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Space mirrors
An Iridium communication satellite's Main Mission Antenna is a silver-coated Teflon antenna array that mimics near-perfect mirrors and are angled at 40-degrees away from the axis of the body of the satellites. This can provide a specular (direct) reflection of the Sun 's disk, periodically causing a dazzling glint of reflected sunlight from their 492 mile-high orbits.
At the Earth's surface, the specular reflection is probably less than 50-miles wide so each flare can only be viewed from a fairly small area.
Iridium satellites normally traverse the sky on the edge of visibility, at +6 magnitude. On this astronomer's scale, smaller numbers represent brighter objects. Venus outshines all stars and planets at magnitude -4.
Iridium satellites can provide reflective flares of magnitude -9. That's up to 100 times brighter than Venus, based on how the brightness scale works. The flares can last anywhere from 5 to 20 seconds before the satellite once again becomes almost invisible to the naked eye.
In fact, it is even possible to see such flares during the daytime, if you know exactly where to look.
Where to look
If you wish to see such brief flares for yourself, you will first have to know your exact latitude, longitude and local time zone. Then, log on to a web page that will tell you when the next Iridium flares can be seen. One such site is Heavens Above, hosted by the German Aerospace Center (also known as DLR).
This site and others also provide viewing information for the International Space Station, the Hubble Space Telescope and other satellite.
A bit of caution: Not all Iridiums flare according to the predicted schedules. Some of the Iridium satellites are either tumbling or otherwise not operational so their future movements cannot be reliably predicted. A fully operational satellite should be orbiting the Earth at 14.34 revolutions per day.
A couple of years ago while out observing with the university astronomy club I happened to be looking up near the zenith and saw a moving point of light which increased from barely visible to at least -8 magnitude and then faded out within a few seconds. I figured it must have been one of the no-longer-operational satellites because none of the sites such as "Heavens Above" listed any flares as scheduled to be visible at those geographical coordinates any time that night. No one else happened to be looking at that part of the sky at that particular moment, so mine was the only observation, as it was of another fainter-than-naked-eye satellite which happened to cross the field when I was peering through a 10x60mm finder looking for something to show the group through the main scope . . .
Getting Crowded Up There Maru
-- Ronn! :)
Ronn Blankenship Instructor of Astronomy/Planetary Science University of Montevallo Montevallo, AL
Disclaimer: Unless specifically stated otherwise, any opinions contained herein are the personal opinions of the author and do not represent the official position of the University of Montevallo.
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