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Date: July 14, 2011

Title: Iridium Flares

Podcaster: Kyle Kneisl

Description: Many people do not realize that the brightest objects regularly in the sky besides the sun and moon are not the planets, but rather, but the reflections of the sun off of satellites orbiting the Earth. Some of these can be spectacularly bright. In this episode, we discuss the most important class of “flaring” satellite: why they are up there, exactly how they flare, and how you can observe the flares almost every night with the equipment you already have.

Bio: Dr. Kneisl grew up in Buffalo, New York, and has had a lifelong interest in astronomy that really began in earnest when he stumbled upon his first book about black holes when he was seven years old. He is still an avid amateur astronomer and sky watcher. He holds a Ph.D. in mathematics from the University of North Carolina and lives in the Washington, DC area, where he hopes to introduce his two young children to astronomy in the coming years.

Sponsor: This episode of “365 Days of Astronomy” is sponsored by midnightmartian.com: 3D apps for the iPhone, iPad and iPod Touch.

Transcript:

Few people seem to know that besides the light of the sun and the moon, by far the brightest things we see nightly in the sky are not stars and planets, but rather the reflections of our sun off of man-made satellites. These reflections–called “flares” due to their sudden brilliance and just as sudden disappearance–can be very dramatic! When we add all of the light from the usual visible stars together, the resulting brightness is known as the “total integrated magnitude” of the night sky. Several times per week there are satellite flares that are at least five times brighter than this number; there are even times when such flares can be as much as fifteen times brighter than this number and may even be easily visible during the day. While any satellite may flare to some extent, there is one type of satellite that, due to its design and sheer numbers, are responsible for almost all of the bright flares in the sky. In this episode, we will discuss exactly why these satellites are up there, how and why they flare, and how you can empower yourself to see the flares with equipment you already have: your eyes and the internet.

Groups of related satellites are often termed “constellations.” One of the largest satellite constellations is the one owned by the company responsible for the global Iridium Satellite telephone network. Over sixty such satellites–each about the size and weight of a small automobile–along with some spare satellites in case any of these break–are in orbits at heights between about 400 and 500 miles, that is, about 800 kilometers above the Earth’s surface. Orbiting at close to 9000 meters per second, these satellites complete their intricate dance around the Earth approximately every hour and a half. The large number of satellites in orbit, along with carefully chosen paths and interconnections, assure that a person at any point on the Earth (including at the poles) is covered by the Iridium Satellite Network,
365 days per year, 24 hours per day (as long as there is a clear shot to the sky).

Each satellite can, under the right conditions, carry over 1000 telephone conversations. At many locations on the Earth, such as in the middle of great deserts or deep within uninhabited regions or near the poles, these can be the only reliable means to access the telephone network. For those that can pay the very hefty per-minute price for this network, it is a miracle of modern satellite communications.

But for us as astronomers, we are more interested in their effect upon the night sky. If you know precisely where, and when to look, the natural reflectivities of these satellites when the sun strikes their surfaces produces a light source visible around a modest +6.0 magnitude; that is, just barely visible to the naked eye from a clear location and you’d probably need binoculars! However, what makes Iridium satellites unique are their multiple Main Missions Antennas (MMA’s), each of which is roughly the size of an adult man. The flat surfaces of these antennas are extremely polished aluminum, which has been sprayed with silver and Teflon, making them like a mirror. When conditions are just right, the MMA’s, which are mounted at an angle of forty degrees to the satellite body, can reflect the ways of the Sun in a very focused spotlight that is visible from the Earth. Most often, just the right geometry is only achieved two or three hours after sunset, or, two or three hours before sunrise. Each flare is only visible to those located in a relatively narrow band on the Earth’s surface, only a few dozen kilometers wide, and only when the geometry is exactly correct to catch the rays of the Sun in full face.

Fortunately, the large numbers of Iridium satellites in orbit mean that one or more Iridium flares are visible at almost every location on Earth, almost every night, and that very bright flares are visible at least once per week. As I write this, there will be no less than eight Iridium flares at my location over the next three nights alone.
Satellites 75, 81, 94, 63, 14, 18, 57 and 39 will all flare. Two of these will be exceptionally bright–much brighter than anything else in the sky at the time. Last night, in just the short span of thirty minutes, we had three very bright Iridium flares.

You might wonder if the sudden appearance of quickly moving very bright lights in the night sky has generated all sorts of unusual interpretations. And they certainly have! It is not unusual, for example, for earnestly reported UFO sightings to be traced back to Iridium flares. On the extreme end, there are even some websites that claim that the Iridium satellite constellation is part of a vast mind control conspiracy, and that flares somehow are involved in the individual controlling of minds.

How can you predict and see the flares? As it happens, each satellite is programmed to maintain a particular “attitude”, which, as long as it is still operational, make it possible to predict with very, very good accuracy where and when Iridium flares will be visible. Anyone can get up-to-date information from any number of free websites for their particular latitude and longitude. A quick search on your favorite search engine will surely give you several choices. Most websites provide information not only on Iridium flares, but also on the visibility of any satellite, including the International Space Station, regardless of whether or not it even flares brightly at all. In order to make successful use of these services, you will need to know your latitude and longitude with good accuracy and you will need to know exactly what part of the sky is meant when azimuth and elevation are provided. The information these sites provide is accurate to within seconds.

Even more conveniently, if you have an Android-based smartphone, an app called “Heavens Above” is free to download and use from the Android Market. This program interfaces with your phone’s GPS for a very precise coordinate fix, and allows you to filter for satellites that will only be certain brightnesses or above, generate alerts as an event approaches, and even will interface with your phone’s built-in compass to point to you exactly where, and when, to look.

With all of those satellites in orbit at the same height, you might wonder, has an Iridium satellite ever had a collision with another satellite? In fact, on February 10, 2009, the Russian satellite Kosmos
2251 and Iridium 33 collided over Siberia and destroyed each other.
What’s more, the collision was never predicted, with most estimates having the satellites miss each other by several hundred meters. The US Space Surveillance Network has cataloged as many as two thousand pieces of dangerous debris from this collision, most of which are larger than the size of a baseball according to NASA. All of these are well capable of damaging or even destroying other satellites.

In any case, Iridium flares are easy to follow and observe, and with any luck, you can see your first bright flare in the day or two after you hear this podcast. You may want to search for videos online; there are several videos where people have captured these satellite flares, and these will help you to know what to expect. Dedicated Iridium fans may someday see an unpredicted or unpredictable flare. As satellites in the Iridium constellation go out of service, their attitude is no longer maintained, making it impossible to track or predict any flares. If you see one of these unpredictable flares, there is no way to really confirm what you saw. Maybe, just maybe, it was a UFO!

End of podcast:

365 Days of Astronomy
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