By Dave Furry
Yet another aspect of astronomy that I have recently become interested in is observing “Iridium flares.” What are they? Essentially, they are (typically) the very bright lightening of a certain class of satellite as sunlight reflects off their main antennas. What fascinates me the most about them is that these flares are predictable for a specific area limited to your particular viewing area – higher math at work, no doubt. As explained below and contrary to what I (and you?) originally thought, the term “iridium” has nothing to do with exotic metals causing the flares.
Starting in 1998 the Iridium Corporation began launching a series of communications satellites in a low-earth orbit of about 800 kilometers (about 500 miles), and inclined at a steep angle which makes them pass nearly over the poles. Although many more were launched, Iridium LLC (the current owner) operates more than 60 Iridium satellites currently in orbit.
Iridium satellites possess three highly-reflective, door-sized “main mission” antenna arrays (three antennas per array, spaced 120 degrees apart). Because they are controlled satellites, an antenna occasionally reflects sunlight down to Earth in a predictable way. The illuminated spot on the Earth’s surface is about 10 kilometers (just over 6 miles) in diameter. If you’re looking up at the right time, you can see a bright flash (flare) for a few seconds.
HOW BRIGHT ARE THEY?
I have noticed that most Iridium flares are in the brightness range of “normal” stars (less than about first magnitude – see below), but they can get much brighter than Venus at its brightest! Some sources even report seeing them in the daytime.
The flares occur rather frequently (two to four times each night), although flares of reasonable brightness (around Magnitude -5) occur only three to four times per week on the average. A brilliant flare (about Magnitude -8) may be seen on the order of three to five times per month.
WHAT DO THEY LOOK LIKE?
The flares start out looking exactly like what they are – rather dim satellites tracking along the sky. Suddenly, however, they become very bright for a few seconds before becoming dim again and “disappearing” back to their normal brightness (about Magnitude 6, equivalent to a star on the edge of naked-eye visibility under very dark skies). Sometimes I have seen the same satellite flare, dim, and then brighten a second time before disappearing.
There are YouTube videos on the web (for example, https://www.youtube.com/watch?v=-XE3LuD5htM) that show flares, but to me these pale (pun intended) to the actual experience of seeing one first hand. Try it!
O.K., HOW DO I KNOW WHEN AND WHERE TO LOOK?
There is a web site called “Heavens Above” (http://www.heavens-above.com/ – don’t forget the hyphen!) that lists upcoming flares as well as a host of other astronomical information. Personally, I use an app on my tablet and phone (both are androids) called “ISS Detector” (you can also see predicted apparitions for the International Space Station). I’m sure there is something out there for Apple users, but you’re on your own. Also, I publish the applicable data for brighter flares on my weekly blog for the Pueblo, Colorado area.
Both Heavens Above and ISS Detector predict flares for several days in advance, and give you the time, direction (azimuth) and height (altitude) for the flare, as well as the predicted brightness. Your location should be automatically loaded from your device, but if not you can always input it.
These sites list the “magnitude” of the flare: if you’re not familiar with this term, it is a measure of the relative brightness of the flare as well as other celestial objects. It was set up as a logarithmic scale so each whole-number change in magnitude is about a 2-1/2 times change in brightness (brighter or dimmer). Somewhat confusingly, the more negative a magnitude the brighter the object; conversely the more positive a magnitude the dimmer the object. Here is a relative scale of the magnitude of objects for help in determining how bright you may expect a particular Iridium flare to be (on the right-hand side it also shows the magnitude limit of various sizes of telescopes).