For most of the history of the planetary program, the outer solar system has been accessible only by flagship missions characterized by large multidisciplinary payloads, relatively high costs (typically over $2 billion), and a requirement for high reliability. Such large missions—e.g., Galileo and Cassini—could be flown infrequently (approximately one per decade), and many new technologies and techniques were developed to support these projects.
As budget pressures on the space science program have increased, NASA and the science community have been motivated to find ways to enable outer solar system exploration within smaller, cost-constrained mission programs such as Discovery and New Frontiers. These missions are characterized by smaller spacecraft, highly focused science goals, lower costs, and more frequent launches. Flagship missions to the outer solar system continue to be a high priority both within the NASA strategic plan and in the planetary science decadal surveys,1,2,3 but they will be cost-constrained and somewhat less ambitious than in the past and probably less frequent. This distinction between flagship and competed missions is important because competed missions are typically selected only approximately 3 to 4 years prior to launch. With such a short planning horizon, there is little time for NASA and the science community to debate and agree upon planetary protection requirements and standards on a mission-by-mission basis; thus, it is imperative that such standards be developed well in advance and that they are not unnecessarily burdensome or overly conservative.
As discussed in this report, the icy bodies of greatest potential concern from a planetary protection perspective are Europa, Enceladus, Titan, and Triton. All four are objects of high scientific priority for both astrobiological and non-astrobiological reasons. As such, it is instructive to review the plans for spacecraft missions to these objects, which have been developed in recent years, primarily by NASA and the European Space Agency (ESA). Exploration plans for other icy bodies—i.e., comets, Ganymede, the satellites of Uranus, other small satellites, Trojan asteroids, and Kuiper belt objects—are not discussed because these bodies are not expected to pose any significant planetary protection concerns. The missions described here should be considered as examples only, since none of them are currently funded or scheduled.
A Jupiter Europa Orbiter (JEO) was identified as a top science priority in the 2011 planetary science decadal survey (Box B.1).4 It has been planned to launch in 2018-2020 as part of a joint NASA-ESA project known as the Europa Jupiter System Mission (EJSM), which would comprise the Europa orbiter as well as an ESA-developed
Jupiter Europa Orbiter
Jupiter Europa Orbiter
– Systems engineering for electronics vault repartitioning
– “Fail operational” fault management to handle environment
– Uncertainty in instrument and shielding mass
– Low launch margin for this development phase
– Overall sensitivity of system mass to changes
– System impacts of changing number and design of radioisotope power system units
– Availability of plutonium-238
– Uncertainties in design of model payload
• Explore Europa to investigate its habitability
• Key science issues addressed:
– Characterizing the extent of the europan ocean and its relation to the deeper interior
– Characterizing the ice shell and any subsurface water, including the nature of the surface-ice-ocean exchange
– Determining global surface compositions and chemistry, especially related to habitability
– Understanding the formation of surface geology, including sites of recent or current activity, and characterizing sites for future in situ exploration
– Understanding Europa in the context of the Jupiter system
• Model Payload
– Ocean: Laser Altimeter, Radio Science
– Ice: Ice Penetrating Radar
– Chemistry: Vis-IR Imaging Spectrometer, Ultraviolet Spectrometer, and Ion and Neutral Mass Spectrometer
– Geology: Thermal Instrument, Narrow Angle Imager, Wide and Medium Angle Imager
– Particles and Fields: Magnetometer, Particle and Plasma Instrument
• Five Multi-Mission Radioisotope Thermoelectric Generators
• Launch Mass: 4,745 kg
• Launch Date: 2020 (on Atlas V 551)
• Orbit: 100-200 km Europa Orbit + Jovian Tour
Ganymede orbiter. JEO would place a spacecraft equipped with remote sensing and radar investigations into a close orbit around Europa for a period of at least 1 year. Prior to insertion into Europa orbit, JEO would complete a 2-year tour of the jovian system using the Galilean satellites for gravity-assist flybys. Given the complex gravitational environments of the jovian system, the long-term stability of JEO’s orbit about Europa cannot be guaranteed. Therefore, to meet planetary protection requirements at the end of its mission, JEO would be either commanded to impact onto the surface of Europa in a controlled manner at a selected site, or ejected from Europa orbit and placed on a collision course with Jupiter. The combination of this controlled end-of-mission scenario, along with standard clean-assembly procedures, selective application of dry-heat microbial reduction, and the sterilizing effect of the jovian radiation environment, would allow JEO to meet planetary protection requirements. The integrated cost of these requirements, while not a primary driver of the mission budget, is nonetheless significant.
As of this writing, budget pressures have led to a descoping and replanning of JEO and probably of the entire EJSM program. Current studies are focused on developing less costly JEO mission concepts. Once those studies are complete and the budget picture is clarified, NASA will decide whether and how to proceed with Europa explora-
Enceladus Orbiter Spacecraft
• Planetary Protection
– Potential modifications to design required if planned Enceladus impact disposal is not acceptable for planetary protection
• Particle Impact Damage
– Potential for spacecraft damage from Saturn E-ring or Enceladus plume particle impact
– Primary concern: high-gain-antenna surface quality
• System Power
– Some potential for reduced science operations with assumed Advanced Stirling Radioisotope Generators (ASRG) degradation
• Investigate the internal structure, geology, and chemistry of Enceladus and plumes discovered by Cassini
• Prepare for a potential future landing
• Observe interactions between Enceladus and the Saturn system and explore the surfaces and interiors of Saturn’s moons
• Key science issues addressed:
– Investigating the nature of Enceladus’s cryovolcanic plumes
– Providing improved measurements of plume gas and dust
– Measuring tidal flexing, magnetic induction, static gravity, topography, and heat flow
– Medium Angle Imager
– Thermal Imaging Radiometer
– Mass Spectrometer
– Dust Analyzer
• Three ASRGs
• Launch Mass: 3,560 kg
• Launch Date: 2023 (on Atlas V 521)
• Orbit: Enceladus Orbit (100 km x 267 km, 62 deg inclination) Plus Saturn Satellite Tour
tion; in the meantime, ESA is continuing its studies of the Ganymede orbiter element of EJSM. When a Europa mission is flown, a key aspect of mission affordability will be adoption of the streamlined planetary protection decision framework recommended in this report.
The 2011 planetary science decadal survey also recommended that NASA consider studying a flagship mission to Enceladus (Box B.2).5 The Enceladus Orbiter would investigate the satellite’s cryovolcanic plumes, habitability, internal structure, chemistry, geology, and interaction with other bodies within the saturnian system. As is the case for a Europa orbiter, the complex gravitational environments of the saturnian system imply that the long-term stability of an orbiter about Enceladus cannot be guaranteed. Thus, special measures would be needed to ensure that the ultimate fate of the Enceladus Orbiter is consistent with planetary protection provisions.
An Enceladus Orbiter mission was accorded a lower priority than either the Europa Orbiter or a proposed Uranus Orbiter and was recommended for flight only if those other two missions could not be accomplished for
Titan Saturn System Mission
Titan Orbiter + Balloon and Lake Lander
• In Situ European Space Agency-Supplied Elements
– Uncertainty in accommodation, pending element maturation
– Element operations and communications relay using Orbiter
– Uncertainty in instrument mass
– Low launch margin for this development phase
– Battery recharge time in Titan orbit
– Impact of switching to Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs) from Advanced Stirling Radioisotope Generators (ASRGs)
• Explore Titan as an Earth-like system
• Examine the organic chemistry of Titan’s atmosphere
• Explore Enceladus and Saturn’s magnetosphere for clues to Titan’s origin and evolution
• Key science issues addressed:
– Exploring organic-rich environments
– Determining the origin and evolution of satellite systems
– Understanding dynamic planetary processes
• Model Payload
– High-Resolution Imager and Spectrometer
– Titan Penetrating Radar and Altimeter
– Polymer Mass Spectrometer, Sub-Millimeter Spectrometer, Thermal Infrared Spectrometer
– Magnetometer, Energetic Particle Spectrometer, Langmuir Probe, Plasma Spectrometer
– Radio Science and Accelerometers
• In Situ Elements: Balloon and Lake Lander
• Radioisotope Power Sources: 5 ASRGs + 1 MMRTG
• Launch Mass: 6,203 kg
• Launch Date: 2020 (on Atlas V 551) Gravity-Assist Solar Electric Propulsion
• Orbit: 1500 km Titan Orbit + Saturn Tour Including Enceladus Flybys
cost or technical reasons. Thus it is likely that a flagship Enceladus Orbiter will not take place until after 2025, and possibly not until the 2030s.
It should be noted that, given the high science priority of both Europa and Enceladus, it is possible, although far from certain, that a midterm update to the decadal survey could include one or both of those targets in the list of candidates for future New Frontiers missions. That could provide an earlier pathway to flight for missions to those bodies, albeit with reduced costs and more constrained science goals. Thus, even in the absence of a clear plan for near-term flagship missions to Europa and Enceladus, it is important to be cognizant of their unique planetary protection requirements so that the community can be prepared to propose such missions should the opportunity arise.
NASA and ESA have sponsored extensive studies of missions to Titan over the years, and a large multiplatform Titan Saturn System Mission (TSSM) was considered as a possible near-term flagship mission (Box B.3).6,7 For
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.
3. National Research Council, Vision and Voyages of Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011.
4. National Research Council, Vision and Voyages of Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011.
5. National Research Council, Vision and Voyages of Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011.
6. Jet Propulsion Laboratory, Titan Saturn System Mission Study—Final Report on the NASA Contribution to a Joint Mission with ESA, Jet Propulsion Laboratory, Pasadena, Calif., 2009.
7. European Space Agency, TSSM In Situ Elements—ESA Contribution to the Titan Saturn System Mission, ESA-SRE4, European Space Agency, Paris, France, 2008.
8. National Research Council, Vision and Voyages of Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., 2011.