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Last Updated: Thursday, 1 July, 2004, 04:12 GMT 05:12 UK
Cassini probe enters Saturn orbit
Bob Mitchell and Ed Weiler, AFP
Bob Mitchell (l) is congratulated by Nasa's associate administrator Ed Weiler (r)
The international mission to Saturn - Cassini-Huygens - has successfully entered an orbit around the planet.

The $3.3bn probe fired its main engine for 95 minutes on Thursday to slow it sufficiently to be captured by the gravity of the sixth planet.

The spacecraft has travelled for more than six years and covered over three billion km to get to Saturn.

The joint US-European mission can now start a four-year study of the ringed planet and its 31 known moons.

It was kind of a nail-biter throughout but what you saw was the result of a lot of work by a lot of people and it all paid off just perfect
Bob Mitchell, Cassini programme manager

There were cheers and clapping in the mission control at the US space agency's Jet Propulsion Laboratory in California when confirmation came through that Cassini was in its correct orbit - at 0412GMT.

The spacecraft had been programmed to perform a series of manoeuvres, turning its high-gain antenna to shield against particles as it ascended through the rings, turning end-on-end to point its engine forward and fire, then spinning around once more to put the shielding dish forward again for a descent back through the rings.

Tracking data revealed the long burn came to an end just a minute earlier than expected.

In 'good shape'

All the while scientists and engineers on the mission had to follow events in delay. The huge distance to the ringed planet means signals take more than 80 minutes to arrive at Earth.

1. Antennas enabling communication with Earth
2. Boom carrying instrument to measure magnetic fields
3. Two cameras will take 300,000 pictures of the planet
4. Infra-red spectrometer analyses Saturn's temperature and composition
5. Radioisotope thermoelectric generators supply 750W of power
6. Cassini has two engines - one is a back-up
7. Thrusters used for small changes of direction or speed
8. Huygens probe will land on Saturn's largest moon, Titan
9. Plasma spectrometer measures charged particles and solar winds

So there were some nerves in the control room - if the programmed sequence had failed, Cassini could have been thrown out of the Solar System by Saturn's gravity.

"It was kind of a nail-biter throughout but what you saw was the result of a lot of work by a lot of people and it all paid off just perfect," said Bob Mitchell, Cassini programme manager.

"There wasn't a single thing that we might have asked to be done differently that would have made anything any better."

Jerry Jones, Cassini's chief navigator, reported on the spacecraft's flawless performance. "Our current orbital period is estimated to be about 116.3 days," he said.

"We were targeting for 117.4 - so we're right there. We'll probably do a clean-up manoeuvre on Saturday but there's some chance we may even call that off because we're in such good shape."

David Southwood, the head of science at the European Space Agency, saluted his American colleagues for executing a "brilliant" orbit insertion.

"This is a world mission - certainly the US and Europe working together," he said. "But I have to say this was the Americans' evening. Thank you USA, thank you Nasa."

Much to learn

Cassini-Huygens - the main spacecraft carries the Huygens probe intended for delivery to Saturn's biggest moon, Titan - is the first human-made object to orbit around the ringed planet.

Scientists hope the mission will provide important clues about how the planets formed.

They want to learn more about what Saturn is made of - its atmosphere, whipped by ferocious storms; its molten core; and its mysterious rings, thought to be the remains of shattered comets, asteroids and moons.

Nasa/JPL/Space Science Institute
Cassini had to pass through a gap in Saturn's rings
Cassini's instruments measure magnetic fields and radio waves, charged particles, cosmic dust, infrared and ultraviolet light. There are also wide-angle and narrow-angle cameras.

In December, the spacecraft will release Huygens on to Titan, the only moon in the Solar System with a thick atmosphere.

Cassini is set to fly past Titan about 36 hours after orbit insertion, giving scientists a better view of this little-known world before Huygens is despatched.

The Cassini-Huygens mission is a cooperative venture of the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency.

The mission carries the names of the 17th and 18th-Century astronomers Jean Dominique Cassini and Christiaan Huygens.

The BBC's Sue Nelson
"It has taken seven years to reach this stage"


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