Sunday, August 15, 1999 Published at 09:32 GMT 10:32 UK
Saturn probe swinging by Earth
Cassini: Heading for a rendevous with Saturn
The Cassini spacecraft is preparing to swing by the Earth on the outward journey in its seven-year mission to Saturn.
"The fact is space technology can and does fail," said Bruce Gagnon of the Global Network Against Weapons and Nuclear Power in Space.
"And when you start using nuclear materials in increasing numbers, the odds of an accident increase."
But Nasa officials insist Cassini's return poses virtually no risk. It will come within 1,165km (725 miles) of the Earth on Wednesday and use the planet's gravity to gain momentum for its trip to Saturn.
The chances of an accidental re-entry of Cassini are about 1 in 1.2 million, according to a Nasa estimate.
"We've been flying this thing for two years now and we've got a lot of practice," said Cassini programme manager Bob Mitchell.
Scientists have been using planetary "gravity assists" since 1973 to fling probes around the outer Solar System.
"It's purely gravity and no more sophisticated than the Moon moving around the Earth," said Mr Mitchell.
Speeding at 17,700 kph
The probe will approach Earth at about 35,000 mph (56,325 kph). Its speed will increase by about 17,700 kph (11,000 mph) after the swingby.
At its closest point over the South Pacific, the probe might be visible from Pitcairn or the Easter islands.
A Nasa planetary swingby has never missed its target beyond an acceptable range, said Mr Mitchell.
The Galileo spacecraft flew by Earth twice on its way to Jupiter. In 1990, it flew within 952km (592 miles) of the surface and was within eight kilometres (five miles) of total accuracy. In 1992, it flew as close as 300km (189 miles) and came within less than a kilometre of total accuracy.
The $3.4bn Cassini probe was launched two years ago. It is Nasa's largest and most expensive unmanned spacecraft. The spacecraft will photograph Saturn's rings, atmosphere, and moons.
It will also deploy a sub-probe on to Saturn's largest moon, Titan.