If we didn't know what we had just seen, we might have suspected a UFO. But that isn't what it was. What we saw was an Iridium flare. This column will describe how you can see them, too.
An Iridium flare, also called an Iridium flash, is a reflection of sunlight at certain angles of incidence from the very reflective surface of an Iridium satellite. There are currently 79 Iridium satellites in low earth orbit, providing cellular telephone service to any point on Earth. Given this many satellites in only six orbital planes, it isn't hard to see an Iridium flash, if you know where to look. On average, in our area, we can expect to see one or two per day. Occasionally you can see several within a few seconds of each other. On November 15 in the Akron area, we should be able to see two at the same spot at the same instant. One photo posted on the web shows five trails in one image during a 90 second exposure.
The magnitude scale used for brightness is a logarithmic scale: large negative values are the brightest. We were fortunate to see a flash of magnitude -7, which is thought to be the maximum brightness for this phenomenon. For comparison, the normal brightness of an Iridium satellite is +6; the brightest star, Sirius, is -1.5; Venus at its brightest is -5; the moon is -12 and the sun is -27. Our flare was roughly six times brighter than Venus when we saw it, about 100 times less bright than the moon. The response of the eye is not linear (fortunately!), so while it looked very bright, it wasn't dazzling. The flash brightness is about 150,000 times brighter than the normal light reflected by an Iridium satellite.
How did we know where to look for the satellite? We used the web, of course. If you send your browser to http://www.gsoc.dlr.de/satvis/ , you will go to the German Space Operation Center Satellite Visibility home page. It's in English and isn't completely devoted to Iridium flares; it also covers other satellites and space artifacts that can be easily seen from the earth's surface. From this page, you need to specify your observation point's latitude and longitude and then you can look at a list of satellites that will be visible from your location in the near future (or which were visible in the recent past). The site also describes the phenomenon of Iridium flares and has a link to http://www2.satellite.eu.org/sat/vsohp/iridium.html , which has a very good, detailed description of many aspects of Iridium flares and the Iridium satellite program.
Specifying where you are located isn't as straightforward as you might think. The GSOC can provide you with generic latitude and longitude values for the city you live in, but these values are really not precise enough to allow an accurate prediction of flare intensity. The flare effect falls off rapidly with distance from flare center. We were less than two miles from flare center and we saw it at maximum brightness. If we had used the generic Akron city coordinates (positioned roughly at the intersection of Copley Road and White Pond Drive), we would have expected a less bright display. That location is about 2.5 miles from our location. You could easily be a lot farther away from that point, even if you live in Akron. So how do you get a precise position for the program? Read on.
It's easy if you have a global positioning system receiver that generates latitude and longitude; you just read your position from it. I don't have one myself and you probably don't either. I do have the US Geological Survey 7.5 minute topographic map for West Akron. With a precise ruler and a calculator, I determined a pretty good value for our position. You probably don't have that map however, and since this is a web column, I set about finding how to get the same information using the web.
The first site I found was http://www.etakguide.com . All you have to do is type your street address and it will show you a map of the region you are in. You can then click on a radio button and get a latitude and longitude value for your address. Make sure not to move the marker from your address when you click. An alternate source for this information is http://tiger.census.gov . You'll have to zoom in on a map of your area and click on the spot for which you want the latitude and longitude value. It's a bit more work than the etak site, but you can use it for locations that don't have an address. It seems to be a faster site, too, so maybe the bit of extra work would be worth the quicker response. The two predictions varied by less than 500 feet; quite adequate for our purposes.
You know what you need now; go see some Iridium flares!
CATBAR - Brain Candy 16 - Iridium Satellite Spotting / Brian Rock / November 9 1998