Artist's image depicting a hot air balloon on Titan, floating over a lake of liquid hydrocarbon.
The next mission to Titan could provide a bird's-eye view of that intriguing Saturn moon, if scientists agree to send a balloon or blimp cruising through its skies for months.
NASA's Cassini spacecraft has been eyeing Titan from afar, and the Huygens probe beamed back data during its brief descent to the surface in 2005. But a long-term aerial survey would provide much more and much different information about Titan, a frigid world with liquid-methane lakes, complex surface features and a thick, nitrogen-rich atmosphere.
Exactly what type of aerial probe would work best remains an open question, and a recent study assesses the contenders. Here's a rundown of some of them, with the study's author discussing their advantages and drawbacks.
The leading aerial-probe concept, according to the study, is probably a hot-air balloon and gondola powered by a radioisotope thermoelectric generator. RTGs take the heat emitted by the decay of radioactive materials, such as plutonium, and convert it into electricity.
This balloon would use an RTG to heat Titanic air for its envelope and to power its scientific gear, which would be fitted to the attached gondola.
The balloon would cruise with the wind at an altitude of about 6 miles (10 km), well below the obscuring haze layer Huygens detected in Titan's atmosphere 13 miles (21 km) up. It would stay aloft for at least six months, long enough for two circumnavigations and a lot of remote sensing and atmospheric sampling.
This concept has a head start because it was chosen for the Titan Saturn System Mission, a joint proposal by NASA and the European Space Agency. TSSM would consist of an orbiter, a lander and the balloon. The mission was originally slated for a 2020 launch but it has been indefinitely postponed, possibly opening the door to other options.
"At present, I think the debate could be opened again," said Graham Dorrington, the author of the study, which was published online this September in the journal Advances in Space Research. Dorrington is an aerospace and aeronautical engineer at Queen Mary University of London.
Other high-altitude balloons
One alternative Dorrington discusses is a balloon filled with hydrogen rather than ambient air. Hydrogen wouldn't need to be heated, freeing up more radioisotope power for the probe's scientific gear. But the escape of its buoyant gas could pose problems for this design.
Dorrington calculates that hydrogen leakage would be significant. The balloon would need to be enormous to stay aloft for very long, or it would have to employ some kind of hydrogen-replenishment system. Alternatively, it could fly very high, where the winds are stronger, and make a complete trip around Titan before too much of its gas leaked out.
According to Dorrington, a cruising altitude of 20 miles (32 km) could work, getting the balloon all the way around Titan in about 16 days. But from that height, Titan's surface would be obscured by its haze layer. So the balloon's gondola, with all of its scientific instruments, would have to be suspended at least 7 miles (12 km) lower, on a long tether.
Another option, which Dorrington came up with, is to use a hydrogen-filled balloon, but to replace the radioisotope generator with a wind turbine attached to the gondola. Winds would be strong enough at the gondola's altitude about 12 miles (20 km) to power the balloon's instruments, and a turbine could be lighter than a radioisotope generator. But lowering the gondola for the first time would require some sort of battery.
The above concepts would scrutinize Titan from a lofty and relatively consistent height. But some designs would allow aerial probes to get down in the dirt as well perhaps a key priority of the next Titan mission.
One of these is a balloon filled with a mixture of hydrogen and methane, Dorrington says. This balloon would move in a series of "hops." Initially, it would rise to an altitude of 6 miles or more; then, as the methane inside the balloon condensed, the probe would gradually fall back to the ground, starting the cycle over again. This concept could therefore combine remote sensing with some dirt sampling.
Another option would be to stay low. A balloon could stay close to the surface a few hundred feet or so and trail an instrument-laden drag rope through the dirt. If the probe needed to stop to make useful measurements, it could use harpoons to anchor itself temporarily.
However, a low-flying craft comes with its own drawbacks, chiefly a lack of mobility.
"Surface wind speeds are low," Dorrington told SPACE.com in an e-mail interview. "It would work well above the lakes and shore regions for ranges of tens to hundreds of kilometers. However, to get a balloon right around Titan within a few months, it's necessary to fly at high altitudes."
Titan's weather also poses a problem. The moon's methane rain might weigh down low-flying balloons, Dorrington says, perhaps dragging them to the surface. Fabric tears could occur upon impact if the balloon envelope became brittle in Titan's freezing conditions (minus 288 degrees Fahrenheit, or minus 178 Celsius).
However, Dorrington says, a similar design the low-flying, climate-monitoring Aeroclipper balloon proved to be robust in field trials on Earth a few years back. The Aeroclipper survived cyclone conditions as it trawled its rope through the Indian Ocean, so the concept may work on Titan.
Not all Titan probes would have to move passively with the moon's winds. Hydrogen-filled blimps could use RTGs to control their flight, according to the study, though gas leakage would still be an issue.
Helicopter-like probes capable of powered hovering are another option, though not immediately practicable, Dorrington said. Hovering demands more power than directed flight so much more that new technology would have to be devised to make such craft work.
A high-altitude jet airplane is one more possibility. A jet engine burning liquid oxygen with the methane in Titan's atmosphere could power a fixed-wing plane around the moon at Mach 0.85, Dorrington calculated. Such a craft could make it all the way around Titan, at an altitude of 6 miles, in just 30 hours.
But this design, like all the others, has uncertainties and drawbacks, according to Dorrington. For one, carrying the liquid oxygen to Titan could be difficult. And the scheme won't work if methane concentrations in Titan's atmosphere are lower than the values Huygens measured.
The final analysis
Dorrington discusses these and several other options without recommending any of them to the planners of the next Titan mission. That's because knowing the best probe concept depends upon the priorities and details of the mission.
The key decision, he said, is whether scientists want the probe to stay high in the sky or not. If mission planners want their probe to make low-altitude excursions and even sample Titan dirt, for example, a balloon with a trailing drag rope might be a good option.
But it's not a given that any of these concepts will make it on the next Titan mission. All aerial probes would have a significant drawback compared with rovers or landers: payload capacity. Because they need to stay airborne, balloons, blimps and planes simply can't carry as much scientific gear.
Dorrington said scientific instruments could comprise no more than 3 to 4 percent of the total weight of most of these probe concepts. For surface craft such as landers, that number is 15 to 20 percent. So scientists wanting the most scientific bang for their exploration buck may go with a lander instead.
Of course, aerial platforms have their own selling point: They could scan much more of Titan's surface, yielding a broad view of the moon. And staying aloft for months could deliver good bang for the buck, too.
"If you consider cost per useful information bit, then long-endurance missions will pay," Dorrington said.
- Video: Target Titan: The Titan Saturn System Mission
- Video: Cassini at Saturn: 4 Years of Discovery
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You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall.