WASHINGTON — An accelerated test schedule for the U.S.-German Stratospheric Observatory for Infrared Astronomy (SOFIA) was the unintended consequence of the telescope-equipped jetliner being forced to land in California last month with its large retractable door half-open, according to NASA officials.

“The door wouldn’t close one day when it was up there flying and we had to land with the door open,” Ed Weiler, NASA’s associate administrator for science, recalled during a March 10 speech at the Goddard Memorial Symposium in Greenbelt, Md.

NASA planned to wait until this summer to land the modified Boeing 747-SP jetliner at the agency’s Dryden Air Operations Facility in Palmdale, Calif., with the flying observatory’s telescope-cavity door partially open. But during a Feb. 11 flight test, a malfunction forced SOFIA’s pilots to make an open-door landing much sooner than expected.

“Some call it an anomaly. I call it an acceleration of schedule,” Weiler said. “The good news is it landed successfully with no problems.”

Though SOFIA was previously scheduled to start making limited initial science runs in late 2009, NASA is holding off on taking astronomers aloft until SOFIA completes flight tests aimed at proving the modified Boeing 747-SP can fly with its roughly 4-meter-tall retractable telescope door open.

SOFIA program executive Ray Taylor says NASA has planned all along to conduct a series of 20 open-door flight tests, including a series of landing tests to ensure the safety of both the aircraft and telescope if the door malfunctions in flight.

During the Feb. 11 test, Taylor said the flight crew “was able to activate the pre-planned procedures for landing with the door open and that’s exactly what took place.” He said the malfunction was caused by a “loose connector in the telescope cavity door” and that the problem has been corrected and has not reappeared.

NASA has conducted five open-door runs since flight testing began Dec. 18. Taylor said NASA no longer plans to conduct a test simulating the conditions of the Feb. 11 incident in which the telescope cavity door remained roughly half-open during landing.

“Now that it’s been done, we don’t have to do that particular test,” Taylor told Space News.

However, despite the happy accident in SOFIA’s test schedule, Taylor said no acceleration in the overall program would occur. In fact, he said the telescope-equipped jetliner’s initial science flight, planned for this spring, has been pushed back by various technical difficulties to the end of this year.

In development for the past 15 years, SOFIA is designed to fly high above the clouds to gain glimpses of the universe not visible to ground-based telescopes. The heart of SOFIA is a 2.5-meter infrared telescope provided by the German Aerospace Center, DLR, and designed to be outfitted with a changing lineup of instruments.

SOFIA’s projected life cycle cost — a figure that includes development plus 20 years of operations — have increased to $2.98 billion. As of this year, NASA has spent about $614 million on SOFIA — more than double the $265 million NASA expected to spend when it started the program in 1995.

Taylor said SOFIA’s observatory should make its so-called first light observation “roughly before summer,” though the schedule depends heavily on good weather. “And then we’re going to work very hard to get our first actual science observation before the end of the year.”

Taylor attributes the program delay to the complexity associated with system development, integration and test.

“This is a never-before-attempted complex system, and it just takes time to work everything out,” he said.

Additional science observations will be folded into SOFIA’s flight plans as the aircraft is cleared to retract its door at increasing altitudes. SOFIA will fly with one instrument fitted to the telescope for each airborne observation period. For science missions, the telescope will typically fly three nights per week for between 800 hours and 900 hours of observing time per year.

“The early science is a significant milestone,” Taylor said. As development continues, NASA expects to begin commissioning the observatory’s two German and six American science instruments, most of which are designed to study the universe in the infrared spectral band across ultraviolet, visible, infrared and sub-millimeter ranges, operating at wavelengths that ground-based observatories cannot use to penetrate Earth’s dense atmosphere, he said.

“These instruments fall into two broad categories: imagers and spectrometers. … Imagers take the pretty pictures and spectrometers look at the chemical composition of the interstellar medium,” he said, describing the dusty areas between solar systems that SOFIA’s instruments will observe.

Taylor said the instruments include the Echelon-Cross-Echelle Spectrograph (EXES), a mid-infrared spectrograph designed to provide high-, medium- and low-spectral resolution to acquire data that cannot be replicated by other ground or space-based mission currently available or planned for future missions. The project is currently undergoing testing at NASA’s Ames Research Center, Mountain View, Calif., and is slated for its first flight on SOFIA in 2013.

Another instrument is the high-resolution Ames Research Center,designed to observe star formation and stellar clusters in the far-infrared. The instrument is being developed in collaboration with NASA’s Goddard Space Flight Center, Greenbelt, Md., and Rochester Institute of Technology, N.Y.

The CalTech Submillimeter Interstellar Medium Investigations Receiver will be used to study a wide range of astrophysical problems ranging from the evolution of galaxies to the birth and death of stars. The Faint Object Infrared Camera for the SOFIA Telescope will obtain multicolor images of protostellar environments, young star clusters, molecular clouds and galaxies. And the High-speed Imaging Photometer for Occultation is designed to provide simultaneous high-speed time resolved imaging at two optical wavelengths.

Two German contributions to SOFIA’s instrument suite include the German Receiver for Astronomy at Terahertz Frequencies, which would measure the submillimeter and far-infrared spectral range, and the Field Imaging Far-Infrared Line Spectrometer, a tool for astronomical 3-D imaging that would monitor merging and interacting galaxies.