LE BOURGET, France —

Ending more than two years of debate in Washington, NASA

signed an agreement June 18 to launch the U.S

space agency’s

showcase astrophysics laboratory, the James Webb Space Telescope (JWST), aboard a European Ariane 5 rocket. The launch of the Hubble Space Telescope’s successor is tentatively scheduled for 2013.

NASA Administrator Mike Griffin and European Space Agency (ESA) Director-General Jean-Jacques Dordain signed the final cooperation accord here during the Paris air show. Griffin said the memorandum represents the formal commitment of the United States to the collaborative arrangement with Europe.

ESA is providing the launch as part of its long-planned contribution to the mission. ESA and its member states have budgeted about 400 million euros ($536

million) for JWST. Current estimates are that JWST will cost about $4.5 billion.

ESA also contributed to the cost of the Hubble telescope as a junior partner to NASA, but its contributions were spread over different elements on board the satellite. Having Europe launch JWST had sparked a behind-the-scenes debate in the United States about the political acceptability of launching such a high-profile satellite on a non-U.S. rocket. The decision ultimately required the approval of the U.S. State Department.

Speaking at a press briefing here, Griffin denied that NASA had faced any political or other pressure to launch JWST aboard a U.S. rocket. “All launch service suppliers would like all satellites launched on their rockets,” Griffin said, but ultimately the logic of the NASA-ESA arrangement prevailed.

Other U.S. and European officials said that while JWST long ago was designed to launch aboard Ariane 5, the actual decision to commit to the arrangement was viewed as a sensitive issue in Washington.

ESA also faced pressure from some European scientists to craft Europe’s JWST contribution to favor scientific instruments rather than provision of the launch vehicle.

But for both ESA and NASA, having a good portion of Europe’s JWST contribution come in the form of a launch vehicle removes technology-transfer complications inherent in close collaboration on scientific instruments. ESA Science Director David Southwood said that while the signing of the agreement took much longer than expected, “the ESA-NASA collaboration on JWST has been extremely smooth.”

ESA has made a down payment of about 2.5 million euros to the Arianespace consortium of Evry, France, for an Ariane 5 ECA rocket for JWST. The agency will not be required to convert the agreement into a formal launch contract until three years before the launch, ESA officials said.

ESA and NASA also signed on June 18 a cooperation accord on ESA’s Lisa Pathfinder technology-demonstration mission, set for launch in 2010. The satellite is designed to prove technologies for a later spacecraft, called Lisa – Laser Interferometer Space Antenna – that will test the theory of general relativity and look for gravity waves in space.

For Lisa Pathfinder, it is ESA that is in charge, with NASA providing instruments. Southwood said ESA is spending 247 million euros to design, build and launch Lisa Pathfinder, with an additional 70 million euros provided by individual European governments acting outside the ESA context.