SOFIA Flies!
Published on Apr 27, 2007 at 2:27 pm.
1 Comment.
Filed under aeronautics, airborne telescopes, observatories, telescopes.
Visual light is only a small part of the electromagnetic spectrum. While historically astronomy started with observations of visual light, that really doesn’t see everything that there is to see out there. Many objects emit primarily infrared light, radio waves, X-rays, or any of the other wavelengths of electromagnetic radiation. And, a lot of things simply can not be seen with visual light alone. In fact, modern astronomers would like to make use of non-visual wavelengths at least as often, if not more, than visual light. Unfortunately, most wavelengths of electromagnetic radiation don’t make it through the atmosphere. Visual light, makes it through. Radio waves also make it through the atmosphere. Some infrared light makes it through much of the atmosphere. But, the lowest portions of the atmosphere interfere with infrared astronomy. For this reason, many telescopes are placed on mountain tops. But, you can get even higher than a mountain — on an airplane. The higher you go, the less that the atmosphere distorts and blocks your view. Ultimately being in space, far above the atmosphere’s effects, is the best place for a telescope, but it is also very expensive.
But, an aircraft can fly high enough to get above most of the Earth’s atmosphere. So, why not put a telescope in an aircraft? NASA actually did that. In 1974, the Kuiper Airborne Observatory (KAO) took flight. The KAO was a converted C-141 aircraft (seen above). It carried a 91.5cm diameter telescope optimized for infrared astronomy. The KAO could operate at altitudes far above most of the water vapor in the atmosphere that interferes with infrared astronomy. But, the telescope could not just look out a window into space. Rather, the telescope had to be open to the outside, so a shutter had to be fitted on the aircraft that could be opened in flight, exposing the telescope. Looking through a window would have induced distortions, but worse, the material of the window, not being at absolute zero itself, would be radiating in the infrared, and would have completely outshone the objects trying to be studied, so the telescope bay had to be open. This presented a series of engineering and aeronautical issues that had to be overcome. While optimized for infrared, the KAO was not limited to infrared, and it could be used as any other telescope, only being mobile. That meant that it could be flow into position when no other observatory was in the area to observe some astronomical event. Being able to fly higher than mountains, KAO was able to observe in the infrared to make discoveries that could not have been made otherwise at the time. The list of discoveries that KAO made is too long to go into here, but it is very impressive, and includes such things as discovery of the rings of Uranus, mapping of the galaxy, discovery of Pluto’s atmosphere, and discovery of new organic molecules in star forming regions. But, the KAO was retired in 1995, and now sits moth-balled at Moffett Field in California.
So, why retire KAO if it was doing such a wonderful job? Well, NASA wanted to build a bigger and better one! The new airborne observatory was called the Stratospheric Observatory for Infrared Astronomy (SOFIA). The new airborne observatory is a 2.7 meter telescope in a converted Boeing 747SP (the aircraft at the top of this post). Though the mirror is 2.7 meters in diameter, you might sometimes see SOFIA listed as a 2.5 meter telescope. That is because the optical design of the telescope uses only 90% of the mirror’s surface at any one time, making the effective aperture only 2.5 meters. This is much the way that the Hobby Eberly Telescope in west Texas works.
The aircraft was originally delivered to Pan American Airlines in 1977. The airplane then was purchased by United Airlines in 1986. United retired the aircraft in 1995, and it was purchased by NASA in 1997. The aircraft modifications to remake it from an airliner into an observatory were performed by L-3 Communications Integrated Systems in Waco, Texas, (where I got to see it while the plane was gutted). Unlike the KAO, SOFIA is an international effort, with NASA cooperating with DLR (the German Aerospace Center) in building and operating SOFIA.
This aircraft is really a feat of aeronautical engineering. Engineers had to figure out a way to make a 747 fly with nearly a quarter of its airframe remove just forward of the tail. Not only that, but there is a huge opening just forward of the tail! SOFIA will be operating at altitudes of up to about 14 kilometers, well above 99% of the water vapor in Earth’s atmosphere. The forward section of the aircraft is pressurized, and there are computers and accommodations for students and astronomers. The telescope cavity is pre-cooled to operating temperatures using liquid nitrogen. The telescope and everything around it need to be as cold as possible so that they don’t emit any more infrared radiation than necessary. Observing IR with a warm telescope would be like using an optical telescope with lights on in the observing dome. And, just in case the door stick open while in flight, the aircraft has to be able to land with the doors in the open position — another aeronautical problem.
The idea of taking a 747 and putting a telescope into it is so impressive that it even catches the attention of non-astronomers, as seen in this posting by Aviatrix at Cockpit Conversations. But, SOFIA will have unique capabilities that sets it apart from any other telescope on Earth, and that is important. It will even have unique capabilities different from orbiting telescopes. So, this will truly be a one-of-a-kind instrument. And, SOFIA is about ready to start work. Yesterday the aircraft had its maiden flight after the modifications. From what I hear, everything went OK. The aircraft will soon be flown to Edwards Air Force Base for extensive flight tests before it goes to its future home with NASA Ames Research Center at Moffett Field, near San Francisco, in California.
-Astroprof
(Images courtesy of NASA)
Astroprof’s Page » SOFIA Aircraft Dedicated on May 21, 2007 at 1:17 pm: 1
[…] The Stratospheric Observatory for Infrared Astronomy (SOFIA) took to the air a month ago. Now, the aircraft carrying the telescope will be rededicated. This particular aircraft had been dedicated as “Clipper Lindbergh” in 1977 by Anne Lindbergh, Charles Lindbergh’s widow, when it began service for Pan American Airlines.  Now, Erik Lindbergh will rededicate the aircraft to his grandfather on the 80th anniversary of Charles Lindbergh’s historic solo trans-Atlantic flight, according to a NASA press release.  The rededication is this morning in Waco Texas at the Texas State Technical College’s airport. […]