With NASA deciding to press ahead with the SOFIA airborne astronomy observatory, another mission-an expensive planet-hunting spacecraft in development for more than a decade-is being scaled back to a mere technology-development program.

The restoration of the Stratospheric Observatory for Infrared Astronomy (SOFIA), a U.S.-German effort to field a Boeing 747 jetliner equipped with a 2.5-meter telescope, had been widely anticipated. NASA's Program Management Council, a senior body chaired by the agency's third in command, all but made it official when it ruled in mid-June that there were no insurmountable technical or programmatic hurdles standing in the way of SOFIA's completion. While NASA Associate Administrator Rex Geveden said at the time that the agency still needed to consider "funding options and sources before we decide whether to continue the mission," the direction the agency was heading was obvious.

NASA Administrator Mike Griffin, addressing the NASA Advisory Council's science subcommittee here July 7, ended what little suspense there was by announcing SOFIA will go forward. However, Griffin did have one surprise for the group: in order to pay for SOFIA's completion, the Space Interferometry Mission (SIM) would be reduced to a research and technology program.

SIM was conceived in the early 1990s as a way to measure the distances between stars with unprecedented accuracy and identify solar systems likely to harbor Earth-like planets. The multibillion-dollar mission is managed by the Pasadena, Calif.-based Jet Propulsion Laboratory with Northrop Grumman Space Technology under contract as the NASA lab's industrial partner. While always envisioned as a powerful observatory in its own right, SIM also was intended to lay important technical ground for the Terrestrial Planet Finder missions NASA hopes someday to conduct to image Earth-like planets around neighboring stars.

Griffin said NASA's decision to refocus SIM on technology development was based on guidance from the National Academy of Sciences' decadal surveys of astronomy and astrophysics priorities that in 1990 and again in 2000 ranked SOFIA higher than SIM. He also pointed out that National Research Council's Space Studies Board in May warned that NASA's astrophysics program was starting to tilt too heavily toward big, expensive missions, such as the $4.5 billion James Webb Space Telescope.

Griffin did not say what SOFIA would cost to complete, and NASA spokeswoman Erica Hupp did not have such details immediately available. However, Griffin did say that as a condition of SOFIA going forward, responsibility for flight testing the telescope-equipped aircraft would shift from Ames Research Center near San Francisco to the Dryden Flight Research Center, which is co-located with Edwards Air Force Base outside Los Angeles.

Under the new plan, according to NASA sources, Ames would retain scientific management of SOFIA, but the aircraft would continue to operate out of Dryden once science flights begin in 2009.

NASA has spent about $500 million to date on SOFIA, which is several years behind schedule for a variety of technical and budgetary reasons. Still, when NASA sent Congress a 2007 budget request in February that included no money beyond this year for SOFIA, the apparent defacto cancellation of the program shocked astronomers and angered NASA's German partners who had spent nearly $100 million on SOFIA's state-of-the-art optics. Under fire from SOFIA backers, their allies in Congress, and the Germans, NASA insisted it had made no final decision on SOFIA.

By reducing SIM from a full-fledged flight program to a technology-development effort, NASA should free up enough money to finish SOFIA, which is on track to begin flight tests in the year ahead.

Since early last year, NASA has cost-capped SIM's yet-to-commence hardware development phase at $1.2 billion and pushed its launch date back from 2011 to 2015, and then again to 2016. But NASA's latest publicly available budget shows the agency still is expected to spend $1 billion on the program through 2011.

The high annual carrying cost of a mission still so far from launch was a concern to astronomers coming to grips with NASA's tightening science budget. The Astronomy and Astrophysics Advisory Committee, a body established in 2003 to advise NASA, the National Science Foundation and the Department of Energy, singled out SIM in its 2006 Annual Report as a mission NASA should consider funding at a lower level in the absence of additional funds for the agency's Science Mission Directorate, which is expecting a flat budget through the end of the decade.

"NASA has sequenced the large Astrophysics missions as [Hubble Space Telescope, James Webb Space Telescope], then SIM. Yet, the current SIM budget is high, and the funding profile is steep, for a mission that is 9-10 years from launch," the report said. "[The Astronomy and Astrophysics Advisory Committee] strongly recommends increasing the overall budget for [the Science Mission Directorate], but if the increases are inadequate, some consideration should be given to a more appropriate, lower level of current support for SIM."

One Washington source who closely tracks NASA's space science programs said reducing SIM to a technology effort could ultimately lead to the mission's demise. "If this goes through as NASA intends, SIM is dead," the source said.

SOFIA is managed by the Columbia, Md.-based Universities Space Research Association.