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By and in
| Space on May 16, 2015

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

We can’t always 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 is standing there next to you, we have no way of knowing exactly what you saw. So we can’t say for certain. It’s possible you saw a flare from an iridium communications satellite. Or you might have seen a satellite or rocket body tumbling as it moves in its orbit around Earth. Follow the links below to learn more.

Flashes from iridium satellites

Other flashes in the night sky

Nature & Man: Iridium Flare, Milky Way, Clouds and Light Pollution by Mike Taylor.  Visit Mike Taylor Photography.

Iridium flare by Mike Taylor Photography.

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

Flashes from iridium satellites. In a dark location, especially just after night has fallen or is about to end, you often see ordinary satellites crossing the night sky, appearing as dim steadily moving “stars.” Sometimes one of them will produce a flash! That may happen when an antenna from an iridium satellite 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.

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.

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

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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.

Other flashes in the night sky. Other objects that produce flashes in the sky include tumbling objects orbiting our planet. After launching a satellite, most rocket bodies continue to orbit Earth for weeks, months or even years. As they are tumbling, they may be visible as a dim moving “star” that might produce bright flashes as the object reflects more sunlight. This will happen when the reflective areas of the rocket are properly reflecting the sunlight.

Dimmer rocket bodies may confuse observers, because they will remain invisible to the unaided eye while moving across the sky, then erratically “flash” toward those watching below.

A rather interesting sighting is a flash that appears to come from a static point in the sky. How is this possible? Geosynchronous satellites need to be placed in a very high orbit, some 22,200 miles (35,727 km) above Earth’s surface. After the launch of this sort of satellite, the rocket body that carried the satellite to orbit will also remain orbiting our planet. Instead of having a circular orbit it will have a highly elliptical orbit (elongated, like a circle someone sat down on). This may produce some peculiar flashes, which might be visible from Earth.

How does it happen? Depending on the location and perspective of the observer, the erratic flashes of the rocket body from a geosynchronous satellite might appear to originate from an apparent static place in the sky. This rare occurrence is possible if the reflecting rocket body is more or less moving towards, or more or less departing, Earth as it pursues its stretched-out (elliptical) orbit. This is the same principle that occurs when a distant airplane appears to be static as it moves towards you.

To the unaided eye, the subsequent flashes may give the impression that they are originating from the same area of the sky. If you are fortunate enough to witness this kind of flash in the sky, grab your binoculars! Magnification may confirm that the object is indeed moving slightly instead of remaining static.

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.

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. Or it might be a tumbling rocket body.