Imagine a beam of light from space, white, bright and focused, aimed at Earth, appearing without warning, arcing along the ground at two or three miles per second before suddenly disappearing; at night sometimes nearly bright enough to read by, if you read very quickly, sometimes even visible in the sky by day. And imagine this has been going on for years, in almost total secrecy, known to only a few people with certain inside information. What could it mean?
Well, you don’t have to imagine it. It’s real. I’ve seen it. And a marvelous thing it is.
It’s called an Iridium flare. Now, Iridium can refer to a number of things. A very hard, brittle metal. Or a fleet of communications satellites. Or what is possibly the single most expensive mistake in the history of private enterprise. (Only one of these things doesn’t belong here.)
Iridium satellites have finely-polished aluminum plates serving as antennae. The plates are just a few meters long, but if everything lines up just right— the position of the sun, the position of the satellite in its orbit, the precise tilt of a plate, and your location on the ground— the satellite will flash a brilliant glint of sunlight at you.
It’s an amazing sight. Early on a particular morning, while it’s still dark, you might drive to a certain parking lot that will be at just the right spot under the satellite. You get out of the car and look up to watch just the right patch of sky. At precisely the right time, a faint point of light appears, moving among the stars. You follow that point of light. Suddenly it explodes into a brilliant white ray, and it can be piercingly bright, much brighter than any star or planet, still moving on its unerring trajectory. And just a quickly fades out, leaving only the dim satellite still moving among the stars. For a second or two, a few square meters of aluminum, hundreds of miles overhead in full sunlight while you were on the ground in darkness, flashed at you from orbit like a boy-scout signal mirror.
But if you don’t know where to stand, where to look, and when, you will never see it. Which is a shame. Because chances are it happens several times a month very near where you live, and really bright ones several times a year.
Enter The Calculated Sky. This is a privately-run website that connects you with some phenomenal computing in celestial mechanics. It will compute for you the next Iridium sighting near you, show you the path it will follow on a map, when it will happen, and where to look.
The site will calculate passes of the International Space Station, and if you’re familiar with the ‘magnitude’ system tell you how bright it will be. If you have a telescope, you can even watch the familiar ISS silhouette race across the disk of the sun (if you have a sun filter!) or moon. (If you don’t use a well-designed sun filter, it’s likely you will never see anything again. Buy one.)
If it’s in the sky, The Calculated Sky is an excellent place to find it. Check it out.