Saturn Mission Ignites Safety Debate

No doubt about it, the element plutonium has a bad name. It’s the stuff of nuclear bombs. It causes cancer. It’s named after the god of hell--Pluto. It was even given the chemical call letters Pu quite deliberately, “as a joke,” said its 1941 discoverer, UC Berkeley Nobel laureate Glenn Seaborg.

Recently, plutonium has gained new notoriety as NASA prepares to launch the Cassini mission to Saturn, which will carry 72 pounds of plutonium as a power source. Critics of the mission say that 5 billion people could suffer ill effects if Cassini blows up on the launch pad or crashes into Earth during a planned 1999 fly-by.

Some South Florida residents say they are ready to leave town--or even the country--before the Oct. 13 launch. And some anti-Cassini activists have threatened to send boats or parachutists to Cape Canaveral, Fla., in efforts to stop the launch.

Proponents say the risk carried by Cassini is minimal--less than the normal background radiation that people receive from natural radioactivity in the atmosphere, rocks and soil. At worst, if it burns up in the atmosphere, 120 people could die over 50 years, according to NASA.

The risk certainly is less than people receive routinely from such mundane activities as jet travel and mammograms, supporters say. “Cassini is a glorious mission that justifies whatever small risks there might be,” said engineer Steven Aftergood of the Federation of American Scientists.

Either way, plutonium is an explosive issue, muddled by a certain amount of paranoia on both sides. Opponents believe that the Defense Department, in cahoots with NASA, would like to put plutonium reactors on asteroids and dominate space with plutonium-powered rockets. Supporters think that anti-Cassini activists want to stop all use of, and research on, plutonium.

Both may be right.

“It’s a complicated matter,” said physicist Philip Morrison of the Massachusetts Institute of Technology. “In general, plutonium’s danger is enormously exaggerated. On the other hand, I think it’s fair to keep people [at NASA and Defense] honest and concerned. There is a thinkable catastrophe.” If Cassini weren’t a scientifically spectacular mission, he said, NASA shouldn’t take the risk.

Cassini critics and supporters disagree on at least three issues central to the debate. First, is plutonium really “the most toxic substance known to man,” as it is often described? Second, if Cassini’s launch should fail or if the spacecraft burns up in Earth’s atmosphere when it swings by to get a gravity boost, would it release enough plutonium to cause significant harm to people? Finally, what are the risks that Cassini will fall to Earth in the first place?

In bulk, plutonium is a brittle, silvery metal, denser than lead. According to anti-nuclear activist and pediatrician Dr. Helen Caldicott, it is so toxic that a single pound, evenly distributed, “could hypothetically induce lung cancer in every person on Earth.”

In a study frequently cited by parties on both sides of the dispute, beagles who inhaled tiny specks of plutonium all died. It is generally agreed that a millionth of a gram of plutonium can cause fatal cancer in humans.

But other kinds of poisons, researchers point out, can be far more deadly--for example, germs or viruses. “If you’re talking about being exposed and keeling over and dying, micrograms of botulism organisms are far more dangerous,” said Lawrence Berkeley Laboratory biophysicist Patricia Durbin.

Indeed, describing plutonium as the most toxic substance known is “hogwash,” according to the lab’s Darleane Hoffman, a nuclear chemist and expert on plutonium. “That’s utterly ridiculous,” she said.

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Plutonium got a bad name, she said, because people associate it with weapons, but that doesn’t make the element the most toxic stuff on Earth. Viruses can be used to make weapons too, she points out.

Caldicott, a pediatrician, adamantly disagrees. “They’re bloody idiots,” she said of people who dismiss the dangers of plutonium. “They’re not biologists. . . . You give someone a flu virus, and it may eventually kill them, but eventually it mutates itself out of existence. Botulism may affect someone who eats a bit of bad food, but it doesn’t hang around.”

But the cancer that results from radiation remains for the most part incurable, she said. “When you have an incurable disease, the only recourse is prevention--not making more.”

Even if plutonium deserves its bad reputation, it still has to get into living tissue to cause harm. NASA repeatedly points out that’s unlikely to happen, even if Cassini should somehow vaporize in Earth’s atmosphere during a catastrophic accident.

Cassini stands a far greater chance of failing on the launch pad than plunging into the atmosphere during its swing by Earth in two years. However, the consequences of a fly-by accident would be far greater, because the spacecraft would probably burn up, producing tiny particles that could be inhaled.

The plutonium on Cassini is compressed into ceramic-like pellets, more likely to break into large chunks than vaporize. The pellets sit inside concentric shells designed to protect them from heat, impacts and corrosion. “There is no explosion that would break open the outside blocks,” said Beverly Cook of the U.S. Department of Energy, who is responsible for the plutonium power sources on Cassini.

Moreover, the radiation can’t get through skin or clothing or even paper, so the plutonium needs to be inhaled or ingested to cause harm. And even 72 pounds of plutonium dispersed over, say, the Northern Hemisphere, would be extremely diluted, Hoffman said. “If it dispersed in space, the amount that would get back to us would be negligible.”

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Even if every speck of plutonium on Cassini were released, it probably wouldn’t cause much harm to humans, said Thomas B. Cochran of the Natural Resources Defense Council. Cochran said that although the total plutonium would be the equivalent of several trillion lethal beagle doses, spreading the particles over the entire globe isn’t like “putting an experimental device up a dog’s nose.” It’s much more scattered and diffuse.

Caldicott thinks this is wishful thinking. Sure, the plutonium could disperse throughout the oceans and atmosphere, she said. “But it might land in London. Or Africa. Or America. Who knows where it’s going to land? It’s Russian roulette.”

City University of New York physicist Michio Kaku calls NASA’s prediction of 120 deaths over 50 years unrealistically optimistic. His estimate is closer to 200,000. “I’ve gone through a lot of the mathematics, and I don’t think NASA has looked at the worst-case scenario,” he said. “Coming in at 40,000 mph--if it’s a fraction of a degree off--it will vaporize in the Earth’s atmosphere, and all of it will come out.”

And what are the chances of a fatal Cassini accident? NASA has calculated that the chance of Cassini plunging back to Earth during its 1999 swing-by is less than a million to one. But Kaku and others argue that NASA’s numbers are impossibly precise, designed to “make the figures seem authoritative and accurate, when in fact they are largely created by fiction. They’re pulling numbers out of a hat.”

NASA based its numbers on the reliability of previous missions that swung closely by planets for gravity boosts, and factored in the probability that Cassini could malfunction.

But Kaku said the agency hasn’t adequately factored in human failure.

“You can design a car with air bags and anti-lock brakes, and some bozo is going to drive it over a cliff,” he said. A human programming or command error, he added, could make all the estimates moot.

As with all risk assessment, numbers alone don’t tell the whole story. Even if the risk is very small, said health physicist Cochran, the real question is: Small for whom? And small compared to what?

“We’re all going to die of something. The risk of dying from cancer is 20%. If someone adds on [a tiny amount] more risk, people will say, ‘That’s too small, I don’t have to worry about that.’ But the issue isn’t whether any one individual has a high risk of getting cancer,” Cochran said, “it’s whether the total cancers you create through this accident outweigh the benefits.”

Cochran’s organization, which takes on a variety of environmental issues, has not taken a position on Cassini, mainly because it pales compared to other risks. “It’s a matter of picking our fights,” he said. A much more pressing priority, he said, is “getting rid of nuclear weapons; there are a lot more of those and they do a lot more damage.”

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Similarly, the Bulletin of Atomic Scientists, another nuclear watchdog group, supports Cassini, while strongly opposing Defense Department plans to put nuclear reactors in space. The group worries that Cassini protesters could actually hurt its cause by focusing so much attention on a relatively benign issue, thereby undermining the credibility of future protests.

At its heart, said Aftergood, the question of whether Cassini should fly is not amenable to mathematics. Science cannot tell whether the relatively small risk of increased cancers over the next 50 years is worth trashing a $3.4-billion journey to perhaps the most beautiful and mysterious planet in the solar system--ring-encircled Saturn.

“That is a subjective question,” he said. “There’s no scientific answer to that.”

And whatever happens to Cassini, NASA’s plutonium issue is not going to go away until the space agency figures out other ways to power spacecraft in the dark corners of space where sunlight is scarce and solar power may not be an option. Kaku said NASA should postpone future missions requiring plutonium until it finds the answer.

“I love the space program,” he said. “But sooner or later, one of these is going to blow up. There’s going to be a backlash. Saturn is not going to go away.”

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Controversial Mission

The plutonium-powered Cassini spacecraft to be launched next month is set to swoop by Earth again in August 1999 on its way to Saturn. If the spacecraft plunges into Earth’s atmosphere (a million-to-one chance, says NASA), it might release plutonium in a form that could be inhaled.

Cassini Trajectory

Saturn is so far away-almost a billion miles-that even an enormous Titan IV rocket lacks enough oomph to throw a spacecraft there in one shot. So NASA engineers will make use of thegravitational pulls of Venus, Earth and Jupiter to slingshot the Cassini spacecraft to the ringed planet.

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Plutonium: The atom is so big that it’s unstable. The nucleus of plutonium contains 94 protons and about 140 neutrons.

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Plutonium 239-used in nuclear bombs-has 94 protons and 145 neutrons. When it captures an extra neutron, it turns into a highly excited state splits quite easily. The reaction also spits out copious extra neutrons, setting off a sustainable chain reaction.

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Plutonium 238 does not capture neutrons as effectively-or split as easily-so it is not used in making bombs. It does radiate far more nuclear particles, however, and these particles can cause cancer if inhaled. The strong radiation of Pu 238 provides power and heat to Cassini.

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How Fission Works:

Neutron

Atom

Neutrons: Fission Energy

Fission Products

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Launch: Planned October 1997

Saturn arrival: July 2004

Sources: Amarillo National Research Center for Plutonium; staff reports.