Although many Boeing 747s are slowly being retired, the version that sits in a hangar about 100 miles south of Dallas is getting a face-lift of astronomical proportions.

The former Pan Am passenger jet, which first flew in 1977 as the Clipper Lindbergh, has been set aside for a stratospheric mission - to look at the universe.After engineers cut a hole in the side of the aircraft, it will house the largest airborne telescope ever built - more powerful than many ground-based telescopes and larger than the Hubble Space Telescope.

The Stratospheric Observatory for Infrared Astronomy, or SOFIA, will allow astronomers to peer deep into vast dust clouds to witness the birth of stars, observe galaxies and quasars billions of light-years away and study the very early universe.

Infrared is an invisible part of the electromagnetic spectrum that is sensed by humans as heat. Clouds of gases and dust, where new stars are born, are often difficult if not impossible to see in visible light.

Among other missions, scientists plan to fly the jet to the Southern Hemisphere to observe the center of the Milky Way, satellite galaxies of the Milky Way, or phenomena that require urgent investigation in the infrared spectrum, such as supernovas.

Eric Becklin, an astronomy professor at the University of California at Los Angeles and the project's lead scientist, is not shy about trumpeting the prospects of the new observatory.

Becklin said it will ``open up the universe' and be as big a leap in observational astronomy as was Galileo's telescope.

NASA and its German counterpart expect to spend about $375 million developing SOFIA. Slightly more than 100 missions per year are planned over 20 years at an average of about $200,000 a flight.

As a joint project between NASA and the German Aerospace Center, scientists and engineers are modifying the airplane at a Raytheon Air Integration Systems hangar in Waco.

To do its job, SOFIA will rise above 99 percent of the atmosphere. Cruising between 41,000 and 45,000 feet in the dark of night, astronomers will open a huge portal behind the aircraft's left wing to expose the telescope's mirror and mounting.

Raytheon program manager Dennis Boroczk said although his teams have converted passenger planes to cargo configurations, they have been challenged by the task of cutting a large door intended to be opened in flight.

``From a production standpoint, this is like our baby,' said Frank Walker, a Raytheon production manager for SOFIA. ``And, yes, we worry about it from the day we start on the project until the day it's delivered.'

SOFIA will cruise at 550 mph with the door open, and the minus-40 degrees of the atmosphere should match that of the unpressurized telescope compartment, which is refrigerated before takeoff to prevent condensation from forming on the optics. A 30-inch thick pressure bulkhead will protect scientists from the near vacuum and fatal cold of the stratosphere.

Once operational at the end of 2002 or early 2003, SOFIA will be the largest airborne observatory, replacing the Kuiper Airborne Observatory, which flew aboard a Lockheed C-141 from 1974 to 1995.

At 8.25 feet in diameter, the mirror of the SOFIA telescope is slightly larger than Hubble, almost the same size as the Hooker telescope at Mount Wilson, Calif., and about eight times larger in surface area than Kuiper.

The largest optical telescope, the Keck telescope at Mauna Kea, Hawaii, is 33 feet in diameter and four times that of SOFIA, but moisture in the atmosphere severely limits Keck's ability to study infrared targets, a problem for all ground telescopes.

``Water just loves to absorb infrared radiation,' Becklin said. ``SOFIA flies above most of the absorbing water and catches the photons before they get absorbed.'

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