BOSTON — To planetary scientists, Cassini represents humanity's most ambitious attempt yet to explore another planet - seeking answers to some of the most basic questions about the origins of the solar system and of life.
To a vocal group of scientists, peace activists, environmentalists, and backers of alternative energy, Cassini represents a disaster waiting to happen.
At issue is the craft's 72 pounds of plutonium oxide - the largest quantity ever flown on an interplanetary mission. The highly radioactive ceramic is divided among Cassini's three electrical generators, known as radioisotope thermoelectric generators (RTGs). The devices convert heat from plutonium's radioactive decay into electricity.
The mission's critics hold that the dangers and reach of radioactive contamination from an accident are too great to allow Cassini to go forward. Those opposed to the launch include Helen Caldicott, a physician and founder of Physicians for Social Responsibility, and former NASA employees responsible for implementing some of the emergency response procedures should an accident occur at launch.
Even if Cassini arrives at Saturn safely, critics say, more missions are being planned that use RTGs, increasing the odds of a mishap that will release plutonium into the environment. From protests at the gates of the Kennedy Space Center to Web sites and mail-in campaigns to Congress and the White House, they've been actively pressing their case. (Last Friday, the White House signed off on the launch, as is required by law when nuclear materials are involved.)
Neither Cassini's supporters nor its critics deny that something could go wrong on the launch pad, or that it could plunge back to Earth, releasing plutonium oxide, when it returns during a flyby in August 1999. The debate centers on the assumptions used for the risk estimates, the thoroughness of the RTG testing, and whether suitable alternatives exist for supplying electricity.
NASA acknowledges that the RTGs won't survive reentry if Cassini is knocked off course during its flyby. But using what it considers the most probable set of scenarios, NASA's "best estimate" puts the overall risk of adverse health effects from a prelaunch accident at 1 in 19,200; from a mishap early in the launch phase at 1 in 1,490; later in the launch phase at 1 in 476; and in an Earth flyby at 1 in a million.
Critics argue, however, that by shifting some assumptions, the likelihood of adverse effects grows substantially. They also argue that RTG testing has failed to simulate the simultaneous conditions the units would experience in several scenarios.
Michio Kaku, a physicist at the City University of New York, says he has used NASA's approach to calculating risk as well as a different but plausible set of assumptions (including the shortcomings he says he sees in the RTG testing program), and he estimates as many as 500,000 fatalities from a worst-case accident. Others, he says, put the number as high as 1 million. NASA estimates 120.
"The point is not which numbers are right," Dr. Kaku says. "The point is that if three PhDs can use the same method and come up with such a wide range of estimates, the uncertainties are much greater than NASA admits. We're all being asked to take part in a big experiment."
The debate isn't new, nor is the RTG concept. The US has sent RTGs into space 23 times during the past 30 years. The Apollo craft carried them to the moon. Deep-space probes such as the two Voyager craft and Galileo, now orbiting Jupiter, use them as well.
During that time, the RTGs performed as designed. In the 1960s, an early version designed to burn up on reentry from Earth orbit did so. Subsequent RTGs were designed to survive the heat of reentry and the effect of other accidents, without releasing their plutonium oxide. Apollo 13's RTGs are said to be resting intact at the bottom of the Pacific Ocean. After another mission failed, its RTGs were used again.
Otto Raabe, president of the Health Physics Society, has spent 40 years studying plutonium and its effects. For it to pose a significant human health risk, he says, it must be breathed in as small particles less than 10 microns in size, or about 1/10th the thickness of a human hair. And those particles must be present in high concentrations.
In NASA's worst-case scenario, where Cassini reenters the atmosphere, about one-third of its plutonium-oxide fuel would vaporize or be reduced to fine dust and spread in the stratosphere. The rest, NASA estimates, would fall as pieces too big to inhale. NASA says the most likely trajectory would disperse much of the debris across the ocean-dominated Southern Hemisphere, reducing the chance the debris would fall on rocky regions where it could be smashed. Even so, NASA estimates that the "footprint" of contamination could range from 600 to 2,000 square miles, although it projects negligible health effects.
Kaku says that one alternative is to break Cassini into two smaller craft, so that solar panels can be used for electricity. He says the craft could be built and launched in time to allow Cassini's minimum science objectives to be met.
ON THE WEB
* www.jpl.nasa. gov/Cassini - NASA's official site for the mission. Chose "Nuclear Safety" for information including the environmental impact statement.
* www.crl.com/~gherbert/Space/Cassini - This site, run by an aerospace executive who supports Cassini, contains links to both pro and con Web sites.
Cassini is the most complex interplanetary craft ever launched. Standing two stories tall, it weighs just over 6 tons at launch. Cassini's size and weight are dictated by the large number of instruments it carries and by the nearly 3-1/2 tons of rocket fuel the craft must take along to conduct its gravity-assisted maneuvers, slow itself down once it reaches Saturn, and make other maneuvers during its four years orbiting the planet.
Cassini's instrument package includes:
Magnetometer: Sitting at the end of a boom, the instrument will study the magnetic fields of Saturn and its moons.
Radar: Housed in the radar bay, Cassini's radar will map surface features on Titan and, if conditions permit, on Saturn's other moons and rings.
Fields and particles experiments: Designed to study how charged particles and dust interact with magnetic fields in Saturn's system.
Radio-plasma wave package: Experiments range from measuring masses within the Saturnian system to searching for gravity waves from beyond the solar system.
Remote sensing: A range of instruments from a high-resolution camera to spectrometers covering a variety of wavelengths will capture images of Saturn and its rings and moons, and measure their chemical compositions.
Huygens probe: The European-built probe will be released in November 2004 to enter the atmosphere of Titan, and if all goes well, return data for a few minutes from the surface. It has instruments to measure everything from atmospheric composition to the depth of liquids.
Radioisotope thermoelectric generator: Cassini carries three, which convert decay heat from plutonium oxide into electricity for the spacecraft and its instruments. The RTGs carry a combined 72 pounds of plutonium oxide, the largest amount ever flow on an interplanetary spacecraft.