NUKES IN SPACE

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.