Scientists dismiss atom risk from Saturn probe

The Cassini mission to the ringed planet Saturn is the first for 20 years. But will environmental opposition prevent it taking off on 13 October? What risks does the plutonium on board really pose? Charles Arthur, Science Editor, reports.
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"I'm quite happy to be at the launch in Florida," said Paul Murdin of the British National Space Centre. "I think it's more far likely that I'll be killed in an aircraft accident or car crash on the way to Heathrow than by the spacecraft blowing up." Mr Murdin was one of a team of British scientists who lined up yesterday to talk about their hopes, and well- controlled fears, for the launch of the pounds 2.2bn Cassini spacecraft (artist's impression above) on 13 October.

Along with many other British scientists, he hopes to be at the Kennedy Space Centre in Florida next month, to see off the spacecraft, which will carry precision equipment to investigate the magnetic field around the planet Saturn, and land on Titan, one of its moons. It will arrive in November 2004.

But after more than a decade of planning, the Cassini mission - the first to the ringed planet for more than 20 years, and very possibly the last during our lifetimes - is threatened by environmental groups and some scientists.

They say that the 33 kilograms of plutonium which power the scientific instruments on the probe pose an unacceptable risk to Florida and the rest of the Earth. If the spacecraft blew up on its launch, they contend that the deadly radioactive metal would be dispersed over a huge area and, in time, lead to deaths from radiation poisoning. Raw plutonium is highly poisonous: one millionth of a gram is thought to be a lethal dose. The amount on Cassini is the largest ever sent on a space mission.

However, the US space agency, Nasa, points out that the plutonium is held in a ceramic matrix so that even in an explosion, it would not be spread widely, and would be easy to detect and collect. It is needed to heat and power instruments in deep space. Solar panels are still too inefficient to operate so far from the Sun.

The final permission to launch must come from the US President, Bill Clinton. But it seems increasingly likely that despite last-minute lobbying the mission will go ahead.

Mr Murdin said, "The risks posed are those that we have learnt to accept in modern life; it's like sitting in London and having 747s flying near us into Heathrow. The bottom line is that everybody has done a good job on the risk management assessment."

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