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Sky Watching: Iridium Flares + More

Saturday, October 22, 2011


When satellites light up: look for iridium flares

Do you remember the night skies of the mid to late 1950s when, except for the “occasional” airplane, the only objects moving among the constellations were shooting stars? If you do remember those nights, you’re just a little older than I. Suddenly the skies above the Earth weren’t so empty anymore.  

Satellite watching

I don’t remember the remarkable achievement of Sputnik I on October 4, 1957, or the panic it thrust upon the world. A new age of mankind, the Space Age, dawned. Every 96 minutes this tiny sphere would circle the globe, sometimes passing directly over the United States. The implications were alarming. Our security was threatened. The same missile technology that could launch a scientific package could also conduct surveillance missions. 

Our first attempt to orbit an artificial satellite came to a fiery end on December 5, 1957, when Vanguard exploded as it just barely rose off its launch pad. We finally got a payload into orbit on January 31, 1958, when Explorer I lifted successfully from Cape Canaveral. 

Though Sputnik did pass over this area of the United States, I do not recall ever seeing it as a young child. However, we soon began launching many communication satellites into orbit. One of the first memories I have of looking intently at the sky was one night in the Shawomet section of Warwick just off West Shore Road. (The sky was still dark in those days.) The newspaper would publish the time of passage for Echo I. It was the late summer, early fall of 1960. 

Every 114 minutes Echo I, a balloon 100 feet in diameter located 1,000 miles high in orbit, would circle the earth. During late twilight you could spot the balloon from the ground because, at that altitude, it was still bathed in sunlight. Echo I would look like a bright tiny star moving against the background of stars. Soon it would blink out as it moved into the Earth’s shadow. Other satellites would follow in the years to come, and likewise the newspapers would print a timetable of their passage over our region. 

Soon there were so many satellites in orbit that on any clear night one could see a dozen or more. My record 32 years ago was 17 satellites in one night. I’m sure someone could surpass that feat these days from a dark sky location. But I’m often amazed that folks have somehow forgotten this fact. On nights of favorable passes of the International Space Station (ISS) over Seagrave Observatory or Ladd Observatory we usually stop our telescopic observing to watch it traverse the sky. (Hey, if the ISS can be seen from Ladd Observatory on Providence’s East Side, then you shouldn’t have any difficulty viewing it from elsewhere in Rhode Island.) 

Many of the visitors did not know they could see this large and reflective orbiting laboratory when it made favorable passes over Rhode Island. And when the Space Shuttle was still flying, not only could you see it as a separate spacecraft, but also, when it docked with the ISS, the combined structure appeared even brighter. 

Basically, though, observing these modern earth orbiters is no different than when I first spotted Echo I, 51 years ago. Though the local newspapers do not publish favorable passes of the ISS today, there is a Web resource where you can determine when it will be visible from your location. More about that resource shortly.

Iridium flares

But there is an added dimension to satellite watching that can excite even a casual sky gazer. These events are known as Iridium flares. 

Iridium satellites are low-earth orbit communications satellites that were designed to be the “cell phone” technology of its day (1997). The service was too expensive for the general public and the technology quickly changed, so the business venture went bankrupt. The satellites were purchased by private investors for cents on the dollar and were spared a fiery re-entry into Earth’s atmosphere. 

Here’s what happens. Each Iridium has three main mission antennae, which are highly reflective. The orbit of each satellite and its orientation to the earth are precisely known. Therefore, for any location on the ground, it is possible to calculate the angle between the satellite, the sun and an observer. When you get a perfect reflection of the sun off one of the antennae, the observer gets flashed by the satellite. 

The brightness of the flash or “flare” depends on how perfect that angle is. From one location the flash may appear 30 times brighter than Venus, while at a location just 27 miles away the same satellite may flash no brighter than Saturn. And farther away from that perfect reflection location the flare won’t be visible at all. As always, it’s location, location! 

It’s incredible when you get flashed! You can usually spot the satellite just a few seconds before a flare occurs. The satellite will first appear dim, and then all, of a sudden, it dramatically increases in brightness, only to fade just as quickly. It’s exciting to watch and makes the experience a little more interactive. 

How can you know when an Iridium flare will be visible from your (or any) location? Visit the Web site Heavens Above where you can input your country and city, town, village, state, or GPS coordinates to calculate satellite viewing opportunities over your airspace. Once that is done, you can select the satellite you want to observe from a menu. That selection also includes the Space Station with predictions for ten days. You can ask for Iridium predictions for the next 24 hours, as well as for the next seven days.

The predictions are presented in a tabular chart. Key terms in blue are actually clickable links to definitions that will help you to determine where to look in the sky, as well as how bright the flare will be.

So keep informed about what is happening in the sky, and you will certainly see evidence of humankind’s presence in low Earth orbit. Check an entire week’s worth of predictions and plan to observer the brightest ones that will appear highest in the sky. 

More details are provided at the Heavens Above Web site.

Keep your eyes to the skies.


Dave Huestis has been an amateur astronomer for almost 40 years. He enjoys educating the public about what can be seen in the sky. Dave is also the historian for Skyscrapers, Inc., The Amateur Astronomical Society of Rhode Island.


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