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Image: NASA/JPL/Caltech The Huygens probe separating from the Cassini spacecraft
Saturn - Lord of the Rings

Questions and answers

Programme transcript

With its famous rings, Saturn is the most distant planet clearly visible to the naked eye. But how did the rings get there and when were they formed? To study the planet in detail, scientists needed to get closer. So on 15 October 1997, the Cassini-Huygens spacecraft was launched.

The Cassini-Huygens is one of the most ambitious spacecraft ever launched, taking seven years to reach Saturn. The mission itself consists of two separate probes. The first is the enormous Cassini probe, designed to gather information about all aspects of the Saturnian system, from its many rings to its 33 moons. The second is the Huygens probe, a smaller wok-shaped craft, attached to the side of Cassini. Its task is to plunge through the atmosphere of Titan, Saturn's largest and most mysterious moon.

The project is a joint NASA, European Space Agency (ESA) and Italian Space Agency venture. It has cost $3.27 billion and involves over 17 countries. It was inspired by another successful mission- the launch of the two Voyager Deep Space probes. These left Earth in 1977, and arrived separately at Saturn in 1980 and 1981. They sent back revolutionary data, changing what scientists thought about the Saturnian system.

They revealed that Saturn's rings are far more complex and dynamic than any one had ever imagined. They also suggested that the rings had been formed after the planet itself. Why? And how old were they? But the Voyager probes had to move on, past Uranus and Neptune and beyond, leaving these fundamental questions about the rings unanswered.

Voyager also raised another mystery - Titan. Titan isn't just Saturn's largest moon, it is also shrouded in a thick orange atmosphere, composed mainly of nitrogen - similar to the Earth's atmosphere. Finding a place so far away which shared features with our own world was exceptionally tantalising.

The building of the Cassini-Huygens project began in 1990. The Cassini probe was named after the French-Italian astronomer Jean-Dominique Cassini (1625-1712). Its job was to fly to Saturn, and remain in orbit around the planet for four years. The Huygens probe, named after Christiaan Huygens (1629-1695) who first discovered Titan, was to detach itself from Cassini and gather crucial information about the chemical composition of the moon's atmosphere and to reveal what kind of landscape lay beneath the clouds.

Not surprisingly, the probes were armed with a formidable array of instruments. Cassini is capable of measuring everything from the planet's huge magnetosphere to tiny particles of cosmic dust. At the heart of the mission are the cameras - the so-called Imaging Science Subsystem. These have both a long and short focal length, which gives both high resolution pictures and the wider context. Cassini also carries a set of spectrometers which look at the same things as the cameras, but which can see wavelengths beyond those visible to the human eye. These wavelengths allow the spectrometer to deduce the chemical composition of whatever is being looked at.

The combined weight of the two probes was a massive 5,712kg and it was to travel across a billion miles of space. This created a whole new set of problems - how to get such a massive craft to its destination.

The mission planners turned to a technique known as a 'gravity assist'. The probe was to be launched by the largest rocket in the world, the Titan IVB. But that was just enough to get it out into space. Cassini was then routed by other planets, 'stealing' energy from their orbital momentum. The result is a kind of gravitational slingshot. After launching in 1997, Cassini passed by Venus twice, then Earth in 1999 and finally Jupiter at the end of 2000.

On 30 June 2004, the spacecraft finally arrived at Saturn, having clocked up a speed of over 80,000km/h (approximately 50,000mph). But to get into orbit around the planet, Cassini-Huygens had to pass between Saturn's F and G rings. Mission planners had plotted a course through what appeared to be a gap, but if Cassini was hit by even a tiny particle the size of a grain of rice, the probe could be destroyed.

At 19:36 Pacific Daylight Time (PDT), scientists waited anxiously at the Jet Propulsion Laboratory in California, as the manoeuvre (called Saturn Orbit Insertion) began, and Cassini fired its main engine to get into orbit. All eyes followed the signal given out by Cassini's low-gain antenna. At 21:12 PDT, the signal flattened out, indicating that Cassini-Huygens had successfully got into orbit around Saturn.

The mission will last for at least four years, with the possibility of going on for longer. Cassini will use its wealth of instruments to study the whole Saturnian system.

It's already providing intriguing information. One of the instruments onboard Cassini, the Ultraviolet Imaging Spectrograph (UVIS), will help scientists answer the question of how old the rings are. By looking at the reflected light in ultra-violet wavelengths, this instrument can tell what parts are icy and what parts have been contaminated. By measuring this over time, UVIS will be able to use the pollution like a clock. The dirtier the rings get, the older they are.

As Cassini passed over the rings during Saturn Orbit Insertion, UVIS took images of the rings which showed a significant difference in the levels of water ice and contaminants. It will take scientists time to decipher this data, but along with the information from Cassini's other instruments, such as the Visible and Infrared Mapping Spectrometer (VIMS), they will be able to build up a picture of the composition of the rings over time, and so work out how old they are.

But while the Cassini mission is due to last for years, the Huygens mission is much more short-lived. It will be key to our understanding of Titan, Saturn's largest moon. Like the Earth's, Titan's atmosphere is mainly made up of nitrogen, but there are other compounds such as methane which open up exciting possibilities for science.

Because Titan is so cold (as it is so far away from the sun), methane can exist in all three states - gas, liquid and solid. Indeed methane might adopt the role water has on earth in a weather cycle, so there could be clouds of methane, methane rain, and possibly even lakes, or seas. The surface itself is probably made not of rock, but hard water ice surface. Scientists expect it to be pitted with impact craters and even volcanoes, spewing liquid water, rather than lava.

So in some ways, Titan's landscape might seem strangely familiar. But crucially, Titan may also offer a window on the distant past, as scientists believe that its atmosphere may resemble the early Earth's atmosphere, before life began. It's all because the methane and nitrogen in Titan's atmosphere combine together to create a compound called tholins, which form the impenetrable orange haze that blocks Titan's surface from view.

Experiments on Earth have shown that tholins, when combined with liquid water, can form amino-acids, which can then form proteins - the building blocks of life itself. While Titan is too cold to have liquid water for long on its surface, if there are volcanoes these could hurl water onto the surface of Titan from the liquid mantle beneath the surface. This may stay liquid for long enough for complex organic molecules to form. The hope is that these conditions could make Titan a planet-sized laboratory in which we can study the processes by which life might have begun on our planet.

But scientists can only hypothesise at this stage since so little is known about Titan. But on 14 January 2005, we will know much more. Huygens will separate from Cassini on Christmas Day 2004, and after a 22-day coast it will enter Titan's thick atmosphere. Its enormous heat shield will protect it from searing temperatures of up to 1,700ºC, and a series of parachutes will slow its descent. After the heat shield is ejected, the instruments will begin to do their work - the onboard camera will take the first ever images from beneath the haze, and the chemical analysers will sample the atmosphere, sending back precious information about this unexplored moon.

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 Horizon - last series

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 Elsewhere on bbc.co.uk

News: Mission to Saturn
Watch how the Cassini mission to the ringed planet is unfolding.

News: New ring discovered around Saturn
...but does it extend all the way around the planet?

News: Exploring Mars
Not enough cash, not enough time. Why the Beagle 2 mission failed.

Science: Saturn
Its density is so small it would float on water - if there was an ocean large enough.

Science: Cassini-Huygens
Huygens' landing on Titan will be the furthest from the Earth ever attempted.

 Elsewhere on the web

Cassini-Huygens Mission to Saturn & Titan

European Space Agency - Cassini-Huygens

CICLOPS: Cassini Imaging Central Laboratory for Operations

NASA: Solar System Exploration

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