With increasingly ambitious missions planned to Mars and beyond, and with solar energy having demonstrated its limitations, engineers and managers inside and outside NASA say nuclear energy may be powering its way to a new dawn in space.

It's a possibility made more likely by recent shifts in U.S. energy and military policies. It's also a move anticipated by antinuclear activists, who are already planning their opposition to any effort to use nuclear power in space.

Fueled by the desire to go farther and faster with fewer dollars, managers at NASA's Jet Propulsion Laboratory -- where many of the agency's robotic missions are conceived and carried out -- are analyzing how to justify the use of nuclear power in space, both technically and in terms of the benefit to science.

"We've been thinking about this, and trying to raise it as a question that warrants some consideration, for a couple of years," says Firouz Naderi, a longtime JPL manager and newly appointed leader of the Solar System's Exploration Programs Directorate. "I think we are going to raise it again and see if the [political] system is amenable to it."

In an interview at his JPL office, Naderi said any such political balloon would have to be floated in Washington by NASA headquarters.

"I believe that if a good case can be made, not only for the science return but for safety, then I could see that [nuclear power] could be in our future," he said.

Others think Naderi may be right. And support could come from the top.

President Bush's recently released energy plan features increased reliance on nuclear power back here on Earth. In several interviews, scientists and mission planners said they were hopeful this might put space-based nuclear power generation back on the table after suffering from years of what they call misinformation.

"The fact that the country is willing again to consider use of nuclear energy for commercial power may improve the prospects of applying this technology to space exploration," said George Schmidt, deputy manager of the Propulsion Research Center at NASA's Marshall Space Flight Center.

The last nuclear-powered spacecraft launched by NASA was Cassini in 1997. Antinuclear activists protested heavily against it, saying a launch accident or potential mishap in a 1999 Earth flyby en route to Saturn could kill billions of people who might develop cancer after contact with radioactive material.

Cassini scientists have called such claims "hogwash," saying that the radiation risk is less than normal background radiation in the air or in rocks.

Before the launch, NASA did admit that "there is a small potential for public health effects." But in 1997, Cassini project manager Richard J. Spehalski said the public was "badly misinformed by alarmists."

Spehalski said that even in the highly unlikely event that the 73 pounds of plutonium on board were somehow released into the atmosphere in a breathable form, and ingested, "the radiation dose an individual would receive over a 50-year period from that exposure would be ... 15,000 times less than a natural lifetime exposure."

In the end, there were no Cassini accidents. Yet the dangers still are debated.

Few debate the potential benefits of nuclear power in space. The life of a Mars rover could be extended from days to years. Maneuverability would be measured in miles instead of feet. And many engineers agree that a human trip to Mars would go from highly impossible to practical.

Further, if humanity is ever to leave this planet permanently and set up colonies on the Moon or Mars, a nuclear power station would be nearly indispensable, most space industry experts agree.

As talk of space-based nuclear power increases, so do the efforts of opposition groups.

Bruce K. Gagnon is a coordinator for the Global Network Against Weapons and Nuclear Power in Space. The organization represents 150 groups around the world with, Gagnon says, millions of members.

Gagnon said the groups have been expecting space-based nuclear power initiatives to resurface, and they've been making plans to mount a concerted effort against all uses, from planetary exploration to military. The U.S. military would benefit from having nuclear generators in space to power huge orbiting radar stations for reconnaissance. And Bush is also pushing for a fresh look at the so-called Star Wars missile defense system.

"We see a deadly connection between each of them," Gagnon said, arguing that the nuclear industry views space as a new market and would love to get a foot in the door any way it can.

Gagnon said the missile defense system would use nuclear energy aboard satellites to refuel lasers that would shoot down foreign missiles. Bush has not committed to any specific system, however, and some expect the weapons would ultimately be ground-based or mounted on ships or aircraft.

Gagnon also argues that relying on nuclear power tends to kill research into alternatives, such as solar power. "When you go with nuclear power you're basically saying nothing else works," he says.

Meanwhile, those who plan missions to Mars and beyond have a more modest goal: getting there. And they say that a new generation of nuclear propulsion systems is safe.

Radioisotope thermoelectric generators, or RTGs as they are called, use the natural decay process of plutonium to generate heat needed to protect a spacecraft in the cold environment of space. Some of the heat is converted to electricity, which can be used for flight propulsion or to power a surface rover.

In future plans, conventional chemical rockets would be used to launch spacecraft powered by RTGs, and the reactors would not be turned on until after they are launched. Still, critics fear a release of plutonium during launch or in the atmosphere, when a rocket is accidentally or intentionally blown up.

In the case of such an accident "the radioactivity in the reactor is nil, less than that received by laying on the beach in sunny California or in Long Island," says Mohamed S. El-Genk, director of the Institute for Space and Nuclear Power Studies at the University of New Mexico.

Gagnon, however, says the greatest danger might come well before a spacecraft is even launched. In order to use nuclear power in space, he points out, the Department of Energy would have to ramp up plutonium production.

"As you contemplate expansion of the use of nuclear power in space, you'll have a dramatic escalation in worker contamination," Gagnon said.

And finally, Gagnon said a launch accident is inevitable, due to faulty parts, human error, or sheer odds.