His job was necessary because both spacecraft run on batteries made of plutonium, a potentially deadly radioactive element. Kohn planned for disaster. He ordered bulldozers that could bury radioactive debris. He planned how to turn buildings into fallout shelters. He arranged for gas masks, protective suits and stations where cars and people could be cleansed of radioactive dust.

At the launchings, which went smoothly from Florida, he sat at the radiation control center, ready to put his team into action. Now, as a retired safety expert, Kohn is criticizing the National Aeronautics and Space Administration for its scheduled launching of the Cassini mission to Saturn. He asserts that the risks are higher than stated and that an accident involving its plutonium batteries could doom thousands or perhaps millions of people to death by cancer.

The space agency denies that the launching will place the public at great risk. Instead, it says a dangerous mix of ignorance and hyperbole is threatening to halt not only the Cassini mission but also a significant part of the nation's space program.

Kohn, a 30-year NASA veteran, explained his criticism in an interview:

"Men and machines are fallible. If you keep launching these things, eventually you're bound to have an accident. It's inevitable. NASA says this whole thing is safe. Nobody can make such a statement. I've seen too many rockets blow up."

Kohn is part of a growing anti-nuclear movement that wants to stop the launching of the $3.4 billion Cassini mission, which carries about 72 pounds of plutonium -- more than ever lofted into space. The Ulysses spacecraft carried 24 pounds of plutonium and Galileo 48 pounds.

The Cassini, under development for eight years, had been scheduled for launching on Oct. 6. But a recent accident on the launching pad damaged the spacecraft's protective cover, beginning rounds of emergency inspections and repairs that will probably delay the start of the mission by a week or more.

Anti-nuclear protesters have seized on the incident as fresh evidence of dangerous fallibility. Monday, they and Kohn are to present their case at the National Press Club in Washington. And if they fail to stop Cassini, protesters plan a major demonstration in Florida just before the spacecraft's launching.

NASA disagrees with the protesters. The recent accident is "no reflection on safety," Richard J. Spehalski, Cassini's program manager, told reporters on Wednesday. "It does not represent anything that could compromise" mission safety.

Officials of NASA and of the Energy Department, which makes the plutonium batteries, also said no possible Cassini accident could pose significant risks to the public. Big fears are unfounded, they insist.

"Our safety analysis report is two feet thick," said Beverly A. Cook, program director for radioisotope power systems at the Energy Department. "It's been thoroughly reviewed. There's absolutely no accident sequence that results in huge amounts of plutonium being released. People have misunderstood the risk. They've been presented numbers that have no basis in fact."

The gulf between the two sides is significant because plutonium batteries are not likely to go away soon. NASA plans to peer into many dark recesses of the solar system: beneath the deserts of Mars, below the icy surface of the Jovian moon Europa and down at the obscure terrain of Pluto. Sunlight is dim or nonexistent in such places. So the solar cells that produce electricity for most spacecraft are weak or useless, increasing the allure of plutonium. The nuclear batteries of Cassini and other spacecraft are made of plutonium 238 and are known as radioisotope thermoelectric generators. As the plutonium 238 dioxide undergoes natural radioactive decay, it gives off heat, which is converted into electric power.

Dozens of such generators have been launched into space over the decades to power satellites as well as lunar and planetary spacecraft. Two of them had accidents: Nimbus-B in 1968 and Apollo 13 in 1970. But the sturdy cases that held the plutonium withstood the blows, as designed, space officials said, and no plutonium was released.

Both accidents occurred on re-entry to Earth's atmosphere. The plutonium battery from the Nimbus satellite was recovered intact from the Santa Barbara Channel, off California, and the one from Apollo 13's lunar module rests unopened in the Tongo Trench of the Pacific Ocean.

Precautions are clearly warranted. Plutonium is highly toxic. If inhaled or ingested in minute amounts, it can cause death by cancer.

To reduce risks, the plutonium is cast in a ceramic form that makes it insoluble in water and hard to break into a fine dust that can be inhaled. Energy Department officials compare it to ceramic cups and plates, usually breaking into chunks and shards. The plutonium is encased in layers of iridium, a very hard metal, and graphite, a form of carbon. Both are highly resistant to heat.

A single battery is nearly 4 feet long, holds 24 pounds of plutonium and puts out 296 watts of electric power, which decreases over the years as the plutonium decays.

Ulysses has one battery. Galileo has two. Cassini has three that are to generate 675 watts of power when it reaches Saturn in 2004.

Experts say the Cassini mission has three risky phases: the launching, the orbit around Earth for a booster-rocket firing and the swing past Earth in August 1999 at an altitude of 500 miles. Earth and its gravitational pull are to raise Cassini's speed so the craft can reach Saturn in the solar hinterlands 890 million miles from the Sun.

Experts also say this final phase is the most dangerous. Cassini will be moving very fast. If control is lost during the Earth flyby, a crash into the atmosphere might scatter some of the plutonium to the winds.

For each phase, NASA has estimated the odds of a serious accident that would release plutonium and hurt people. At launching it is 1 in 1,400, with the amount of plutonium released being 10 percent of the dose needed to kill one person by cancer.

At orbit it is 1 in 476, with 4 percent of the fatal dose for one person. And when it flies by Earth it is one in a million, and would release enough plutonium to kill 120 people.

Dr. Michio Kaku, a physicist at the City University of New York, who is among those scheduled to appear at the National Press Club, says the space agency's estimates are dangerously false. The 120 deaths estimated by NASA in the worst possible accident were reduced from an earlier estimate of 2,300 deaths, Kaku said, and the true casualties might reach more than 200,000.

Cassini, he said, "should be downsized and made into smaller, more frequent solar-powered missions." Other experts estimate cancer deaths in the millions.

NASA rejects the high death estimates, saying that opponents not only overstate the dangers but also underestimate the difficulty of producing solar cells. Even if advanced new arrays of solar cells could be made, they would have to be greater in size than two tennis courts to turn feeble sunlight into adequate power.

The spacecraft, said Spehalski, Cassini's manager, would be "too massive for launching."

Kohn, the retired safety expert, views the whole debate with skepticism, saying both sides are lost in a fog of scientific uncertainty.

"I don't care which side is making the estimates, pro or con," he said. "There are too many variables for accurate prognostication. You have to make a series of assumptions." Kohn added: "Who is right? That's not the relevant question. The point is that Cassini is part of a series. Eventually you're going to have an accident. We're trying to stop not only Cassini but the whole doggone plutonium idea. It's insane, criminally insane."