With a launch time frame from 2021 to 2023, the missions would most likely be unable to receive a gravity assist from Jupiter. Another challenge facing these missions is the lack of plutonium-238 development, a major constraint. Although the mission involved risks, such as multiple ASRG failures, none of the risks were believed to be significant.
Different levels of risk were assigned for each mission architecture. The highest-risk missions were those that would either land on Enceladus or conduct sample return. Challenges for landers could occur because of unknown terrain, which could result in loss of opportunity to reach science objectives or the loss of the lander. The main risk for a sample return mission was related to planetary protection requirements and associated technological developments.
A variety of mission options for exploring Enceladus’s plume were examined. The consideration of science benefit versus cost and development risk made an orbiter more attractive for the first mission that would focus on Enceladus. A simple-payload Enceladus orbiter with a 12- or 6-month orbital tour was deemed particularly promising because it would provide a global picture of Enceladus. The proposal was sent for additional study at NASA’s Jet Propulsion Laboratory (see Appendix C).
TITAN LAKE PROBE
The Titan Lake Probe mission concept study was performed by NASA’s Jet Propulsion Laboratory.
The purpose of this RMA study was to develop mission architectures for the in situ examination of a hydrocarbon lake on Titan. To this end, the study considered one large-class mission (to be delivered to Titan as part of a larger flagship Titan mission, which was not part of this study) and three stand-alone, medium-class, mission architectures. Distinguishing design trade-offs among these missions included the use of direct versus spacecraft-relaying communications and submersible versus floating probes, as well as the application of Advanced Stirling Radioisotope Generators, instrument selection, and trajectory design. The subsolar and sub-Earth points are in Titan’s southern hemisphere from 2025 to 2038, and the largest lakes are near the north pole. Therefore, it was important to understand the feasibility of different mission architectures as a function of launch date.
• Understand the formation and evolution of Titan and its atmosphere through measurement of the composition of the target lake, with particular emphasis on the isotopic composition of dissolved minor species and on dissolved noble gases.
• Study the lake-atmosphere interaction in order to determine the role of Titan’s lakes in the methane cycle.
• Study the target lake as a laboratory for both prebiotic organic chemistry in water (or ammonia-enriched water) solutions and nonwater solvents.
• Determine if Titan has an interior ocean by measuring tidal changes in the level of the lake over the course of Titan’s 16-day orbit.
The large-class mission would consist of both floating and submersible probes. The stand-alone mission options include the following: a lake-lander using a direct-to-Earth (DTE) communications link, a submersible-only probe with a flyby relay spacecraft, and a lake lander with a flyby relay spacecraft. All missions would require the use of ASRGs, either on the lander for the flagship and DTE options or on the relay spacecraft with battery-powered in situ segments for the remaining options.