Iridium Craze

This summer I have spent a bit too much of my time searching for Iridium Flares. For the uninitiated, the flares are sun glints from Iridium Communication Satellites which pass overhead at regular intervals. Each satellite is one of a constellation launched by Motorola in the late 1990s. Motorola's idea was that mobile, wireless, global communication would be the wave of the future... and that satellites would make the wave even more future-y. They got half of the equation right, mobile communication is big, but the satellite constellation was too late, too expensive (more than a dollar a minute!), and just too inconvenient for consumer use. Motorola's venture eventually failed, but was resurrected to service the U.S. Military (for which late, expensive and inconvenient are ideal conditions).

But aside from largess, it turns out that Iridium satelllites have another cool property. The communication antennae of these birds are shiny and large enough that a specular reflection of the sun is possible, that make wonderfully bright flares as seek from the ground. For the geeks out there, the satellites are at an altitude of about 780 km. The sun subtends about a half a degree of arc, which means the specular reflection should be spread out about ~6-7 km on the earth's surface. Since the satelllites travel at about 7 km/s, the maximum flare lasts just a few seconds. In reality, there is a more diffuse reflection which spreads out a larger area and makes the flare last for about 20 seconds total.

The flares are very bright, very specific to your location, and very short. It's quite possible for you to see a bright flare, and a friend just 10-20 miles away sees barely anything. Thus, when chasing flares, it's important to know three important things: where you are, the precise time, and where to look.

I took the photo above with my little digital camera (no tripod!). The streak is an Iridium flare with a 15 second exposure. It is chopped because I started the exposure late. The black lines are power lines next to the road.

Catching a Flare

I knew in advance where and when to catch the flare because I knew those important three items (my position, the exact time, and where/when to look).

I use the web service called Heavens Above to predict flares times. It is very straightforward to create an account. One of the first things when you create an account is to enter your coordinates (latitude and longitude).

You can determine your location most easily using Jef Poskanzer's ACME Mapper (or, you can use a GPS device if you have one). Just find your self in the Mapper by clicking the link, and panning and zooming (or searching). Be sure to zoom in close enough so that you are within a kilometer or less. Then read off your latitude and longitude from the indicator. Here is what you need to look for:

Then it's a simple matter of plugging those examples into the coordinate boxes of Heavens Above. For example, the latitude box for the coordinates above would look like this:

and the same for the longitude box. Ignore the "Minutes" and "Seconds".

Once you enter your coordinates, you should be at the Heavens Above main page. Click on the Iridium "next 7 days" link to get predictions of flares near you for the next week.

Handy tip: focus on the -5, -6, -7, -8 or -9 flares. Because of the crazy astronomical magnitude system, more negative means brighter. Thus, the most spectacular flares are -8 or -9 (but these are rare).

Now you have your list of flare predictions. The last thing you need is the precise time. If you have a GPS, use it. Flares predictions are exact to the second, so you need a clock accurate to one second as well. The easiest thing to use is your cell phone clock, which is actually synchronized very accurately by the cell phone company. Unfortunately, your cell phone usually gives minutes and no seconds, so you will have to guestimate to the number of seconds. You can also synchronize your wristwatch to an official time server (such as the US server) and use that.

Now go out and watch. I have two final tips. First, the time given in the prediction is the time of flare maximum, so be sure to be looking a few seconds early. Usually you will be able to see a faint, fast-moving dot before it starts to get really bright. Second, if you are looking for flares at 50, 60 or 70 degrees altitude above the horizon, this will be much higher on the sky than you think. Be prepared to crane your neck. For lower altitudes, you will need to find an open space away from tall trees.

I guess my fascination is two-fold. For one thing, the really bright flares are quite spectacular. The brighter flares are easily the brightest objects in the sky for those few seconds. For another thing, there is something wonderful in how regular and predictable the universe is.