NASA Administrator Mike Griffin said his decision to delay indefinitely the Space Interferometry Mission (SIM) in order to pay for pressing ahead the SOFIA airborne astronomy observatory came down to respecting the stated priorities of the National Academy of Sciences and honoring the agency’s commitment to its German partners on the nearly completed project.

Griffin said SOFIA was the obvious choice once it became clear that no major technical roadblocks stand in the way of getting the flying observatory airborne.

“The issue in my mind on SOFIA was always whether or not we could get to the finish line technically,” Griffin said at a July 12 Capitol Hill luncheon sponsored by the Aerospace Industries Association. “An independent and expert panel has come back to me and decided that yes we can get to the finish line technically, and since we can I think therefore we should.”

SIM, for its part, has yet to enter the hardware-development phase, which NASA cost capped at $1.2 billion after restructuring the program last year and pushing the launch date back from 2011 to perhaps 2016. Even with the later launch date, NASA still expected to spend more than $1 billion on SIM between this year and 2011.

With NASA’s decision to go forward with the Stratospheric Observatory for Infrared Astronomy, or SOFIA, Griffin said the agency cannot afford to keep SIM on track for a 2016 launch given everything else the agency needs to do within its limited astrophysics budget.

“Given that a Hubble [Space Telescope] repair mission — if we can do it technically — is a matter of law, and given that James Webb Space Telescope is the National Academy’s first priority in astrophysics, that basically uses up the astrophysics money, and SIM, as I have said a number if times in past months, must be delayed,” Griffin said. “It’s not that we don’t think SIM is worth doing. It’s not that we won’t do it eventually and will — but not everything can be our first priority.”

Griffin said that SOFIA was chosen over SIM based on a combination of international political considerations and the stated recommendations of the National Academy of Sciences, which in 1990 and 2000 ranked SOFIA ahead of SIM in its decadal surveys of astronomy and astrophysics priorities.

“With regard to SIM versus SOFIA, those decisions have been made in accord with the — not only the original but subsequently reiterated — decisions and priorities of the National Academy of Sciences and in respect, frankly, of agreements with our international partner on SOFIA, Germany,” Griffin said.

SOFIA, had been in limbo since the beginning of the year when it first became clear that NASA would be requesting no money for the U.S.-German program beyond 2006.

NASA officials struggled to convince SOFIA supporters here and abroad that the program’s apparent de facto cancellation was only a strategic pause to allow for a thorough review of any technical issues still standing in the way of the telescope-equipped 747 jetliner completing test flights and beginning science operations.

In June, NASA announced that an outside panel headed by former NASA Dryden Flight Research Center Director Kenneth Szalai found that SOFIA faced no insurmountable technical challenges. NASA’s Program Management Council concurred with the findings and recommended proceeding with SOFIA provided the necessary funding could be found.

Continuing SOFIA also has the support of the Senate Appropriations Committee. The report accompanying the NASA spending bill the committee approved July 13 urges NASA to fund SOFIA now that it has passed its technical review. However, the same report sharply criticizes the NASA Science Mission Directorate’s handling of the SOFIA issue, however, following so closely the cancellation and subsequent resurrection of the Dawn comet-chasing mission. The pair of flip-flops, the report says, “calls into question the credibility of the science directorate in making budget decisions and determining scientific priorities.”

Despite 10 years of stop-and-go development effort and approximately $500 million spent to date, SOFIA is still two or three years from conducting its first science flights. NASA estimates the total cost of the program, including 20 years of flight operations, at around $1.6 billion.

While NASA’s decision to go forward with SOFIA has pleased German space officials, the contractors working on the flying observatory, and the scientists looking forward to using it, SIM boosters feel maligned.

“The investment in SIM will reap long-term benefits for science. The investment in SOFIA will be important and beneficial but not of the same magnitude. In some ways SOFIA was funded because of politics over scientific considerations,” said a member of the SIM scientific community.