Cassini runs rings round Saturn

Dropping into orbit this week

At 01:12 on Thursday morning, the Cassini spacecraft will begin its orbital insertion around Saturn, after its six and a half year journey from Earth. (By the time this signal reaches Earth, it will be 02:36) It will go into a highly elliptical orbit from which it will tour the rings and moons of the gas giant for the next four years, dropping the Huygens probe off at Titan on one of its passes.

As the craft approaches the planet from below the ring plane, the main engine will swing around to face the direction of travel. Facing that way, when the engine fires, it will act as a brake.

As Cassini passes through the rings - it is aiming for the large gap between the F and G rings - the engine will burn will 96 minutes. This manoeuvre should drop the craft into its orbit 18,000km about the clouds of Saturn: its closest view of the planet during its visit to the system.

Over the next few days, timing will be crucial. Note: The following timings are given in Coordinated Universal Time (UTC), and are of events as they will be observed on Earth, ie, they include the 84 minute delay of a signal travelling from Saturn.

At 01:11am on 1 July, the signal from Cassini will be lost. An hour later it will pass through the ring plane. At 02:21am, the spacecraft will make its 'turn to burn' manoeuvere, and regains its signal. at 02:36, the braking burn begins. Near the end of the burn, at 04:03, the craft is at its closest approach to Saturn. It will be in its orbit by 04:12am. The first images should be with the ESA/NASA team by 12:39 that afternoon.

During its 74 orbits of the planet, Cassini will make 45 close flybys of Titan, the second largest moon. This serves two purposes: for the gravity kick the move will give the craft, and for scientific study. On Christmas day 2004, on approach to Titan, Cassini will release the ESA's Huygens probe. The probe will coast for 21 days before parachuting to land on the surface of the moon.

Huygens is equipped with six instruments, with which it will sample and analyse the atmosphere of the gassy moon. It also has a 'Surface Science' package. Unsurprisingly, the ESA says, this is "a suite of sensors to determine the physical properties of the surface at the impact site and to provide unique information about its composition".

Meanwhile, NASA's Cassini will continue to collect data on its tour of the system, snapping pictures as it goes. Instruments on board the spacecraft will conduct studies on waves, particles and imaging data. ®

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