ON 1 July, the Cassini spacecraft will reach Saturn. Its 3.5-billion kilometre journey from Earth has taken nearly seven years, but an hour-and-a-half will decide the fate of the spacecraft as it zips through Saturn鈥檚 rings, turns around and fires its engine to insert itself into orbit around the planet.
In the hours after the burn, Cassini will be at its closest ever to Saturn, giving it a glorious view of the planet鈥檚 rings. The images it sends back will tell us about the rings鈥� structure, age and possible future. Mapping the planet鈥檚 magnetic field will tell us about its interior. Cassini will also study the chemical make-up of Saturn鈥檚 atmosphere, its fierce winds and its many moons 鈥� including the most enigmatic of all, Titan.
But all this depends on what happens on Thursday. Before inserting itself into orbit around Saturn, Cassini will have to negotiate its way through a gap in Saturn鈥檚 rings, using its main antenna as a shield against shrapnel. An impact with debris just a few millimetres across could be fatal, but no particles have been observed in this gap, and with the antenna as a shield 鈥渢he risk is essentially zero鈥�, says mission planner David Seal of the Jet Propulsion Laboratory in Pasadena, California. 鈥淵ou would probably have to have something like a large marble to get through the antenna and still hurt Cassini.鈥�
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After passing through the rings, Cassini will fire its rocket for 96 minutes, slowing down by about 2200 kilometres per hour. The spacecraft must manoeuvre unaided, because signals from Earth take more than an hour to reach Saturn. 鈥淚f something goes wrong, there is nothing you can do,鈥� says Andrew Coates of Mullard Space Science Laboratory in London, an investigator on one of Cassini鈥檚 instruments. To leave little to chance, the designers have built a lot of redundancy into Cassini鈥檚 systems, including a spare rocket motor.
Less than two hours after the burn, as the probe heads back towards the plane of the rings, its cameras will focus on the A-ring. Like the other main rings, it is about 100 metres thick and made of loose ice and rock, ranging in size from boulders down to microscopic dust, and is divided into thousands of ringlets. Carolyn Porco of the Space Science Institute in Boulder, Colorado, says that Cassini will get close enough to see features as small as 100 metres across.
The most popular theory suggests that the rings are a recent acquisition. They may be the remnants of an interloper from the outer solar system that strayed too close to Saturn and got torn apart by its gravity. Or they might be an old moon that was smashed to bits by an asteroid.
The rings are expected to drift inwards, and fall into the planet within a few hundred million years. But this depends on how the individual particles behave. Porco hopes to see striations within the ringlets, whose size and shape could then be used by modellers to work out how the particles bounce off each other. From this researchers would be able to calculate the age of the rings and how long they are likely to survive.
By mapping Saturn鈥檚 magnetic field, Cassini鈥檚 magnetometers will let scientists see into the planet鈥檚 interior. The field is thought to be produced by convection currents in a layer of liquid metallic hydrogen that lies above a small rocky core. At the moment that structure is little more than guesswork, according to Michele Dougherty of Imperial College London, who is in charge of Cassini鈥檚 magnetometer. The craft could give us a much clearer picture by studying the magnetic field in detail 鈥� and data collected just after the insertion into orbit will be vital.
Cassini will then go into a series of elongated orbits that will take it past the planet鈥檚 eight largest moons. The grand prize among them is Titan, which is larger than the planet Mercury and the only known moon with an atmosphere. Cassini鈥檚 radar will cut through the orange haze that fills the moon鈥檚 dense atmosphere to map its surface, and drop the Huygens probe onto Titan in January 2005. Among the lesser prizes are Enceladus, whose brilliant surface may be covered with pure water ice, punctured by vapour volcanoes; and Iapetus, with its mysterious deep red hemisphere.
In between exploring the moons, Cassini will test the make-up of Saturn鈥檚 atmosphere, where multicoloured clouds are blown by 1500-kilometre-per-hour winds, some of the fastest in the solar system. These winds are driven by the heat generated within the planet, but the heat source is uncertain. It may be released by helium condensing and raining down on the core, but Cassini should tell us for sure.
Researchers are in for a flood of Cassini data. 鈥淲e could get surprised by the data in ways that might be profound,鈥� says Richard Young of the NASA Ames Research Center in Moffet Field, California. 鈥淭hat鈥檚 why we have to do these missions. Our theories don鈥檛 always hold up very well.鈥�
