But the mission is not without risk.

Because the sun is little more than a bright star when seen from Pluto's distance - a quarter beyond the far end zone in the Rose Bowl - a solar power system would require arrays on the order of 1,200 square yards in size to collect enough light, to generate enough electricity, to power the craft's systems and instruments.

Instead, New Horizons relies on a single radioisotope thermoelectric generator, or RTG, which converts the heat produced by the radioactive decay of plutonium 238 dioxide into electricity. At launch, the RTG will produce about 240 watts of power. By the time the craft reaches Pluto, the power output will have dropped to about 200 watts, enough to drive two household light bulbs. But that's enough to keep New Horizons alive and well.

RTGs have no moving parts and are extremely reliable. The compact generators have powered all of NASA's deep space missions, including the Pioneers and Voyagers, the Ulysses sun-study probe, the Galileo Jupiter orbiter and the Cassini spacecraft currently in orbit around Saturn. RTGs also were used on six Apollo moon missions and the two Viking Mars landers.

In all, 25 RTG-powered missions have been launched by the United States. Three of those missions failed. In one case, the RTG burned up in the atmosphere as designed at that time. In another, the RTG was later recovered. And an RTG aboard the ill-fated Apollo 13 lunar lander now rests at the bottom of the Tonga Trench in the Pacific Ocean.

New Horizons is equipped with a single RTG loaded with about 24 pounds of highly toxic plutonium dioxide. RTGs are designed to withstand the sort of forces one could expect during a launch mishap, but NASA's environmental impact statement concludes there is a 1-in-350 chance of a launch mishap in which plutonium could be released into the environment.

Even so, NASA claims no additional cancer deaths could be expected over the next 50 years as a result of any failure that has a reasonable probability of occurring. In some extremely unlikely cases - the rocket's self-destruct system fails after a major malfunction, for example - the agency concludes there is a more significant biological risk. But the odds of a release after such multiple failures, NASA engineers believe, are in the realm of 1-in-1 million or lower.

Kurt Lindstrom, a senior manager at NASA headquarters in Washington, said major U.S. rockets have a 93.8 percent overall success rate. Of the failures, 5.8 percent would not involve the kind of forces necessary to damage an RTG enough to release plutonium into the environment. Of the 0.4 percent of the remaining mishaps, Lindstrom said less than half would result in 0.1 latent cancer deaths. A "latent cancer fatality" in this case is the statistical probability of developing cancer over 50 years.

"So there's a very low probability of having an accident generally or particularly having an accident that has a radiological release for this mission," he said.

But anti-nuclear activists argue the risks, however small, outweigh the scientific payoff. At a small demonstration Jan. 7 just outside the Cape Canaveral Air Force Station, about three dozen activists, several with children, urged area residents to protest New Horizons' launch.

"We distrust their reasons for needing to launch plutonium," said Maria Telesca-Whipple of the Global Network Against Weapons and Nuclear Power in Space.

"Their impact statement says a 1-in-300 chance of an accident. I've lived here 22 years and have seen accidents with launches that had no plutonium but which released toxic fumes over Cape Canaveral and Cocoa Beach where the school children were told to stay inside. ... We think it's all smoke and mirrors and we think that it has a lot more to do with the military applications (of nuclear power in space). ... We really just doubt that the general population is being told the truth about why we're actually doing this."

The activists also challenge the NASA-Department of Energy environmental analysis and argue alternative technologies might permit a Pluto mission without the need for RTGs.

NASA managers say that isn't true and the White House has officially approved the New Horizons launch. There are no other known political or legal obstacles to the flight. The environmental impact statement is available online.

PREVIEW REPORT PART 3 --->