April 11, 2003
Mystery Meteors
Credit: Don Pettit, ISS Expedition
6 Science Officer, NASA
Explanation: They look like meteors flying in formation–three
stars with stubby 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 6, 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 is 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 Sun
setting with it’s center on the horizon, the uppermost limb
of the Sun has, in fact, already just set. Refraction lifts the
upper part of a low-hanging Sun by 0.25º –half its apparent
diameter. From orbit, light rays enter 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, looking like 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." |
