In the weeks before the flyby, however, critics of the Cassini mission warned of the potential for a nightmarish accident. The spacecraft contains three radioisotope thermoelectric generators (RTGs), which produce electricity from the heat emitted by the radioactive decay of plutonium 238 dioxide. RTGs have provided power for about two dozen spacecraft, including the Voyager and Galileo probes; the devices are particularly useful in the outer reaches of the solar system, where sunlight is too weak to generate much electricity. Critics have focused on Cassini because it holds a record amount of plutonium fuel: about 33 kilograms (72 pounds). More than 1,000 people demonstrated against the mission in Cape Canaveral, Fla., before the spacecraft's successful launch from there in October 1997. In June of this year anti-Cassini groups organized smaller demonstrations against the Earth flyby.

Image: Global Network Against Weapons and Nuclear Power in Space ANGRY PROTESTS against the Cassini spacecraft's flyby of Earth have irked space agency officials, who insist there is no danger of an impact.

The protesters claimed that if the spacecraft hit Earth instead of swinging by it, much of the craft's plutonium fuel would be pulverized into fine particles that would spread throughout the atmosphere. The fuel pellets are enclosed in iridium capsules and two layers of graphite shielding, but the modules were not designed to withstand an ultrahigh-speed reentry. The harm that would be done by such a release is virtually impossible to predict--estimates vary from 120 fatal cancers worldwide to hundreds of thousands of deaths. Although far more plutonium has been released into the atmosphere by nuclear bomb tests, plutonium 238 is about 280 times more radioactive than plutonium 239, the material in bomb fallout. According to John Gofman, professor emeritus of molecular and cell biology at the University of California at Berkeley, a single micron-size particle of plutonium 238, if inhaled, could cause lung cancer. "It's pretty hot stuff," Gofman says.

Fortunately, the chances of an impact on August 18 were calculated to be minuscule: less than one in a million, according to the National Aeronautics and Space Administration. Because Cassini is so heavy (more than 5,000 kilograms), it would take a mighty push--an explosive leak, for example, or a collision with a large meteor--to alter the spacecraft's trajectory significantly. As an extra precaution, the mission team at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., biased Cassini's trajectory so that it would miss Earth by at least 5,000 kilometers if the ground controllers lost contact with the craft.

Even some of Cassini's opponents acknowledged that the flyby would probably be uneventful. Only 60 people showed up at the Cape Canaveral protest in June. "People are still concerned, but it's really out of our hands," explains Bruce Gagnon, who organized the demonstration. Michio Kaku, a physicist at the City University of New York who has been the most prominent Cassini critic in the scientific community, says NASA should not draw the wrong lesson from the anticipated success of the flyby. "Sooner or later," Kaku maintains, "the odds will catch up with us."

Over the next 10 years NASA is planning three more missions that are expected to use plutonium fuel for electric power: Europa Orbiter, which will travel to Jupiter's fourth-largest satellite; Pluto-Kuiper Express, which will whiz past the farthest planet; and Solar Probe, which will go into an elongated orbit to study the sun. John McNamee, project manager for the missions at JPL, says that all three spacecraft will journey too far from the sun to rely on solar power. The probes would have to carry oversize solar panels to generate enough electricity for their needs. Besides adding weight to the craft, the large panels would be difficult to deploy and control. "Solar power just isn't technically feasible for these missions," McNamee remarks.

Image: Sarah Donelson GRAVITY-ASSIST FLYBYS are needed to speed Cassini to Saturn (planets' orbits not drawn to scale).

Unlike Cassini, the three planned missions will not fly by Earth, but McNamee says this is not because of any concerns that the probes might hit our planet. The future spacecraft will be several times lighter than Cassini, so they will not need as many gravity-assist flybys to reach their destinations. For the same reason, the probes will not need giant rockets to blast them into space. Cassini was launched by a powerful Titan 4 booster--the reliability of which has been questioned after some recent spectacular failures. The future missions will most likely be launched by the space shuttle or by updated Delta or Atlas rockets, McNamee says.

This prospect frightens Kaku. With a spacecraft carrying plutonium, the launch is by far the most dangerous moment. "If Cassini had blown up at launch, it would've been the end of the space program," he says. "We're putting a lot of hope on a firecracker." According to NASA, however, even a catastrophic launch accident would not release any plutonium fuel. The U.S. Department of Energy (DOE), which builds the RTGs, has subjected them to extensive tests that simulated the conditions of a rocket explosion. The testers fired .30- and .50-caliber bullets at RTG components to determine if they could be pierced by shrapnel. They also slammed rocket sleds against the devices, exposed them to propellant fires and detonated explosives to mimic blast waves.

Most of the tests did not damage the plutonium-fuel capsules, but some of the more severe impacts created fissures that would have released small amounts of fuel. NASA officials assert that such intense impacts would be unlikely during a launch accident. Kaku, though, looked at the same test results and came to the opposite conclusion. "The worst case," he says, "is if it explodes high in the atmosphere and the winds blow the plutonium around. Whole areas of Florida would have to be quarantined. And you could kiss Disney World good-bye." Aerospace engineers dispute this claim: Jerry Grey, a mechanical and aerospace engineer at Princeton University, says RTGs proved their survivability in 1968, when a military satellite carrying two generators was destroyed in a launch explosion in California. The RTGs landed in the Santa Barbara Channel and were retrieved intact from the seabed. "Nothing has a zero hazard," Grey notes. "But the hazard from RTGs is so small it should not bar their use."

In the debate over RTGs, however, perceptions are sometimes more important than facts. NASA officials admit that the Cassini controversy may threaten the chances of any future space mission that would carry radioisotopes. "I think it may be a problem," concedes Robert Mitchell, Cassini's program manager. "The amount of effort needed to get missions like this approved will increase."

Meanwhile the DOE is developing a more efficient generator for spacecraft called the Advanced Radioisotope Power System (ARPS). If successful, ARPS would require 50 percent less plutonium fuel than a comparable RTG does. ARPS would also be about 25 percent lighter, no small consideration for a spacecraft component. NASA is paying the DOE $75 million to develop the generators, and JPL's McNamee says flight units could be ready for the planned 2003 launch of Europa Orbiter. The spacecraft would then need to carry as little as five kilograms of plutonium fuel.

But this effort has not satisfied the Cassini protesters. "It doesn't matter to us, because it takes so little plutonium to create havoc," Gagnon argues. Kaku would prefer that NASA spend its money developing better solar power technologies for its spacecraft. "NASA is saying that solar is difficult and nuclear is easier," he states. "I'm saying that solar is difficult but not impossible." Kaku acknowledged that solar power is currently not a viable option for a probe to Pluto, but technical advances may eventually make such a mission possible. "The technology is not there yet," Kaku says. "But that's okay. Pluto is not going to go away."

Mark Alpert