Gagnon and his group, Global Network Against Weapons and Nuclear Power in Space, envision a ship carrying a nuclear reactor exploding on launch -- or, as with Columbia, disintegrating in the upper atmosphere -- and scattering radioactive material over a wide swath.

A release of fallout would require the removal of "people, buildings, vegetation," Gagnon said. "They'd have to store the soil for thousands of years. You're talking about a nuclear wasteland here. Is Florida ready for that risk?"

NASA engineers say the chances of such a disaster and the potential threat are ridiculously low. The space agency already has launched rockets with nuclear powered payloads, and now it is dusting off long-shelved plans for nuclear-powered spacecraft with plans to spend billions in the next few years. Officials say they can keep risk levels at or below those for nuclear power plants. And they say any long-range exploration, including manned trips to Mars and beyond, must be powered by something other than the thrust power used by the Chinese thousands of years ago.

Several missions early in the space program carried nuclear generators or fuel. But the program was scrapped in the early 1970s over safety concerns. NASA now plans to spend $1 billion in the next five years to expand its nuclear propulsion research. And Project Prometheus, which would use nuclear-powered spacecraft to explore the moons of Jupiter, is marked for $3 billion, $279 million of that in 2004 alone. Of course, all of that must get past a Congress that may ask tough questions in the wake of last month's disaster -- among them, whether it will spend $2 billion to $3 billion to replace Columbia.

NASA is looking for new power sources because the heavier the spacecraft, the more fuel you need to lift it off. But the more fuel you carry, the more the spacecraft weighs. Eventually it becomes impractical to build a rocket that can carry enough fuel to get itself off the ground.

Astronautical engineer Robert Zubrin says he has the answer, and it's nuclear power.

"Like it or not, mankind is going nowhere, astronautically speaking, without the power of the atom," Zubrin says. He is president of the 6,000-member Mars Society, which lobbies for a manned mission to Mars. He also argues that even unmanned missions need nuclear power to be effective. Such a spacecraft could travel a lot farther, he contends. More importantly, he said, a nuclear reactor would deliver perhaps 100 times more power to transmit data.

But it was such a craft -- the Cassini -- that sent Gagnon and his group into the streets and into court. It left for Saturn in October 1997, launching with conventional propulsion but carrying 72 pounds of deadly plutonium to power its scientific instruments. Demonstrators threatened to chain themselves to the pad and filed lawsuits to try to stop the mission.

"Things can go wrong if things are not engineered correctly, but that's true of any bridge, any skyscraper, or any aircraft or water treatment plant," Zubrin said from Colorado.

Nuclear power only way to go

Robert L. Park, Washington director of the American Physical Society, agrees that nuclear power is mandatory for any Mars or beyond missions. Solar power won't cut it; ships in deep space are just too far from the sun. But Park, who's also a professor of physics at the University of Maryland, says any nuclear-related space flight should not include people.

"My guess is we'll never send them," Park said. "The radiation levels between here and Mars are very high and, if there's a solar (radiation) storm while you're on your way, you're toast." Engineers agree a nuclear reactor is many times more deadly after it's powered up. The Mars Society's Zubrin says a Mars mission should use a conventional rocket with a nuclear reactor that would activate only after a spacecraft is successfully orbiting Earth.

Another alternative would be to build a ship, perhaps including a nuclear propulsion or power source, in space. But that's not easy. NASA has spent the past five years assembling the international space station, and it's not finished. On top of that, while the fuel needed to get off the ground would no longer be required, the fuel for the trip to Mars and back would still have to be lifted from the surface and loaded.

Zubrin's solution: have a rocket deposit a nuclear reactor on the Martian surface that would generate power. It would produce breathable air, purify water, recharge fuel cells for vehicles, and most importantly, create the fuel that will power the return.

Park, doubts a reactor could make enough fuel for a return trip and wouldn't gamble lives on it. He said there's still no proof astronauts would be able to harvest hydrogen in large enough quantities and in convenient enough places -- if at all.

Whether used for propulsion, on-board power, or as a Mars-based generator, any nuclear reactor would not be coming back, Zubrin said.

Right now, anti-nuclear activists such as Gagnon are more worried about a different piece of machinery already in space. Radioisotopic thermoelectric generators, which were employed on Cassini, convert heat from decaying plutonium into electricity. While reactors are least dangerous at launch, RTGs are dangerous pretty much all of the time. But NASA stresses that no RTG safety design has ever failed. Park said it's most likely the canister containing the plutonium would survive reentry and strike the Earth in one piece. But he said it would be as pellets, rather than a cloud of radioactive dust, so the risk to humans, while palpable, wouldn't be significant.

But Gagnon believes there's still too much risk. The former Air Force sergeant points out that spacecraft carrying nuclear material already have been involved in mishaps, some occurring in reentry, as did Columbia.

In May and June, NASA plans to launch two Mars rovers from Kennedy Space Center that will contain heaters powered by plutonium. Gagnon claims NASA documents conclude a 1-in-30 chance of small quantities of "radionuclides" being released into the atmosphere. NASA spokesman Robert Mirelson says "it's virtually no chance." He said the quantities are "minute" and NASA doesn't consider them dangerous.

The rover launches are still on schedule during the Columbia investigation. In light of the disaster, Global Network has called on NASA to cancel them and plans a protest at Kennedy Space Center on May 3.

"See, that's the thing," Gagnon says, his voice rising with passion. "There have already been accidents. It's not just some theoretical thing."