<P dir=ltr align=left><STRONG>Space probe set for Saturn orbit</STRONG></P>
Space probe set for Saturn orbit
The Cassini space probe is all set to begin its four-year mission to Saturn - to study the ringed planet and the many moons that move under its influence. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
Cassini will pass through a gap in those famous rings before making an engine burn to slow it down enough to enter an orbit around the gas giant
The burn starts at 0336BST on 1 July, with the probe's main engine turned towards the direction of travel.
The braking manoeuvre ends at 0512BST, with Cassini in an elliptical orbit.
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The Saturn orbit insertion (SOI) will mark the end of a seven-year journey across the Solar System for the $3bn spacecraft.
On its voyage, Cassini performed four gravity-assist fly-by manoeuvres to help carry it to the sixth planet from our Sun.
As it prepares to insert into Saturn's orbit, the probe will approach the planet from below the plane of its ring system and cross it through a large gap between the "F ring" and the "G ring".
Before passing through, Cassini will point its high-gain antenna in the direction of travel. This 4m-wide solid disc will shield the spacecraft's delicate components from any particles of debris it may encounter on the way through.
"This gap between the F and the G ring is very clear of debris. The Voyager and Pioneer spacecraft have already been through there," Claudio Sollazzo, the European Space Agency's (Esa) head of operations for the Huygens spacecraft, told BBC News Online.
"But to increase our safety margin, the spacecraft is aligned so that any small particles that may be there will impact the high gain antenna, which is fairly strong and resilient."
When Cassini emerges above the ring plane, it needs to turn its main engine towards the direction of motion and fire it for 96 minutes. The resulting thrust will slow the spacecraft down by 640 m/s - enough to be captured by Saturn's gravity.
Professor David Southwood, Esa's director of science, told BBC News Online: "For 90 minutes, the motor will fire. Just as it switches off at Saturn, we'll get the first signal that it is firing at Earth.
"It takes 90 minutes for the signal to travel from Saturn to Earth. This is going to be real nail-biting stuff."
The spacecraft will then cross back below the ring plane - once again using the gap between the F and G rings - and turn its high-gain antenna towards Earth to transmit its new data to mission controllers. This should start arriving late on Thursday (BST).
The burn will bring Cassini closer to Saturn than it will ever be again during its four-year tour. During the manoeuvre, the probe's distance from Saturn will be about 18,000km (about 11,200 miles). Cassini will use its proximity to Saturn to take close-up photographs of the planet and its rings. But scientists stressed that Cassini will not be able to resolve individual ring particles and will be flying too fast to image large areas.
The spacecraft will gather scientific data for about an hour while it is above the ring plane.
"We will measure the strength and the direction of Saturn's magnetic field. Because we are so near, small irregularities in this field can tell us a lot about the interior and core of Saturn," said Dr Sollazzo.
"Also, being so near to the planet, we'll be able to determine events such as lightning in the atmosphere using radio emission from the lightning strikes."
Cassini gathered a goldmine of scientific data during its flyby of Saturn's outermost moon Phoebe on 13 June. But mission scientists say that was just a curtain-raiser
"We're about to embark on a delicious smorgasbord of scientific opportunities," Dr Dennis Matson, Cassini project scientist told a news conference in Pasadena, California, US.
In December, the spacecraft will release the piggybacked Huygens probe into the thick atmosphere of Saturn's moon Titan.
Cassini will fly past Titan about 36 hours after orbit insertion, giving scientists a better view of this little-known world before Huygens is despatched.