The head of NASA's nuclear push says a scientific mission to the inner solar system -- perhaps to the moon, Mars or an asteroid -- will be used to demonstrate a new propulsion system in place of a mission to Jupiter's icy moons.

But the Jupiter trip is only delayed, not canceled, Ray Taylor said. It could fly in 2017, a few years later than the demonstration mission.

In the meantime, "we have a range of options being looked at in the analysis," he said. "They're all in the inner solar system . . . and they're all shorter mission duration."

Nuclear power has been deemed important for the exploration vision that would send astronauts back to the moon and on to Mars, though Taylor said it wasn't clear yet whether nuclear propulsion would be used for a human Mars mission. Analyses within the agency are ongoing, he said.

Meetings in Cocoa on Tuesday, featuring Taylor and other officials, will allow the public to comment on the nuclear initiative.

A recent report by the Government Accountability Office (formerly the General Accounting Office) suggested NASA was smart to put off the complex Jupiter mission.

It also said the agency might have trouble defining the project's technical requirements and cost estimates by this summer. That's when a review determines which mission will demonstrate the nuclear propulsion system developed by NASA's Project Prometheus.

Taylor said the agency was looking at alternatives to the Jupiter mission in October, before a draft of the GAO report was handed to NASA. After NASA got the report, the agency's 2006 budget request deferred the Jupiter mission, citing "concerns over costs and technical complexity."

Even if it's not clear how future human missions will be powered, nuclear propulsion could be invaluable for scientific missions such as the Jupiter Icy Moons Orbiter.

Though deep-space missions have flown by bodies in the outer planets, they have not been able to orbit a moon that distant because of a lack of power. That far from the sun, solar power doesn't help much.

JIMO is supposed to study one of Jupiter's moons, Europa, which has an icy crust thought to conceal oceans potentially hospitable to life.

"Now the scientists know that Europa is extremely high priority, extremely important from a science standpoint," Taylor said. ". . . They need to have much more up-close operation. In other words, they need to be in orbit."

Nuclear power would lend instruments more juice, too, potentially giving a spacecraft better radar and other capabilities.

NASA is looking at a range of technologies for a nuclear-
powered drive, Taylor said. Electric thrusters would be larger versions of some that have flown already. A reactor system would be new.

It's challenging to test such a system on Earth, especially for a trip as ambitious as the Europa mission, said former Project Prometheus director and consultant Alan Newhouse, who left NASA in December.

A mission to the outer planets might have to run a decade or more, Newhouse said. "How do you prove you can make it run 10 years when you haven't even built it?" he said.

That challenge is one reason NASA decided to choose a different mission.

Taylor said the agency agreed with the GAO's recommendations.

"The good thing is that we have a very disciplined technical and business analysis process," he said.

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