reasons of cost and technology readiness, the recent planetary science decadal survey deferred that mission to the subsequent decade (after 2022). Titan science, however, remains a high priority due to the unique characteristics of the satellite’s atmosphere and the discovery of hydrocarbon lakes on its surface. Titan is believed to represent an environment in which prebiotic chemical processes, similar to those that were active on early Earth, can be studied in depth.
Within the low-cost Discovery program, NASA is currently evaluating the so-called Titan Mare Explorer (TiME) as a candidate for launch in 2016. This would be a highly focused investigation of the composition and characteristics of a northern hemisphere Titan sea using a floating platform. The decision on whether to fly this mission or one of the other two candidates will be made in mid-2012. Titan may also be considered as a potential New Frontiers candidate during a midterm update to the planetary science decadal survey.
As discussed previously in this report, planetary protection is not a major consideration for missions to Titan because of the cryogenic temperatures, limited or no access to liquid water, and lack of phosphorus to support cell growth. Standard clean-assembly procedures and bioburden assays are expected to be sufficient for all future Titan missions. It is important to note that Titan missions with a strong focus on prebiotic chemistry will likely face rigorous constraints on organic cleanliness analogous to those placed on the biological cleanliness of missions carrying life-detection experiments.
Missions to Neptune and its large satellite, Triton, have been identified in prior NASA strategic plans as high priorities for the long term. Like the TSSM, a Neptune Orbiter and Probe mission was identified in the recent planetary science decadal survey as a high science priority. For reasons of cost and technology readiness, however, it was not recommended for development in the coming decade.8 A dedicated Triton mission was not included in the decadal survey recommendations, although it is anticipated that a flagship Neptune Orbiter would also conduct extensive Triton science. Planetary protection planning for Triton would thus focus on ensuring that a Neptune Orbiter was developed with appropriate safeguards, including standard clean assembly, bioburden assays, and selective dry-heat microbial reduction.
As with the Europa, Enceladus, and Titan missions, it is possible that future Discovery or New Frontiers missions may propose investigation of the Neptune/Triton system, and these may represent earlier launch opportunities than would be possible within a flagship mission paradigm. Such a mission to the Neptune/Triton system would be very challenging within the current cost caps and would likely be enabled by new technologies that are only now under study. Thus it is expected that Triton missions are far enough in the future as to not be appropriate drivers for specific planetary protection recommendations at this time.
MISSIONS TO OTHER ICY BODIES
The outer solar system is home to a large number of icy bodies that are scientifically interesting for reasons other than astrobiology. These include comets, Trojan asteroids, trans-Neptunian objects, and the small satellites of Uranus and the other giant planets. It is generally expected that missions to these bodies will undergo standard clean assembly procedures as are followed in all planetary missions but will not be required to meet any other planetary protection requirements owing to their lack of liquid water, sources of energy, and/or chemical constituents that can promote cell growth. Thus eventual missions to these targets will be governed under the decision rules contained in this report and should impose no unique requirements.
1. NASA, 2011 NASA Strategic Plan, NP-2011-01-699-HQ, NASA, Washington, D.C., 2011, p. 13.
2. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, p. 196.