Months before the spacecraft Cassini was launched from Cape Canaveral nearly seven years ago, controversy flared across the country over the plutonium fuel that would generate power for the spacecraft's long journey to Saturn.

Critics feared the possibility of a nuclear "meltdown" and widespread radioactive contamination of the environment should the nuclear core of the generators fail during launch or explode early in flight, or when Cassini swung past Earth on its looping voyage two years later.

Nothing like that occurred, and the miniature power plants, fueled by plutonium dioxide, are now generating 750 watts of power to run the spacecraft's 12 scientific instruments and the radio that is beaming a stream of signals back to Earth.


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But while Cassini was still standing on its launch pad in 1997, environmentalists and anti-nuclear forces formed a "Stop Cassini Coalition" that held demonstrations in the Bay Area and other cities.

Three local members of Congress at the time -- Ron Dellums, D-Oakland; Pete Stark, D-Hayward; and Lynn Woolsey, D-Petaluma -- urged President Bill Clinton to postpone the launch so independent scientists could review the possibility of using solar power instead.

But Cassini is flying so far from the sun -- nearly 900 million miles away -- that it receives only 1 percent of the solar energy that reaches Earth. Scientists had long since determined that solar-powered batteries like those used on missions to nearby planets would have to be so large that the spacecraft could never be launched.

The power plants aboard Cassini are called "radioisotope thermoelectric generators," or RTGs. The fuel is plutonium-238, a form of the manmade element whose atoms decay to yield radioactivity but cannot split to explode like plutonium-239, which is used in atomic bombs.

The plutonium's radioactive decay produces heat, which is converted to electricity by devices called thermoelectric converters.

Cassini has three main RTGs to run its dozen scientific instruments, while 82 similar but much smaller devices each provide a single watt of power to keep the spacecraft's electronics warm against the deep cold of space around Saturn, where temperatures reach more than 200 degrees below zero.

The European Space Agency's Huygens probe now riding piggy-back on Cassini, which will reach the surface of Saturn's mysterious moon Titan, is also being warmed and powered by another 35 one-watt RTGs. Altogether, the Cassini's generators can produce a total of 885 watts of electricity.

Earlier versions of the plutonium-fueled RTGs were used successfully on the two Voyager spacecraft that flew past Jupiter, Saturn and Neptune on planetary missions that ended in 1989. Similar devices also powered the Galileo mission that ended last year after exploring the neighborhood of Jupiter and its icy moons.