Flying Telescopes

Mercury, May/June 2002 Table of Contents

Courtesy of L3 Communications, Integrated Systems.

by Sally Stephens

Following nearly four decades of bigger and better airborne observatories, NASA’s 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 airplane’s 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 Earth’s 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 it’s 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 Earth’s 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. That’s 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 that’s 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. "It’s 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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