THE nearly 50 pounds of plutonium that will produce electricity on the Galileo probe make this space shuttle launching the most disputed ever.
A trio of anti-nuclear groups has sued to block the launching, saying that a shuttle disaster could produce a rain of radioactive debris that would cause thousands of cancer deaths.
NASA, pointing with pride to a $50 million, decade-long program to make the power pack as safe as possible, says the groups are wildly exaggerating the risks and overlooking the benefits of the mission.
Suit Seeks to Halt Launching Today in Federal District Court in Washington, a judge is scheduled to hold a hearing that centers on the groups' charge that the danger of plutonium contamination from a shuttle disaster is far greater than NASA has admitted.
''After the Challenger explosion, Chernobyl and the Valdez accident we have learned that technology can go terribly wrong,'' said Bruce Gagnon of the Florida Coalition for Peace and Justice, one of the groups suing to stop the mission.
Some of the anti-nuclear activists have said they would sit in on the launching pad to block the launching.
''They indicated to us and to the press an intent to do some back-country-type activities to try to stop the launch,'' said Gary Wistrand, deputy director of the space center's security office. ''We are postured to try to prevent that.'' The space center is patrolled by armed guards and by helicopters and boats. Power From Radioactivity
Because it will travel too far from the sun to be powered by solar panels, Galileo carries two radioisotope thermoelectric generators, each weighing 124 pounds. As the plutonium-238 dioxide inside undergoes radioactive decay, it produces 285 watts of electrical power for the robot craft. A similar power source was used on the recent Voyager probe of Neptune.
Safety precautions are clearly needed because plutonium is highly toxic if inhaled or ingested, causing cancer that can be fatal.
According to the Energy Department, which made the power generators, the ways the risk has been reduced start with the plutonium itself, which is pressed into a pellet about the size of a marshmallow. The plutonium is in ceramic form, which makes it insoluble in water and unlikely to break into a fine dust that could be inhaled.
''It's similar to your ceramic kitchen cook wear,'' said James A. Turi, director of the office of special nuclear applications at the Energy Department. ''It breaks into large chunks.''
The plutonium pellets are encased in iridium, an extremely hard metal that can withstand high heat. Iridium is used in pen points. A pair of iridium-clad pellets are then enclosed in a capsule made of graphite, which has a high tolerance to heat. Pairs of capsules, in sleeves, fit into graphite fuel modules. The modules are set end to end, then surrounded by 70 layers of insulation, made alternately of molybdenum, a metal, and astroquartz, which is like fiberglass. Plutonium Heat Units
In addition to the two power generators, the spacecraft will carry 129 tiny plutonium heater units to protect instruments from the cold of space. The plutonium in these units is the size and shape of a pencil eraser, locked inside a metallic case as well as multiple layers of graphite. Throughout the long journey, each heater unit will produce one watt of heat, about the same as that given off by a miniature Christmas tree bulb.