Appendix D

Other Missions Considered

The mission concepts (some of which are discussed in Chapters 4 through 8) that were among the 24 studied at the committee’s request by leading design centers (including the Jet Propulsion Laboratory, Goddard Space Flight Center, the Johns Hopkins University Applied Physics Laboratory, and Marshall Space Flight Center) but that were not selected by the committee for analysis applying the Aerospace Corporation’s cost and technical evaluation (CATE) methodology (Appendix C) included the following (in approximate order of distance from the Sun):

• Mercury Lander

• Venus Mobile Explorer

• Venus Intrepid Tessera Lander

• Lunar Polar Volatiles Explorer

• Mars Sky Crane Capabilities

• Mars Geophysical Network

• Mars Polar Climate Mission

• Ganymede Orbiter

• Enceladus Rapid Mission Architecture

• Titan Lake Probe

• Chiron Orbiter

• Neptune-Triton-KBO Mission.

These missions were not subjected to the CATE process because the committee considered them to have lower science merit and/or to be less technically ready than the missions discussed in Appendix C. Each of the 11 missions is described in more detail in the sections below.

In addition, four technology studies were also carried out in support of this report:

• Near-Earth Asteroid Trajectory Opportunities in 2020-2024

• Small Fission Power System

• Saturn Ring Observer

• Comet Cryogenic Sample Return.



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Appendix D Other Missions Considered The mission concepts (some of which are discussed in Chapters 4 through 8) that were among the 24 studied at the committee’s request by leading design centers (including the Jet Propulsion Laboratory, Goddard Space Flight Center, the Johns Hopkins University Applied Physics Laboratory, and Marshall Space Flight Center) but that were not selected by the committee for analysis applying the Aerospace Corporation’s cost and technical evalu- ation (CATE) methodology (Appendix C) included the following (in approximate order of distance from the Sun): • Mercury Lander • Venus Mobile Explorer • Venus Intrepid Tessera Lander • Lunar Polar Volatiles Explorer • Mars Sky Crane Capabilities • Mars Geophysical Network • Mars Polar Climate Mission • Ganymede Orbiter • Enceladus Rapid Mission Architecture • Titan Lake Probe • Chiron Orbiter • Neptune-Triton-KBO Mission. These missions were not subjected to the CATE process because the committee considered them to have lower science merit and/or to be less technically ready than the missions discussed in Appendix C. Each of the 11 missions is described in more detail in the sections below. In addition, four technology studies were also carried out in support of this report: • Near-Earth Asteroid Trajectory Opportunities in 2020-2024 • Small Fission Power System • Saturn Ring Observer • Comet Cryogenic Sample Return. 355

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356 VISION AND VOYAGES FOR PLANETARY SCIENCE The full text of the 11 mission concept studies that did not undergo CATE analysis (as well as the full text of the 13 mission concepts that did), plus the full text of the four technology studies, is provided in the CD accom- panying Appendix G and included with the printed report. The design centers conducted two types of mission study: rapid mission architecture (RMA) studies and full mission studies (FMSs) of mission point designs. RMA studies considered a broad array of mission architectures to find a promising approach. The resulting “point design” could then, as an option, be subjected to a full mission study. MERCURY LANDER The Mercury Lander mission concept study was performed by the Space Department of the Johns Hopkins University Applied Physics Laboratory in partnership with NASA’s Marshall Space Flight Center and Glenn Research Center. Overview The purpose of this RMA study was to determine the feasibility of a landed mission to Mercury. This mission concerned fundamental science questions that can be best, or only, addressed by conducting surface operations such as those for determining Mercury’s bulk composition, the nature of the planet’s magnetic field, surface his- tory, internal structure, and surface-solar wind interactions. Science Objectives • Characterize major and minor elements of the chemical composition of Mercury’s surface. • Characterize the mineralogy and structural state of the materials at Mercury’s surface. • Investigate the magnitude and time dependence of Mercury’s magnetic field, for at least one location on the surface. • Characterize geologic activity (e.g., volcanism, tectonism, impact cratering) at scales ranging from regional to local. • Determine the rotational state of Mercury. Mission Design The architecture for the flight system of the Mercury Lander was based on a three-stage concept consisting of a cruise stage, a braking stage, and a final descent and soft-landing stage. Because of the complexities associ- ated with a landed mission to Mercury, analysis of scientific instrumentation was not included in this study, which focused instead on the viability of flight-system and landing elements. The 2018-2023 time frame was chosen for launch, with landings planned approximately 5 years after launch; specific dates would be dependent on trajectory type. Landed operations would be modest in scope, with 22 contigu- ous days planned for science operations and a possible extension of 68 more days, depending on mission success. Mission Challenges Because a landed mission to Mercury is extremely challenging with respect to a launch energy and relative velocity, two trajectory approaches were considered: • A ballistic/chemical approach fitting on the edge of Atlas V 551 constraints; and • A low-thrust option using a solar-electric propulsion (SEP) cruise stage that would be dependent on high- temperature solar cell technology that has yet to be developed beyond the cell level.

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357 APPENDIX D Using an Atlas V 551 launch vehicle, a ballistic/chemical option with a reduced payload and favorable trajectory performance assumptions was estimated at the lowest cost. More expensive options utilized SEP and a similar launch vehicle or a chemical propulsion system using a Delta IV Heavy. Additional challenges identified included the availability of an Advanced Stirling Radioisotope Generator (ASRG) and the plutonium-238 to fuel it. Conclusions Because of the complex and challenging nature of this mission, a more detailed characterization study is needed before moving forward with the Mercury lander concept. Both SEP and ballistic trajectory approaches and concepts should be further explored with a more detailed mission design and concept definition in order to determine the preferred mission implementation approach. Currently each approach has benefits and risks that could not be fully characterized at this level of study. VENUS MOBILE EXPLORER The Venus Mobile Explorer mission concept study was performed by NASA’s Goddard Space Flight Center. Overview The purpose of this RMA study was to determine whether a Venus mission with surface or near-surface mobility and realistic operational lifetime could achieve meaningful surface science at two or more independent locations separated by several kilometers. Of particular interest was a metallic bellows concept for aerial mobility. Science Objectives • Determine the origin and evolution of Venus’s atmosphere, and determine the rates of exchange of key chemical species between the surface and atmosphere. • Characterize fundamental geologic units in terms of major rock-forming elements, minerals in which those elements are sited, and isotopes. • Characterize geomorphology and relative stratigraphy of major surface units. Mission Design This mission’s space segments consist of a probe and a flyby carrier spacecraft that is also used as a com- munications relay. The probe consists of two top-level elements: the entry and descent element, which includes the aeroshell and parachute systems; and the lander. The lander has two major systems—one being the gondola that carries the science instruments and subsystems inside a thermally protected pressure vessel and the other one being the bellows aerial mobility system, including the bellows and the inflation subsystems. Two 20-day launch windows were considered, in 2021 and 2023, with an initial flyby and a second Venus encounter approximately 112 days later. Mission Challenges Significant development risks with respect to this mission include bellows concept development; safe landing assurance; test facilities for large test articles to simulate Venus’s high temperature, high pressure, and chemical environment; critical events timing; and Raman/laser-induced breakdown spectrometer development. Operation risks include bellows mobility, safe landing, and aeroshell operations. Uncertainty exists in the technology devel- opment cost owing to the relative immaturity of some of the essential technologies.

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358 VISION AND VOYAGES FOR PLANETARY SCIENCE Conclusions Based on analyses of the mechanical, thermal, power, avionics, and communication designs for the probe and the carrier spacecraft, a Venus mission using the metallic bellows architecture for short-lived (approximately 5 hr) aerial mobility is technically feasible. The cost estimate for the nominal baseline mission was estimated to be in the flagship range. The cost is driven by the metallic bellows and supporting mechanisms for its operation. Technology development, accommodation, and complex integration also contribute to the high cost of the probe. VENUS INTREPID TESSERA LANDER The Venus Intrepid Tessera Lander mission concept study was performed by NASA’s Goddard Space Flight Center and NASA’s Ames Research Center. Overview The purpose of this enhanced RMA study was to investigate a mission capable of safely landing in one of the mountainous tessera regions on Venus. This mission concept provides key measurements of surface chemistry and mineralogy and imaging of a tessera region, as well as new measurements of important atmospheric species that can answer fundamental questions about the evolution of Venus. Science Objectives • Characterize chemistry and mineralogy of Venus’s surface. • Place constraints on the size and temporal extent of a possible ocean in Venus’s past. • Characterize the morphology and relative stratigraphy of surface units. Mission Design The mission design elements include a carrier spacecraft to be used as a communications relay and a two- element probe. The probe elements are the Venus lander and the entry and descent element including aeroshell and parachute systems. The lander’s design focuses on enabling a safe landing in the rough tessera terrain. The launch opportunity considered was for 2021, with flyby and descent/landing of mission elements in 2022. Mission Challenges The most significant challenges posed by this mission were related to the development of a high-technology- readiness-level (TRL) Raman/laser-induced breakdown spectroscopy (LIBS) system, safe landing, and testing at Venus environmental conditions. To reduce risk, advancements in two key technology areas are needed: first, verification of the Raman/LIBS implementation, calibrated operation, and sizing for the Venus surface environ- ment, including high entry loads on the laser; second, additional analyses and testing to ensure safe landing in potentially rugged terrains (at lander scales). Conclusions Venus’s tessera provide fundamental clues to the planet’s past, but the terrain has been viewed as largely inaccessible for landed science owing to the known roughness. Based on analyses of the landing dynamics and the mechanical, thermal, power, optics, avionics, and communication designs for this mission, a robust space- craft capable of landing safely in the tessera terrain, conducting surface science, and transmitting all data back to Earth by way of the telecommunications-relay spacecraft is technically feasible. However, because the Venus In Situ Explorer and the Venus Climate Mission were judged of higher priority and also fit more favorably with

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359 APPENDIX D considerations of program balance, the Venus Intrepid Tessera Lander mission was not considered further as a decadal option. LUNAR POLAR VOLATILES EXPLORER The Lunar Polar Volatiles Explorer mission concept study was performed by NASA’s Marshall Space Flight Center in cooperation with the Johns Hopkins University Applied Physics Laboratory. Overview The purpose of this RMA study was to determine the feasibility of a mission to investigate putative volatiles in permanently shadowed areas of the lunar poles. Whereas previous orbital missions have provided data that sup- port the possibility of water ice deposits existing in the polar region, this concept seeks to understand the nature of those volatiles by direct in situ measurement. Science Objectives • Determine the form and species of the volatile compounds at the lunar poles. • Determine the vertical distribution and concentration of volatile compounds in the lunar polar regolith. • Determine the lateral distribution/concentration of volatile compounds in the lunar polar regolith. • Determine the secondary alteration mineralogy of the regolith. • Determine the composition and variation in the lunar exosphere adjacent to cold traps. Mission Design The mission concept explored involves placing a lander and instrumented rover in a permanently shadowed crater near one of the Moon’s poles. The rover would carry a suite of science instruments to investigate the loca- tion, composition, and state of volatiles. Rovers powered by batteries and radioisotope power systems (RPSs) were considered. A battery-powered option, designed to support 4.4 days of surface operations, could achieve some but not all of the mission’s top science goals. The development of the mission was assumed to start in 2013 to support an October 2018 launch. Mission Challenges Although this study identifies several mission components at TRLs less than 6, the required technology advance- ments are believed to be achievable and consistent with the outlined mission schedule. Technology development was found to be required for the rover lidar, the drill and sample acquisition system, the RPS, and terrain-relative navigation. Additional identified risks to the mission include the following: drill performance; thermal environment effects; high thrust-to-weight bipropellant thruster qualification; soft-landing precision guidance, navigation, and control; low-mass and low-power avionics development; the availability of plutonium-238; and battery mission- mass growth. Conclusions A lunar polar volatiles mission represents an important opportunity to study the nature, composition, and dynamics of volatiles trapped in the frigid interiors of lunar polar impact craters. It also provides an opportunity to investigate polar volatiles, especially water ice, as a potential resource for the future human exploration of the Moon and destinations beyond. Although such a mission retains a high science value, the polar crater environment presents a number of technical challenges, including rover survivability, sample collection and characterization,

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360 VISION AND VOYAGES FOR PLANETARY SCIENCE and navigation. Although some technical maturation is required, there remain no major impediments to such a mission within this decade. MARS SKY CRANE CAPABILITIES The Mars Sky Crane Capabilities study was performed by NASA’s Jet Propulsion Laboratory. Overview The purpose of this special study was not focused on a mission but on exploring the full range of science capabilities that could be delivered to the surface of Mars by a Mars Science Laboratory (MSL)-derived Sky Crane entry, descent, and landing system. Of particular interest were options to address pathways to three broad classes of surface science and the differences between delivering mobile and fixed systems. In addition, the study investigated the potential capabilities of the Sky Crane with respect to landing ellipse, landing altitude, and landed mass. Special attention was paid to options for the 2018 launch opportunity. Science Pathways • The Surface Fieldwork—Astrobiology/Geology Pathway emphasized the geological and geophysical evolu- tion of Mars; the history of its volatiles and climate; the nature of the surface and subsurface environments, now and in the past; the temporal and geographic distribution of liquid water; and the availability of other resources (e.g., energy) necessary to sustain life. • The Subsurface—Geology/Astrobiology Pathway emphasized conducting several experiments (including subsurface sampling) at sites where records of recent climate, geologic processes, and organic molecules might well be preserved and accessible in the near subsurface. • The Network Science Pathway emphasized strategies for the deployment of network investigations using seismological and meteorological measurement methods to study both the martian atmosphere and subsurface characteristics. Mission Design An initial assessment of more than a dozen potential design elements was performed. After favorable study elements were identified and placed in combination, these new groups were also examined for feasibility and effectiveness in meeting the stated science pathway objectives. All study architectures assumed an MSL-derived Sky Crane descent system to deliver the science payloads to the surface; however, the main trade-offs for the study were the sets of science payloads that could be delivered in the 2018 Mars opportunity. The individual mission elements studied for this investigation were the following: the Mars Astrobiology Explorer-Cacher (MAX-C) rover, ExoMars, Network Pathfinder, Seismic Drop Package, and Subsurface Station. It was determined that the science pathway objectives could be fulfilled by a combination of these mission ele- ments, as described below. Mission Options • Baseline—MAX-C and ExoMars. Pathways addressed: Surface Fieldwork—Geology/Astrobiology and Subsurface—Geology/Astrobiology. • Option 2—MAX-C, ExoMars, and Network Pathfinder. Pathways addressed: Surface Fieldwork— Astrobiology/Geology, Subsurface—Geology/Astrobiology, and Network Science. • Option 3—MAX-C, Network Pathfinder, and Seismic Drop Package. Pathways addressed: Surface Fieldwork— Astrobiology/Geology, and Network Science.

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361 APPENDIX D • Option 4—MAX-C and Subsurface Station. Pathways addressed: Surface Fieldwork— Astrobiology/Geol- ogy, Subsurface—Geology/Astrobiology, and Network Science. Conclusions All of the mission options listed above—with the exception of Option 2—are technically feasible based on considerations of technical maturity and mass margins. Despite these conclusions, further assessments of relevant planetary protection requirements and specific instrumentation development must be performed before further recommendations on the implementation of the Mars Sky Crane system can be made. MARS GEOPHYSICAL NETWORK The Mars Geophysical Network mission concept study was performed by NASA’s Jet Propulsion Laboratory. Overview The purpose of this full mission study was to investigate a geophysical network mission for Mars. After an initial trade-off-study, a two-lander mission concept was selected. Science Objectives • Characterize the internal structure of Mars to achieve a better understanding of the planet’s early history and internal processes affecting its surface and habitability. • Characterize the thermal state of Mars to develop a better understanding of the planet’s early history and internal processes affecting its surface and habitability. • Characterize the local meteorology and provide ground truths for orbital climate measurements. Mission Design The mission studied includes two independent, identical spacecraft. Each spacecraft consists of a lander, an entry system, and a cruise stage. These elements would be combined in a Phoenix-like architecture with a powered- descent lander. The main instrument on each lander is a seismometer. The mission would nominally launch on a single Atlas V 401 launch vehicle. Both horizontal (stacked) and vertical (parallel) launch configurations were considered. Although the latter configuration would mitigate the risk that separation failure of the first lander could affect deployment of the second lander, the former configuration was ultimately chosen to simplify development and to forgo the need for a larger launch vehicle. September 2022 was chosen as the nominal launch date for the purpose of this study, followed by typical cruise duration of some 6 months. Mission Challenges The Mars Geophysical Network concept was susceptible to common risks associated with Mars in situ vehicles. The most prominent risks identified involved failure of the entry, descent, and landing (EDL) system of one or both landers. Owing to the concept’s design heritage, most notably derived from the Phoenix mission, no significant technology development program was deemed necessary. Conclusions For this mission concept, both instrumentation and spacecraft architecture benefit from an established technol- ogy base and therefore are considered at a high level of technological maturity. Most mission-related risks stem

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362 VISION AND VOYAGES FOR PLANETARY SCIENCE from the simultaneous launch and deployment of two spacecraft requiring separate EDL systems. The mission was given lower scientific priority, however, than the Mars Astrobiology Explorer-Cacher mission recommended in Chapter 9 of this report. MARS POLAR CLIMATE MISSION The Mars Polar Climate Mission concept study was performed by NASA’s Jet Propulsion Laboratory. Overview An RMA study was conducted to explore which science objectives related to the study of the martian climate through the record preserved in the polar-layered deposits (PLDs) could be pursued by a small to moderate-size mission. Five concepts were studied: two orbiters, two stationary landers, and a mobile lander. Science Objectives • Develop an understanding of the mechanism and chronology of climate change on Mars. • Determine the age and evolution of the PLDs. • Determine the astrobiological potential of the observable water-ice deposits. • Determine the mass and energy budget of the PLDs and how volatiles and dust have been exchanged between polar and nonpolar reservoirs. Mission Design The two-orbiter mission scenarios were for a small orbiter with two payload options and for a medium-size orbiter combining most of the payload options from the small orbiter. Also investigated were a small station- ary lander and a small/medium-class stationary lander with a meter-scale drill. The final mission scenario was for medium-class rover with an ice sampler/rock corer, similar to the one envisioned for the Mars Astrobiology Explorer-Cacher, as well as spectrometry instruments. Mission Challenges No formal risk assessment was conducted for this study, but it identified several areas of necessary or benefi- cial technology development. For orbiters, Ka-band telecommunications were envisioned, pending implementation with the Deep Space Network. All lander options assumed the availability of telecommunications relay orbiters. In addition, all landers considered in the study were likely to require precision-guided entry, pending demonstration by the Mars Science Laboratory. Some of the missions considered would also benefit from the Advanced Stirling Radioisotope Generators, which are currently under development and may be subject to reliability and logistics issues regarding the availability of plutonium-238. Conclusions The variety of mission concepts discussed covered a significant breadth of options for exploring Mars’s polar-layered deposits. No prioritization among these options was detailed, but the study served to illustrate the trade-off-space studies and instrumentation options for each of the concepts. GANYMEDE ORBITER The Ganymede Orbiter mission concept study was performed by NASA’s Jet Propulsion Laboratory.

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363 APPENDIX D Overview The purpose of this full mission study was to develop an architecture suitable to perform a scientifically viable Ganymede orbiter and to determine the feasibility of a NASA-only Ganymede mission in case the European Space Agency (ESA) Jupiter Ganymede Orbiter is not realized. Increased mission duration and modest enhancements to the flight system were also considered in order to accommodate enhanced payloads for a “baseline” and an “augmented” mission. Science Objectives • Further characterize Ganymede’s subsurface ocean. • Investigate Ganymede’s geology, including its history, tectonism, icy volcanism, viscous modification of the surface, and the nature of surface contact with the ocean. • Characterize Ganymede’s unique magnetosphere and determine the methods by which the magnetic field is generated. • Investigate Ganymede’s origins and evolution. • Characterize Ganymede’s gravity anomalies and place constraints on the size and composition of its core and rocky and icy mantles. • Study Ganymede’s interaction with the rest of the Jovian system. • Characterize the variability of Ganymede’s atmospheric composition and structure in space and time. • Further characterize Callisto’s interior and subsurface ocean. Mission Design A mission with three distinct scientific phases was considered: the Ganymede and Callisto flyby phase; a pump- down phase in an eccentric Ganymede orbit; and an orbital tour phase in a circular polar orbit around Ganymede. For the “floor” mission option, the spacecraft would spend 3 months in Ganymede orbit, with orbits of 6 months and 1 year for the baseline and augmented mission options, respectively. The floor mission option payload included wide-angle and medium-angle cameras, a magnetometer, radio science, a laser altimeter, a visual/near-infrared imaging spectrometer, and a plasma package. The baseline mis- sion added a mass spectrometer and an ultraviolet spectrometer. The augmented mission included all of the above instruments plus radio and plasma wave instruments, a narrow-angle camera, and sounding radar. Each of these options employed a three-axis-stabilized, solar-powered spacecraft with conventional bipropellant propulsion. The preferred launch date for this mission was in May 2021, with Ganymede arrival in 2028; two other launch opportunities are available, in 2023 and 2024. Mission Challenges There are no significant technological risks associated with the Ganymede Orbiter mission, and there is no further technology development required. For the spacecraft, the key required Ganymede flight system elements are being developed and demonstrated for Jupiter applications on the Juno mission. The instruments are all based on technology that is either highly developed or has already flown; for the instruments, the only engineering developments necessary are in response to Ganymede’s high-radiation environment. Conclusions A Ganymede Orbiter mission appears to be technically feasible with no required technology development for the spacecraft, propulsion system, or instrumentation. For all moderate mission risks, mitigation strategies have been incorporated into the final mission architecture. The mission was not given further consideration because of the likelihood that the ESA Jupiter Ganymede Orbiter would achieve most of the same science goals.

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364 VISION AND VOYAGES FOR PLANETARY SCIENCE ENCELADUS RAPID MISSION ARCHITECTURE The Enceladus Rapid Mission Architecture (RMA) concept study was performed by NASA’s Jet Propulsion Laboratory. Overview The RMA study investigated a set of missions to Saturn’s moon Enceladus. Several different architectures were considered, including flybys, orbiters, sample returns, and the Titan-Enceladus Connection that would extend a proposed explorer flagship mission to Titan. The study assessed comparative science return as well as mission and development risk in order to make its recommendation. Science Objectives • Further characterize the molecular composition of organic material in Enceladus’s plume; • Investigate the nature of Enceladus’s geologic history; • Study the nature of Enceladus’s cryovolcanic activity including its source of heat, delivery mechanisms, mass loss rate, and temporal variability; • Investigate the internal structure and chemistry of Enceladus, and search for indications of global or regional subsurface oceans; • Study how Enceladus interacts with the rest of the saturnian system; • Examine possible future landing sites; and • Assess the life potential of Enceladus. Mission Design The RMA study examined 15 mission architectures, including one Enceladus flyby, nine Enceladus orbiters, four Enceladus sample returns, and the Titan-Enceladus Connection. The different types of orbiters proposed varied in their secondary payload, instruments, and mission duration. The sample return missions differed in their power sources, instruments, and sample collection speed. The Titan-Enceladus Connection would modify and use the Titan flagship mission spacecraft to orbit Enceladus after completing its mission at Titan. All but one mission would launch on an Atlas V-class vehicle from the years 2021 to 2023. The Titan-Enceladus Connection would launch on a Delta IV-Heavy-class launch vehicle within the same time frame. Each mission would take advantage of flybys of the inner planets in order to get to Saturn. Subsequently, all would go into orbit around Saturn and conduct flybys of other saturnian moons in order to make their final approach to Enceladus. Every orbiter, except one short-operations proposal, would conduct a 1-year science-based mission in orbit around Enceladus. After completing their science objectives, the orbiters would be crashed onto the surface of Enceladus. A different approach was also proposed, a sample return mission that would take 4.5 to 5.5 years to return from the saturnian system and then make use of an Earth Entry Vehicle (EEV). This approach, however, was not exam- ined in detail and would need further study. The mission durations varied from a period of 10 to 16 years in total. Mission Challenges As a result of the complex nature of the mission, there were a variety of inherent risks involved that could occur before or during flight operations. One of the main risks was planetary protection, including forward and back contamination. Planetary protection requirements for crashing an orbiter onto Enceladus would put the mission into Category IV of the COSPAR planetary protection requirements. This could require full spacecraft sterilization; a cost of between $100 million and $200 million. For sample return missions, back contamination from Enceladus to Earth would have to be considered, placing the mission in Category V. Facilities that would receive the collected samples would have to be developed 10 years before the samples would be returned because of certification and regulatory requirements.

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365 APPENDIX D 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. Conclusions 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. Overview The purpose of this RMA study was to develop mission architectures for the in situ examination of a hydro- carbon 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. Science Objectives • Understand the formation and evolution of Titan and its atmosphere through measurement of the compo- sition 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. Mission Design 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.

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366 VISION AND VOYAGES FOR PLANETARY SCIENCE The large lander would carry a comprehensive suite of instruments capable of carrying out in situ measure- ments of Titan’s atmospheric evolution, lake-atmosphere hydrocarbon cycle, and prebiotic lake chemistry, and of checking for the presence of a subsurface ocean. This list was reduced for the DTE mission, eliminating the submersible instrumentation as well as a few instruments on the lake lander. The submersible-only mission would carry just the gas chromatograph-gas chromatograph/mass spectrometer (GC-GC/MS), a lake properties package, and a Fourier transform infrared spectrometer. Finally, the lake lander mission with a flyby relay spacecraft would represent the science floor and would contain only the GC-GC/MS and lake properties package. Lake landers for all architectures would be capable of sampling gases and liquids. In addition, both the large and stand-alone submersibles would be able to sample solids from the lake bottom as well as liquids. Mission Challenges Moderate risks identified as affecting all mission concepts included the availability of plutonium-238 for the ASRGs and the long-term reliability of the ASRG. Furthermore, the concepts would require significant technol- ogy development to become viable, including instrument development for the cryogenic operating environment. Limitations on the current understanding of the Titan atmospheric and lake behaviors would make landing in the small southern lakes challenging; all architectures thus assumed landings on the much larger Kraken Mare in the north. A requirement to target Kraken Mare constrained the trajectory of the DTE mission, since the likely launch opportunity left little time until Earth would no longer be in view from the lake surface. Consequently, a high-energy trajectory was required for this architecture in order to reduce travel time to Titan, thus increasing launch mass and launch costs. Conclusions The exploration of Titan’s hydrocarbon lakes has high scientific potential, and the Titan lake lander concepts appear feasible. However, because of the costs and the relatively limited science scope of a stand-alone lake probe without the orbiter and balloon elements, the stand-alone lake probe concepts were judged to be of lower priority than a lake probe that would be an element of a flagship mission, or some of the other mission concepts studied. The cryogenic environment of Titan and lack of heritage in lake probe design would necessitate strategic invest- ment in technology development, including cryogenic sample acquisition and handling. CHIRON ORBITER The Chiron Orbiter mission concept study was performed by NASA’s Goddard Space Flight Center. Overview The purpose of this RMA study was to determine several options for delivering a useful payload into orbit around Chiron. The five options discussed focused mainly on the propulsion and trajectories needed to place a spacecraft, with a given science package, into orbit around Chiron. Science Objectives • Observe the current geologic state and composition of the surface and infer the past evolution and relative importance of surface processes. • Observe and measure the sporadic outgassing activity and determine the composition of outgassed volatiles. • Characterize bulk properties and interior structure.

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367 APPENDIX D Mission Design The majority of the engineering work for this study was spent on propulsion, power, and trajectory trade-offs to define how the science payload could be delivered to Chiron within the given constraints, leaving fewer resources for the definition of the science package. Several trajectories for flights between Earth and Chiron, including both direct and gravity-assisted flyby trajectories, were examined. Launch was determined to occur between 2019 and 2025 depending on the propulsion option, with a cruise-phase duration of between 11 and 13 years. None of the preliminary propulsion solutions could deliver an acceptable mass to Chiron with an 11-year transit time; however, five propulsion options were determined that could deliver acceptable masses into Chiron orbit with a 13-year transit time as the baseline. Mission Challenges Because of the inherent complexities in reaching Chiron, the primary challenges discussed in this study relate to propulsion and the trajectories needed to orbit Chiron. Budgetary assumptions made in the mission study cost assessment do not include the mission launch vehicle. Additional challenges are posed by the availability of plutonium-238 for the ASRGs and the long-term reliability of ASRGs. Conclusions Regarding the five propulsion options considered for trajectories into Chiron orbit: the all-chemical option did not deliver a viable payload; the two solar-electric and chemical propulsion options delivered useful masses with reduced science payloads; finally, the two radioisotope-electric propulsion (REP) options delivered a viable payload capable of meeting all science requirements. However, the REP system will likely need more than the two ASRGs assumed available for this mission. This study demonstrated the need for continued investments in long-term communication infrastructure and propulsion technologies before such missions could be attempted. NEPTUNE-TRITON-KBO MISSION The Neptune-Triton-KBO mission concept study was performed by NASA’s Jet Propulsion Laboratory, and a follow-on full mission study of a Neptune Orbiter with Probe was conducted by the Johns Hopkins University Applied Physics Laboratory. Overview This RMA study investigated a set of missions to the Neptune system, including some with the potential for continued travel to a Kuiper belt object (KBO). Several mission architectures were considered, ranging from relatively simple flybys to complex orbiters. This study initially examined a robust orbiter with an atmospheric probe each for Neptune and Uranus, to assess and develop an understanding of the feasibility and technological differences between the two targets. Neptune is discussed here; Uranus is discussed in Chapter 7 of this report. Science Objectives • Determine temporal variations of Neptune’s atmosphere. • Characterize the chemistry of Neptune’s atmosphere. • Develop an understanding of the structure, dynamics, and composition of Neptune’s magnetosphere. • Develop an understanding of the chemistry, structure, and surface interaction of Triton’s atmosphere. • Develop an understanding of the interior structure of Triton. • Determine the age and geologic processes that shape the surface of Triton/KBOs.

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368 VISION AND VOYAGES FOR PLANETARY SCIENCE • Develop an understanding of the spatial distribution of surface composition and how the composition is coupled to geologic processes on a given KBO. Mission Design The RMA study examined seven flyby architectures with varying degrees of complexity and focus on Neptune, Triton, or KBOs: a “minimal” orbiter of Neptune, five simple orbiter concepts (one including a shallow atmo- spheric probe, and another separate KBO-flyby spacecraft), as well as a “high-performance” orbiter. All flyby options relied exclusively on chemical propulsion; all other options included a solar-electric propulsion system. The most complex of the simple orbiters and the high-performance orbiter would insert themselves into orbit around Neptune by means of aerocapture. The remaining orbiter concepts employed chemical propulsion for this purpose. Most architectures included a 25-kg or 60-kg primary instrument payload (predominantly based on New Horizons heritage). The high-performance architecture allocated up to 300 kg in payload mass. All missions called for the use of three to six ASRGs, depending on the mission architecture. The follow-on point-design, full mission study focused on an orbiter mission with limited payload and a shallow atmospheric probe (1- to 5-bar terminal pressure). The studies were limited to trajectories without Jupiter gravity assists in order to assess the difference between identical Uranus and Neptune missions without narrow launch window constraints. Mission Challenges All of the concepts studied had moderate reliability risk due to the long duration of the missions. Furthermore, a failure of multiple ASRGs was deemed a moderate risk for all of the simple-orbiter concepts. The availability of plutonium-238 and other logistical issues associated with ASRGs also incurred moderate implementation risks for all options. For the most elaborate of the simple orbiters and the high-performance orbiter, the use of aerocapture was identified as necessitating further technology development and therefore posed a moderate programmatic and technical risk. Scheduling constraints were identified for all but the high-performance option by the availability of a Jupiter gravity assist maneuver, which would favor a launch between 2016 and 2018, with reduced-performance opportunities sporadically thereafter. Cost increases proportionally from that for the flyby missions to that for the simple and high-performance orbiters. The point design was terminated before a full evaluation of risk, cost, and schedule was completed, as it was deemed less technically feasible than a comparable Uranus mission (see Appendix C). A Neptune mission without a Jupiter-flyby gravity assist requires aerocapture for orbit insertion. Aerocapture itself not only adds complexity and risk but also makes probe delivery and orbits that allow Triton encounters more challenging. Even with a SEP system, a mission to Neptune has a long duration and thus higher risks for instruments and spacecraft components. Conclusions The flyby mission architectures were deemed capable of achieving significant science progress beyond that from Voyager 2’s visit of Neptune and offer the potential for new KBO science. Even the simplest of the flyby missions exceeded the cost cap of a New Frontiers mission and offered low science return relative to its cost; it was deemed not compelling. More complex missions and orbiters provided a vast gain in science objectives that would be unavailable to flyby missions, but at increased cost; the highest-performance option yielded a modest increase in estimated science value for its higher cost. More detailed design work of a “sweet spot” mission design identified technical risks that make a Uranus mission more favorable for the coming decade. Technology develop- ment will increase the feasibility of a future Neptune orbiter mission.