Of the roughly 3,000 spacecraft in Earth orbit, nearly 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).

In financial circles, the Iridium "constellation" of satellites stands apart because it was built at a cost of roughly $5 billion, only to be sold for $25 million when its first corporate owner, Iridium LLC, went bankrupt in 1999. The spacecraft (and the ground stations supporting them) were intended to enable owners of special portable telephones to communicate from any point on the surface of the globe. However, Iridium LLC never obtained the millions of customers needed to make the project profitable. The U.S. Department of Defense and Federal Emergency Management Agency are among the principal customers of the satellites' current corporate owner, Iridium Satellite LLC of Leesburg, Virginia.

In skywatching circles, the Iridium satellites stand apart because their flat, shiny, door-size antenna arrays (three per spacecraft) periodically reflect sunlight toward the ground, causing brief (seconds-long) but brilliant flares that can momentarily reach an apparent magnitude of –8 — outshining the planet Venus. What's more, these flares are predictable, thanks to the satellites' publicly available orbital elements and to software and Web sites that satellite-watching aficionados have made available free of charge.

How To Plan Your Watch

Most would-be Iridium-flare watchers need go no further than Heavens Above. This fascinating Web site is maintained by Chris Peat, a physicist and space-industry veteran who now works for the German Space Operations Center. A few mouse clicks should produce a list of any Iridium flares occuring over your location in the next several days. Each listing will tell you where in the sky to look — and when! — to see sunlight glint off an Iridium spacecraft's antenna panel. The site also will tell you how bright the flare should appear (in magnitudes), and where to go to see that particular flare at its very brightest (traveling just a few kilometers can make a big difference in the flare’s brightness). Note that you can see some Iridium flares in daylight if your skies are very clear and you look in precisely the right direction!

Heavens Above tabulates the altitude and azimuth of each flare event, but you also can 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 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.

To explore the Iridium-flare phenomenon in more detail, check out the Visual Satellite Observer's Home Page, whose Iridium section provides links to orbital elements, aficionados' custom flare-predicting software, and other resources. More information — launch history; financial and political developments — on the Iridium satellite constellation (and others) is available on a Web site maintained by University of Surrey (U.K.) communications specialist Lloyd Wood.

While most flare-seekers won't need these numbers, the so-called "two line elements" that describe an Earth-circling satellite's orbit can be obtained from Thomas S. Kelso's frequently updated celestrak.com Web site.