Iridium Flares

Transient Events

There are 66 active Iridium satellites providing worldwide coverage for satellite phones. Each satellite has large flat surfaces that can reflect sunlight towards locations on Earth when everything is lined up properly. Some of these flat surfaces are antennas, and some are solar panels. The first picture below shows an Iridium satellite. The gold colored surfaces covered with squares are "door sized antennas" according to Wikipedia. You can also see one of the solar panels on the left.

When you are in the right observing location at the right time of day it's possible to see a temporary bright "flash", or flare, from one of these satellites when sunlight reflects from a flat surface towards you. Some of these flares are very bright. I've seen a particularly bright one appear in blue sky before sunset!

On the evening of November 19th I made the first attempt to capture an Iridium flare with my phone camera. Since flares usually last only a few seconds, and the maximum phone camera exposure time is 10 seconds, the shutter release needed careful timing. I tried to have the exposure begin 5 seconds before the predicted flare time. The timing was fairly good as you can see in the first image below which shows the flare as a short bright streak left of center.

This flare occurred against the background constellation Aquarius. Aquarius and Capricornus are most prominent in the image. Moonlight illuminated patchy clouds. The brightening appears asymmetrical. Either the exposure didn't include both the increase and decrease in brightness, or there was a particularly sudden increase of brightness. This flare was from Iridium satellite 64 and was predicted to be magnitude -7.4, brighter than everything normally in the sky except a full Moon and the Sun.

On December 10th another dimmer Iridium flare occurred near the same section of sky in the constellation Cetus. In this case it was Iridium satellite 47, and the flare magnitude was only 0.4. The next image shows the brightest part of the flare I was able to capture. You can see how dim it was compared to the Iridium 64 flare on November 19th.

This time I was able to get a sequence of three images over a span of 43 seconds showing the satellite's trail as it dimmed and moved from Cetus into Sculptor toward the southern horizon.

Iridium flares would look great in a continuous video. Unfortunately, as I mentioned in the last post, phone camera videos aren't sensitive enough to show stars. A phone video would show only a moving temporarily brightening and dimming light against a blank black background.