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Space Station Science

Picture of the Day

May 21, 2003

Double Iridium Flare
Photo credit: ISS Expedition 6 science officer Don Pettit, NASA

May 21, 2003: On April 30, 2003, a geomagnetic storm was underway on
Earth. The skies beneath the space station were glowing red and
green, but science officer Don Pettit wasn't looking out the window.
He was checking his email. 

Fortunately he got one from space scientist Rob Matson. A pair of
Iridium satellites would soon appear outside the ISS, the email said.
Don flew to the window--literally--and captured today's picture. His
approximately 30-second exposure recorded two bright Iridium flares,
lots of star trails, and some vivid green aurora australis. 

Iridium flares are flashes of sunlight glinting from the antenna
panels of Iridium communications satellites. They're dazzling. You
can see Iridium flares a few times a week from your backyard on
Earth--some in broad daylight. 

There are 90 Iridium satellites orbiting Earth. "The ones Don saw
were Iridium 67, which flared at 06:19:12.2 UT, and Iridium 14 at
06:19:28.5 UT. The latter is an on-orbit spare." The spares are in a
slightly lower orbit, explains Matson, so they are constantly
"lapping" the operational satellites. "Closely-spaced double flares
are rare and almost always involve a spare." 

Sunbeams glinting from Iridium satellites are narrow--only 10 or so
km wide. Predicting when two such beams would shine through a single
window on the fast-moving space station in quick succession ...
that's impressive. Matson, who works at SAIC as an orbital
dynamicist, did it using the same software he's been using for years
to predict Iridium flares for sky watchers on Earth. "I did have to
make some modifications," he says, "to handle the more complex case
of an orbiting (rather than stationary) observer."

Would you like to see some Iridium flares yourself? Visit Heavens
Above or download Matson's software to find out when to look. 
 
Credits & Contacts
Author: Dr. Tony Phillips 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Catherine Watson

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