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[cdn-nucl-l] plutonium power for Titan helicopter
A more detailed version of this concept development study by Ralph Lorenz
appears in the current issue of the Journal of the Brit. Interpl. Soc.
For more information: www.lpl.arizona.edu/~rlorenz
Ralph Lorenz is a planetary scientist at the Lunar and Planetary Lab at the
University of Arizona in Tucson
>From New Scientist magazine, 15 July 2000.
Titan here we come
WAY OUT beyond the icy rings of Saturn there's 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's 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's exotic landscape from above, Huygens will take a
detailed look at the complicated organic chemistry in its hazy red
Unfortunately, one of the most reliable sources of power--solar
panels--isn't 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
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's 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.
Equip the craft with a small amount of plutonium, an alkali metal converter
and a rechargeable battery and it can use the battery's 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
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's battery and give the craft up to 24 hours of flying time every
Titan day--which lasts the equivalent of 16 Earth days.
It's too early to say what this machine will actually look like. It's 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
Now comes one of the mission's 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).
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's low gravity. "This kind of mission is unique," says Joel
Levine, an atmospheric scientist at NASA's Langley Research Center. "You can
decide where you want to explore." Wendy Calvin, a geophysicist at the
University of Nevada, Reno, agrees. "If the helicopter is smart enough to
say: "Hey, there's organics over there," it can go over and drill the stuff.
That's a very compelling concept."
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.