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AstroBob: Iridium satellites keep fireworks coming -- but not forever

Time exposure of an Iridium 75 flare from 2010. The star Vega is to the right and below the trail. Bob King / rking@duluthnews.com 1 / 4
A spectacular flare from the Iridium 3 satellite occurred July 4 that was more than 15 times as brilliant as Venus. It happened at 11:39 p.m. below the bright star Arcturus at upper right under a partly cloudy sky. In this 25-second time exposure, the satellite first appears as a faint star (the narrow trail at left), then quickly flares for several seconds before fading back below naked eye visibility (narrow right half of trail). Bob King / rking@duluthnews.com2 / 4
Iridium satellite with three reflective Teflon-coated antennas. Credit: Seesat-L3 / 4
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The crackle of firecrackers and thump of cannons echoed through the neighborhood on July 4. But a whole different kind of fireworks was happening 483 miles overhead — silent but showy flares from Iridium satellites.

The Iridium extravaganza's been going on every night somewhere on Earth since the mid-1990s. A quick check of what flares happen at heavens-above.com reveals at least a couple a night for as far into the future as you might care to look.

The one I saw was probably the brightest reflection from a satellite I'd ever seen at magnitude -7.6, or more than 15 times brighter than Venus. It commanded attention. Anyone else who saw it might have dismissed the outburst as an especially bright bottle rocket in the context of the night. What I saw and what anyone can view, even from downtown Chicago in some cases, was an Iridium flare.

The satellites are equipped with antennas that reflect sunlight really, really well. When the angle between the observer, satellite and the sun is just right, an antenna can reflect sunlight back to the ground along a track like a moving spotlight. If you're standing in that track, you'll see a brilliant flare of light that looks like a supernova, only it lasts just 5 seconds or so. You can anticipate the flare because the satellite will begin to slowly brighten from invisibility about 10 seconds beforehand. After the flare, it quickly fades from view.

The brightest flares, those similar to Sirius or Venus, are incredible, almost shocking in their intensity. The Iridium "constellation" of 66 satellites with additional spares was launched in the 1990s to provide global telephone service for subscribers. Inadvertently, they've also been a constant delight to skywatchers. If you've never seen one, you'll learn how in a moment.

Now the bad news. They're all in the process of being replaced by a new constellation called IridiumNEXT. The antenna design is completely different, so the new generation doesn't provide the flares we've grown to love. Matter of fact, they're barely visible with the naked eye except under the darkest skies. Recent estimates put them around magnitude +6-7. Already, 20 have been launched and are being moved into position to replace the older units.

Seventy-five total will be in orbit sometime in mid-2018, at which point all the older satellites will be de-orbited. De-orbiting is a controlled procedure where engineers on the ground command the spacecraft to fire rockets to bring it down into the atmosphere, where it safely burns up. Since most or possibly all the older birds will be removed from orbit, that gives us only a limited amount of time to get out there and enjoy these shiny reflections.

Let's get to it.

Go to heavens-above.com, login and select your city. On the home page under Satellites, click on the Iridium flares link and you'll be taken to a page showing the times (in 24-hour time) and lots of other information about upcoming flares. If you click on the time link, a handy map pops up showing you the satellite's path across the sky with the flare position marked.

Now you know when and where to look. Returning to the flare list, you'll also see the following:

• Brightness — It's given using the magnitude scale and can be a bit counterintuitive. Just remember that the greater the negative number, the brighter the flare.

• Azimuth — The compass direction where the flare occurs.

• Satellite number — Which Iridium in the constellation is flaring

• Distance to flare center — Flares are brightest if you're standing right in the center of the reflection. Stand some distance away, and you'll catch less of the sunlight, making for a fainter display. The map at the bottom of the page shows the red centerline, where the flare would be best. If you'd like to see it, jump in your car. Kind of like driving to a total solar eclipse, only much closer to home!

• Brightness at flare center — The peak brightness for flares is magnitude —8 or about 30 times brighter than Venus.

• Sun's altitude at flare time — The numbers are negative because the sun is below the horizon.

The brightest Iridium dazzles can even be seen in the daytime sky. Check the Include Daytime Flares box to search them. Let the fireworks continue!

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