WAY OUT beyond the icy rings of Saturn there鈥檚 a mysterious world called Titan. The cloud-shrouded surface of this huge moon is one of the largest unexplored regions in the Solar System. Somewhere here, in the icy soup of organic molecules that coats its surface, scientists believe they will discover primitive proteins, or better still, living cells that could help them solve once and for all the mystery of the origin of life.
Our first glimpse beneath Titan鈥檚 clouds will come in 2004, when the Cassini orbiter arrives at the moon and releases a small probe called Huygens. While Cassini maps out Titan鈥檚 exotic landscape from above, Huygens will take a detailed look at the complicated organic chemistry in its hazy red atmosphere.
This visit could be the first of many. NASA already has a second visit to Titan in the pipeline, known simply as 鈥淭itan Organic Explorer鈥. Only a probe with remarkable abilities will do for this ambitious mission. But time is short. Since the journey lasts up to 8 years, work must begin soon on the challenging new technologies that this explorer will need.
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A second mission to Titan should be well worthwhile, though. Researchers already know how to build simple organic molecules in the lab by blasting mixtures of nitrogen, methane and ammonia with sparks or ultraviolet light. Add a little oxygen from water vapour and you can even create amino acids. But no one really knows what else you need if you want to trigger life-what mix of compounds, catalysts or conditions will turn simple amino acids and the like into self-replicating molecules such as RNA or DNA. Earth-bound labs simply aren鈥檛 up to the job.
Enter Titan. High in the moon鈥檚 atmosphere, sunlight breaks up molecules of nitrogen and methane, leaving hydrogen molecules and atoms of carbon and nitrogen that recombine to form all sorts of heavier organic molecules-telescopes have already spotted about 20 different compounds. The most abundant of these is ethane, which may collect on the surface as huge lakes or seas. But in a world with only carbon, nitrogen and hydrogen, it鈥檚 impossible to make most of the components we associate with life. You need that other vital ingredient, oxygen.
Titan is too cold to permit anything but a whiff of oxygen-containing compounds in its atmosphere, and all the oxygen in its surface is locked up in ice. Yet occasionally the water melts. In 1992, Carl Sagan and his colleague Reid Thompson at Cornell University, New York, suggested that meteor impacts could melt Titan鈥檚 icy crust and allow organic molecules from the atmosphere to react with oxygen in liquid water. They calculated that organics in the crust around impact sites may have been exposed to oxygen for anything up to 2 million years; enough time perhaps for simple amino acids to form. And what then: proteins? sugars? Maybe even primitive living cells? The only way to discover how far along the road to life this chemistry has gone is to sample these oases on Titan.
Tracking down these sites will throw up some sticky problems: for one thing, no one knows exactly what the surface of Titan looks like (New Scientist, 12 April 1997, p 34). Whatever vehicle we send has to cover huge distances-too far for a rover, especially over uncertain terrain broken up by lakes of liquid ethane.
An aerial explorer is the only sensible solution. Recording Titan鈥檚 doubtless spectacular landforms is best done from above, but Titan鈥檚 thick, hazy atmosphere will make it difficult to get good close-up pictures from orbit. An aeroplane flying at lower altitudes would be ideal for surveying the surface, but landing to collect samples would be impossible.
The cheap and simple answer is a balloon or airship. But anchoring them against the wind and generating enough downward force to dig samples from the hard surface might be a problem.
This leaves a helicopter. Although on Earth these craft need huge amounts of power, according to my calculations an electrically powered helicopter is the most practical proposition for Titan. It could cover large distances, but also land exactly on top of the most interesting chemical deposits and be stable enough to drill into them. And since Titan鈥檚 atmosphere is four times denser than Earth鈥檚, it is much easier for a helicopter鈥檚 rotors to push against it. Better still, the moon鈥檚 gravity is just one seventh of Earth鈥檚, so the craft wouldn鈥檛 need to generate so much lift. All in all, a helicopter on Titan would need 38 times less power to take off than the same helicopter on Earth.
For example, I calculate that a 100-kilogram helicopter with big, fold-out rotors would need about 500 watts to fly, little more power than you need to run a domestic vacuum cleaner. Which is just as well, because even powering a small vacuum cleaner would be a challenge on Titan. A one-shot battery would drain in a day or so, not long enough to get decent results.
Unfortunately, one of the most reliable sources of power-solar panels-isn鈥檛 practical either. Titan is 10 times as far from the Sun as we are, so it gets only 1 per cent of the sunlight the Earth receives. Worse, the thick, hazy atmosphere that surrounds the moon absorbs most of the light that reaches it, so that only a tenth of that amount arrives at the surface. This is far too little to generate power. So that leaves reliable, but expensive and politically unpopular, power supplies containing radioactive material.
Plutonium power
These devices are known as radioisotope thermoelectric generators, and they create electricity from heat given off by the decay of the short-lived isotope plutonium-238. Their efficiency depends on how the unit works. Most RTGs, such as those on Cassini, channel the heat into a thermoelectric converter made from a semiconducting material. This kind of RTG is just 5 per cent efficient. For example, Cassini鈥檚 RTG would need 17 kilograms of plutonium to generate 500 watts of power. Newer converters that use alkali metals instead are almost 15 per cent efficient.
Build a power supply with an alkali metal converter, for instance, and the helicopter would need only 7 kilograms of plutonium to generate 500 watts. But the best way to reduce the amount of costly plutonium required is to limit the time the helicopter spends in the sky.
Space hopper
Equip the craft with a small amount of plutonium, an alkali metal converter and a rechargeable battery and it can use the battery鈥檚 power to take long, leisurely leaps through the atmosphere-like a frog in slow motion. After a few hours of flight it lands and recharges its batteries ready for the next big hop. As it flies, it can study the ground beneath. When it spots an interesting feature such as an ice sheet around a crater, the craft can hover over it long enough to take detailed measurements, or land directly on the site.
Operating this way, the power supply should need no more than 1 kilogram of plutonium to generate 70 watts or so, enough power to charge the helicopter鈥檚 battery and give the craft up to 24 hours of flying time every Titan day-which lasts the equivalent of 16 Earth days.
It鈥檚 too early to say what this machine will actually look like. It鈥檚 not clear whether a conventional helicopter layout with a small tail rotor, a pair of contra-rotating rotors, or even something like a tilt-rotor aircraft-with propellers mounted on tilting wings-would work best. Ease of control and power efficiency are important, but so is packaging.
To get the craft to Titan, it would need to be stowed in a small entry vehicle with a heat shield. As this enters the atmosphere, a parachute will extract the helicopter from the heat shield and the rotors will spring into action.
Now comes one of the mission鈥檚 greatest challenges: how do you control this helicopter in flight? It takes over an hour for radio signals from Earth to reach Titan, so the craft will have to fly itself. NASA engineers are already working on smart software to control space probes and spot signs of life (New Scientist, 22 April, p 22).
Meanwhile, a team at the Robotics Institute at Carnegie Mellon University in Pittsburgh are building their own autonomous helicopter that could provide the role model for Titan鈥檚 robotic explorer. First, though, the researchers hope it will be used to map out the topography around a future Mars base.
This project has begun to solve the problems of designing a helicopter to operate on alien worlds. 鈥淗elicopters are very unstable objects,鈥 says Takeo Kanade, director of the Robotics Institute. 鈥淭hey must be actively controlled all the time.鈥 For that, he says, you need two things: first, you must understand the way it performs at different speeds and in different environmental conditions.
Secondly, and much more demanding, the helicopter needs detectors such as gyroscopes that feed information on the helicopter鈥檚 speed, heading and altitude back to the craft鈥檚 computer. 鈥淚f you want the helicopter to do a task such as follow a certain kind of terrain,鈥 says Kanade, 鈥測ou must also have an additional sensor, most likely a camera, which can automatically recognise and track terrain.鈥
In 1998 the researchers flew their craft over one of the most Mars-like places on Earth: an ancient crater at Devon Island in the Canadian Arctic. During these flights, an onboard computer steered the Carnegie Mellon helicopter using signals from the Global Positioning System satellite network. But without the luxury of GPS, how could a robotic helicopter explore Titan and not get hopelessly lost?
Kanade and his team seem to have the answer. They have built a vision-based navigation system or 鈥渧isual odometer鈥 that uses distinctive features on the ground as markers so the craft always knows where it is. To work out how fast it is moving, the odometer uses an onboard camera to record the scene, and the computer measures how fast it is changing. From this, the craft calculates where it is and where it鈥檚 going.
The researchers have even shown that it鈥檚 possible to pack the odometer into a unit small enough for use on an interplanetary mission. 鈥淥ur self-navigating system weighs about 20 kilograms,鈥 says Kanade. But by miniaturising the components, you can slash its weight even more. 鈥淓ven a package weighing a tenth of that is conceivable,鈥 he says.
With a total weight of 100 kilograms, the helicopter should be able to carry about 15 kilograms of sensors. These can be designed to take samples from the surface and deliver them to an onboard lab where detectors will test for specific biomolecules. Mix a sample with molecular 鈥渓abels鈥 which fluoresce when they bind to DNA or other proteins, for example, then pass the whole lot under an ultraviolet light, and you could easily spot the target molecules. The instrument could also test what forms these molecules take. Many biomolecules such as amino acids and sugars exist as two forms, structurally the same but mirror images of each other. No one knows why, but all the important amino acids on Earth are the left-handed form. Knowing whether this selection occurs on Titan too could help solve this mystery.
The explorer will also need a suite of cameras and spectrometers to find interesting deposits and document where they formed. During the long periods the helicopter spends on the ground, it can act like a regular planetary lander, monitoring the weather and beaming the scenes around it back to Earth.
We should be in for a treat-this world has some bizarre sights. Large raindrops of methane, almost a centimetre across, drift slowly from the red haze. Geysers spout pale plumes of ethane high into the sky. Careful planning could even land the craft at a cliff edge from which it could watch giant waves breaking on the shores of an ethane lake, in the slow motion mandated by Titan鈥檚 low gravity. 鈥淭his kind of mission is unique,鈥 says Joel Levine, an atmospheric scientist at NASA鈥檚 Langley Research Center. 鈥淵ou can decide where you want to explore.鈥 Wendy Calvin, a geophysicist at the University of Nevada, Reno, agrees. 鈥淚f the helicopter is smart enough to say: 鈥淗ey, there鈥檚 organics over there,鈥 it can go over and drill the stuff. That鈥檚 a very compelling concept.鈥
Making any new space mission happen is not a trivial exercise, and it is certainly not for the impatient. Cassini was first proposed some 18 years ago. So now I must get other scientists interested and persuade NASA to perform a detailed technical study. Is 100 kilograms enough? Will the helicopter need a relay satellite? What will all this cost? It could be a long slog, but just imagine what might be waiting for us on Titan.