This blog entry continues to look at White Papers being prepared for the current Decadal Survey effort. In keeping with the theme of previous post on a proposal for a Titan network mission, this entry looks at the case for exploration of Titan by balloon. (Future in situ balloon exploration of Titan’s atmosphere and surface.)
Balloon missions to Titan have been extensively studied as part of the Billion Dollar Box study and the Titan Saturn System Mission analyses and proposals. This white paper provides a good summary of the scientific rational and engineering options for a balloon mission. I did not see anything, however, that struck me as new.
Titan is the best atmosphere in the solar system for ballooning. The atmosphere is cold and dense, making a hot air montgolfiere balloon simple and providing a long descent time to inflate the balloon. The sun is far away, meaning that little solar heat arrives at Titan, reducing the diurnal heating of the balloon and the subsequent stress on the balloon material. However, a number of technical issues have been identified that require further work before a mission could be flown. These include balloon deployment and inflation and packaging of the balloon and it radioactive power source. These issues are being worked with a goal to achieve flight readiness by 2015. The goal is a system that can circumnavigate Titan at least once (3 to 6 months) but that might have a lifetime of years.
The white paper lists a number of investigation firsts that would be enabled by a balloon carrying a sophisticated (i.e., large) payload:
"1. First analysis of the detailed sedimentary record of organic deposits and crustal ice geology on Titan, including the search for porous environments (“caverns measureless to man”) hinted at by Cassini on Xanadu.
2. Direct test through in situ meteorological measurements of whether the large lakes and seas control the global methane humidity, which is key to the methane cycle.
3. First in situ sampling of the winter polar environment on Titan, a region expected to be vastly different from the equatorial atmosphere explored by Huygens.
4. Compositional mapping of the surface at scales sufficient to identify materials deposited by fluvial, aeolian, tectonic, impact, and/or cryovolcanic processes.
5. First search for a permanent magnetic field unimpeded by Titan's ionosphere.
6. First direct search for the subsurface water ocean suggested by Cassini.
7. First direct, prolonged exploration of Titan’s complex lower-atmosphere winds.
8. Exploration of the complex organic chemistry in the lower atmosphere and surface liquid reservoirs discovered at high latitudes by Cassini."
The paper lists the one major problem with a Titan balloon mission. Imaging of the surface with cameras and spectrometers would require a relay craft in orbit around Saturn or Titan. (Chemical analyses of the atmosphere, atmospheric structure (e.g., pressure) measurements could probably be acheived with direct to Earth communications. Perhaps even ice penetrating radar measurements could be taken without a relay orbiter; I'm not sure how much data this instrument would generate.) The white paper assumes a highly capable orbiter similar to the Titan Saturn orbiter that would cost approximately $3B. As long as a balloon mission is tied to an expensive flagship mission, I don't believe this mission will arrive at Titan until the 2030s. In the next blog entry, I'll do a thought experiment about an alternative approach.