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- The Cassini spacecraft is preparing to swing by the Earth
on the final leg of its seven-year mission to Saturn.
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- But anti-nuclear activists have raised fears that the
probe, with its load of plutonium, could malfunction and crash into the
Earth.
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- "The fact is space technology can and does fail,"
said Bruce Gagnon of the Global Network Against Weapons and Nuclear Power
in Space.
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- "And when you start using nuclear materials in increasing
numbers, the odds of an accident increase."
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- The spacecraft uses plutonium for its propulsion and
to power its dozen scientific instruments.
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- But Nasa officials insist Cassini's return poses virtually
no risk. It will come within 725 miles (1,165km) of the Earth in the early
hours of Wednesday morning and use the planet's gravity to gain momentum
for its trip to Saturn.
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- 'Purely gravity'
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- The chances of an accidental re-entry of Cassini are
about 1 in 1.2m, according to a Nasa estimate.
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- Mission officials say that for re-entry to occur, a failure
aboard the probe would have to cause an exact change in its speed before
the flyby. Then something would have to happen to prevent Nasa from transmitting
corrective orders.
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- "We've been flying this thing for two years now
and we got a lot of practice," said Cassini programme manager Bob
Mitchell.
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- Scientists have been planetary "gravity assists"
since 1973 to fling probes around the outer solar system.
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- "It's purely gravity and no more sophisticated than
the moon moving around the Earth," said Mr Mitchell.
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- Speeding at 35,000mph
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- The probe will approach Earth at about 35,000 mph (56,325
kph). Its speed will increase by about 11,000 mph (17,700 kph) after the
swingby.
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- At its closest point over the South Pacific, the probe
might be visible from Pitcairn or the Easter islands.
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- A Nasa planetary swingby has never missed its target
beyond an acceptable range, said Mr Mitchell.
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- The Galileo spacecraft flew by Earth twice on its way
to Jupiter. In 1990, it flew within 592 miles (952km) of the surface and
was within five miles of accuracy. In 1992, it flew as close as 189 miles
(300km) and came within a half mile of the point.
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- The $3.4bn Cassini probe was launched two years ago.
It is Nasa's largest and most expensive unmanned spacecraft.
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