An Iridium flare is a sudden burst of light in the sky caused by sunlight reflecting off an Iridium satellite. While any satellite can produce a reflection flare, the Iridium network is particularly prone to do so because of its satellites' low orbits, large numbers and unique design. Iridium satellites use large, flat reflective panels to bounce signals to and from Earth, and these panels readily reflect the Sun's light.
Iridium Communications began launching these satellites in the late 1990s to provide satellite phone and data communication worldwide. The network became operational in 1998, and as of 2014, there are 66 active satellites in the constellation that orbit around 485 miles above the surface of Earth. The network is designed to cover the entire Earth at all times, which is why these satellites pass overhead so frequently.
The bright Iridium flare is caused by the large, door-shaped antenna at the base of the satellite. Its configuration focuses the Sun's light into a single bright point as the satellite passes overhead at certain times of the day, making it look extremely bright for a moment when observed from the ground. Several astronomy groups track these satellites, providing information about the times and locations where the brightest flares may occur.