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Space Station Science
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.
25º --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.

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