11/21/2005: Iridium flare

This was the Earth Sci Pic of the Day recently, but I have it in a different crop. Anyway, what if you looked up at the sky one evening and saw this? It would freak you completely balls out, would it not?

And yet there's a simple explanation. These are flares from two different Iridium satellites. Quoth the EpoD,

Iridium flares are brief but often bright reflections of sunlight off of low-Earth satellites. The Iridium satellites span a world-wide mobile phone network. Originally it was intended that a fleet of 77 satellites would encircle the globe -- 77 is the atomic number of the iridium. The idea being that if the Earth is the nucleus and the satellites are the electrons, the entire system would take on the appearance of an iridium atom. The reflected sunlight from one of these satellites' main antennas or solar panels can attain a magnitude of -9. This is approximately as bright as the half illuminated Moon and nearly 100 times brighter than Venus when it's at its brightest!

But why do they look like long streaks and not bright flashes?
Much better than the EPoD picture, UT. Nice job.

The way the shot is exposed almost makes it seem like a time-lapse photo. I dunno...

The stars are streaks so i must be at least a short time lapse.
Maybe the satillites are moving up or down.

Time exposure in much lower light than it would appear to be in the final photo.

I've spent a lot of time outside looking for predicted Iridium flashes, but haven't managed to see one yet.

One can see the even brighter International Space Station per a schedule for most world cities here: ISS

I've looked for and seen a few iridium flashes. It's very easy. It's been a while since I've done it, but if you go to http://www.heavens-above.com/ they can tell you when satellites will pass over your co-ordinates.

Anyway, the flashes look a lot like airplanes coming in for a final approach with their "headlights" on. Of course they look slightly different because they travel in a straight line. They start out dim, and get brighter as they move across the sky, then they get dim again. It takes about 60 seconds or so for them to travel across the sky if they are directly overhead. They are only visible around dusk and dawn, because of their low orbit.

I think this picture captured two different irridium flashes at the satellites took turns going by, several minutes apart. Iridium satellites don't travel right next to each other, as far as I know. But they do travel in predictable orbits, and I can imagine one being a few minutes behind another in a similar orbit. This is either a double exposure, or a time lapse exposure, or a combination of the two.

Now that I think about it a little bit, I am pretty sure this is a couple minute long time lapse photo. The streak of the stars proves it's a time lapse photo, and iridium flashes are usually only about ten seconds or so long, even if it takes a full minute for them to cross the entire sky. They are very predicitble, so the photographer wouldn't need to take a several minute long exposure to catch one. It's not like shooting lightning, where you take a long exposure in the hopes of getting something on film. You know when and where the flare will be. A 20 second exposure should be enough to capture a single flare. A longer exposure would be needed to catch both flares. The trickiest part is having a syncronized watch to know when to open the shutter.

Based on the length of the star streaks, I would think that the exposure is only something on the order of 20-30 seconds. Even a full minute exposure should have longer streaks than are shown in that picture. Not positive, but I think so.

I'd guess 30 seconds, partly because that's the longest on a lot of SLRs without going into full bulb mode, particularly DSLRs.

Interesting point. I think it depends on where the stars are. Stars near the North Pole (Polaris) will move slower and streak less than stars near the celestial equator. The stars in the picture appear to be moving parallel to the horizon, which means they are to the north. If they were to the east or west, they would be moving perpendicular to the horizon. Stars to the north will be moving more slowly.

There isn't enough information in the picture to easily determine the length of the exposure. You could measure the radius of the star streaks and then the length of each star streak to figure out the exact length of exposure, but the radius is so large (the streaks appear to be straight lines) that the error would be huge.

Yes, stars near the pole will have shorter streaks than those near the ecliptic. But those
"polar" streaks will also have a higher curvature than "ecliptic" ones. Thus, with the combination of length and curvature, the exposure may be estimated. The lines in the picture above seem to have little or no curvature, suggesting that they are far from the pole.

I'm such a dork. I actually tried to find the exposure time in the metadata in the original jpg. Couldn't find any.

Perhaps this was a rare coincidental double flash occurance? I mean, it *is* a single time exposure, as evidenced by the lack of a break in the star trails.

Because inquiring minds want to know.

Iridium satellites can be seen in pairs but it is rare.

Most Iridium satellites have similar orbital parameters so they are usually well separated. The Iridium satellites are groups of satellites spaced around the same orbit. A few Iridium satellites have orbits at a slightly different altitude. There used to be spare satellites in orbit at a different altitude, so if one satellite died there was a ready replacement already in orbit. Now I think that's no longer the case, but there are a few Iridium satellites that have different orbital periods to the rest. In either case, what happens is the lower satellite orbits faster than the higher one. Every now and again two satellites will pass each other. If you are lucky enough to be in the right spot it is possible to see twin flares.

I witnessed a twin Iridium flare about 3 or 4 years ago, after it was predicted on heavens-above.com. Two very bright satellites visible simultaneously is really an amazing sight.