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Last Updated: Friday, 14 January, 2005, 10:53 GMT
Huygens begins its Titan descent
By Paul Rincon
BBC News science reporter in Darmstadt, Germany

Titan: An atmosphere not unlike Earth's billions of years ago

The Huygens spacecraft has sent back its first signal on its historic descent to Saturn's moon Titan.

Early on Friday Huygens began its dive through Titan's atmosphere, taking images and readings as it parachuted towards the surface.

Its scientific investigation of this mysterious world could yield clues to how life first arose on Earth.

Ground controllers detected Huygens' signal showing it was alive via a network of sensitive radio telescopes.

"It looks like we have heard the baby cry," said Jean-Pierre Lebreton, Huygens mission manager.

"It tells us the probe is alive, that everything has been successful and that we are on the parachute."

Huygens' systems should now be preparing to transmit data to the Cassini spacecraft. The probe still has a couple of hours to go on its journey to the moon though.

Huygens has been coasting silently towards the exotic world for 20 days since being released from its mothership Cassini.

If all goes well, it will be the furthest from Earth a spacecraft has been landed.

"We're doing something today which will last for centuries. It's not my name that matters it's what we're doing," said Professor David Southwood, director of science for the European Space Agency (Esa).

Unknown surface

"I'm afraid if Columbus hadn't decided to cross the Atlantic ocean, and he'd said it's going to be a two-month trip, Queen Isabella would have saved a lot of money," he added.

"You have to take risks, otherwise, nothing ventured, nothing gained."

Titan is veiled by a thick orange haze which obscures its surface features. Huygens could land with a thud on ice and rock, squelch into tar-like gunge, or splash down in an oily sea.

Huygens diagram (Nasa/Esa)
1. HASI - measures physical and electrical properties of Titan's atmosphere
2. GCMS - identifies and measures chemical species abundant in moon's 'air'
3. ACP - draws in and analyses atmospheric aerosol particles
4. DISR - images descent and investigates light levels
5. DWE - studies direction and strength of Titan's winds
6. SSP - determines physical properties of moon's surface

The spacecraft will take a total of 750 images during its two-and-a-half-hour descent, shedding light on this cosmic enigma.

"This should provide a spectacular new view of Titan and hopefully a much greater understanding of this mysterious world," said Marty Tomasko, principal investigator on the Descent Imager/Spactral Radiometer instrument on Huygens.

Professor John Zarnecki, principal investigator on the surface science package on Huygens, has made no secret of his wish to land on an extraterrestrial ocean.

"I'm pleased that my instrument has got something to measure a liquid surface, a solid surface and something in between," he told the BBC News website.

"Despite the flybys of Titan by Cassini we still don't know [what its surface is like]."

Data gathered by the spacecraft should give detailed information on the moon's weather and chemistry.

The sounds of Titan's stormy atmosphere will be recorded with an onboard microphone, and scientists hope that they will even hear lightning strikes.

When the European-built probe entered Titan's atmosphere at an altitude of 1,270km (789 miles) from the surface, it was travelling at over Mach 20 which is 20 times the speed of sound.

Once friction slowed the probe's descent to about Mach 1.5, it deployed the first of three parachutes, pulling off the rear cover that protects Huygens from the fierce heat as it enters Titan's atmosphere.

Dominated by nitrogen, methane and other organic (carbon-based) molecules, conditions on Titan are believed to resemble those on Earth 4.6 billion years ago.

As such, it may tell scientists more about the kind of chemical reactions that set the scene for the emergence of life on Earth.

Huygens has spent the past seven years tethered to the Cassini spacecraft, which arrived at Saturn in July 2004.

See a demonstration of the probe's descent to Titan

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