May 21, 2003

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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.

This is NASA photo number ISS-006-50247.