J. Brit. Astron. Assoc., 108, 4, 1998, p.187-188

Iridium satellites light up the sky

by Nick James

Maurice Gavin captured two passes of Iridium satellites on 1998 May 7 in the same Cygnus starfield. Left: Iridium 10, 21.48 UT; Right: Glint from Iridium 9, 21.57 UT. 35mm f/2 camera lens + SX-M CCD, 10s exposures. M. V. Gavin.

Motorola's Iridium satellite system is the largest and most ambitious of a set of competing satellite-based mobile phone systems. Motorola's objective is to allow handheld mobiles to be used from anywhere on the planet, with the call being routed directly from handset to handset via one or several of the satellites. After a bad start when the first Delta launch failed, Iridium spacecraft have been launched up to five at a time and the system is due to go operational late this year. The system is based on a constellation of 66 identical satellites. (Its name, from the element Iridium with an atomic weight of 77, betrays the changes and losses since the design was first proposed - but understandably no-one has suggested changing it to Dysprosium!) Each satellite is in a 790km circular orbit with an inclination of 86.5. The system is designed so that, wherever you are on Earth, there will always be at least one satellite available to route your call. To achieve this the spacecraft are distributed between six equi-spaced orbital planes with each plane containing 11 equi-spaced satellites.

Even before the first satellite was launched Iridium was the subject of considerable controversy. In order to communicate with small handheld mobiles each spacecraft has to transmit a powerful radio signal and the frequency of that signal is very near to the 1612 MHz resonance of the hydroxyl molecule. In theory this band is reserved for radio astronomers, but since the Iridium wavelength is nearby it has been impossible to prevent interference. Some agreements are now in place restricting Iridium transmissions but the situation is not very satisfactory from a radio astronomy point of view.

While radio astronomers knew that Iridium was bad news most optical astronomers were unaware that the satellites were a potential source of optical image interference. In order to transmit its radio signal each satellite has three Main Mission Antennae (MMAs) spaced at 120 intervals around the base of the spacecraft. The radio antennae on most spacecraft are the familiar parabolic dishes that we associate with satellite TV. Iridium uses a much newer technology and its antennae are flat. For thermal reasons the MMAs are highly reflective and at optical wavelengths they are effectively 1.6m2 plane mirrors. If the viewing geometry is right these mirrors will reflect sunlight to an observer on the Earth and a bright glint will be seen. Effectively we are seeing a reflection of a small part of the surface of the Sun against the night sky. Given that we know the size of the MMA it is simple to calculate how bright the specular reflection could be. The Sun is half a degree across and shines at magnitude -27.This corresponds to a mean surface brightness of -11m per square arcsecond. A 1.6m2 mirror at range of 790km has an apparent area of just under 0.2 square arcsec. The brightest glint assuming a perfect specular reflection at normal incidence with the spacecraft overhead is thus around -9m. In practice most glints are considerably fainter than this, but they can still be spectacular. In a typical pass the satellite will first be seen as a faint fast-moving object. A bright glint lasts for only a few seconds but, during that time, the satellite can become many times brighter than Venus.

The calculation above also allows us to make another startling prediction. It should be possible to see a glint from Iridium caused by moonlight. The full Moon is about 15 magnitudes fainter than the Sun, so the glint would be about magnitude +6 under optimum conditions. Such glints are very rare since the Iridium spacecraft and the observer must be in darkness and the Moon should be near full. Incredibly an American observer has already reported a glint from an Iridium spacecraft and this was near to the predicted magnitude. This makes it only the second reported visual observation of a satellite by moonlight (the other was a very difficult observation of the Mir space station). In principle I feel that, as astronomers, we should object to the optical interference caused by Iridium. In practice however the glints can be predicted accurately and they are interesting to watch. It is the radio astronomers who really suffer since they have to contend with Iridium's continual radio interference and in the scale of commercial priorities, radio astronomy comes some way below the ability to make a phone call from the north pole!

Very accurate glint predictions for your particular location can be obtained from the Web site maintained by the German space agency DLR at
http://www.gsoc.dlr.de/satvis. (This is the correct URL - it was wrongly printed in the August Journal.) Be sure to enter your position accurately. The magnitude of the glint can change dramatically over only a few kilometres.

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