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I just saw the term "Double Iridium Flare" and I'm wondering how close two Iridium satellites might actually be (in distance (km) and angle). I found this video as an example.

Iridium 94 (00:05) and Iridium 23 (00:51)

Since they are flying in a spread-out formation for more uniform coverage, why would they become so close?

uhoh
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Iridium satellite constellation consists of 72 satellites, 6 of those on-orbit spares, and since May 1997 to June 2002, in total of 95 Iridium satellites were launched to orbit. Wikipedia currently lists 17 defunct Iridium satellites. Two of them decayed, Iridium 33 collided with Kosmos 2251, 6 in uncontrolled orbit, one presumed partially operational, and additional 6 in their nominal orbit. That list seems a bit outdated though, and searching through NORAD catalog, at the time of writing this answer, I managed to find only 70 Iridium satellites that aren't tracked as debris and haven't yet decayed.

All that means that not all of their orbits will have identical orbital period, i.e. some will be in slightly higher orbits than others. Operator might have also moved some of the on-orbit spares to closely matching (trailing or leading) orbits to some of by now defunct satellites to ensure global coverage. So the Iridium satellites, defunct on-orbit ones and active ones, can appear to converge over a small area of the sky as observed from a location on Earth, and would produce a flare at a specific angle to the observer at nearly the same time, due to same or similar orbital geometry, small angular separation as observed from the ground, and reflecting light from the same distant source.

How close can they get? Well, theoretically, angular separation between two Iridium satellites as observed from the ground can be 0. So, really close.

Mind that not only Iridium satellites produce satellite flares, and there's a lot of junk up there, from tumbling upper stages, to other defunct satellites, many of them in similar orbital inclinations, so chances that two or more of such object produce a flare close to each other increases. For example, just at this exact moment, there's this many object in orbit over the area I live at and directly overhead (there's many more at shallower angle to the horizon):

                     enter image description here

  Objects in orbit currently appearing directly overhead to an observer in central Europe. Made with SatFlare

TildalWave
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    I really like the graphic - it helps to see the physical reality of stuff "right above our heads" at any given moment. Aren't the really bright Iridium flares reflections from the solar panels, and quite directional and localized on the ground? If two satellites in (nearly) the same point in the sky both produce strong flares for a fixed observer, does it suggest that at least the attitude control is still working on both of them? – uhoh Feb 15 '16 at 14:23
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Heavens above told me there would be an Iridium flare tonight. It would be a bit to the left of Arcturus.

I found a bright point at the approximate location : Okay, that must be Arcturus. And here, to the left, that must be the Iridium satellite. Wait, why are both moving, in the same direction and the exact same speed? Wow, they're both flaring at the same location, a few seconds apart!

I didn't notice it at first, but Heavens above sent 2 notifications :

enter image description here

They flared at the exact same location as seen from Earth, and only 7s apart. Both were visible for much longer than 7s and could be seen moving parallel to one another.

Iridium 6 and 51 seem to have very similar orbits

Here's another video showing them flaring at almost the same time.

SkySafari indicates there were 54km apart from each other during the flares, with a ground speed of approximately 7.5km/s.

Eric Duminil
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  • Heavens Above is a great site. We use it for ISS passes since they last a bit and are usually bright enough to see this close to L.A. And, darn it, I didn't know that SkySafari was already in use. Now everydayspacer.com may have to come up with a different name for its event. – ShadoCat Apr 07 '17 at 23:30
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    That must have been thrilling! It's good to have a first hand report here, but wow what a bonus to see it without expecting it. – uhoh Apr 08 '17 at 01:42
  • @ShadoCat: If you can see ISS passes, you'll surely be able to see the brightest Iridium flares. They're much brighter than ISS passes, albeit much shorter. I must say I love SkySafari 5 Plus on my Android phone. It's a mix of Stellarium/HeavensAbove/Wikipedia/Celestia. You can see the point of view from inside Iridium 51, track Iridium 6 and fly around Earth. It's pretty expensive for an App, but it's worth it if you have a small telescope. The normal version is much cheaper, but you cannot change POV to orbits. – Eric Duminil Apr 08 '17 at 09:44
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    @uhoh : Indeed, I laughed like a kid looking at the sky. Last time, it was the first time I saw a very sharp moon transit on Jupiter. It was so sharp in comparison to my usual views, I thought someone had drawn a point with a black marker on my eyepiece. When I noticed it was real, I laughed out loud in excitement. Finally, seeing the whole structure of ISS in my telescope and being able to track it manually for a few seconds probably was the best moment with my telescope. – Eric Duminil Apr 08 '17 at 09:49
  • Sounds like a nice telescope, mount, and drive! For fun you can try this next. I'm looking at these two in detail using Skyfield, will wait for the next TLE update, then post a plot for fun. – uhoh Apr 08 '17 at 15:21
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    @uhoh: Nope, that was with a small dobsonian (Orion XT4.5). :) With a bit of training, it becomes quite easy to track the ISS inside the eyepiece (82° at 6.7mm helped with a big AFOV). It's easier with a friend looking through the finder scope, to find the ISS again if it ever goes out of the eyepiece view. – Eric Duminil Apr 08 '17 at 15:34
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    @EricDuminil, I know Iridium flairs are brighter but the ISS pass is a much better "event." We are often out there long enough to point it out to neighbors to hopefully spark some interest. The are also good at star parties because of their duration. – ShadoCat Apr 08 '17 at 17:04
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Aren't you lucky! I was chasing them yesterday, but thick clouds ...

It's not a coincidence that they flared together. Iridium 6 and 51 were paired several years ago, each of them supposedly providing part of the functionality, with no. 51 officially being labeled a spare satellite. They travel at the same altitude in nearly identical orbits. They were as close as 6 seconds apart last summer (when I saw two "massive" double flares), then moved apart to app. 20 seconds and have been moved closer up again recently.

eerie
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    I had no idea that flare-watching had evolved to such a level of sophistication! OK I'm going to start watching the predictions and see if I can get in on the fun as well. Thanks! – uhoh Apr 20 '17 at 01:11
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    You better do so! It is a lot of fun and with the old satellites now being replaced by the IridiumNEXT sats that won't flare, the fun-promising days are unfortunately numbered. There are several smartphone apps that give you all the information you need - that's the easiest way to get started. And apart from Heavens Above there's CalSky and SatFlare – eerie Apr 20 '17 at 12:59
  • Yikes! I've just asked this follow-up question. Thanks for the advice! – uhoh Apr 20 '17 at 13:07