Last chance to see Iridium flares

For years, amateur astronomers have been watching for and photographing Iridium flares, which are caused by a particular group of Earth satellites used for satellite phones. But these flares will soon end. 

The first generation of these satellites had polished surfaces which could reflect sunlight, causing a predictable bright flare in the night sky, often brighter than any star or planet. Typically, you see a faint moving satellite appear, then over a matter of seconds brighten up and then just as quickly fade away. In a photo, these look very much like a meteor trail, except that they have a very symmetrical appearance when brightening and fading.

However, the first generation of these satellites are now being replaced by a new generation of a different design, which will no longer produce flares. As each first generation satellite becomes redundant, it is taken out of service by altering its orbit so that it enters the atmosphere and burns up. There are now fewer than 20 out of the original 66 satellites, and most of those are being kept as spares rather than being operational.

As a result, the number of Iridium flares is now drastically reduced, with typically only three or four a week at the moment instead of maybe two or three a night. So if you want to see one, get looking now.

Iridium flare
An Iridium flare near Altair. Photo: Robin Scagell

For each flare there is an ideal location on the Earth’s surface where it is brightest, so for the best results you need to travel,  but from nearby locations you will still see a flare. You can get predictions of the timing and positions of flares free of charge from the website heavens-above.com, which gives star maps showing the positions of flares as seen from any location. You don’t need to register to do this, but doing so avoids the need to enter your exact location each time you use the service. It will also give you plenty of other predictions, including passes of the International Space Station. For help with understanding these predictions, refer to our help page on predicting the ISS and other satellites.

Even when the Iridium first-generation series of satellites have all been de-orbited, flares may still occur from other satellites, though unpredictably. If you see a completely symmetrical meteor-looking trail on your sky photos, suspect a satellite rather than a meteor. Heavens-Above also gives predictions for all satellites brighter than magnitude 5, so check whether a satellite was on that track at the time of the exposure. Some Iridium satellites that are currently still in orbit but no longer controlled can still give flares, though unpredictably. Occasionally one also sees or photographs flashes or short trails in the sky, which are often due to tumbling satellites that just happen to produce a brief reflection of the Sun.

Photographing Iridium flares

To photograph a flare, mount your camera on a tripod, point it at the predicted location using a standard or moderately wide-angle lens setting and begin a 30-sec time exposure about 15 seconds before the predicted time of the event. The ISO setting will depend on your sky brightness – in badly light-polluted sites you will need an ISO of 200 or lower to avoid the shot being overexposed, but in dark skies you can get away with higher ISO settings without the sky background becoming unpleasantly bright.

These settings should guarantee you a shot, but with practice you should be able to refine your technique by choosing a flare that is fairly low down in the sky and including foreground details, and getting the exposure details spot on so that you get both the flare and good star images with a dark background.

Using a camera mount driven to follow the stars will give you sharp, untrailed star images but will cause foreground landscape to blur slightly during the exposure.