Q&A: Iridium Flare

Question: I was stargazing last night and saw this one star get really bright, then it dimmed out again. The whole event lasted maybe 5-10 seconds. Any ideas what it was? — DH, Albuquerque, NM

Answer: What you saw was most likely an Iridium flare, not a nova (exploding star), which typically lasts on the order of days or months, or a meteorite, which typically lasts for less time and moves across the sky. Your description suggests it stayed in the same place, so that’s my best guess.

Orbiting high overhead is a string of 66 Iridium™ satellites in low Earth orbit. They provide satellite phone service anywhere on the planet, a cool but expensive technology.

Their antenna plates are on the bottom end of the satellite, which always points toward Earth. When the Sun has just set, and up to an hour or so after sunset (depending on your latitude), the angles can be just right for the antenna to reflect sunlight back down to the night side of the planet. If you’re in the right place at the right time, you’ll see an Iridium flare that can last from 2-20 seconds.

Under the right conditions, these flares can be 30 times brighter than Venus, normally the brightest object in the sky (next to the Sun and Moon). If you see one, you’ll definitely notice it. It doesn’t blink like aircraft lights — it smoothly brightens and dims only once.

If you want to see one, there’s free software and prediction tables to help you know when and where to look. Do an internet search for “iridium flares prediction” (25,000 hits).

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