PASADENA, Calif. -- A two-story, $3.4 billion spacecraft carrying a load of deadly plutonium will zoom within 725 miles of Earth this week to gain momentum for the final leg of its meandering, seven-year voyage to Saturn.
Cassini's return, two years after NASA launched the largest and most expensive unmanned spacecraft ever, poses virtually no risk, mission officials say.
But anti-nuclear activists, concerned over the 72 pounds of carcinogenic cargo, aren't so sure.
"The fact is space technology can and does fail," said Bruce Gagnon of the Global Network Against Weapons and Nuclear Power in Space. "And when you start using nuclear materials in increasing numbers, the odds of an accident increase."
The flyby at 8:28 p.m. PDT Tuesday will use Earth's gravity to change the probe's direction and speed relative to the sun. Without the "gravity assist" and two previous close encounters with Venus and a future flyby of Jupiter, the probe would never reach its destination in 2004 to study Saturn's rings and moons.
The probe will approach Earth at about 35,000 mph. Its speed will increase by about 11,000 mph after the swingby. At its closest point over the South Pacific, the probe might be visible from Pitcairn or the Easter islands.
NASA has used planets' gravity to fling its probes through space since 1973. The plutonium-powered Galileo probe to Jupiter twice swung by Earth in the early 1990s at altitudes much lower than Cassini's closest point.
The chances of an accidental re-entry of Cassini are about 1 in 1.2 million, according to a NASA estimate.
"It's just not a credible event," said Bob Mitchell, Cassini's program manager at NASA's Jet Propulsion Laboratory in Pasadena. "I'm not telling you it's impossible, but it's just not credible."
Activists fear that some sort of navigation or human error could cause the craft to burn up in the Earth's atmosphere, showering the planet with deadly plutonium dioxide.
Gagnon's group organized protests in the United States, England, Germany and elsewhere in June, but he admits there is little that can be done to change the spacecraft's course. A handful of anti-Cassini Web sites also have been set up.
The protests pale in comparison to events leading up to Cassini's October 1997 launch, when demonstrators threatened to chain themselves to the pad and filed lawsuits to stop the mission.
The spacecraft requires plutonium not for propulsion but to power its dozen scientific instruments. The probe's three radioisotopic thermoelectric generators, or RTGs, convert heat from the naturally decaying plutonium into electricity.
The units were built especially strong in case of an accident during launch or flyby. Each pellet is boxed in layers of heat- and corrosion-resistant iridium and graphite.
Mitchell said for re-entry to occur, a failure aboard the probe would have to cause an exact change in its speed before the flyby. And then something would have to happen to prevent NASA from transmitting corrective orders.
"We've been flying this thing for two years now and we got a lot of practice," he said.
Even if the capsules were to vaporize during an accidental re-entry, the effects on Earth's population over 50 years would be less than the amount of radiation from dental X-rays or a round-trip flight across the United States, according to NASA.
Anti-nuclear activists, who dispute the numbers, say the space agency should be using safe solar energy to power all its probes.
But scientists point out that Saturn is 10 times as far away from the sun as Earth and its solar panels would have to be the size of two tennis courts to harness enough energy.