NASA Administrator Mike Griffin’s pledge to put a Europa orbiter mission back on the table has drawn praise from scientists and planetary exploration advocates, even if some are wondering where such an undertaking will fit in the U.S. space agency’s list of priorities.

NASA canceled the mission to Jupiter’s moon Europa in 2002 as part of a broader dismantling of the agency’s Outer Planets program. The following year, NASA announced that it intended to build a nuclear reactor-powered spacecraft that would orbit not just Europa, but two of Jupiter’s other icy moons as well. But that program, the Jupiter Icy Moons Orbiter (JIMO), has since been done in by questions of cost and relevance to NASA’s exploration vision.

Sending an orbiter to explore Europa is a high scientific priority. The National Research Council’s 2002 decadal survey for solar system exploration ranked a mission to Europa, even one that would have to get by on solar power alone, a top priority for planetary science.

Griffin appears to have taken that recommendation to heart. He told a Senate Appropriations panel May 12 that he supports doing a Europa orbiter mission that is not tied to the development of a nuclear reactor or nuclear propulsion system. He said that NASA may propose such a mission “in a year or two as part of our science line, but we would not, again would not, favor linking that to a nuclear propulsion system.”

Griffin’s unexpected announcement of a possible Europa orbiter mission followed a stinging critique of the JIMO program. “JIMO was in my opinion too ambitious to be attempted,” Griffin said. “It was not a mission in my opinion that was well formed.”

Griffin pointed out that by the time NASA decided late last year to defer the JIMO mission for a less challenging in-space demonstration of a nuclear electric propulsion system, it was apparent that JIMO had become so big that it would take at least two heavy lift launch vehicles to put the massive spacecraft in orbit. Furthermore, it would require more than twice the world’s annual xenon production to fuel it.

Since coming aboard in April, Griffin has initiated a dramatic restructuring of NASA’s nuclear systems program that would postpone indefinitely development of the type of propulsion system needed for JIMO.

Joseph Alexander, director of the National Research Council’s Space Studies Board, said the scientific community will be pleased by Griffin’s support for taking the development of new nuclear systems off the critical path of a Europa orbiter mission.

“As the scientific community watched the evolution of JIMO, they were apprehensive that the approach had become so ambitious and costly that it might never take off,” Alexander said. “They were worried that with the JIMO approach, they were seeing a very high scientific priority disappear over the horizon.”

For Louis Friedman, executive director of the Pasadena, Calif.-based Planetary Society, Griffin’s Europa pledge is vindication.

“Doing a Europa mission prior to a nuclear electric mission was something we had been calling for, saying that making any high-priority mission like Europa dependent on the development of a nuclear reactor is not prudent,” Friedman said. “Mike saying it’s back on the table is certainly welcome, although I have to point out that there is no Outer Planets program at NASA to speak of and, as far as I know, not even any serious studies being done on this.”

Friedman said that given all the competing pressures on NASA’s budget, the only way he can see NASA doing a Europa mission that does not skimp on science objectives is by making it an international undertaking.

“If it’s going to be squeezed to fit within NASA’s science budget, it is going to be unduly constrained,” said Friedman, one of Griffin’s longtime associates. “If it’s broadened to an international mission, there are definitely more possibilities.”

Before it was canceled in 2002, Europa Orbiter was expected to cost around $1.2 billion and launch by 2008. Friedman predicted that NASA could launch a Europa mission as soon as 2012, provided the agency gets the program restarted in 2006.

Brian Berger is editor in chief of and the SpaceNews magazine. He joined in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...