They saw it as a ball of poison.
Across the world, they yelled and held up banners to protest its flyby of Earth.
They said the craft, careening through space at 42,000 miles per hour, had a good chance of slapping into the Earth's atmosphere and spreading dangerous radiation throughout the planet.
In protests, on Web sites, in papers, and in speeches, they claimed that thousands, even millions, could die.
No one did.
On early Wednesday morning the Cassini probe, carrying 72 pounds of radioactive fuel, whizzed by Earth -- without incident -- on its way to Saturn.
But as long as there's been a Cassini, there have been vocal anti-Cassini activists. For two years, they've spoken out on the craft's plutonium dioxide power, made with the radioactive element used in nuclear weapons. (The plutonium used on Cassini, though, is not weapons-grade.)
Come launch time -- in October 1997 -- the protesters staked out the Cape Canaveral launch site to try to stop the probe.
The launch went off without a hitch.
And when the craft was on its way back to Earth after spiraling around Venus for a gravity boost, the opponents once again unfurled their banners.
Jonathan Mark was one of the protest leaders. His website, "Stop Cassini Earth Flyby," was a kind of Protest Central for opponents of the craft.
On it, there are impassioned pleas to stop the mission with headlines like "NASA Misleads the World," and "The Cassini Gamble."
There are links to scholarly papers on the risks of the mission to the planet and its inhabitants and to political dispatches decrying the military's role in space research.
You can sign up for legal action and send off thoughts to world leaders.
And you can view pictures of the worldwide protests: of grandmothers making their voices heard, and Bangladeshis taking a stand.
But now that Cassini passed by safely, was it all for naught?
"Protesting against Cassini wasn't a mistake," Mark said. "It's significant, and I'm glad there wasn't any accident."
Mark, who sends his anti-Cassini newsletter to over 1,000 subscribers, still believes that plutonium-powered spacecraft could one day play a role in destroying life on Earth.
"We're living at a time when human beings don't have to survive on this planet," he said. "We can go extinct like the dinosaurs."
On the other side of the world, the General Secretary of the Bangladesh Astronomical Society had a similar reaction.
"Our calculation was not wrong, because we did not say that it would definitely fall on the Earth during its closest approach with Earth," F.R. Sarker said. "We were more concerned about the radioactive materials Cassini has been carrying on board."
NASA, on the other hand, couldn't resist a little gloating.
"We told you so," chimed in Mary Beth Murrill, spokeswoman for NASA's Jet Propulsion Laboratory in Pasadena, California, when asked what she would say to the anti-Cassini protesters.
The space agency had said all along that a disaster was extremely unlikely, with a probability of less than one-in-a-million.
Another NASA spokesman, Douglas Isbell, said the protesters' concerns were legitimate, but that mission scientists had taken extra precautions to prevent an accident.
NASA is planning eight more missions that will rely on the same kind of plutonium power generators used on Cassini, Isbell said.
Radioisotope thermoelectric generators, or RTGs, power Cassini. They are primarily used on missions to the outer solar system, where the sun's rays are too weak to be a usable source of power.
RTGs rely on the heat produced as plutonium, a radioactive element, decays. A converter changes that heat into electricity, which powers the craft.
The next launch of an RTG-powered craft is scheduled for the end of 2003, the Europa Orbiter, which will study a moon of Jupiter.
Those future launches are keeping protesters from packing up their signs.
"Concern about future plutonium launches is my main concern," Mark said.
He plans to leave his website up as a reminder of the Cassini protests, though daily updates will probably stop.
And in Bangladesh, the Cassini probe's success has left a residue of worry.
"It is true, we are lucky as there was no accident with Cassini," said Sarker, whose group led an anti-Cassini protest march on June 18. "But we are unlucky because this would encourage NASA to launch more space probes in the future with plutonium dioxide until an accident really takes place."