A Telescope Takes Flight

Instruments will see newborn galaxies from behind the wing of a plane

Photo: Stratospheric Observatory for Infrared Astronomy

EYE IN THE SKY: The Sofia space telescope [not visible], lodged on a Boeing 747 jumbo jet, makes its first test flight above Waco, Texas, on 26 April.

A landmark moment in the exploration of the deep cosmos occurred recently. A powerful flying telescope, SOFIA--the Stratospheric Observatory for Infrared Astronomy--made its first checkout flights, having survived a bureaucratic near-death experience only a year ago.

SOFIA is a joint project of NASA and DLR, Germany's space agency, based in Bonn and Stuttgart. An infrared telescope with remarkable spectral range, it picks up the mantle of the Kuiper project, which took to the skies in the 1970s and 1980s, peering from the side of a Lockheed C-141 Starlifter transport. Kuiper was groundbreaking, but SOFIA, seated aft of the wing of a Boeing 747 jumbo jet, has more scope: it will scan the deep heavens, where stars are forming and the universe is still being born.

SOFIA flies at 13 300 meters, above 99 percent of the atmospheric water vapor that obstructs infrared observation from the ground. A hatch in the plane, which is traveling at 800 kilometers per hour, opens the telescope to the elements; its mirror is 2.7 meters across. The astronomers sit comfortably behind a pressurized bulkhead and monitor any of a number of observation instruments, which can be switched out to serve different purposes.

Because of its size, SOFIA can carry large, power-intensive instruments and high-resolution spectrometers, NASA program executive Ray Taylor says. That gives astronomers a wavelength range for observation rare in its breadth, from 0.3 micrometers, which is near the visible range, through the far infrared, out to 1600 µm.

Infrared telescopes cut through the gases and clouds in the universe that obscure optical observation. Project scientists plan to use the broad spectral range across nine instruments--seven designed in the United States, two in Germany--to explore what powered the luminous galaxies that lit up the early universe, peek into ”dark” clouds to take a census of new stars, see near the center of black holes, look at the physical conditions of the universe's infancy, and map out chemical architectures in the interstellar medium.

SOFIA can do it all while exposed to the winds and turbulence of a 747 in flight, chief engineer Nans Kunz says. Project technicians spent years in custom- building and modifying Kuiper-era ideas to keep the 14 500-kilogram apparatus stable through turbulence.

To that end, they built a kind of dumbbell structure that keeps the center of gravity in the very middle. On one side are the cabin area, counterweights, and observation instruments, pressurized and warm, and on the other is the telescope, chilled to stratospheric temperatures, with no infrared leakage from inside the aircraft. Connecting the two is a carbon-fiber tube, and in the very middle of the tube is a 1.2-meter-diameter cast-iron bearing with a hole in it that channels the light from the big primary and smaller secondary mirrors of the telescope. The bearing, which rests on a 15-µm-thick film of warmed oil, mechanically isolates the telescope from the aircraft and allows it to point independently.

The science instruments and counterweights balance the telescope, and the result is an apparatus that can be aimed and kept still, mostly on account of its own inertia. Torque motors and spherical brakes keep the telescope tracking the desired stellar target, while gyros keep track of the telesope's absolute position.

German astrophysicists, engineers, and designers built the US $100 million telescope in Mainz and Stuttgart, forging and polishing the big mirror in Paris before flying the entire apparatus to Waco, Texas, in 2002. It sat there while engineers worked on modifying SOFIA's carrier airplane and advocates lobbied in Washington to keep funding alive for the project. Its cost is now estimated at $570 million.

Complicating matters was the 2003 launch of the Spitzer observatory, a space-based infrared telescope that is already producing great results. Critics raised eyebrows at spending so much money to turn a jumbo jet into a flying infrared observatory while a space-based infrared telescope was beating it into operation. Citing repeated delays, slipping schedules, and ballooning budgets, NASA announced last year that SOFIA would lose its funding.

That produced a swift reaction from astronomers, who wondered why a project would be abandoned so close to being finished. SOFIA was not meant to be a rival to Spitzer, they argued, but a complement. Spitzer, at supercooled orbital temperatures, sees clearly. But it carries only three observation instruments, and they are small because of space constraints. SOFIA will work with its nine instruments, and their size will allow a broad spectral range for observations.

Besides, SOFIA's advocates pointed out, it would be operational longer than Spitzer. Academic and amateur astronomers orchestrated a letter-writing campaign, and surely there was a quiet intervention on the part of Germany's government, which is led by Chancellor Angela Merkel, a physicist by training. NASA backtracked, putting the project on review and finally reinstated its budget. NASA slated $75 million for the project this year, with $3.2 billion expected over SOFIA's life through 2030.

SOFIA made its first flight on April 26, its hatch closed for the first of several shakedowns this summer that will generate systems specs for the 747 and the telescope. It is a sensitive piece of machinery, and caution is still the rule. After two checkout flights, Taylor says, SOFIA will move to the Dryden Flight Research Center, at Edwards, Calif., in the Mojave Desert. Plans call for gradual checkout of the science instruments--some of which are still being developed--through next year, with the first real science missions to follow.

The project's plans have been stretched before. But with a budget in place, SOFIA is close now. Close as the heavens.

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