April 3, 2003

Orion's Hazy Feet
Credit: Don Pettit, ISS Expedition 6 Science Officer, NASA

Explanation: On January 6, 2003, International Space Station science officer Don Pettit peered out station's Destiny Lab window and photographed the constellation Orion as it set behind Earth's limb. The snapshot shows something rare: a constellation seen both above and through Earth's atmosphere--at the same time. Orion's shoulders are bright and clear against the black of interplanetary space. Orion's belt is surrounded by a faint hazy-red glow. Orion's feet are ... stuck in the mud?

Not really. The brownish hazy layer is a phenomenon called "airglow." During the day, ultraviolet (UV) radiation from the Sun hits Earth's upper atmosphere. These energetic UV photons break apart molecules and knock loose electrons from their atoms. Later, at night, the molecules and atoms and electrons get back together again. Sometimes they return to their original state. Sometimes they form new molecules and compounds. These nighttime chemical reactions taking place 80 km or more above Earth's surface cause the air there to glow--hence "airglow." Although the airglow in this picture looks rather dull, airglow can be colorful: greens and reds come from reactions involving atomic oxygen, yellows from sodium and hydroxyl (OH). In fact, says NASA atmospheric physicist Joe Minow, the red aurora-like band through Orion's belt is probably airglow, too.

Airglow is difficult to see from Earth because it is faint and widespread, but it is ever-present in pictures of Earth's limb from the International Space Station. Click here to view a movie and narrative by Don Pettit describing some of the airglow he has seen.

Editor's note: Sky watchers who are familiar with Orion might have noticed something odd in this photo. The hunter's right foot--the bright star Rigel--looks like a meteor streaking straight down into the airglow layer. Why? We'll answer that question in a future Picture of the Day.