Iridium flares are relatively new ultra bright objects in the sky, produced by the glancing reflection of the Sun's rays off a particular type of satellite - the Iridium satellite. The Iridium satellites form a constellation of communication spacecraft, created to provide a commercial satellite telecommunication network around the Earth. The Iridium consortium is in the process of establishing a network of 66 satellites operating above the Earth. The satellites are in a near-polar orbit at an altitude of 485 miles (780 km). Their orbital period is approximately 100 minutes with a velocity of 16,800 miles per hour.
The uniqueness of the Iridium flares is that the spacecraft emit 'flashes' of very bright reflected light that sweep in narrow focused paths across the surface of the Earth. Whilst the satellites themselves have a relatively weak brightness of magnitude +6 (binoculars are needed to see them at dusk), the Iridium flare has a magnitude of magnitude -8. However you have to be in exactly the right position on Earth at the correct time to see the flare - it's a bit light catching the flash from a lighthouse. By way of comparison, the Iridium flares have a brightness up to 30 times greater than Venus, which is typically magnitude -4.9. The flare duration can last from anywhere between 5 to 20 seconds and can be easily seen by the naked eye around dawn and dusk. Flares can sometimes be seen during daylight, as long as the exact position of the satellite is known.
To understand why these spacecraft produce such bright flares, we need to understand the structure and orientation of the satellite. The Iridium satellites consist of a main structure with a pair of large solar panels outspread from one end. At the other end of the spacecraft, facing towards the Earth, there are three Main Mission Antennae (MMA) positioned 120 degrees apart and angled at 40 degrees from the axis of the satellite. Each antenna is 188 cm long by 86 cm wide by 4 cm thick, fabricated from highly reflective aluminium plates covered with silver-coated teflon. It is these antennae that produce the flares observed on Earth, due to reflection of the Sun's rays. Crucially however, the flares cross the Earth's surface in regular and predictable patterns due to the fact that the axis of the satellite is always maintained vertical to the Earth's surface. The MMA provide a direct reflection of the Sun's disk onto the Earth, which is only tens of kilometres wide at the Earth's surface. Consequently, in order to see the flare, the observer must be within this relatively small area at the exact moment that the reflection passes across the observer.
In order to observe Iridium flares a relatively sophisticated prediction program must be used. Since each Iridium satellite maintains its axial and longitudinal position to a very high precision, software can be used to track flare positions on Earth based on the relative angle of the Sun to the satellites, the reflective positions of the mirrors, and the local observing conditions on Earth.
Unfortunately the prediction of Iridium flares is beyond the capabilities
of our Satellite tracking map. However there is an excellent free Iridium
flare prediction service provided by the Heavens-Above
web site in Germany, supported by the German Space Agency DLR. This
site allows the user to enter their co-ordinates manually, or to choose
from a range of world towns and cities. Because the area of the flare
on Earth effect is fairly small, you need to be able to define your
co-ordinates accurately to get an accurate prediction.