Mercury,
May/June 2002 Table of Contents
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Courtesy
of L3 Communications, Integrated Systems.
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by
Sally Stephens
Following
nearly four decades of bigger and better airborne observatories,
NASAs SOFIA mission will carry infrared astronomy to new heights.
The
airplane looks like any other 747, until you notice the large, gaping
hole in its left side, just behind the wing. The hole is no accident.
It will give astronomers an unobstructed view of the universe through
a 2.5-meter telescope mounted inside the plane. When the airplane,
nicknamed SOFIA, finally takes flight in 2004, it will become the
latest in a small but distinguished series of flying observatories.
For
nearly 40 years, astronomers have battled air turbulence, engine
vibrations, and complicated aerodynamics of wind blowing past a
hole in an airplanes side to gain a view of the universe impossible
to achieve from the ground. Airborne astronomy has shown scientists
the secrets of star formation, planetary rings, and the energetic
cores of our own and other galaxies. But why go to all the trouble?
Ground-based
telescopes work well for studying the sky in visible light. But
Earths atmosphere blocks most other forms of light from reaching
the ground. This is good when it comes to ultraviolet light, X-rays,
and gamma rays, which would otherwise damage skin and other cells
in our bodies. But its bad when it comes to infrared light,
also known as thermal radiation or heat.
Astronomers
are interested in infrared light because, unlike visible light,
its wavelengths are too long to be scattered by tiny dust particles.
That means an infrared telescope can see what is happening deep
inside large dust clouds that appear opaque when viewed in visible
light. Such clouds house the nurseries where stars are born and
they hide the center of the Milky Way Galaxy from view.
Unfortunately,
water vapor in Earths atmosphere absorbs infrared light, keeping
most of it from reaching even the tops of mountains. But water vapor
is concentrated near the bottom of the atmosphere. Thats where
airplanes come in. At 41,000 to 45,000 feet (12,500 to 13,700 meters),
the air contains only about 20% of the molecules present at sea
level, but thats enough air for the wings to generate lift,
enabling an airplane to fly. But the amount of infrared-absorbing
water vapor has gone down by a factor of a thousand. "Its
kind of a happenstance of our atmosphere that you can fly an airplane
and still see in the infrared," says UCLA astronomer Eric Becklin,
chief scientist for SOFIA (Stratospheric Observatory for Infrared
Astronomy).
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