NASA has threatened to ground a vintage Boeing 747SP equipped with an astronomical observatory unless more funding is raised by partner Germany or new sources.

The possible grounding marks the most serious threat to the Stratospheric Observation For Infrared Astronomy (SOFIA) project since NASA suspended funding to the programme for five months in 2006 following a long series of delays and cost increases.

The 2.7m (100in) infrared telescope aboard SOFIA has been operational for nearly four years, but pressures on NASA’s $5 billion science budget have forced the agency to consider shelving the project, says NASA Administrator Charles Bolden in a teleconference with reporters on 4 March.

“We have a $5 billion science budget, but we – even with that large amount of money – we have to make choices,” Bolden says, explaining the decision.

Bolden says NASA already operates the world's “foremost” astronomical observatory with the Hubble Space Telescope. It is scheduled to be replaced in 2018 by the James Webb Space Telescope, which Bolden describes “the most incredible astrophysics instrument” in development by any country.

NASA will ground the SOFIA aircraft after 1 October without additional funding from sources outside the agency’s budget, he says.

Germany’s aerospace research center (DLR) contributes 20% of the cost of operating the SOFIA laboratory and NASA covers the remaining 80%.

Germany’s contributions include the assignment of 15 staff members to the SOFIA base at NASA’s Dryden Research Center in Palmdale, California, according to DLR’s web site. The DLR also pays for spare parts for the telescope and the 747SP’s engines, as well as the fuel cost for the 30% of flights assigned to German research missions.

Conceived in the late-1980s to replace a NASA-owned Lockheed C-141 equipped with a flying telescope, SOFIA has survived many delays and threatened cancellations. The reunification of Germany delayed the DLR’s involvement until 1996, when it signed an agreement with NASA to launch the project.

NASA acquired a used 747SP that originally formed part of Pan Am’s long-range fleet, and was christened the Lindbergh Clipper. But the task of installing the 19t telescope and installing an exterior door in the aft fuselage led to several delays. Plans to launch science missions were delayed from 2004 to 2010, as costs more than trebled.