WASHINGTON — A NASA airborne observatory that enjoys unusual protection from regular reviews resumed science flights recently after an extended maintenance period.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) made its first science flight in more than half a year May 22, a 10-hour flight out of NASA’s Armstrong Flight Research Center in California. SOFIA is scheduled to fly to New Zealand June 1 for a campaign of southern hemisphere observations.

The science flight was the first since late 2017 for the airborne observatory, a Boeing 747SP with a 2.5-meter infrared telescope. The plane flew to Germany in November 2017 for maintenance known in aviation as a “C-check” by Lufthansa Technik. That work was scheduled to be completed in January.

However, during that C-check, workers found a fuel leak in the aircraft where the outer left engine attaches to the wing. In addition to repairing that leak, Boeing directed Lufthansa Technik to inspect all four engine pylons for potential leaks, a process that took several more weeks. That work was finally completed in May, and SOFIA flew back to the United States May 18.

SOFIA provides a unique platform for performing infrared astronomy, flying above most of the infrared-absorbing water in the atmosphere while also accommodating multiple instruments that can be updated or replaced. However, SOFIA is among the most expensive astrophysics missions to operate, second only to the Hubble Space Telescope.

SOFIA, whose development dates back to the 1990s, is exempt from a process called senior reviews where the agency examines the status of science projects that have completed their prime missions to determine if proposed extensions should be funded. A provision in the report accompanying the omnibus spending bill for fiscal year 2018 prevents NASA from including the mission in the next astrophysics senior review in 2019.

Congress made that determination by declaring that SOFIA now has a prime mission of 20 years. “The agreement notes that SOFIA, which began its prime mission in 2014, has a prime mission lifetime of 20 years,” it states. The bill also provided SOFIA with $85.2 million, $5.3 million above the agency’s request.

A fiscal year 2019 spending bill approved by the House Appropriations Committee May 17 again provides $85.2 million for SOFIA, compared to a request of $74.6 million that declines to zero by 2022. NASA said in the request — released prior to the completion of the 2018 omnibus spending bill — that SOFIA would participate in the 2019 senior review. If it passed the review, NASA noted, it would budget funding for future years.

The new House report, though, reiterates that SOFIA is exempt from the next senior review. “The Committee is concerned with NASA’s proposed inclusion of SOFIA in the 2019 Senior Review, given it began its prime mission in 2014 and has 15 years of prime mission lifetime remaining,” it states. “Accordingly, the Committee directs NASA to only undertake a Senior Review of SOFIA at the time SOFIA completes its planned mission lifetime.”

That language has puzzled many in the astronomy community, who note that SOFIA will not have to participate in a senior review until at least 2034. Most other NASA missions, by comparison, have far shorter prime missions, such as five years for the James Webb Space Telescope.

SOFIA’s prime mission was also originally set for five years, and thus would have been part of the 2019 senior review prior to the congressional report language. A March 2017 presentation by Harold Yorke, director of SOFIA science mission operations, noted that five-year prime mission, but added there were plans to operate SOFIA out to 2034, including securing of spare parts from other Boeing 747SP aircraft.

However, agency officials said there will be other mechanisms other than the senior review process to monitor SOFIA’s effectiveness, such as program implementation reviews used by NASA programs in general. “One doesn’t go 20 years without reviewing a program, especially a program as complicated and as costly as SOFIA,” said Paul Hertz, director of NASA’s astrophysics division, during a March meeting of the National Academies’ Committee on Astronomy and Astrophysics.