Fear of fall-out threatens voyage to Saturn

Safety in space: US scientists raise nuclear power alarm as Russian space station hits trouble again

The launch of a Nasa module bound for the planet Saturn, originally scheduled for early next month, may be threatened following claims that the US government is understating the risks associated with its nuclear power source.

The anti-nuclear lobby in the United States is mobilising itself to block the Cassini mission after hearing claims from a former senior Nasa official that an unforeseeable mishap with the module could shower parts of the Earth with radioactive debris that could threaten millions with cancer.

Alan Kohn, who was Nasa's emergency preparedness director for the launchings of both the Ulysses craft towards the Sun in 1990 and the Galileo ship to Jupiter one year earlier, was due to underline his concerns at a Washington press conference yesterday.

The focus of Mr Kohn's worries are the plutonium batteries that will power the Cassini as it penetrates deep space, far away from the rays of the sun that might otherwise have been its energy source. Altogether, the Cassini will bear 72lb (32kg) of plutonium in three batteries.

By comparison, the Ulysses carried 24lb (11kg) of plutonium and the Galileo 48lb (22kg). However, plutonium-based power cells have been built into scores of spacecraft, including satellites and Nasa's Apollo rockets. A battery from Apollo 13 was lost on re-entry to the Earth's atmosphere and at present lies at the bottom of the Pacific Ocean.

Nasa is adamant that every precaution is being taken to eliminate possible scenarios where a disastrous failure could release the plutonium into the Earth's atmosphere. The batteries, for instance, are encased in iridium, a metal of extraordinary density.

Mr Kohn is not impressed. He told the New York Times: "Men and machines are fallible. If you keep launching these things, eventually you're bound to have an accident. It's inevitable.

"Nasa says this whole thing is safe. Nobody can make such a statement. I've seen too many rockets blow up."

There are three moments during the projected mission that fire concern. There is the launch itself. That was set for 6 October, but has since been delayed because of a recent accident on the launchpad that damaged some of the craft's protective layer. That incident has itself rung alarm bells.

Thereafter, however, risk of radioactive pollution will remain during an orbit of Earth by Cassini and later, in August 1999, when it is due once more to fly close to the Earth's atmosphere.

Among those backing up Mr Kohn is Dr Michio Kaku, a physicist at the City University of New York. While Nasa has put 120 deaths on Earth as the worst possible scenario, Dr Kaku believes the number of deaths through radioactive exposure following a catastrophe could reach more than 200,000.

Anxious that Mr Kohn's campaign should not ground the $3.4bn Cassini mission, the government is moving to offer counter-evidence. "Our safety analysis is two-feet thick," a spokeswoman for the Energy Department told the New York Times. "There is absolutely no accident sequence that results in huge amounts of plutonium being released."