IN ANCIENT times, it would have guaranteed you godlike status. Today, pointing to a dark patch of night sky just before it flares up like a supernova is merely an impressive party trick. The shining spectacle known as an Iridium flare is neither a warning from the heavens nor a stellar explosion, but sunlight glinting off an artificial satellite just a few hundred kilometres above your head.
The system of satellites called the Iridium constellation was conceived in the late 1980s as a way of providing cellphone coverage for remote parts of the planet, like the North and South Poles. The original plan was to place 77 satellites in orbit and route signals between them rather than exclusively to base stations on the ground, reducing both the cost and the lag time of calls, as the signals would have less distance to travel. The constellation took its name from the element iridium, which has 77 electrons orbiting its nucleus.
By the time the constellation was up and running in 1998, though, design changes and cutbacks had reduced the number of satellites to 66. Even though 66 corresponds to the element dysprosium, the name Iridium stuck.
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Both the name and the project proved to be overly ambitious. With few potential customers in the most remote areas of the planet, the company behind the project was nearly forced to take its satellites out of orbit and allow them to burn up in the Earth鈥檚 atmosphere. In a last-minute deal, the $6 billion constellation was sold for $25 million to a group of private investors and the satellites were saved. The US Department of Defense became the constellation鈥檚 main customer.
So what makes the Iridium satellites flare? Each satellite has three flat antenna panels made of highly polished aluminium covered with silver-coated Teflon. The panels act as mirrors, and at certain points in the satellite鈥檚 orbit they reflect sunlight towards the ground, forming a bright spot a few tens of kilometres across. From within that relatively small circle, you can see the satellite flare up as it passes overhead.
By astronomical standards, the flares are spectacular. They last for up to 20 seconds with a magnitude of up to 9 鈥� that鈥檚 somewhere between the brightness of Venus and a full moon. At times, they can be the brightest objects in the night sky. Iridium flares are a menace for astronomers, however. An unexpected appearance can ruin sensitive astronomical measurements.
Where can you see a flare for yourself? For a handy guide, visit . You can practise your godlike party tricks on most days from nearly anywhere on the planet.
