Student Features

Mystery Meteors

Three stars of Orien's Belt

Left: Picture of the Day -- April 11, 2003.

Explanation: They look like meteors flying in formation -- three stars with upward-pointing tails shining through Earth's airglow layer. Using a digital camera, ISS science officer Don Pettit caught them in a 15-second exposure on January 3, 2003. What are they?

The three stars of Orion's Belt.

NASA scientist Rob Suggs explains: "Orion was setting behind Earth's limb when Don took this picture. His camera was locked on Orion, so Earth's atmosphere moved upward during the exposure. As the Belt stars were covered, they were also deflected upward by atmospheric refraction -- hence the illusion of three meteors in this long exposure. We know that the descent of the setting Sun can be slowed by refraction; this is the same effect."

"The atmosphere acts like a giant lens," agrees expert Les Cowley. "Here on Earth, when we see the setting Sun with its center on the horizon, the uppermost limb of the Sun has, in fact, already set. In such cases, refraction has lifted the upper part of the Sun by 0.25ยบ -- half its apparent diameter. From orbit, light rays enter the Earth's atmosphere and then have an equally tortuous journey out again. Refraction is almost doubled. The setting Sun and setting stars are lifted twice as much."

In a Picture of the Day last week we saw the same thing: Orion's foot, the bright star Rigel, posing as a meteor as it set behind Earth's limb. "A magnified image of Rigel reveals a streak dimming and reddening in the upward direction," notes Cowley. "The reddening is caused by the atmosphere scattering more blue light than it does red."

Editor's note: There are many stars in this image shining through the atmosphere. Why aren't more of them streaked? The answer: Earth has an exponential atmosphere -- its density increases very rapidly as you sink into it. Orion's belt stars are the lowest of the bright stars in today's image, hence they are refracted most. A few other stars are at about the same low altitude, but they are too dim to display the faint tails.