Date:25/12/2004 URL: http://www.thehindu.com/2004/12/25/stories/2004122500621500.htm

International

Saturn mission nears climax

By Tim Radford

LONDON, DEC. 24. A little flying saucer not much bigger than a washing machine will separate from its mother ship early tomorrow morning on the last stage of a long journey to oblivion.

If all goes to plan, the European lander Huygens will ease away from a lorry-sized NASA orbiter called Cassini for a date with death and glory 1292 million km from home.

On January 14, Huygens will plunge into the freezing orange-coloured skies and atmospheric tempests of Saturn's moon, Titan, the biggest moon in the solar system and the only one with a dense atmosphere.

In its two and half hour descent, it will measure everything it can about the Titanic world and transmit the data back to Cassini for just as long as its batteries hold out, and before it is overwhelmed by the conditions on the mysterious moon.

Everything about the ride will be a nerve-racking test of plans and technology fashioned a decade ago.

Suspended animation

Cassini-Huygens, a joint NASA-European mission, has taken seven years to cross the divide between Earth and Saturn.

For almost all the journey, Huygens has been in a state of suspended animation, destined only for a brief awakening on a moon with an atmosphere thought to most closely resemble that of Earth before the dawn of life 3.8 billion years ago.

Scientists in Pasadena, California, and Darmstadt, Germany, will have a tense moment as Cassini — temporarily out of touch with Earth — points itself towards a precise spot on Titan's surface and releases the lander at a speed of 35 cm a second, spinning seven times a minute to keep it stable. Then Cassini will turn and signal back to Earth that its little passenger is on its way.

But the next 20 days will also be crucial. Cassini will have to alter its course very precisely, so that as Huygens begins its death dive it will be placed at the correct distance and at the precise fly-by location to pick up every precious digit of data from its little hitchhiker. Then, as it disappears over Titan's horizon and away from any possible contact with Huygens, it must turn and relay everything it has learned back to Earth.

Even at the speed of light, the signals will take 67 minutes to reach the mission scientists.

None of them know quite what to expect. Cassini-Huygens has now made three passes over Titan, two of them skimming over the moon at an altitude of only 1332 km.

But Titan is a dark world: it receives only 100th of the sunlight experienced on Earth. Temperatures are around -180C and the clouds of organic chemicals may obscure a surface of freezing methane, or an ice sheet dotted with lakes of hydrocarbons, or even a range of low ice hills emerging from seas of chemical slush. Cassini's cloud-penetrating radar has detected signals that no one can interpret with any certainty. Scientists still do not know what Huygens will finally hit, as it parachutes slowly to oblivion.

Back on Earth, radio astronomers expect to measure the probe's position within a mile rather than a billion miles. ``That is like being able to sit in your backyard and watch the ball in a ping-pong game being played on the Moon,'' said Leonid Gurvits of a joint European radio-astronomy research base in the Netherlands. —