Handling public perception about danger from nuclear systems is part of NASA's overall strategy in moving forward on Project Prometheus and JIMO.

"I appreciate the fact that NASA's papers that have been put out on Prometheus have said it will be a very open, transparent process for the public to see into," said Kristine Svinicki, Senior Policy Advisor for Republican Senator Larry Craig of Idaho. That state is home for the U.S. Department of Energy's Idaho National Engineering and Environmental Laboratory, and the future site of advanced nuclear reactor work.

Svinicki said that much of the solar system remains impossible to adequately explore without new nuclear propulsion systems. That is a significant limitation, she said, and the public needs to make a candid assessment of that constraint.

"It's a question for the American people," said NASA's Hartman. "The laws of physics can't be broken. If you want to go to the outer planets, to be an explorer of this solar system, to understand comparative planetary biology…the American public pays for it. So I believe it is their choice," she said.

"You want to have time on target. If you are whizzing by Las Vegas, you're not going to gamble very much. If you sit and stay…that's basically the idea here. You can investigate what's there. You can have serendipity take place," Hartman said.

Putting the pieces together to make JIMO real is a massive tradeoff between many knotty technical issues. And there is also need to keep an eye on the nuclear engineering workforce.

"We are very short of graduate nuclear engineers that know anything about aerospace and vice versa," said NASA's Newhouse. The space agency has begun to come to grips with this situation, he said.

Newhouse said that ground checkouts of JIMO hardware means use of large chambers that mimic the thermal and vacuum environment of space. "You have to be very careful in making a decision to do a ground test or not do a ground test," he said.

Also there's need for ground shakeouts of JIMO's ion engines -- or Hall thrusters now used extensively in commercial applications for satellite station-keeping -- essential hardware that must be long-lived to push the spacecraft to Jupiter over years of thrusting. Departing no earlier than 2011, JIMO's Earth-to-Jupiter transit time might be as long as seven years.

Reducing the overall mass of the spacecraft is what Newhouse labels as the "magic mantra" for the JIMO effort.

In the nuclear design process, Newhouse added, certain testing called "criticals" will be necessary, making use of nuclear fuels to assure reactor performance is up to snuff. Any nuclear testing is to be done under the auspices of the Department of Energy.

"I certainly don't want to have a reactor that's never been run before…never been tested…then turned on en route to Jupiter. That's not a good idea," Newhouse said. "We will probably want to do a full-up test of something that looks like the reactor we want to fly in space," he said.

Newhouse said that one of the most difficult issues is ground testing. On the nuclear side of the business, facilities have aged and, in some cases, are no longer operable. Industry contractors, as part of their JIMO plans, are outlining ground testing plans and facility requirements.

Despite being faced with years of hard-nosed engineering, Newhouse points to one maxim he embraces. "It is easy to go nowhere. It requires no energy and has no risk except that of being left behind. To go forward and run ahead is a supreme test."