Physics Photo of the Week

November 4, 2011

Satellite Flare
This is the reflection of sunlight off a polished antenna section of a communications satellite called an "Iridium Satellite".  The position and orientation of the satellite is very well-known allowing computer programs to accurately predict when and where the mirror-like surfaces will briefly reflect the sunlight to a spot on the Earth.  Once such prediction program is available at the website that is developed and maintained by Chris Peat:  At Warren Wilson College several flares were predicted each week in late October and early November, 2011. 

We often see the sunlight reflected off approaching car windshields if the Sun is in the proper position so that the windshield briefly emits a flash toward on-coming traffic.  Other common situations occur when the setting Sun is reflected in windows of a distant house.  With the stationary window reflection, the flash lasts a couple of minutes.  The satellite flare is a similar phenomenon.

The image above consists of two 15-second time exposures at about 6:15 am on October 25, 2011.  The gap in middle of the streak is caused by the camera shutter switching between exposures.  The flash from an Iridium satellite lasts about 10 seconds.  Physics assistant Mayuri Patel processed the above image by stacking about 10 images.  The bright star just to the right of the satellite streak is Polaris - the North Star.  Occasionally satellite streaks can be mistaken for meteors - See last week's Physics Photo of the Week

At right is another flash event obseved on October 30 from the back yard of a WWC colleague Dean Kahl who lives in Asheville, NC, several kilometers from WWC.  East Asheville area was the center of the maximum brightness for the October 30 flash at about 7:24 pm.  At the Asheville location the flash was bright enough that we could photograph the flare with only 1/2 second shutter times.  About 10 images are played back in real time in the animation.  The flash lasts for several seconds and is about 100 times brighter than the brightest stars visible.

The Iridium satellites serve "satellite phones" - cell-type telephones that can be used in any location in the world using the network of about 60 Iridium satellites instead of stationary cell towers.  The satellites orbit the Earth about 500 km above the ground level.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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