It's been a long piggyback ride. For seven years, a dish-like probe named Huygens (HOY-gens) has been riding through space on a bus-size spacecraft named Cassini. Following a 1997 launch from Cape Canaveral, Florida, the duo cruised more than 3.4 billion kilometers (2.1 billion miles) on a meandering route to reach Saturn last July (see map, p. 14). But that's only the beginning of their adventures.

On December 25, 2004, the two will finally part ways. Cassini will spend the following four years orbiting Saturn (see Nuts & Bolts, p. 15). It will use a trove of sensitive tools to examine the ringed planet. As for Huygens, the probe will coast toward one of the oddest places in the solar system: Titan, the largest of Saturn's 33 known moons (icy and/or rocky bodies that orbit a planet).

What makes Titan so strange? It's the only moon in the solar system that has a thick atmosphere (protective layer of gas that surrounds a planetary body). No instrument has been able to see clearly through this smoggy haze. Could there be--as some scientists predict--lakes of oil or even volcanoes with watery eruptions on Titan's surface? If all goes as planned, scientists will soon find out.

On January 14, 2005, Huygens will plunge into Titan's atmosphere. During its descent, the battery-packed probe will have enough power to collect and transmit three hours' worth of data to Cassini. It will then relay the information to Earth. Candice Hansen-Koharcheck of NASA's Jet Propulsion Laboratory will be waiting eagerly for news from the oddball moon. This Titan expert orchestrated the scientific observations of the Cassini-Huygens mission. She talks to Science World about Huygens's journey into the unknown.

FIRST, WHAT DO YOU PREDICT IS ON TITAN'S SURFACE?

It will be very weird looking. If you look at Mars, as alien as it is, there are deserts here on Earth that look kind of similar. As for Titan, there are just no comparisons.

HOW DO YOU KNOW THAT? YOU CAN'T SEE THROUGH THE HAZE.

We have a few clues. The last time we flew by Titan was with the Voyager mission in 1980, and we took a few measurements. So far, we know Titan is extremely cold--about 93 Kelvin (-180.15[degrees]C, or-292.27[degrees]F). We've also learned that it has a rocky core with a thick, water-ice crust (outermost layer). We have detected large linear (linelike) features on the surface; this suggests tectonics (movement of the slabs that make up the crust). Unlike Earth's crust, which floats on a partly molten rock mantle (layer between the crust and the core), we think it's possible that Titan's icy crust may be floating on a watery mantle. If that's the case, Titan may have volcanoes spewing icy water instead of lava as they do on Earth. (For more on volcanoes, see SW 11/22/04.)

UNLIKE VOYAGER, WHICH SAW TITAN FROM A DISTANCE. HUYGENS GETS TO DIVE IN. DESCRIBE THIS TRIP.

The probe will enter the atmosphere at about 1,270 km (790 mi) above Titan's surface (see Splash Down!, p. 15). From there to an altitude of 300 km (186 mi), it will be traveling toward the moon's surface at mach 20 (20 times the speed of sound, or approximately 22,300 km/h, or 14,000 mph). You know those movies showing a spaceship speeding down, and it has a glowing trail, like it's burning? That actually happens. Friction (rubbing force that resists movement) between the high-speed spacecraft and the atmospheric gas molecules (particle of two or more atoms, the smallest units of an element, joined together) generates high heat. So we have a heat shield protecting the probe.

WHAT HAPPENS NEXT?

We wait for the probe to decelerate (slow down). When it slows to roach 1.5, which happens at around 180 km (112 mi) above Titan's surface, we deploy the pilot chute. This slows the probe some more, helping it stabilize. Then, at an altitude of 150 km (93 mi), we get rid of the heat shield and deploy the main parachute. This allows the probe to descend through the atmosphere slowly and start collecting data. At about 100 km (62 mi) above the surface, we get rid of the main chute and deploy a smaller chute to adjust the probe's descending speed. Overall, we get roughly 2.5 hours of data on this trip.

WHAT KIND OF DATA WILL HUYGENS COLLECT?

It's designed to measure the temperature, pressure (force over an area), wind speed, and chemical composition at every level of Titan's atmosphere. We also have cameras that will be looking downward and to the sides, snapping more than 1,100 pictures throughout the probe's descent.

HOW DOES LEARNING ABOUT TITAN HELP HUMANS?

It helps us get a better understanding of our own planet. For example, Titan's atmosphere, like Earth's, is mostly nitrogen. But it's 10 times thicker; it's packed with more gas molecules. We've also detected hydrocarbons (chemical compounds that contain only the elements carbon and hydrogen)--like methane--in Titan's air. Scientists believe the composition of Titan's atmosphere resembles Earth's before life existed here. Comparing them will be like looking at "before" and "after" snapshots of Earth. This helps us learn how Earth's atmosphere might have evolved.