Titan has long been a favorite for science-fiction tales. But hardly anything is known about it except that it's the only moon in the solar system with a discernable atmosphere and is larger than the planets Mercury and Pluto.
Scientists aren't even sure about its surface -- or whether the probe will crash onto hard ground or splash into a methane lake.
Andrew Coates, a member of the Cassini-Huygens team at Mullard Space Laboratory at London's University College, told RFE/RL: "It's really going to be an alien world, we think. It will be lit by an orange sky, as we go down. Then the surface could be in a state where it's solid, or liquid or whatever. And there could be stuff coming up from the surface -- geyser-type activity with material spewing up from a liquid subsurface. And there could be stuff going down on to the surface in terms of rain of hydrocarbons."
"Huygens" has spent the past seven years piggybacked on the "Cassini" mother ship, which arrived at Saturn in July. The U.S.-European probe separated from "Cassini" on 25 December and will parachute through Titan's atmosphere on 14 January, gathering data about the moon during a two-hour descent.
For now, the probe is coasting with its systems shut down to preserve the battery power required for its study of Titan. A timer will switch the probe back on just before it reaches Titan's atmosphere, said Mark Leese, "Huygens" project manager at Open University in the English city of Milton Keynes.
"The probe is in hibernation, and all that is happening is there are three counters within the master timing unit just counting down. They were set just a couple of days ago now, and they'll wake the probe up around four hours before it reaches the upper atmosphere of Titan. Mainly to help the communication system to get to the best operating temperature, and give us a better link," Leese said.
Leese said the probe's landing will begin with a two-hour descent through Titan's dense atmosphere, believed to consist of nitrogen and methane. "The probe hits the atmosphere at something like 1,270 kilometers. Within about 4 1/2 minutes of that time, the probe detects that the atmosphere is slowing it down sufficiently, and it fires the parachute system," he said. "Shortly after that, within maybe 20 or 30 seconds, the first experiments are switched on and start to transmit data back to 'Cassini.'"
The 319-kilogram probe's three cameras are expected to provide 20 panoramas during the descent, as "Huygens" rotates on its parachute. The final pictures should reveal surface details as small as a soccer ball. Other sensors will detail the atmosphere's composition, clouds, and mists.
Conditions on Titan are believed to resemble those on Earth about 4.5 billion years ago. Scientists thus hope to see many of the chemical processes that led to life on our planet. But with temperatures at minus 179 degrees Celsius, Titan is considered too cold to host life forms.
Mullard Space Laboratory's Coates noted that in such cold, methane behaves something as water does on Earth. It could exist as a gas, liquid and snow, or ice. If Titan boasts seas of hydrocarbons, Coates explained that the tides could be several meters high with raindrops the size of cherries, due to low gravity.
After landing, the probe will continue its study. But it is expected to remain operational only for a brief time before its batteries die.
Leese explained that "Huygens" will transmit its findings back to "Cassini," which in turn will beam them back to scientists on Earth. "We won't actually get that data on Earth until the entire mission descent is over, because 'Cassini' continues recording that data transmitted from the probe until the probe has reached the surface, and the link is lost," he said. "And then 'Cassini' turns towards Earth and transmits the data, which takes another 66 minutes."
Until then, Coates said it will be nervous waiting for scientists who have spent nearly 15 years on the project. "It will be an extremely exciting, nail-biting time, as we wait, before we actually hear from it," he said.