Cassini launches familiar debate Nuclear foes fear accidents, but NASA insists risk is tiny

With NASA's 6-ton Cassini spacecraft cleared for launch tomorrow on a 6 1/2 -year voyage to Saturn, space agency scientists and anti-nuclear activists have converged on Cape Canaveral, Fla., with their fingers crossed.

The NASA folks are hoping the $3.4 billion mission will lift off without a hugely expensive scientific failure.


Nuclear protesters, meanwhile, are praying that Cassini's electric generators, powered by 72 pounds of plutonium, will not be blown apart in a launch accident that showers radiation over Central Florida, or vaporized later in a fiery re-entry that poisons the atmosphere and puts hundreds of thousands of lives in danger.

NASA insists the risk of such accidents is tiny, and Cassini's potential scientific payoff is worth it.


"I'm taking my wife and youngest daughter and three of my grandkids" to the launch, said Dr. Ellis Miner, Cassini science manager for NASA's Jet Propulsion Laboratory. "If I had any concerns at all, I certainly would not take them along."

Cassini's opponents remember the Titanic. "Nothing humans have ever made has been indestructible," said Bruce Gagnon, coordinator for the Florida Coalition for Peace and Justice, representing church and peace groups.

Gagnon also fears a nuclearization of space, orchestrated by post-Cold-Warriors in the military, weapons labs and academia. Next, he said, they will try to launch nuclear reactors to power moon and Mars bases, and powerful space weapons.

Others just see unnecessary risk. "For pocket change you can downsize it" and go later with solar power, said Dr. Michio Kaku, a professor of theoretical physics at the City University of New York. "Saturn will still be there."

It's a familiar confrontation.

Anti-nuclear groups went to federal court in 1989 and 1990 and tried unsuccessfully to block the launches of the Galileo mission to Jupiter and the Ulysses solar explorer. Both carried plutonium-powered generators like Cassini's.

In fact, these "radioisotope thermoelectric generators," or RTGs, have flown on 23 previous U.S. spacecraft. They were aboard the Apollo lunar landers; the twin Viking Mars landers in 1976; the two Voyager spacecraft that surveyed the planets in the 1980s; weather and spy satellites.

What's new is that Cassini is carrying 72 pounds of plutonium, 22 more than Galileo and the most ever launched at one time. To power its big scientific payload at Saturn, 887 million miles from the sun, Cassini would need solar panels the size of two tennis courts -- too heavy to launch and too big to maneuver in space.


The generators are a reliable alternative, NASA argues. The two Voyager RTGs are still working 20 years after launch and more than 5 billion miles from the sun.

Even the late Dr. Carl Sagan -- a well-known planetary scientist who demonstrated against nuclear weapons testing and warned the deadly "nuclear winter" that would follow an atomic war -- concluded after considerable "agonizing" that RTGs posed an acceptable nuclear risk.

Even taking into account "the past history of government incompetence or worse in matters of public health," Sagan wrote during the Galileo debate in 1989, "my personal vote is to launch."

Saturn, NASA's Miner said, is "probably the richest scientific target we have available anywhere in our solar system."

It tantalizes with its extensive ring system and a wider variety of moons than any other planet. One of those 18 moons, Titan, sports a thick nitrogen atmosphere and hints of a deep ocean of liquid methane -- the only place in the solar system besides Earth with air and ocean.

"Titan may be a sort of Earth in deep freeze," Miner said. "That aspect makes it an exciting place to go." While Cassini orbits Saturn, its 771-pound Huygens probe will parachute to a soft landing (or splashdown) on Titan.


Eager scientists packed 18 instruments onto Cassini, making the two-story craft NASA's biggest planetary mission. Only the Soviet Union's twin Phobos Mars explorers, launched in 1988, weighed more.

Born big in 1989, Cassini survived cancellation attempts by Congress and NASA Administrator Daniel S. Goldin. But it is the last of its breed.

The new mantra for space science is "better, cheaper, faster." Goldin wants mini-spacecraft that can be developed quickly for a few hundred million dollars, with a swift scientific return. The new generation includes the Mars Pathfinder that landed in July.

Critics blame Cassini's size and thirst for electricity for the decision to power it with plutonium.

"This is a gas-guzzling Cadillac with tail fins," said Kaku. "I would send two compacts to Saturn rather than one gas-guzzler that uses plutonium."

Worldwide opposition


Opponents around the world have demonstrated and sent petitions and letters to President Clinton. Many people have encountered the issue on the Internet.

One of them was Baltimore Mayor Kurt L. Schmoke, who was concerned enough last month to ask Vice President Al Gore, Sen. Barbara A. Mikulski and EPA Administrator Carol M. Browner to "question NASA officials about [Cassini's] potentially harmful environmental impact."

After a 45-minute NASA briefing, Schmoke emerged saying that he had "a much higher comfort level with the whole project."

Cassini has won safety clearances from the departments of Energy and Defense, the Environmental Protection Agency, the Nuclear Regulatory Commission and the White House. NASA says all but the slimmest risk have been engineered out of the mission.

But the debate continues.

NASA says the Titan IV rocket is reliable and puts the risk of a launch failure at 1 in 1,400.


But of the 20 Titan IV launches so far, one failed. So critics call it a 1-in-20 risk of failure. They also point to Russia's "Mars '96" mission, which failed last year and dropped an RTG on Bolivia or Chile. It was never found.

NASA's risk estimates are flawed and ignore unpredictable space mishaps, they say. "How do you put a number on human stupidity, on solar flares or a radio that doesn't work?" Kaku said.

Nonfissionable isotope

Both sides agree Cassini's plutonium (Pu) can't blow up. It's not Pu 239, the stuff used in reactors and bombs. It's a different, nonfissionable isotope, Pu 238.

The RTGs aren't reactors and don't propel the spacecraft. The plutonium simply decays, releasing heat and alpha radiation. The heat from Cassini's three RTGs will be converted directly into electricity -- about 750 watts.

The alpha particles -- helium nuclei composed of two protons and two neutrons -- travel less than 3 inches. They can be stopped by paper or the dead, outer layer of human skin. They are hazardous only if the plutonium is inhaled or ingested. Alpha radiation from particles next to living cells can cause cancer.


What Cassini critics fear is an accident that vaporizes Pu 238 in the air. It has happened before.

Early in the space program, the Pu 238 in RTGs was in a metallic form designed to burn up on re-entry. Engineers felt that would disperse and dilute the hazard. In 1964, a falling spy satellite atomized about a pound of Pu 238.

Since the 1960s, U.S. policy has been to contain plutonium in an accident. Today's RTGs carry a ceramic form of Pu 238 sealed in shells of iridium -- a metal resistant at near-solar temperatures -- and graphite such as that used in ballistic missile nose cones.

Cassini's RTGs carry 54 such modules, to cut the chance that all the Pu 238 would escape in an accident. If some fail on impact, the ceramic plutonium would shatter, like a coffee cup, into shards mostly too big to be inhaled.

"That's silly," said Kaku. In an unplanned re-entry at 42,000 mph, "it will burn up and all the plutonium will escape." What isn't vaporized would hit the ground. If it hits rock, the plutonium would smash into a "spectrum" of sizes.

NASA says the shells work. An RTG like Cassini's survived a 1968 launch failure in California. It was recovered intact and reused.


Opponents also fear Cassini may get too close to Earth and burn up during a fly-by in 1999. But Miner said the craft is designed to fly wide of Earth at first, narrowing its aim to within 310 miles only at the last moment. That's calculated to hold the re-entry risk to one in 2 million.

Even if the "incredible" occurs and radiation escapes, Miner said, "we estimate the average dosage for those individuals who get contaminated at 1 millirem over a 50-year period." Over 50 years, we all get 15,000 millirems from natural sources.

NASA estimates such an accident might cause 120 added cancer deaths over 50 years. Critics' calculations predict up to 900,000.

"This mission could kill thousands of people. Why take the risk?" Kaku said. Even a launch accident in which all the plutonium is cleaned up would leave fears that would devastate Florida tourism and property values.

Kaku said he plans to be "as far away from Florida as possible. I'll watch it on TV."

Nonsense, Miner said. RTGs are "one of the safest power sources available to us. And since they are the only available source that can handle a craft the size of Cassini at the distance of Saturn, I don't think there's any question but that the right choice has been made."


NASA's Cassini Internet site is at

The Florida Coalition for Peace and Justice is at www.

Pub Date: 10/12/97