The glint of sunlight known as an Iridium flare. Photograph: nasaimages/org
At its best, the International Space Station outshines every star and can come close to rivalling Venus, the brightest planet. It is, though, occasionally surpassed by the glints of sunlight reflecting from the antennae of Iridium satellites – glints that are popularly known as Iridium flares.
Launched to provide satellite-phone communications, there are 66 active satellites, plus spares, in the Iridium constellation. All are near 778km in altitude and in circular orbits tipped at 86° to the equator, so we see them tracking northwards or southwards as they pass overhead.
Each maintains a stable attitude, making it possible to compute when their highly reflective door-sized antennae, three on each satellite, are in just the right orientation to reflect sunlight directly to any point on the ground.
An observer who is in the right place sees a glint or flare over several seconds that can reach magnitude -8 in astronomical terms – more than 30 times brighter than Venus and bright enough to be seen in broad daylight. Predictions for such flares for any location are available from www.heavens-above.com.
The days of Iridium flares may be numbered, however. Launches to construct a new Iridium-Next constellation are to begin next year, and most of the current Iridium craft are to be pushed to lower altitudes to accelerate their decay from orbit.
For a time, the flares may become brighter but unpredictable as attitude stability is lost. Meanwhile, the design of their replacements suggests that we cannot expect them to generate similar flares.