NUKES IN SPACE

By MICHAEL D. LEMONICK

It's mid-October 1997--about a month from now--and a Titan IV rocket has just lifted off the pad at Cape Canaveral. Perched on top is the Cassini spacecraft, one of the most ambitious probes NASA has ever launched. If the mission goes as planned, Cassini will reach Saturn in 2004 and spend the next four years exploring the giant ringed planet and most of its 18 icy moons.

But suddenly something goes wrong. Maybe the Titan's fuel system springs a leak, triggering a fireball that duplicates the Challenger explosion of 1996. Or maybe the rocket simply wanders off course, forcing ground controllers to blow it up before it can fall back to Earth. In an instant, the Titan and its precious cargo are blasted into a million pieces.

What happens next depends on whom you ask. To hear NASA tell it, the fallout from such a disaster would be primarily emotional. Scores of scientists and technicians would watch bitterly as years of work went up in smoke and the chance to learn valuable information about a distant, mysterious world evaporated. Congress would wring its hands over wasting $3 billion of the taxpayers' money. And NASA's reputation would get another black mark.

According to a small but vocal group of antinuclear activists, however, there's much more to worry about than upset feelings. Like all the deep-space probes that have gone before it, Cassini is powered by radioactive isotopes--in this case about 72 lbs. of plutonium 238. If the spacecraft were destroyed, insist these critics, some of the plutonium could be pulverized and wafted away by the wind. Even worse, Cassini is supposed to swing by Earth in 1999 for a gravity assist that would sling it out toward Saturn. If the probe comes too close, it could re-enter the atmosphere at 42,000 m.p.h. and vaporize, releasing enough plutonium to be inhaled by millions of people. The radiation from P-238 is harmless under most conditions, but breathing in particles of it can be deadly. It is, says cancer specialist and protester Dr. Janice Kirsch, "the stuff nightmares are made of. One exposure can lead to cancer."

The danger is so great, say the critics, that Cassini must be stopped. Last week the Women's International League for Peace and Freedom ran newspaper ads calling Cassini a "nuclear experiment in space" and claiming that NASA has failed to consider safer power sources like solar cells. The group is planning rallies at the U.N., at the White House and, on Oct. 4, at Cape Canaveral in an effort to get Cassini canceled.

NASA is not amused. Snaps Wesley Huntress, the agency's chief of space science: "NASA believes this mission is safe, period. Otherwise we would not be doing it." Contrary to what the critics say, the agency did consider solar power, insists Cassini engineer Richard Stoller. Because sunlight at Saturn is only 1% as strong as it is on Earth, solar cells would not have done the trick. Neither would batteries and fuel cells; they would never last through the 11-year mission.

That left NASA with only one choice: radioisotope thermal generators, which make electricity from the heat of radioactive decay. RTGs have been flying in space since 1961. They were redesigned, on orders from the Atomic Energy Commission, after a satellite fell to Earth and released a small amount of plutonium in 1964. As a result, when another RTG-powered craft was destroyed after going off course in 1968, the plutonium stayed intact--and in fact, was re-used on a later satellite.

Modern RTGs are even tougher. Their plutonium is embedded in marshmallow-size pellets of ceramic--designed to splinter rather than pulverize--so it can't be inhaled. Each pellet is wrapped in iridium, a metal that's hard enough to withstand enormous shocks and remain intact, and that melts at 4450[degrees]F (a re-entering spacecraft reaches only 2565[degrees]). The pellets are then encased in a graphite-fiber shell.

All this protection has proved formidable in extensive Department of Energy tests over the years. Engineers have fired bullets into RTGs. They've put them through explosions. They've simulated atmospheric re-entry with five times the pressure and heat an RTG would actually face. "We have tested beyond failure," says Beverly Cook, the DOE official in charge of RTGs. "We get no release of material."

Still, observes retired NASA engineer Alan Kohn, who is former emergency-preparedness officer for the plutonium-fueled Ulysses and Galileo missions and an anti-Cassini protester, "nothing designed by man is indestructible." The chances of a failure may be slim, but they're not zero.

Even if there was a release, however, it's not clear that millions or even scores of people would be in danger. To cause harm, says Otto Raabe, professor of radiation health biophysics at the University of California at Davis, plutonium exposure has to rise significantly higher than the background level of natural radiation. And that, he calculates, wouldn't happen even if all Cassini's plutonium escaped. "The image of an Earth shrouded in deadly plutonium clouds," he says, "is just total nonsense."

What will be lost if Cassini is canceled? As Galileo's spectacular images of Jupiter and its moons showed last spring, an extended visit is really the only way to study a distant planet. Saturn's rings are perhaps the most mysterious and magnificent objects in the solar system. Its moon Titan has its own atmosphere, filled with organic chemicals; scientists suspect it's just the sort of place life could have gained a foothold. Pulling the plug on Cassini now, when we're on the verge of exploring such a place, would be a missed opportunity of astronomical proportions.

--Reported by Dick Thompson/Washington

With reporting by Dick Thompson/Washington