Science: Unlocking the secrets of Saturn

The Night Sky

The night sky in the coming month will feature the planet Saturn - and that's where a new space probe is headed. Nigel Henbest and Heather Couper have the details in their monthly column.

"Planet of the month" has to be Saturn. On 10 October, it is at its closest to the Earth this year, and you can see it shining all night long in the southern part of the sky. A small telescope will reveal the glorious set of rings that girdle this distant world, along with its biggest moon, Titan, as a speck of light that travels round Saturn in a "month" that is just 16 Earth days long.

Saturn is also in the news for another reason. If all goes according to plan, a Titan 4 rocket will blast off from Cape Canaveral this month carrying the first artificial moon of Saturn, the Cassini-Huygens spacecraft.

So far, our best views of Saturn and its many moons have come from the two Voyager probes which sped past in 1980 and 1981. Their results astounded astronomers. The Voyagers found that Saturn's rings are made of thousands of narrow ringlets, some braided together and "herded" by so-called shepherd moons. They discovered many new moons, bringing Saturn's total to 18 - the largest family of any planet. And Titan turned out to have an atmosphere even denser than the Earth's, laden with orange clouds that frustrated any attempts to see the surface.

The new probe will open a new phase in our exploration of the ringed planet. It is in fact two spacecraft in one: Nasa's Cassini probe will orbit Saturn for many years, sending back close-up views of the planet, its rings and its icy moons. Meanwhile, the European Space Agency (ESA) probe Huygens will parachute down on to Titan's surface to find out what lies below the clouds.

It won't be a straightforward trip. Even before launch, the take-off has been delayed by damage to Huygens's insulation (it was ripped when someone left the air-conditioning set too high).

It may also be affected by misguided activists objecting to the plutonium- powered electricity generators on board: Saturn is so far from the sun that Cassini cannot use solar panels to provide power. But as far as safety is concerned, a similar generator has already survived intact when a Titan rocket exploded under it.

Even the Titan and its Centaur upper stage - the most powerful combination Nasa can call on - can't propel the massive Cassini-Huygens directly to Saturn. Instead, it will be launched "the wrong way", towards Venus. It will fly past Venus twice, and the Earth once (in August 1999), each time picking up extra momentum that will fling it on towards Saturn.

The most exciting discoveries are likely to lie beneath the clouds of Titan. By bouncing radio waves off it, astronomers have already found evidence that it has liquid seas or oceans. It's so far from the sun's heat that these can't be made of water: they probably consist of liquid methane or ethane.

Infrared views from the Earth suggest that Titan may have mountains, too. Most interestingly, the results from the Voyagers suggest that the orange clouds are made of some kind of organic "gunge", caused by sunlight welding together the simple molecules into something more complex. Many scientists think that life on the early Earth began in just such a way: the organic gunge on our planet was washed into seas, where chemical reactions created the first living cells. On Titan, we may find an "early Earth preserved in the deep freeze". But don't hold your breath: it's not due to arrive until June 2004.

Watch out for in the next month...

As well as Saturn, the brighter planet Jupiter lies over towards the south west. Towards the end of the month, look low down in the evening twilight for even more brilliant Venus.

Around 21 October, we will be treated to a shower of shooting stars, best seen soon after midnight (before the moon rises). These tiny grains of cosmic dust, burning up in the Earth's atmosphere, are bits of debris left behind by Halley's Comet.

Diary

(24-hour clock, BST up to 26 October)

1 1752 new Moon

9 1322 Moon at first quarter

10 Saturn at opposition

16 0446 full Moon

21 maximum of Orionid meteor shower

23 0549 Moon at last quarter

26 end of British Summer Time

31 1002 GMT new Moon