Europe will have no choice but to develop nuclear-powered satellites if it wants to continue to explore the outer solar system, European Space Agency (ESA) Science Director David Southwood said.

Several European nations, notably Germany, have strong anti-nuclear feelings and may resist any move to develop radioisotope thermoelectric generators (RTGs), which are currently the preferred method for providing power to satellites traveling too far away from the sun to make solar-electric power feasible.

Europe's Rosetta comet-chaser satellite, launched in February, carries a huge solar-array system that Southwood agreed is about as far as solar-electric power can go.

"Is this where we want to stop? I refuse to believe that," Southwood said in an interview here as he followed ESA's Huygens probe as it descended to the surface of Saturn's largest moon, Titan. Huygens was carried to Saturn orbit by NASA's Cassini satellite, which is nuclear-powered. "The fact is you cannot imagine going to the outer planets without a power source that doesn't depend on sunlight."

Southwood said ESA program managers have been discussing the best way to introduce at least an RTG-development program, if not a nuclear-propulsion program, onto ESA's technology-research agenda.

Sergio Vetrella, president of the Italian Space Agency, said he would favor a broad development program on new forms of nuclear power that focused not only on space applications but on uses in medical science and other areas.

Concerning conventional RTGs aboard future ESA satellites, Vetrella said he would favor not an ESA development program but a policy of buying RTGs from the United States. "We shouldn't get too dispersed in what we develop," Vetrella said, adding that he recognized the need for alternate satellite-power sources. "For conventional nuclear power on satellites, let's buy it off the shelf."

Southwood acknowledged that anything related to nuclear technology "remains very sensitive in Europe, even though several nations, including Britain and France, have mastered nuclear technology for civilian and military purposes. It is an issue we will have to treat delicately, but we've got to put it on our agenda. It's an issue ESA absolutely must address."

Southwood said ESA is happy with its collaboration with NASA on Cassini and Huygens, but sooner or later Europe would need to develop technologies to permit it to lead big-ticket space-exploration missions.

"We don't always want to be the younger brother in our collaboration with NASA," Southwood said. "For a real cooperation, you need two partners fully able to contribute."

Southwood said long-duration rovers on Mars -- currently the subject of low-level research at ESA -- ultimately would need RTGs and that Europe's space-exploration program, called Aurora, may be the most logical avenue by which to start an RTG effort.

The next big cooperative effort scheduled between ESA and NASA in space science is the James Webb Space Telescope, which NASA has tentatively scheduled for launch in 2011.

The two agencies had tentatively agreed that the NASA-led Webb Telescope would feature European participation in some of the instruments, and a launch on a European Ariane 5 rocket.

But no contract for the launch has yet been signed, and Southwood said he is awaiting a decision in Washington on whether the Ariane 5 will be definitively selected. NASA is normally prohibited from using non-U.S. rockets, with waivers being given in the case of international collborations such as that foreseen with the Webb Telescope.

Al Diaz, NASA associate administrator for science, said NASA does not have a formal opinion on whether the Webb should be launched on an Ariane 5 or some other vehicle. He said a final decision likely will take several months to reach.

Southwood said that if NASA determines that it cannot use Ariane 5, ESA would still "have a debt of honor" toward the Webb program to the extent that Europe will be receiving science data from the satellite.

"I went out on a limb to make sure the Ariane 5 is available to JWST [the James Webb Space Telescope]," Southwood said. "But if the Americans want to use one of their own rockets, that's their affair. My only concern is that there comes a point where delays in launcher selection start to cost money, and that point is going to come in mid-2005. We need a decision by then."