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| Space on Dec 08, 2014

I saw a flash in the night sky. What is it?

We can’t ever say for sure what you saw in the night sky. But it might have been a flare from an iridium satellite.

Many people ask us about flashes in the night sky. They see one and want to know, what is it? Unless one of us were standing there next to you, we have no way of knowing exactly what you saw. So we can’t say for certain. But it’s possible that what you saw is a flare from an iridium communications satellite.

It often happens that an antenna from one of these satellites reflects sunlight directly down at Earth. When it does, it creates a quickly moving illuminated spot on Earth’s surface. The diameter of the spot is about 10 kilometers, or about six miles. If you’re standing inside this spot, looking up, you see what appears as a bright flash, or flare in the night sky. The flare lasts a few seconds, then is gone.

EarthSky Facebook friend Simon Waldram, in Spain, saw and photographed what looks like an iridium flare on the night of December 9, 2012. Thank you, Simon.

The RASC Calgary Centre has a great page on what causes iridium flares. This is an illustration from that page. Click here if you want more.

Iridium satellite donated to the Air and Space Museum. Photograph by Flickr user ideonexus via Wikimedia Commons.

Want to see an iridium flare? Try the website Heavens-Above.com

The flares from an iridium satellite can be bright – as bright or brighter than the brightest planet Venus. Some of the flares are so bright they can be seen in daylight.

It’s the unusual shape of iridium satellites, by the way, that causes them to flare on occasion. They have three polished door-sized antennas, 120° apart and at 40° angles with the main bus. The forward antenna faces the direction the satellite is traveling. The flares come from these antennas.

By the way, you can also see these satellites when they’re not producing a flare. Then they simply cross the sky, like any other satellite, appearing as dim steadily moving “stars.”

Bottom line: If you see a flash or flare in the night sky, it might be from sunlight glinting off an antenna of an iridium satellite.