One bizarre trip: a spacecraft will soon plunge into Titan, one of the strangest moons in the solar system. What will it find?

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.

WHAT WILL THE PROBE DO ONCE IT LANDS ON THE SURFACE?

Since we can't see what's on the surface, we don't know what it will land on. If it lands on something really hard and jagged, the probe may not survive. If it survives, we have enough battery power to collect another half an hour's worth of data. One of the coolest things about the probe is that it can float.

YOU'RE EXPECTING AN OCEAN?

Maybe. Because it's so cold there, the clouds of hydrocarbons in the atmosphere could condense (change from gas to liquid) and rain down. Theoretically, this liquid is oil. We use the same stuff to power camp stoves on Earth! In case we land in a lake of oil, we're ready.

DO YOU THINK ALIENS WILL BE SWIMMING IN THE OILY LAKES?

I'm doubtful of finding life there. No Earth-life can survive in that cold of an environment. If there's anything living on Titan, it will be something very unique to that moon.

CASSINI INTERPLANETARY TRAJECTORY 

Cassini--with Huygens on board--blasted off from Florida. For almost seven years, its programmed trajectory (route) sent the craft coasting by Venus (twice), and on to Jupiter, before reaching Saturn.

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Nuts & Bolts

Saturn is the sixth planet from the sun. With a 120,000 km (71,000 mi)-diameter, Saturn is one of the largest planets (second only to Jupiter) in the solar system. Unlike rocky planets, such as Earth, Saturn is a gas giant; it has no solid surface to land on! The planet is approximately 75 percent hydrogen, 25 percent helium, and has traces of methane and water ice. Saturn is famous for its numerous moons and jumbo rings, which are made of countless icy and rocky particles.

Splash Down!

DIVE IN: At an altitude of 1.270 km (790 mi), Huygens begins its plunge into Titan's atmosphere.

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READY: At an altitude of 180 km (112 mi), the heat shield separates, and the first of three parachutes deploys, helping to slow the probe.

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STEADY: At an altitude of 150 km (93 mi), Huygens begins collecting data as it slowly sails downward.

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SPLAT? Non one knows what type of surface Huygens will land on. Will the probe survive the landing? stay tuned ...

WEB EXTRA

Follow the Cassini-Huygens mission at: http://saturn.jpi.nasa.gov/home/index.cfm

One Bizarre Trip

DID YOU KNOW?

* The Cassini-Huygens mission is a joint project of NASA, the European Space Agency, and the Italian Space Agency. More than 8,000 people contributed to the spacecraft's building and design.

* Titan is the largest of Saturn's moons. With a 5,150 kilometer (3,200 mile) diameter, it's even larger than the planet Mercury. Still, Ganymede--one of Jupiter's moons--beats Titan by 112 km (62 mi) as the largest moon in the solar system.

CRITICAL THINKING:

* In the article, NASA scientist Candice Hansen-Koharcheck said that no Earth life could survive under Titan's atmosphere. List five ways that Earth's atmosphere contributes directly and indirectly to your survival.

CROSS-CURRICULAR CONNECTIONS:

HISTORY/LANGUAGE ARTS: Divide students into teams and have them research what Galileo Galilei, Jean Dominique Cassini, and Christian Huygens discovered about Saturn. Then, have each team write and present a play about the work of these scientists.

RESOURCES

* Grolier search term: Saturn

* For more about the Cassini-Huygens mission, check out: www.esa.int/SPECIALS/Cassini-Huygens/

CHECK FOR UNDERSTANDING

ONE: BIZARRE TRIP

NAME: --

DIRECTIONS: On a separate piece of paper, use details from the article to help you write the following:

1. Suppose you're a space alien looking for a new location to call home. Describe why Titan's environment may or may not make this moon a good choice.

2. You're a NASA scientist, and you've been invited to appear on a talk show. Describe to the TV audience why a heat shield and parachutes are important to Huygen's descent. Also, explain what the probe will accomplish on its trip through Titan's atmosphere.

ANSWERS

One Bizarre Trip

Answers will vary, but they should contain the following points.

1. The alien must ponder the following environmental factors: Titan has a thick atmosphere. Like Earth's atmosphere, it's mostly nitrogen. But it's five times thicker. There are also hydrocarbons--like methane--in the air. Since Titan is extremely cold-about 93 Kelvin (-180.15[degrees]C, or -292.27[degrees]F), the clouds of hydrocarbons could condense and rain down, forming lakes of oil. The moon also has a thick water-ice crust that may be floating on a watery mantle, so there could be volcanoes spewing watery eruptions.

2. The probe will enter the atmosphere at about 1,270 km (790 mi) above Titan's surface. From there to an altitude of 300 km (186 mi), it will be traveling toward the moon's surface at mach 20. Friction between the high-speed spacecraft and the atmospheric gas molecules generates high heat, so a heat shield protects the probe. Scientists are aiming to collect roughly 2.5 hours of data--which includes more than 1,100 photos and measurements of the temperature, pressure, wind speed, and chemical composition at every level of Titan's atmosphere. To achieve this, a series of parachutes will help the probe to stabilize, and then descend slowly through the atmosphere. If the probe survives the landing, it contains enough battery power to collect another half an hour's worth of data. If the probe lands in liquid, it's ready: It is even designed to float.

COSMIC CHUTES

Name: --

In "One Bizarre Trip" (p. 12), you learned that a parachute's function is to help slow an object's descent through the atmosphere. What parachute features provide the slowest and smoothest descent? First, review the "Guidelines" (below). Then, embark on a challenge to create the ultimate parachute in your class. *

GUIDELINES

1. Each parachute must carry a payload (cargo carried by a spacecraft) of two small paper clips.

2. All materials for the chute must come from the "Parachute Resources" list (see below).

3. The parachute must be either a circle or square shape.

4. All parachutes must be released from the same height: The bottom of the payload measures 1.8 meters (6 feet) above the ground.

PARACHUTE RESOURCES

plastic grocery bag * sheet of printer paper * tissue paper (76 cm by 51 cm, or 30 in. by 20 in.) * thin string (200 cm, or 80 in. long) * 2 small paper clips * scissors * measuring stick * tape (20 cm, or 8 in. long) * stopwatch * pencil * graph and plain paper

PREDICT

Which type of parachute material do you think will give the slowest and smoothest descent? Will the size of the parachute affect how well it works?

CONSTRUCT A PARACHUTE

1. Form teams of three to four students. Hold a discussion to decide on the material, shape, and size of the parachute.

2. Cut the material to create the parachute. Calculate and record the area of the chute. (Hint: For a square, multiply the length by the width; and for a circle, multiply pi, or 3.14, by the circle's radius squared.)

3. Cut four pieces of string--each 25 cm (10 in.) long. Then, space the strings at equal distances apart and tape the end of each string to the edge of your chute.

4. Slip two paper clips through the ends of the dangling strings. Then, tie the string ends into a single knot with the clips hanging down from the knot. The clips represent the "payload."

5. Have one student hold the chute from an outstretched arm. Adjust the arm so that the bottom of the payload is 1.8 m (6 ft) above the floor. Be careful: The student might need to stand on a chair.

6. Designate one student as the timekeeper. The timekeeper will record the parachute's descent from the release until the payload first hits the floor. The time should be recorded to one decimal point.

7. Release the parachute. Record the time and observations of the parachute's descent. For example: Did the parachute flip over or veer in one direction?

8. Repeat Step 7 two more times. Then, calculate to find the average descent time.

9. As a team, discuss ways to improve the chute so that it descends slower, or stays upright. Should you try a different material, chute size, or shape?

10. Based on your ideas front Step 9, repeat Steps 2 through 8.

11. After completing the activity, your teacher will record the results from the better of your two chutes on the chalkboard. Results will include the parachute's material, shape, area, and average descent time.

CONCLUSIONS

1. How did the size, or area, of your parachute affect its descent time?

2. Did one of your chosen materials lead to better results compared with the other material? Explain.

TAKE IT FURTHER:

As a class, discuss the positive and negative changes your team made to your parachutes. For example, did a larger chute have a slower descent than a smaller chute? Based on the class's conclusions, decide which attributes will create the ultimate class parachute. Build it.

ANSWERS

Activity Tips:

Real parachutes come in many sizes and can be made from different types of materials. Still, the chutes serve the same function: Catching enough air to give an object a softer, and slower, landing. Discuss air resistance (the slowing force of air on a moving object) with your class, and how this force is responsible for a parachute's function. To show how air helps chutes function properly, you could demonstrate the opposite by making a parachute out of a porous material such as cheesecloth. Also, for added variables in the students' parachute experiment, you may choose to add these materials in the "Parachute Resources": newspaper, aluminum foil, and wax paper. Because of their weight and texture, parachutes made of these materials generally flip over during the descent.

Answers

1. Usually, increasing the chute's area--while keeping the same material--will slow its descent. With more area, the chute can catch more air, giving it more air resistance.

2. Depending on its size compared with other chutes, the tissue and plastic bag could give better results than the heavy paper. That's because the paper is too heavy for the lightweight payload.