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Thomas Keilig manages SOFIA’s telescope and science instruments. (Chad Slattery)

A 747 for Star-gazing

How engineers altered a jumbo jet to carry the world's biggest airborne telescope.

On several nights this winter, in the frigid stratosphere over the Pacific Ocean, the pilots of a dramatically modified Boeing 747SP will relinquish control of the aircraft, and astronomical mission controllers will take over, steering the jumbo jet westward along a slightly curved star-tracking course at 520 mph. In the former passenger cabin where rows of seats used to be, two dozen astronomers and technicians at computer workstations will sit poised for an event unprecedented in the history of aerospace.

At a computer command, a giant door in the rear fuselage, just behind the wings—a door nearly as wide and half again as high as one on a two-car garage—will open the aft left side of the aircraft, exposing an enormous cavity to the rarefied atmosphere. Inside this cavity, a reflecting telescope with a mirror nearly nine feet across will point toward an invisible celestial object and gather its light at far infrared wavelengths, while just feet beyond the telescope’s end, air will whip past at nearly the speed of sound.

This airborne observatory is SOFIA, the Stratospheric Observatory For Infrared Astronomy. SOFIA is a joint creation of NASA, which oversaw the extensive modification of the aircraft, and the German Aerospace Center DLR, which managed the construction of the telescope, the largest ever borne aloft. The 2.7-meter telescope is bigger than NASA’s famed 2.4-meter Hubble Space Telescope, and even bigger than the 100-inch (2.5-meter) Hooker reflector atop Mount Wilson, California, which reigned as the largest telescope in the world from 1917 until 1948 (when it was surpassed by the 200-inch Hale on Palomar Mountain in San Diego County).

SOFIA also is an unparalleled undertaking in aircraft modification. The aircraft is flying with a hole in its side that extends nearly a quarter of the way around the circumference of the fuselage; the telescope cavity door is not a structural component. Just how was the 747SP reconstructed so that, absent a quarter section of its fuselage, it could withstand normal aerodynamic loads and buffeting at Mach 0.85 without the tail twisting off?

IT WAS IN THE EARLY DAYS of NASA that astronomers conceived of flying a telescope in the stratosphere to observe the universe at far infrared wavelengths. Across a broad swath of the electromagnetic spectrum, from visible light to wavelengths measured in millimeters (1,000 micrometers), astronomers are effectively blind: Atmospheric water vapor blocks much of the infrared radiation from reaching Earth. To be sure, cryogenically cooled detectors on telescopes at mountaintop observatories can observe short infrared wavelengths, such as heat radiation. But even at the highest and driest locations, such as in Chile’s Atacama desert, residual atmospheric water vapor precludes observations in most of the far infrared.

An aircraft flying in the stratosphere, however, is above 99.8 percent of the vapor. The stratosphere’s dry cold (as low as –60 degrees Fahrenheit) can keep the entire structure and primary mirror at cryogenic temperatures without condensation or frost, although liquid nitrogen is needed when the airplane is on the ground just before and during takeoff to pre-cool the telescope to stratospheric temperatures before the cavity door is opened. Unlike a spacecraft, an aircraft periodically returns to earth; when it does, its cryogenics can be replenished, so an airborne observatory has a lifetime as long as that of the aircraft itself—about 20 years or more. (Most space-based infrared observatories don’t last as long; the Spitzer Space Telescope, for example, was launched in 2003 and ran out of cryogenics in 2009.) And unlike a ground observatory, an airborne one can be flown anywhere in the world for the best view of important celestial events.

In the early 1960s, NASA modified and flew several aircraft carrying telescopes to observe in the infrared; the last was the Gerard P. Kuiper Airborne Observatory (KAO), a modified Lockheed C-141A Starlifter carrying a 36-inch reflector. From its first science mission, in 1974, the KAO, operated from NASA’s Ames Research Center at Moffett Field, California, revealed the structure of Pluto’s atmosphere, the gas and dust between stars, and stunning cosmic processes involved in the birth of stars. It also discovered rings around Uranus.

The infrared marvels it revealed were so exciting that “astronomers immediately began wondering what they could do with a telescope that could collect roughly 10 times more light,” recalls Edwin Erickson, KAO’s facility scientist at Ames and later SOFIA’s first project scientist (now retired). Erickson described the potential of such a telescope in a 1980 paper, and by January 1986, Ames had established a SOFIA study office. Over the next decade, Ames scientists tested models of several open-port designs in wind tunnels. In 1995, NASA decommissioned the KAO so its budget could be applied to the full development of SOFIA, which began in 1996.

“As a replacement for the KAO, the science community wanted the largest possible telescope that could be flown on the largest possible aircraft at 41,000 feet or higher for as long as possible,” says Ames’ Nans Kunz, SOFIA chief engineer from its inception until 2007. Like a family on a budget shopping for the best used car, NASA was restricted by its funding to a used aircraft. SOFIA’s basic mission specs immediately narrowed the aircraft candidates to a handful of then-available military cargo planes and large commercial airliners. After scientists compared the finalists for fuselage size, weight-carrying capability, and ability to cruise for hours at high altitude, the hands-down winner was a Boeing 747 jumbo jet. The 747 had a record of reliability in commercial service, readily available spare parts, and an estimated lifetime of at least another 20 years.

Most promising was the unusual short-body 747SP, or Special Performance model, produced in the 1970s and 1980s. Only 45 were built, primarily for airlines wanting to offer long-range, nonstop service between cities nearly half the globe apart, such as New York and Tokyo. The 747SP has the same engines and wingspan as a full-length 747-100, and carries the same amount of fuel, but it is 48 feet shorter, making it lighter and thus longer in range. To compensate for the handling changes that shortening the fuselage produces, designers made the 747SP’s tail two feet taller and 10 feet wider than a standard 747’s.

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