Seen from the exterior with its rear canopy closed, the Stratospheric Observatory for Infrared Astronomy (SOFIA) aircraft looks much like a typical jumbo jet you might see at any airport-- perhaps even reminiscent of another well-known, heavily modified Boeing 747, Air Force One.

"SOFIA has a lot of characteristics that are very similar to a normal passenger airplane," explains Erick Young, Director of the SOFIA Science Center at NASA/Ames Research Center in Mountain View, CA. "Except for the fact that we’ve completely gutted the insides, and there’s a hole in the side of the airplane the size of a garage door, and there’s a 17-ton telescope mounted in the back. But other than that it’s pretty much like a regular airplane."

It may appear modest from the outside, but SOFIA is more than a telescope tucked into a re-purposed commercial airliner. Obscured from view is a complete flying astronomical observation platform centered around a 2.7 meter-wide reflecting telescope, which carries a dozen or more astronomers, observers and crew above the clouds to observe objects and phenomena too cold to be seen in visible light.

WS301_SOFIA_young — Erick Young is the Director of the SOFIA Science Center, and is responsible for the airplane crew and science operations of the observatory. Credit: NASA / Tom Tschida.

"The appearance of things that we can see in visible light is primarily because things are hot enough to give off light at visible light wavelengths," says Young. " If you get things too cold, then things look redder and redder and eventually they’re so red that the human eye can’t see them anymore. And then what we are actually sensing is a different kind of light and it’s called infrared. And that’s basically the heat radiation that’s coming from objects. And so what we can look at are not things that are thousands of degrees hot, but things that are hundreds or tens of degrees above absolute zero. And it turns out that there’s a lot of material in the universe- the dust, planets like the Earth, clouds in space. They’re all too cold to normally emit in visible light, but by looking in the infrared, we’re able to sense them, detect them, and measure their properties."

Our most familiar instrument for observing and measuring the electromagnetic radiation emitted by objects, the human eye, senses a narrow range of energy wavelengths. These are waves with wavelengths of 380 nanometers (violet) to about 740 nanometers (red). Although astronomy has its origin in observing the universe in this part of the spectrum, there are also telescopes and instruments that measure incredibly short, energetic wavelengths such as Gamma rays and X-rays, as well as extremely long wavelengths such as radio waves. Infrared or "IR" astronomers study that expansive swath of wavelengths just below the visible range, but still above the radio end of the spectrum.

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Source: NASA

But why go through the trouble of observing objects in the IR spectrum from the stratosphere, 12+ kilometers above the surface of the earth? There are several major infrared telescopes operating both on the ground and in space: at the Mauna Kea Observatories in Hawaii, or Kitt Peak National Observatory in Arizona, for example.

"We put a telescope on the airplane because there are parts of the spectrum which are completely blocked and completely opaque in the Earth’s atmosphere. This was primarily in the infrared part of the spectrum," explains Young. "The main thing that blocks the infrared light from reaching the ground is water vapor in the Earth’s atmosphere. And if we want to observe many of these wavelengths, we have to get to some place where there’s no water vapor and SOFIA will fly above more than 99% of the water vapor in the atmosphere."

Flying telescopes may not be commonplace, but are hardly a new idea. SOFIA is merely the latest and largest in a line of airborne observatories going back to the 1920's, when eclipse chasers first carried a modest instrument aboard a 2-seater biplane. SOFIA's most recent ancestor is the the Kuiper Airborne observatory (KAO), a converted C-141 aircraft with a 36-inch mirror that flew missions from 1974 to 1995, and can be seen peacefully enjoying its retirement on a nearby patch of Moffett Field tarmac. The idea within NASA to use a 747-SP as a telescope platform goes back at least to the late 1970's. Technical challenges, years of delays and cost overruns nearly ended the project more than once. But SOFIA' construction eventually prevailed, test flights began in 2010, and it made its first scientific observations this past year.

Although all science operations are managed here in the Bay Area within Moffett Field's NASA Ames Research Center, SOFIA's primary home is near Palmdale, California, at Dryden Flight Research Center. SOFIA is a collaboration between NASA and the German Aerospace Center, DLR (Deutches Zentrum fur Luft-und Raumfahrt). The Germans provided the telescope; NASA provided the airplane and crew responsible for the science operations.

Another operational advantage of flying the instruments on a plane is flexibility. Unlike IR telescopes that we have launched into space, such as the Spitzer Space Telescope, SOFIA comes home every night. It can swap out, repair, or update existing instruments as needed. To take advantage of this flexibility, SOFIA has available a collection of 9 specially-designed primary instruments to attach to the telescope, that each cover a specific range of wavelengths across and around the IR spectrum. Lastly, these instruments need not hew to the stringent weight requirements of their space-based counterparts, which saves money and allows use of instrumentation normally too big to launch on a rocket.

The nine "first-light" SOFIA instruments plotted on the axes of spectral resolution and observing wavelength. Source: NASA.

After a successful run of several science test flights in 2011, SOFIA is currently in the shop for a major upgrade to its avionics systems. Upon returning later in 2012, the observatory plans to ramp up to 3 missions week by 2014, a schedule it hopes to keep for the next 20 years.

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Erick Young expects that the future of IR astronomy with SOFIA is very bright - or warm, as the case may be. In fact, as the 1st generation of instruments are just being put through their paces, the call for the second generation has already begun. "The instrumentation that’s available on the infrared is still rapidly evolving, particularly at the very long wavelengths that SOFIA operates at. The technology is still relatively in the infancy. And so one can expect that as the years go by, we’ll have huge increases in our capabilities as the technology improves. And SOFIA will definitely be able to take advantage of that."

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