Yesterday’s discussion of the AVIATR mission to Titan inevitably brought up another prominent Titan mission concept: Titan Mare Explorer (TiME). I’ll have more to say about this one next week, as today I want to continue talking about AVIATR, but you can once again see how Titan enthralls us with its ‘Earth-like’ aspects. Need a thick atmosphere around a moon? Titan is your only play, and if, as with TiME, you want to put an instrument package into an off-planet surface lake, you’ll be hard pressed to do it anywhere else, at least in this Solar System.
These two mission concepts fire the imagination — they’re the kind of thing kids like me used to dream about when we plunked down our money at the newsstand for copies of Galaxy or Fantasy & Science Fiction. It’s unlikely, though, that both missions will fly. If TiME, which is a Discovery-class mission finalist and thus cost-capped at $425 million, is chosen, then the odds on AVIATR probably drop. AVIATR, a New Frontiers-class mission, ups the cost to $715 million, and because it didn’t make it into the National Research Council’s ‘Decadal Survey,’ the mission will be pushed back into the next decade at the earliest even if TiME loses out to a competing proposal.
Soaring in Titan’s Skies
Nonetheless, this is one intriguing idea. One of the benefits of using an actual aircraft — as opposed to the balloon concept envisioned in the Titan Saturn System Mission (TSSM) concept — is that AVIATR would not be carried at random by the local winds, but would have the ability to fly directly to a location and to remain at that site for as long as needed to study it. It would be capable of descending for close-in, high-resolution views (3.5 kilometers) and then returning to a cruising altitude somewhere in the neighborhood of 14 kilometers. There would be, in other words, no latitude restrictions for AVIATR, which unlike a balloon would be able to visit high-latitude lakes and polar regions as easily as the equatorial zone.
In terms of maximizing science, then, AVIATR is holding plenty of cards, and I haven’t even mentioned the fact that because the airplane would be flying at a significant fraction of Titan’s rotational velocity, flying west could keep our instruments on Titan’s daylight side for longer periods, maximizing the scientific return as well as the communications link with Earth. The airplane could also be conceived as fitting into a broader pattern of Titan exploration:
In addition to its own science, AVIATR could be a pathfinder for future landed missions at Titan – one of our objectives is to “Identify potential landing sites of scientific interest and constrain their safety”. Using our full instrument complement would allow us to discover places on Titan at which future missions may elect to land. These places may be of interest for geological, geophysical, chemical, meteorological, or astrobiological reasons. The best spatial resolution of the AVIATR cameras will be of the same order as that of HiRISE at Mars (25 cm/pixel), and this data will be able to constrain the engineering safety of surface environments for landers.
This could be a critical factor in future missions:
Such landing-site candidate analysis may include characterization of rock hazards, slope determinations, the persistence of liquid, and rover trafficability. Our global wind field measurements can reduce the size of the landing ellipse for those future missions as they descend through the atmosphere to their destinations.
Image: An artist’s conception of the parachuted descent portion of the AVIATR EDD sequence. Credit: Mike Malaska.
And while a large imaging instrument like HiRISE on the Mars Reconnaissance Orbiter can map the Martian surface in detail, we lack the same kind of imaging capability for Titan because of the opacity of the atmosphere and because orbiters need to keep to altitudes higher than 1000 kilometers here, so the kind of close-in look AVIATR can provide is otherwise unavailable.
A ‘Gravity Battery’
Taking advantage of AVIATR’s ability to climb and descend as needed, the team has worked out an ingenious method of power management that uses excess power — available when the vehicle is not transmitting data back to Earth — to climb to high altitude, storing these energies as gravitational potential energy in a process the team refers to as a ‘gravity battery.’ During periods of data transmission, the power to the propeller is drastically reduced and the extra power made available is then used to boost transmitted power for the uplink to Earth.
The plan the team discusses in their paper is to have one 8-hour downlink session to the Earth each day:
At the start of a downlink we cut power to the propeller, using that power instead to improve the total radiated power from the telecommunications dish. The airplane then glides for the entire duration of the roughly 8-h downlink (Fig. 34). Once we arrive at our lowest safe altitude of 3.5 km we return to straight and level flight and reduce the transmitter power if the downlink session is still ongoing. Otherwise at the cessation of the downlink we would again pour the extra energy into the propeller, beginning a new climb. We call this mode of operation “climb/glide”.
The team figures AVIATR can manage a one-year mission, but the paper also notes that there are no expendables here that limit the aircraft’s total lifetime beyond the gradual decay of the plutonium-238 that powers the Advanced Stirling Radioisotope Generators (ASRGs). We’ve gotten used to missions that extend well past their intended lifetime, and the AVIATR vehicle’s electrically powered pusher propeller could keep it airborne until the onset of some kind of mechanical failure. If that happens, or if it runs out of funding, AVIATR would descent to an area of soft dunes on Titan, possibly producing still more science for at least a brief period.
While we wait to see whether AVIATR makes it as a New Frontiers mission into the next planetary science decadal survey, a variety of matters still need attention, especially the question of the power output of the ASRGs in the atmosphere of Titan. Also needed: A tune-up of the instrumentation to Titan-specific conditions. All told, though, the AVIATR team sees no showstoppers here and strongly urges Titan airplanes as a part of future missions to the moon. See yesterday’s entry for the full citation of the paper on this work.