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Will Nuclear Robot Ship Sail Saturn's Moon?

One of NASA's next great adventures could take place with a raindrop-flecked camera bobbing around on extraterrestrial waves. Or at least, that's the hope of several researchers who want to sail an unmanned, nuclear-powered capsule on Saturn's moon Titan.

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Titan eerily resembles Earth with characteristics such as wind, rain and lakes, but all within the bounds of a frigid environment where liquid methane and ethane replace water. The many lakes dotting the moon's surface suggested an alternative mission proposal compared to the usual rovers and hard surface landers that NASA has sent to other destinations.

"We got funded to look at the possibility of sending a lake lander to Titan," said Ellen Stofan, a geologist with Proxemy Research in Maryland. "Scientifically, it's sort of a beyond obvious thing to do."

The concept may hold water as far as tickling scientific fancy, but must still await final selection for one of NASA's upcoming Discovery missions. But it may receive a technical boost from the space agency's development of a more powerful nuclear generator in recent years.

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A lake lander forms part of NASA's proposed Titan Saturn System Mission, an ambitious plan to send three probes — including a balloon-mounted vehicle and an orbiter — to explore the cloudy Saturnian moon.

Landing on Earth's strange twin

Titan practically beckons to a boat-shaped mission with the many lakes crowding its northern hemisphere. Average temperatures hover around -292 degrees Fahrenheit (-180 Celsius), and create weather patterns such as methane drizzling down from the clouds into liquid ethane lakes.

The moon's only previous visitor came when NASA's Cassini spacecraft dropped Europe's Huygens probe in 2005. Huygens' successful parachute landing raises hopes that a future mission could manage the feat again.

"It's very cold, but the technological challenges aren't as big as you might think," Stofan told SPACE.com. "Landing in liquid is a lot more forgiving than on land."

A successful splashdown requires a good lake target. Many of Titan's thousands of lakes reach lengths of just several kilometers, but two of the largest northern lakes extend hundreds of kilometers and compare in size to the North American Great Lakes or Europe's Black Sea. Both Ligeia Mare and Kraken Mare represent "targets we can hit with high confidence," Stofan said.

Titan's thick atmosphere also makes descent and landing slightly easier for would-be landers compared to the thinner atmosphere of Mars. And a Titan mission may also prove less technically challenging than the unprecedented step of putting landers or submersible drones on Jupiter's icy moon of Europa.

Nuclear vessels

The Titan lake lander already has a long list of scientific goals beyond snapping images of lake and sky. Stofan hopes the mission can measure the chemical composition of the lake, gauge lake depth, and study the lake-to-atmosphere cycle of exchanging organic material. Sonar could also help create a profile of the lake bottom.

All the necessary instruments require power, and Stofan says that nuclear generators represent the most practical option. Titan's thick atmosphere rules out solar panels that have kept Mars rovers and landers alive for years, while batteries would only provide some hours of life at most.

NASA recently approved Advanced Stirling Radioisotope Generators (ASRGs) for use on the Discovery missions. The new radioisotope generators are not unlike earlier generation models that powered Cassini and other spacecraft by using the heat from radioactive decay, rather than actually using nuclear fission. But the ASRGs take advantage of the heat to power a piston and generate electricity, and use just a quarter of the plutonium normally required.

"ASRGs takes the mission from a type of grab science to answering deeper questions," Stofan noted.

Stofan has already teamed up with the Applied Physics Lab at Johns Hopkins University to shepherd the lake lander through proposal phases. Now the researchers must await a possible year-end announcement of the final selections for NASA's Discovery missions.

If the lake lander does someday splash down on Titan, its mast would hold a camera as opposed to a sail. But a stiff wind and currents would still push the floating capsule around for months, and help convey an oddly familiar sight on an alien world, mission planners have said.