|Iridium Satellite 54 - Photo by Christoph Lohuis|
Of the roughly 3,000 spacecrafts in Earth's orbit, just about 100 stand apart: the Iridium communications spacecraft, which skim the uppermost, most rarefied region of the atmosphere (the exosphere) at altitudes around 800 kilometers in six steeply inclined orbital planes (orbits that nearly pass overhead at the North and South Poles). Known as an iridium flare, the glare from these satellites is well known to many astronomers.
What causes Iridium Flares? Iridium satellites are unique because their flat, shiny, door-sized antenna arrays periodically reflect sunlight toward the ground, causing brief, but brilliant flares that can momentarily reach an apparent magnitude of –8, brighter than the planet Venus. Also, these flares are predictable and their orbital elements are public information. Thanks to websites such as Heavens-Above, satellite-watching enthusiasts are able to witness these brilliant occurrences. The illustration to the left demonstrates how Iridium Flares are seen from the Earth.