

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