Pictures of Iridium Flares

My first flare image! I wasn't sure what exposure to use so tried 15sec at 200ASA: I thought that a lower ASA might not catch the flare and a longer exposure would be affected by the Moon (the bright patch to the top of the image). As it turned out, this was about right photographically but the exposure was too short to catch all the flare - note that it starts somewhat abruptly (top left), indicating that it was underway before the shutter opened. The fade-out at the end was captured in full though. This is Iridium 31.

These flares were just 4mins apart, in almost the same place in the sky. I kept the camera still between the exposures but of course the stars moved, as can be seen in the composite image to the right. Multiple flares in one evening (or morning) are not unusual - there are eleven satellites in each of six orbital planes so it is likely that more than one will become correctly orientated - but they are not often in the same direction because the Sun-satellite-observer geometry changes as the Earth turns thus shifting the angle of maximum reflection. Read on for why these two are an exception! The two satellites here are Iridiums 29 & 95, with the first flare supposedly brighter than the second - seems the extensive high cloud interfered with this though.

Based on my experience with the first flare, the exposure for these two was 30sec at 100ASA. This has clearly caught the entire duration of each without the images being fogged too much by the cloud.

If the flares above were close in time, this pair is exceptionally so. In fact they were near-simultaneous - just 6sec apart! This image is thus a single exposure, not a composite: 30sec at 100ASA again. Cloud, and the fact that they happened at 6:30pm so the sky was still quite bright, meant the flares were not so brilliant but I was very pleased to capture this most unusual double event, involving Iridiums 96 & 28.

The double flares above both involved a "primary" satellite and an "in-orbit spare" from the same orbital plane. Although the spares (nos. 95 & 96 in this case) are in the same orbital plane as the 11 primary satellites they orbit somewhat closer to the Earth. This means they go round the Earth a little faster so will occasionally be in a position to produce a flare at nearly the same time as a primary satellite. The two are never physically close of course, simply in the same line-of-sight.

At last - a cloud-free evening! This image (of Iridium 5) shows what a difference cloud makes - despite being inherently less bright than some of the above flares, the absence of cloud makes this one far more impressive. In fact, when the "flare" part of the pass was over I could still see the satellite moving among the stars quite easily. The duration of the flare must have been in excess of 30secs (the length of the exposure) as close examination of the image shows it was just present when the shutter opened and had not entirely faded when it closed.

Despite some thin high cloud, this image has several unusual features. Firstly, note that the flare itself (of Iridium 53) is asymmetric i.e. it fades (to the right) much more quickly than it builds (to the left). This is because the satellite went into eclipse before the flare itself was over. Then there are several stars of interest. The bright one below the left end of the flare is called Cor Caroli [Charles' Heart in Latin], named by Edmund Halley, of comet fame, in honour of either King Charles I of Great Britain, who was executed following the Civil War, or his son Charles II in recognition of the restoration of the monarchy.

The star in almost the same position above the flare is called Chara, which is very similar to our Sun and was named in 2006 as the star most likely to have a planet with extraterrestrial life! To its left and up a bit we find the extravagantly-named La Superba, one of the reddest (and thus coolest) stars in the sky. To see its colour, move the mouse pointer close to it. Amazing what you can find in an apparently unimportant piece of sky!

It is obvious that, despite the orbits of all the Iridium satellites being at the same angle to the equator (86.4deg i.e. very nearly polar), the flares are not in the same orientation. This is because the observed orientation depends on where the satellite is relative to the observer, in terms of both altitude and compass direction. The geometry involved is quite straightforward but I've included it as part of my article on Artifical Satellites in the Astronomical Theory section of this website to avoid "interrupting the flow". There's a link to this section at the bottom of the previous page. The flares shown here have a large range of orientations, perhaps the most extreme being this one of Iridium 55 descending vertically. The direction of travel of a flare will be determined by whether it is a north-to-south or south-to-north passage by the satellite that caused it. As it happens, except for Iridium 55 all of the above flares were actually from "left to right" and in the eastern half of the compass i.e. the satellites passed from north to south. Conversely, although still "left to right", most of the flares below were in the western half of the compass i.e. the satellites passed from south to north - difficult to tell on a still picture though!

The brightness of a flare as seen by a particular observer depends mainly on how far he/she is from the centre-line of the ground-track of the flare. [ Note that this will be different from the ground-track of the satellite itself, as the "beam" of the flare shines down at an angle because it is caused by reflection of light from the Sun by what is effectively a large mirror ]. You usually need to be within about 5miles of the centre-line to see the maximum effect but if the width of the beam is large when it reaches the Earth (because the satellite is far away) then a flare can be seen even 100miles from the centreline.

Here we have Iridium 25 going to town with the brilliance! HeavensAbove listed it at magnitude -8 but it could actually have been brighter. The effect was rather like a searchlight shining down on you - really dramatic!

We see here what happens if you have to put your trust in a plastic compass when your usual one is playing up - you point the camera in the wrong direction and miss some of the flare! (from Iridium 10). Mind you, it also shows how fate can sometimes even things up - now what exactly is that streak at the top of the frame?? I had the distinct impression it must be another satellite as, when viewing the flare, I also looked in the wrong direction and thus saw nothing but afterwards did observe two faint trails leaving the right part of the sky in opposite directions. Consultation of the Heavens-Above listing showed it must be MetOp-A, a European meteorological satellite, whose track I had caught entirely by chance in precisely the extra field-of-view provided by the mis-alignment! How's that for serendipity?

MetOp-A has a large solar array which can produce flares in the same way as the Iridium antennae, which is what the streak to the top of the picture is. To see two near-simultaneous (non-Iridium) flares like this is highly unusual though. The image below, a greatly enhanced and slightly enlarged version of the one above, shows not only the flare but the faint track of the satellite as it moves upwards from bottom right. The track is faint because the satellite, though brighter than most of the stars in the frame, was moving sufficiently quickly that it didn't have time to affect the camera sensor enough to register a more definite image.

Wow! A triple flare!! No, not really, I'm afraid - I don't think that would actually be possible. This is a composite of flares taken on three out of four consecutive nights (it was cloudy on the other one!) aligned on the background stars. Saturn is to top left, with Regulus lower to its right - the other stars constitute the front right paw of the constellation Leo the Lion. The flares were produced by Iridiums 64, 67 & 62 respectively and while they also moved from left to right (as with the images above) they were in the western half of the compass and so were caused by south to north satellite passages.

The picture illustrates one of the many periodicities in Iridium flare timings - each individual flare happened 6mins per day earlier than the previous one. This is because, remarkably, after this time (24hrs minus 6mins) not only does the track of a particular plane of satellites repeat but there is also a satellite in the correct position for a flare to be produced. [ I presume this was engineered for operational reasons, not for the convenience of flare-watchers! ]. They will thus appear in the same position in the sky - small variations in timing, plus the fact that the Sun-satellite-observer geometry changes slightly each day, do cause the position of the flares to move slowly right (i.e. west) and up though.

These images are composites of flares produced by adjacent satellites in the same plane. They happen about 9min 20secs apart but will not often be visible because the optimum Sun-satellite-observer geometry rarely holds for long enough. We have Iridium 47 & 20 top left, Iridium 3 & 22 top right, Iridium 70 & 62 bottom right (together with the Moon!). The pairs of tracks are different distances apart because each pair of satellites producing them was a different distance away. A satellite further away will produce a lower elevation flare than one nearer (which is why the pairs are separated in the first place) but this effect reduces with increasing distance. A distant pair of satellites will thus produce flares that are both lower and closer together than a pair nearer to you.

I tried to get some sets of three while taking the pairs above, but the third one was usually really faint due to the poor Sun-satellite-observer alignment. I was thus highly delighted to capture the "triple" of Iridiums 10, 54 & 12 shown as a composite here. These just happened to have good enough alignments that all three were not only visible to the naked eye but also to the camera. Not only bright enough but also very close together. This was due to their low elevation (just 13deg) amongst the stars of the feet of the Twins in the constellation Gemini.

There are several other ways that flares can be close together in the sky, all due to the varying geometry of satellite, orbit and observer. Here are two more varieties of "close encounter".

And no, I haven't put the same image in twice by mistake! The satellite on the left is Iridium 94 and that on the right Iridium 31, captured 7mins 18sec apart amongst the stars of Coma Berenices. The really amazing thing is that their tracks relative to the stars are incredibly close to being identical. This is only possible if the first one of the pair is an in-orbit spare, which also accounts for the unusual time interval. I didn't realise in advance that this pair were going to repeat their tracks, so I'm very pleased to have caught this unusual event.

The pair of flares above (Iridium 90 on the left and Iridium 10 on the right, captured just 90sec apart) demonstrate the other sort of equivalence - tracks that are in the same position in the sky relative to the observer (i.e. same altitude and azimuth). Again, this is only possible if the first one of the pair is an in-orbit spare. The image to bottom right is a composite of the two above (which were taken without moving the camera), to show that although the stars have moved the tracks do not. To see this better, hover the mouse pointer over the picture to enlarge the area of interest: the very small difference between the flare positions makes it hard to spot there are actually two tracks otherwise.

As with the ISS, still images don't really give a true impression of what a flare looks like as it happens. Most are insufficiently bright to register properly when the camera is in movie mode so I had to wait until a very bright one occurred (Iridium 60 at magnitude -8, in fact, and at just past 2am!). Even so, the camera struggled slightly to decide whether there was anything there or not at the beginning and end so the track flashes a bit when not very bright - real flares don't actually do this! The extreme brightening at maximum is very obvious. To view the movie, click here.

While pleasant enough, pictures of Iridium flares can get a bit similar after a while so I decided to be a bit "arty" with these two and put them into a landscape context. While perhaps slightly optimistic with the visibility of the stars (just pretend it was a very clear night!), they are actually quite accurate as concerns both the length and brightness of the flares.

Here we have Iridium 26 flaring at magnitude -5 against the stars of Coma Berenices - the bright star to top left is Arcturus and Saturn has just sneaked into the bottom right corner. If you hadn't guessed, the light-yellow thing is the back wall of my house!
And this is Iridium 72 at a magnificent magnitude -7 over our garage/outbuildings, with house to the left. Mars is just to the right of the flare, with Pollux & Castor further right, and Saturn is above Regulus at top left.

As mentioned on the Iridium information page, the first generation satellites were replaced with more capable versions which do not have the same type of antennae and so do not produce flares. The above images are thus now merely a part of space history - it was good to catch them while I could though!

Back

	My first flare image! I wasn't sure what exposure to use so tried 15sec at 200ASA: I thought that a lower ASA might not catch the flare and a longer exposure would be affected by the Moon (the bright patch to the top of the image). As it turned out, this was about right photographically but the exposure was too short to catch all the flare - note that it starts somewhat abruptly (top left), indicating that it was underway before the shutter opened. The fade-out at the end was captured in full though. This is Iridium 31.

These flares were just 4mins apart, in almost the same place in the sky. I kept the camera still between the exposures but of course the stars moved, as can be seen in the composite image to the right. Multiple flares in one evening (or morning) are not unusual - there are eleven satellites in each of six orbital planes so it is likely that more than one will become correctly orientated - but they are not often in the same direction because the Sun-satellite-observer geometry changes as the Earth turns thus shifting the angle of maximum reflection. Read on for why these two are an exception! The two satellites here are Iridiums 29 & 95, with the first flare supposedly brighter than the second - seems the extensive high cloud interfered with this though. Based on my experience with the first flare, the exposure for these two was 30sec at 100ASA. This has clearly caught the entire duration of each without the images being fogged too much by the cloud.
	If the flares above were close in time, this pair is exceptionally so. In fact they were near-simultaneous - just 6sec apart! This image is thus a single exposure, not a composite: 30sec at 100ASA again. Cloud, and the fact that they happened at 6:30pm so the sky was still quite bright, meant the flares were not so brilliant but I was very pleased to capture this most unusual double event, involving Iridiums 96 & 28.
The double flares above both involved a "primary" satellite and an "in-orbit spare" from the same orbital plane. Although the spares (nos. 95 & 96 in this case) are in the same orbital plane as the 11 primary satellites they orbit somewhat closer to the Earth. This means they go round the Earth a little faster so will occasionally be in a position to produce a flare at nearly the same time as a primary satellite. The two are never physically close of course, simply in the same line-of-sight.
At last - a cloud-free evening! This image (of Iridium 5) shows what a difference cloud makes - despite being inherently less bright than some of the above flares, the absence of cloud makes this one far more impressive. In fact, when the "flare" part of the pass was over I could still see the satellite moving among the stars quite easily. The duration of the flare must have been in excess of 30secs (the length of the exposure) as close examination of the image shows it was just present when the shutter opened and had not entirely faded when it closed.
	Despite some thin high cloud, this image has several unusual features. Firstly, note that the flare itself (of Iridium 53) is asymmetric i.e. it fades (to the right) much more quickly than it builds (to the left). This is because the satellite went into eclipse before the flare itself was over. Then there are several stars of interest. The bright one below the left end of the flare is called Cor Caroli [Charles' Heart in Latin], named by Edmund Halley, of comet fame, in honour of either King Charles I of Great Britain, who was executed following the Civil War, or his son Charles II in recognition of the restoration of the monarchy.
The star in almost the same position above the flare is called Chara, which is very similar to our Sun and was named in 2006 as the star most likely to have a planet with extraterrestrial life! To its left and up a bit we find the extravagantly-named La Superba, one of the reddest (and thus coolest) stars in the sky. To see its colour, move the mouse pointer close to it. Amazing what you can find in an apparently unimportant piece of sky!

We see here what happens if you have to put your trust in a plastic compass when your usual one is playing up - you point the camera in the wrong direction and miss some of the flare! (from Iridium 10). Mind you, it also shows how fate can sometimes even things up - now what exactly is that streak at the top of the frame?? I had the distinct impression it must be another satellite as, when viewing the flare, I also looked in the wrong direction and thus saw nothing but afterwards did observe two faint trails leaving the right part of the sky in opposite directions. Consultation of the Heavens-Above listing showed it must be MetOp-A, a European meteorological satellite, whose track I had caught entirely by chance in precisely the extra field-of-view provided by the mis-alignment! How's that for serendipity?
MetOp-A has a large solar array which can produce flares in the same way as the Iridium antennae, which is what the streak to the top of the picture is. To see two near-simultaneous (non-Iridium) flares like this is highly unusual though. The image below, a greatly enhanced and slightly enlarged version of the one above, shows not only the flare but the faint track of the satellite as it moves upwards from bottom right. The track is faint because the satellite, though brighter than most of the stars in the frame, was moving sufficiently quickly that it didn't have time to affect the camera sensor enough to register a more definite image.

Wow! A triple flare!! No, not really, I'm afraid - I don't think that would actually be possible. This is a composite of flares taken on three out of four consecutive nights (it was cloudy on the other one!) aligned on the background stars. Saturn is to top left, with Regulus lower to its right - the other stars constitute the front right paw of the constellation Leo the Lion. The flares were produced by Iridiums 64, 67 & 62 respectively and while they also moved from left to right (as with the images above) they were in the western half of the compass and so were caused by south to north satellite passages. The picture illustrates one of the many periodicities in Iridium flare timings - each individual flare happened 6mins per day earlier than the previous one. This is because, remarkably, after this time (24hrs minus 6mins) not only does the track of a particular plane of satellites repeat but there is also a satellite in the correct position for a flare to be produced. [ I presume this was engineered for operational reasons, not for the convenience of flare-watchers! ]. They will thus appear in the same position in the sky - small variations in timing, plus the fact that the Sun-satellite-observer geometry changes slightly each day, do cause the position of the flares to move slowly right (i.e. west) and up though.


These images are composites of flares produced by adjacent satellites in the same plane. They happen about 9min 20secs apart but will not often be visible because the optimum Sun-satellite-observer geometry rarely holds for long enough. We have Iridium 47 & 20 top left, Iridium 3 & 22 top right, Iridium 70 & 62 bottom right (together with the Moon!). The pairs of tracks are different distances apart because each pair of satellites producing them was a different distance away. A satellite further away will produce a lower elevation flare than one nearer (which is why the pairs are separated in the first place) but this effect reduces with increasing distance. A distant pair of satellites will thus produce flares that are both lower and closer together than a pair nearer to you.
	I tried to get some sets of three while taking the pairs above, but the third one was usually really faint due to the poor Sun-satellite-observer alignment. I was thus highly delighted to capture the "triple" of Iridiums 10, 54 & 12 shown as a composite here. These just happened to have good enough alignments that all three were not only visible to the naked eye but also to the camera. Not only bright enough but also very close together. This was due to their low elevation (just 13deg) amongst the stars of the feet of the Twins in the constellation Gemini.

There are several other ways that flares can be close together in the sky, all due to the varying geometry of satellite, orbit and observer. Here are two more varieties of "close encounter".

And no, I haven't put the same image in twice by mistake! The satellite on the left is Iridium 94 and that on the right Iridium 31, captured 7mins 18sec apart amongst the stars of Coma Berenices. The really amazing thing is that their tracks relative to the stars are incredibly close to being identical. This is only possible if the first one of the pair is an in-orbit spare, which also accounts for the unusual time interval. I didn't realise in advance that this pair were going to repeat their tracks, so I'm very pleased to have caught this unusual event.

The pair of flares above (Iridium 90 on the left and Iridium 10 on the right, captured just 90sec apart) demonstrate the other sort of equivalence - tracks that are in the same position in the sky relative to the observer (i.e. same altitude and azimuth). Again, this is only possible if the first one of the pair is an in-orbit spare. The image to bottom right is a composite of the two above (which were taken without moving the camera), to show that although the stars have moved the tracks do not. To see this better, hover the mouse pointer over the picture to enlarge the area of interest: the very small difference between the flare positions makes it hard to spot there are actually two tracks otherwise.


Here we have Iridium 26 flaring at magnitude -5 against the stars of Coma Berenices - the bright star to top left is Arcturus and Saturn has just sneaked into the bottom right corner. If you hadn't guessed, the light-yellow thing is the back wall of my house!	And this is Iridium 72 at a magnificent magnitude -7 over our garage/outbuildings, with house to the left. Mars is just to the right of the flare, with Pollux & Castor further right, and Saturn is above Regulus at top left.