Monday, October 11, 2010

Titan Lake Probe Mission Concepts

Note: We appear to be in a quiet time for planning future planetary missions.  In the U.S., we are awaiting the recommendations of the Decadal Survey to be released next March.  In Europe, we are waiting for the decision for the next large mission selection between a Jupiter Ganymede orbiter and two astronomy missions.  I will post as information and ideas become available, but I expect that posts may occur every week or so for awhile.

One of the most exciting concepts for a future planetary mission has been a mission to land on and sample one of the large lakes of Titan.  The lakes are likely to be chemical repositories that contained important clues to Titan's "are repositories, through dissolution of airborne solids, of organics scattered globally on Titan, as well as noble gases, which are a key clue to Titan’s origin and evolution." Sampling the lakes will help answer important questions about Titan:

Cassini-Huygens leaves us with many questions that require a future mission to answer.  These include whether methane is out-gassing from the interior or ice crust today, whether the lakes are fed primarily by rain or underground methane-ethane aquifers (more properly, “alkanofers”), how often heavy methane rains come to the equatorial region, whether Titan’s surface supported vaster seas of methane in the past, and whether complex self organizing chemical systems have come and gone in the water volcanism, or even exist in exotic form today in the high latitude lakes." [NASA/ESA JOINT SUMMARY TITAN SATURN SYSTEM MISSION, January 2009]

As part of its analysis of missions to recommend in the coming decade, the Decadal Survey commissioned a study of possible Titan lake probe missions.  The results of the analysis was presented at a recent conference, and the studies' lead, John Elliott of JPL was kind enough to send me a copy of the presentation and answer some questions.

The study looked at addressing four key scientific questions using variations of two variations of probes that would sample one of Titan's large northern lakes.  The scientific questions were:

  • SGa: Atmospheric evolution (studied during descent through the atmosphere and by analyzing the lake)
  • SGb: Lake and atmospheric interaction to determine how the two exchange material much as the Earth's hydrosphere and atmsphere influence each other (studied by a long-lived floater on the surface of a lake)
  • SGc: Lake chemistry (studied by either a floater or a submersible)
  • SGd: Interior structure (studied by a long-lived submersible on the lake bottom to determine whether or not there is a large ocean deep beneath the surface as there is at Ganymede and Europa)

The study looked at a Flagship class (multiple billions of dollars) and three New Frontiers class (~$650M) missions.  Only the Flagship class mission would be able to address all questions.  It would place a long lived, plutonium-powered (via ASRGs) floater on the lake surface to study long term interactions and would deploy a submersible to the lake bottom for a thirty day stay before popping to the surface for data relay.  The presence of an interior ocean would be explored by measuring the depth of the lake from the lake bottom using an echo sounder.  The amplitude and phases of the lake tides would be used to infer the presence of an interior ocean.

The three New Frontiers missions would conduct subsets of the Flagship mission:

  • Long lived floater (ASRG powered) to study atmospheric evolution, lake/atmospheric interaction, and lake chemistry.  Communications would be direct to Earth and the carrier craft would be a simple stage attached to the entry shell, much like the carrier craft for NASA's Martian landers. 
  • A battery-powered submersible to study atmospheric evolution and lake chemistry.  The submersible would remain on the lake bottom for only six hours before returning to the surface to relay its data back to the carrier stage for retransmission to Earth.  (The presentation notes that this option would provide the most science for the dollar.)
  • A battery-powered floater that would survive on the lake surface for twelve hours to study atmospheric evolution and lake chemistry.  (The presentation notes that this would be the cheapest option.)

In addition to options for the lake probe, the mission also had multiple options for arriving at Titan and for relaying data.  One option would be to arrive at Titan by 2026 to enable direct transmission of data to Earth.   This option requires launch by 2020, and to achieve this short (for a Saturn mission) transit, the carrier would require substantial fuel to enable two deep space maneuvers.  A second option would be two launch by 2023 and arrive by 2032.  In this latter case, data relay would have to be through the carrier, which the study assumes would be ASRG powered.  (Elliot told me that solar powered carriers might be an option, but, "but there are a lot of unknowns (e.g. required technology developments) and uncertainties in how much array area we’d need and how well cells would perform at Saturn distances, etc., so we chose for the study to assume ASRGs as the simpler implementation given our current understanding.  This is something that could benefit from a more detailed trade if further studies are performed.")

The TIME Discovery proposal that is been discussed in this blog would most resemble the long-lived floater concept with direct communication to Earth and a dumb carrier.  The TIME mission assumes a launch by the mid portion of this decade, possibly eliminating the need for powered deep space maneuvers.  The Discovery proposal also would benefit from NASA providing the ASRG's outside the mission's PI budget of ~$450M, enabling the mission to potentially fit within that lower budget instead of a New Frontier budget (~$650M).

See Titan Mare Explorer (TiME) and Dive, Dive! Titan Submersible for additional background information.

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