Unless mounting protests in the US and around the world manage to stop it, NASA plans Monday to launch its Cassini probe - containing more deadly plutonium than has ever been used on a space device.

Plutonium has long been described by scientists as the most toxic substance known. Some 72.3 pounds are to be used on Cassini as fuel to generate electricity - just 745 watts - to run the probe's instruments, electricity that could be produced instead by safe solar panels.

The probe is to be launched on a Titan IV rocket, despite the poor record of Titan rockets. In 1993, another Titan IV blew up on launch, blasting an $800 million spy satellite to smithereens.

If Cassini does make it up on the Titan IV, an even more dangerous event lies ahead. Cassini does not have the propulsion power to get directly from Earth to its final destination of Saturn, so NASA intends to hurtle back to Earth for a "sling shot maneuver" - to use earth's gravity to increase its velocity so it can reach Saturn.

NASA plans to have Cassini come hurtling back to Earth in 1999 at 42,300 miles per hour for a "flyby" less than 500 miles overhead.

But, after many millions of miles in space, if there is a rocket misfire or miscalculation and the probe makes what NASA in its "Final Environmental Impact Statement for the Cassini Mission" calls an "inadvertent reentry," it could fall into earth's 75-mile high atmosphere, disintegrate, and release plutonium. Then, says NASA, "Approximately 7 to 8 billion world population at a time ... could receive 99 percent or more of the radiation exposure."

NASA says fatalities over a 50-year period could be 2,300. But scientists I've interviewed - including Michio Kaku, professor of physics at the City University of New York, John Gofman, professor emeritus of medical physics at the University of California, and Ernest Sternglass, professor emeritus of radiological physics at the University of Pittsburgh School of Medicine - estimate far more

Kaku estimates 200,000 deaths, Gofman projects up to 1 million, and Sternglass suggests 40 million in the event of a dispersal of plutonium from Cassini in an earth "flyby" accident.

Three out of the 26 earlier US nuclear space missions have involved mishaps. The worst: In 1964 a satellite with a SNAP 9-A plutonium system aboard fell to earth, disintegrating and dispersing its 2.1 pounds of plutonium. Gofman has long linked that accident to a rise in lung cancer on earth.

After the SNAP 9-A accident, NASA pioneered development of solar photovoltaic power for satellites. Solar power can now substitute for plutonium on space probes.

The European Space Agency has developed new high-efficiency solar cells for space probes and intends to use solar power instead of plutonium to generate 500 watts of electricity on its Rosetta probe that is to go beyond the orbit of Jupiter and rendezvous with a comet. Such solar technology could be used to provide electricity on a redesigned Cassini probe.

But NASA, under pressure from Lockheed Martin, the company that manufactures plutonium systems, and the US Department of Energy and its national nuclear laboratories, insists on sticking with atomic power on Cassini - and other planned space shots.

Also, NASA seeks to stay in coordination with the US military which wants to deploy nuclear-powered weaponry in space.

We're on a countdown to nuclear space disaster - on Cassini or the other atomic space missions. The use of nuclear power on space devices is not worth the risk. Let's explore space but do it safely, not risking the destruction of a portion of life on Earth.

BECAUSE of the danger of nuclear space missions, presidential permission is required to launch. But in spite of an avalanche of calls and letters to the White House, President Clinton signed off on the launch of the colossally and needlessly life-threatening Cassini mission.

The Florida Coalition for Peace and Justice has held a daily anti-Cassini vigil at Cape Canaveral. Some US legislators have called for cancellation of a nuclear Cassini mission.

"It doesn't take a rocket scientist to see the dangers of the Cassini probe," says Rep Jerrold Nadler (D) of New York.

If the launch does happen, the pressure built up in the US and around the world to stop the Cassini launch should continue because the Cassini Earth "flyby" could still be scuttled.

Only if NASA orders it in 1999 will Cassini and its 72.3 pounds of lethal plutonium come flying back at Earth.

"The job of government is to protect the people, not to put the people at risk," says Alan Kohn, a retired 30-year veteran of NASA emergency preparedness operations and an officer for earlier nuclear shots.

"It's time to put a stop," says Kohn, to NASA's "freedom to threaten the lives of the people here on Earth."

* Karl Grossman, professor of journalism at the State University of New York/College at Old Westbury, is the author of "The Wrong Stuff: The Space Program's Nuclear Threat to Our Planet" (Common Courage Press, 1997).