On Monday at 4.45 a.m. NASA will launch a space probe towards Saturn, eight years and 794 million miles away. The $3.4 billion mission, called Cassini, has been 11 years in the making. It is probably the last of its kind. From now on, the agency's voyages of discovery will be smaller, like Pathfinder's recent trip to Mars. If Cassini isn't equally successful, it could be the last in every sense of the world.

I wish I could be sanguine about that "if", but it is hard when Cassini's potential for disaster defies reason. The probe is partpowered by 72.3lb of plutonium, the most toxic substance we know. One pound distributed in the atmosphere would, hypothetically, be enough to induce lung cancer in all of us.

Cassini is offering humankind two opportunities for exposure. The first is lift-off. Cassini and its plutonium powerpack are mounted on a Titan IV rocket, NASA's "workhorse" which has been described by Space News, the industry's trade paper, as "costly to build, inefficient to assemble".

In 1993 a Titan IV exploded 101 seconds after launch, destroying its $800 million spy satellite payload. Cassini has already been delayed once due to a fuel leak.

But the risk at lift-off is nothing compared to Cassini's second possible flashpoint in August 1999. To achieve the velocity of 100 km per second needed to reach Saturn, NASA has to get the rocket to orbit twice around Venus, fire it back for a quick orbit of Earth and then slingshot it out to space.

During this procedure, it will be just 312 miles above Earth's surface, or within 35.36 seconds of what NASA calls an "inadvertent re-entry". If it enters the atmosphere, it will likely burn up and scatter plutonium across the globe in a radioactive "footprint" that would contaminate 50,000 square kilometres. NASA estimates a clean-up cost of $247 million per square mile, nearly $4.8 trillion.

Still, what are dollars compared to the human cost? Even NASA conceded that billions of people could be exposed to at least 99 per cent of the released radiation. Dr Ernest Sternglass, professor emeritus of radiological physics at the University of Pittsburgh, translates exposure into a death toll "as high as 30-40 million".

So what are the risks that either of these "doomsday-ish" scenarios would come to pass. I take my cues from warnings issued by the scientific community, even by people inside NASA, where there is reputedly a swell of resistance to Cassini.

NASA insists that Cassini's three plutonium-fuelled radioisotope thermoelectric generators (RTGs) are "indestructible". Mr Alan Kohn, a veteran safety officer, calls them "indestructible like the Titanic". A physicist, Mr John Gofman, who developed some of the first methods of isolating plutonium, says NASA's confidence is "optimism beyond belief".

I wonder if NASA isn't a little shaken, because the agency has now lowered its initial one-in5,000 chance of Cassini releasing plutonium to 1-in-345. Meanwhile, NASA's jet propulsion laboratory reckons Cassini's engines have a 1-in-20 chance of failure.

So why is NASA risking our lives? According to Lieut Gen James Abrahamson, former head of the Strategic Defence Initiative organisation, "failure to develop nuclear power in space could cripple efforts to deploy anti-missile sensors and weapons in orbit".

Cassini has been developed in the face of a media blackout. The fact that General Electric and Westinghouse, the owners of NBC and CBS respectively, are responsible for 85 per cent of the world's commercial nuclear plants puts a new spin on the notion of vested interests. It is only now that the public is getting to grips with Cassini's implications.

President Clinton can stop it. That's why the White House is being besieged with faxes and emails. It seems reasonable to ask the President to postpone Cassini until the probe can be redesigned using a solar power source.

You can fax Bill Clinton on 001-202-456-2461, or email the Whitehouse at www.whitehouse.gov

Anita Roddick is the founder and part-owner of The Bodyshop.