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  • Local site that tracks iridium flares over the Washington area.
  •   Interview: Iridium Flares

    The Post's Mike Mills conducted an e-mail interview with Chris Peat of Anite Systems, the British software firm contracted to maintain the Web site for the German Space Operations Centre (GSOC). Here is the unedited transcript from that conversation.

    Q. What do the flares look like? How long do they last?

    A.The flares only last for a few (10 to 30) seconds, and the maximum intensity period is even shorter. They are point sources of light and move slowly but perceptibly across the sky. The brightest flares can reach magnitude -8, which is about 50 times brighter than Venus (Mag. -4), the brightest star-like object in the sky. The flare rises gradually in intensity to its maximum, then fades until it is no longer visible to the naked eye (the satellites themselves are only magnitude +6 under normal lighting conditions, and so can only be seen with the aid of binoculars). Although the flare is not as intense as the full Moon (Mag. -12), they can often seem to outshine it because they are point, rather than extended sources.

    Q. Who first noticed them? Has the stargazing community been talking a lot about them?

    A. According to the S. & T. article, the first person to spot them was Brian K. Hunter, a chemistry professor at Queen's University, Ontario, Canada, on August 16th, 1997. The stargazing, or rather amateur satellite watching community has been very actively discussing and observing these flares, and it is safe to say that Iridium flares are the current "hot topic" for these people. Anyone who has seen a magnitude -7 flare cannot fail to be impressed, and it would be easy to think you had seen something else, such as a UFO, if you didn't know what you were looking at.

    Q. Is anyone upset by light pollution, i.e. radioastronomers?

    A. There are two different things here - light pollution for optical astronomers, and radio interference for radio astronomers. The latter is much more serious, although I heard the the people operating the Hubble Space Telescope are taking them seriously and investigating whether a flare presents a significant threat to the telescope's sensitve optical detectors. The side lobes of a radio telescope have a much greater field of view than an optical telescope, and so there will very likely be one or more Iridium satellites within it. The Federal Communications Commission (FCC), has allocated the frequency band 1621.35 to 1626.5 MHz to the Iridium satellites, which will use this frequency to beam signals to handsets on U.S. soil. This is close to the 1612 MHz spectral band emitted by hydroxyl radicals, which is very important to radio astronomers. Motorola Satellite Communications has conceded that the satellites will generate at least some interference in the protected hydroxyl band. See the S&T article for more on this.

    Q. How many hits have you gotten on the Iridium site? From around the world?

    A. The last few days we have been averaging 7000 or so page loads per day. You can see the statistics for yourself on-line under the URL;

    The site has been running since December 1997, and the hit count has been steadily rising since then, with a jump when the Iridium flare article appeared in the May issue of "Sky and Telescope". There is also an interesting map showing the geographical distribution of visitors to our site under;

    Each circle on the world map corresponds to a single location, and the area is proportional to the number of visits from that location. This map is only possible on a site like ours, where visitors must specify their coordinates.

    Normally, geographic information is not available because it is not possible to translate an Internet address into a geographic location.

    On this note, it would be good if you informed your readers that the intensity of the flares is very sensitive to the observer's location, and even a 10km difference in position on the ground can change the intensity by several visual magnitudes. Also, all points west of Greenwich (all of the USA and Canada) have negative, rather than positive longitudes. Many people are still entering positive longitudes which put them in central Asia (as you can see from our map), and then wonder why the predictions are wrong.

    In the next couple of weeks, I will be adding a new page which gives more details for a particular flare, and shows the user a map with their own location and the track of the point of maximum intensity along the ground. Many people have asked for this feature, and it seems some are willing to drive 20 miles or so to get the best view! The map will show them how far to go, and in which direction.

    © Copyright 1998 The Washington Post Company

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