Inquinamento luminoso in Italia
Light Pollution in Italy

by Pierantonio Cinzano



Warning to all friends who deal with meteors and bolids

Rome 08/11/1997

Release 1.2 by Franco Foresta Martin


Meteors and fireballs watchers must be very careful before they officially deliver their observations. Infact a new group of communication satellites, called Iridium, is producing very bright (up to -9 mag !) and short lived flashes that could be misinterpreted as the burn ups of meteoroids in the atmosphere. This absolutely unexpected phenomenon has been repeatedly reported by amateur astronomers all over the world engaged in visual tracking of artificial satellites (SeeSat Group).

The Iridium system consists of a large number of satellites, flying in circular polar earth orbits, 780 km above the ground. Their multiple launches began in the spring of 1997. Until now almost 40 of them have been put into orbit. By the end of 1998, the Iridium constellation will consist of 66 radio-interconnected satellites, enabling cellular phone users to make and receive calls from anywhere on Earth, even in areas with incompatible cellular standards. An international consortium of 17 companies (including the Italian Stet Group) owns the Iridium network. The Motorola Satellite Communication Group, who designed the system, acts a the prime contractor of the consortium.

Since the design of the Iridium satellites is an industrial secret, the mechanism for these flashes can be only conjectural. But it would seem that these flashes are generated by the reflection of sunlight on the surface of the satellite Main Mission Antenna (MMA). A special tiny teflon coating transforms each MMA into a perfect mirror, when observed from Earth, under specific Sun angles. As a matter of fact a few amateur astronomers have succeeded in developing software that predicts the Iridium flashes; and thanks to these computer programs, many others amateur astronomers are now able to observe and photograph phenomena. The usual estimated magnitude of Iridium flashes ranges from between 0 and -5, but exceptional flashes of up to -9 have been recently reported. At the moment the rate of reported flashes is about a dozen per week. In normal conditions the brightness of an Iridium is of about +7, so binoculars are needed to spot it. An Iridium satellite is made up of a prismatic body, about 2 mts long, equipped with a couple of solar panels (not involved in the flashing phenomenon) and three MMAs. The size of each MMA is 86x188cm. It is not a large surface, as you can see, but due to its particular treatment, it is capable of generating "monster glints", as observers define the most powerful flashes.

Although spectacular, the recently observed phenomena have increased the concern of astronomers (professional and non) who fight against light pollution. The frequency of flashes is indeed increasing due to the rising number of new Iridiums put into orbit; and in the meantime the possibility that they could interfere with many astronomical observation programs is also rising. Non-official actions have already been taken by some individuals in order to obtain a revision of the satellite line production, but it is extremely unlikely that any changes will be introduced in the near future. On the other hand , the Iridium constellation will soon be completed with a lifetime of at least 5 years. Fortunately the SeeSat international group has set up a public archive on the Web which collects the observation of all flashes. This will hopefully help astronomers (professional and non) in recognising artificial Iridium flashes from natural phenomenon.


Franco Foresta Martin, Uai member, SeeSat Member.


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