March 28, 2003

Iridium Flare
Credit: Don Pettit, ISS Expedition 6 Science Officer, NASA
with assistance from amateur astronomers Rob Matson and Robert Reeves

Explanation: Two spacecraft circle Earth in different orbits, each traveling faster than 17,000 mph. Sunlight bounces off one and hits the other ... but only for a split second. What are the odds? International Space Station (ISS) Science Officer Don Pettit saw it happen--and photographed it (above)--on March 21, 2003. He was staring out one of the windows onboard the ISS when an Iridium communications satellite passed overhead. Sunlight hit one of the Iridium's mirror-like antenna panels and bounced down to the ISS; the fast-moving Iridium satellite seemed to flare like a supernova! Sky watchers on Earth, using web sites such as Heavens Above to tell them when to look, see these Iridium flares often. The flares are trickier to predict for sky watchers on the ISS, however, because the space station and the passing satellite are both moving. Furthermore, the reflected beam of sunlight is narrow--only ten or so kilometers wide. The chances of the ISS passing through it are slim. Pettit knew when to look thanks to the work of amateur satellite hound Rob Matson, who wrote a computer program to calculate when Iridium flares would beam through the windows of the space station. Using Matson's predictions, Pettit now sees as many as four Iridium flares each day.