IRIDIUM

Iridium is a system of 72 communication sats at a height of 780 km. There's something special going on with this satellites. Normally, they are as bright as magnitude +6 and therefore (almost) not visible with the naked eye. Sometimes, it happens that such a satellite brightens during several seconds, outshining by far all other celestial bodies (eventually except the Moon), to diminish after that with the same speed to its original brightness. Such flares could go to magnitude -8, as much as the half lit Moon! These flares are bright enough to be seen by day.

The satellites

What causes these flare-ups? An Iridium satellite (image left) has a height of about four meters and a triangle base of approximately one square meter. Attached on the lower half of the satellite are three diagonally planes of 1.6 m² each. These do duty as antennae. They are coated with a layer of highly reflective teflon for thermal control. The 'body' of the satellite is always pointing downwards (to the earth's surface) and the antennae are angled 40° from it. On sudden moments, we look into the Sun via this mirrorplanes, wich causes an enormous increase of brightness.

The constellation is called after the chemical element Iridium with an atomic number of 77. Primarily, it was the meaning that there should be so many satellites launched, but the number is decreased to 72. They are devided into six almost polar planes, each finally with 11 operational sats (image below), so there would at least one above the horizon everywhere on earth. The Iridium satellites get launched in small groups, the first on May 5, 1997. At this moment, all planned satellites of this type are in space, but the loss of five of the Iridiums, made two additional launches necessary. They are planned for July. The system should be operational in September of this year. The satellites are quantity-produced by Motorola and Lockheed and will serve for handheld telephone communication.

Five Iridiums don't behave like they used to be. You cannot expect predictable flares from Ir 21 and Ir 24, because their orientation is unknown. Ir 11, Ir 20 and Ir 27 are spinning and for many passes, they show a (regular ?) series short flashes, which can be as bright as the normal iridium flares. To make it yet more complicated, Ir 20 and Ir 24 are mistakenly (and hopefully temporary) indicated as resp. Ir 18 and Ir 46.

History of the observations

Until September last year, nobody knew that these satellites could be so spectacular bright. In the first half of august '97, Canadian satellite observers reported 'outbursts' from Iridiumsats till magnitude -1. Not long after that, we in Flanders also began to look after the satellites. For example, the Brugian satellite watcher Tristan Cools reported a flare of magnitude -4, as bright as Venus. Stimulated by these messages, my father Bram Dorreman and I too decided to look for these curious objects. On September 14, Bram saw a flare of magnitude -6,5 (until then the brightest one) and exactly a week after that, we both saw the first flash of magnitude -8, (see the photograph on the beginning of this page) wich was at that moment probably the brightest satellite ever seen from Belgium. The next day, an American observer did a same kind of observation.

At the beginning of October, in the US there where two satellite observers who, independently of each other, provided the first software to predict the iridium flares. (Before that, we only knew when the satellites passed, but not when they would flare). The satellites appeared to comply very accurate the predicted times. At October 12, 1997, that was proved by Bram, who saw, as the first person in the world (as far as we know), a satellite by daylight, that means Sun above the horizon. In the mean time, there are several others who saw an iridium flare at daytime, including myself. And I can assure you, it is something very special...

Back to homepage