Each of our highest priority targets in the outer Solar System offers something unique, from Europa’s internal ocean to the geysers of Enceladus. But Titan exerts the kind of fascination that comes from the familiar. The imagery of lakes and river channels reminds us inescapably of our home world, even if the temperature on the Saturnian moon averages a brisk 94 K, which works out to -291 degrees on the Fahrenheit scale. But because of its thick atmosphere we have options for exploring Titan that are unavailable on the other icy moons, and we’re working with a landscape that is a compelling frozen doppelgänger of Earth, a landscape we’d like to explore up close.
As we saw yesterday, part of the outer system puzzle is getting supplies of plutonium-238 up to speed, and there is at least some movement on that front. If we want to get aggressive about exploring Titan, one excellent way to deploy that plutonium is aboard AVIATR (Aerial Vehicle for In-situ and Airborne Titan Reconnaissance), a 120 kg airplane that could study the moon’s atmosphere from within and give us views of its geography on demand. Under study by a team led by Jason Barnes (University of Idaho), the airplane could do some things a balloon could not, including not being at the mercy of equatorial winds and being more flexible in studying some of the interesting terrain around high-latitude lakes and in the turbulent weather near the poles.
I’ve been thinking about AVIATR and other Titan options this week because last weekend I had the chance to speak with Mike Malaska, an organic chemist who has been collaborating with members of the Cassini team in research that involves characterizing the Sikun Labyrinth canyon-land region and hydrocarbon channels located near Titan’s South Pole. I discovered that Mike is also a volunteer artist for the AVIATR mission proposal. Like I said, Titan fires the imagination, and the idea of a human aircraft in operations there sounds too fascinating not to attempt. Mike’s image of the unmanned AVIATR vehicle flying over the surface of Titan is below.
Image: A rendering of the AVIATR airplane flying over Titan. Credit: Mike Malaska.
But back to that thick atmosphere, which helps us on Titan in so many ways. In an environment where gravity is seven times less than on Earth, we’re dealing with an atmospheric pressure one and a half times greater than Earth’s. It was Robert Zubrin who suggested, back in the 1990s, that humans with wings strapped to their arms would be able to fly in this thick and soupy environment. We’ve already seen evidence of this atmosphere’s effect on the Huygens probe, which took fully two and a half hours to descend to the surface in early 2005.
A paper on the AVIATR concept notes Titan’s advantages:
With an Earth-like surface shaped by rainfall and atmospheric interaction, a rich and complex chemistry, and the astrobiological potential of complex organic molecules interacting with liquid water, Titan is one of the three most interesting targets in the solar system for planetary exploration. Like Mars, but unlike Europa, Titan can be explored inexpensively. Although cruise times to Titan are long (∼7 years), arrival at Titan is easy. Titan’s dense atmosphere with a large scale height is perfect for decelerating landers and aerial elements cheaply and with low heating and acceleration loads. Orbiters at Titan can be mass-efficient, too, if they utilize aerocapture…
The plutonium-238 issue becomes still more interesting because the new type of radioisotope thermoelectric generators NASA is examining seem made to order for an airplane in Titan’s atmosphere, while they’re unlikely to function in alternative balloon proposals. Called Advanced Stirling Radioisotope Generators (ASRG), the new RTG’s are considerably more efficient than their predecessors, requiring less plutonium-238 and producing less waste heat. Barnes and team argue in their paper that a hot-air balloon would not work on Titan with an ASRG because of its lower heat production — balloon designs would need to function with the waste heat from an MMRTG [Multi-Mission Radioisotope Thermoelectric Generator]. In fact, Barnes’ calculations show that the 500 W of heat from an ASRG would support less than 10 kg in a balloon, which is actually less than the mass of the ASRG itself.
Contrast this with AVIATR, which can use the longevity of twin ASRGs for extended operations on Titan. Setting aside the power requirements for computers, actuators and instruments, Barnes and company find they have in the neighborhood of 80 W to power up the propeller for straight and level flight operations. From the paper:
The propeller’s thrust on an airplane with singly folded wings can keep aloft an airplane with a mass up to ∼120 kg. Other nuclear power sources, such as the MMRTG, cannot satisfy the physical requirements for heavier-than-air flight at Titan subject to appropriate engineering and risk constraints. Thus the use of an ASRG makes the AVIATR mission concept possible…
In fact, once deployed in Titan’s atmosphere, AVIATR should be in a benign environment:
Heavier-than-air flight on Titan is easier than anywhere else in the solar system. With over 4 times more air and 7 times less gravity than Earth, flight on Titan is 28 times easier than it is here (in the sense that a vehicle with the same contours flying at the same velocity on both planets could lift 28 times more mass). It is over 1,000 times easier on Titan than on Mars.
Long-duration flight is the name of the game, and AVIATR is seen as capable of carrying out a 1-year mission. The stability of the design would return the aircraft to straight and level flight in case the autopilot failed, in which case stabilizing vanes would deploy automatically to keep the aircraft in a stable position while pointing its antenna at the Earth to receive safe-mode instructions. The paper notes that the materials for the airframe are conventional and based on designs for unmanned aerial vehicles (UAVs) that operate here on Earth. AVIATR would also be more robust than a balloon in handling atmospheric turbulence and wind shear.
How would AVIATR be deployed, and how would it conduct flight operations? More on this tomorrow, when I want to dig a little deeper into the science and the unusual telecommunications strategy afforded by an aircraft on Titan. The paper is Barnes et al., “AVIATR—Aerial Vehicle for In-situ and Airborne Titan Reconnaissance: A Titan airplane mission concept,” Experimental Astronomy, Volume 33, Issue 1 (2011), pp.55-127 (full text).