3
Solar System Exploration

The panel finds that overall the proposed missions and timescales in The Solar System Exploration Strategic Roadmap1 have significant scientific merit, and the near-term recommendations are consistent with those of the NRC solar system decadal survey New Frontiers in the Solar System: An Integrated Exploration Strategy.2 The solar system roadmap appropriately recognizes the importance of timely technology development to support cost-capped missions and emphasizes the contributions to NASA’s education and outreach program and the need to continue this vital effort.

SCIENCE GOALS AND OBJECTIVES

One major area of concern this panel has with the solar system roadmap is that the concept of “habitability” is central to the roadmap but is not well developed in regard to scientific lines of enquiry and appropriate missions to respond to those scientific questions. A second, overarching concern is that habitability is the basic premise for scientific exploration in this roadmap. Although habitability is clearly of great importance, the panel thinks that it is inadequate as the sole theme for solar system exploration and thus it undermines the role of fundamental discovery science.

The roadmap addresses the issue of habitability across two broad themes: (1) habitability in planetary environments and (2) habitability associated with planetary system architecture. Within that context five objectives are identified: (1) learn how the Sun’s family of planets and minor bodies originated, (2) determine how the solar system evolved to its current diverse state including the origin and evolution of Earth’s biosphere, (3) explore the space environment to discover potential hazards and search for resources that would enable permanent human presence, (4) understand the processes that determine the fate of the solar system and life within it, and (5) determine whether there is or ever has been life elsewhere in the solar system. The fundamental issue of how planetary systems become habitable is addressed in this roadmap from two complementary perspectives—comparative exploration of worlds and exploration of planetary architecture. Both threads connect this roadmap to other strategic roadmaps through the exploration of Mars as a once habitable world, the exploration of the Moon as a preserved record of the earliest evolution of Earth and its impact environment, and the potential variety and habitability of planetary systems around other stars.

The panel agrees with this breakdown of goals and notes that it is consistent with, and supports, the goals outlined in the NRC solar system decadal survey. However, the panel cautions that the question of whether the basic requirements for the origin and persistence of life were present at some time during the history of a planet is complex. It is generally accepted that the requirements for life are reduced to three basic components: (1) liquid water, (2) sources of basic building blocks (carbon, hydrogen, nitrogen, oxygen, phosphorus, plus minor elements), and (3) sources of energy. Initial steps to evaluating the habitability of a planetary environment require a determination of whether or not this combination of requirements was present long enough to allow life to originate, evolve, and persist. The roadmap uses the concept of habitability as its basic premise for scientific exploration, yet in its current form it does not clearly articulate how the planned investigations will address these requirements for life and how each mission will build on previous mission results. In addition, nowhere is objective 3, which is concerned



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Review of Goals and Plans for NASA’s Space and Earth Sciences 3 Solar System Exploration The panel finds that overall the proposed missions and timescales in The Solar System Exploration Strategic Roadmap1 have significant scientific merit, and the near-term recommendations are consistent with those of the NRC solar system decadal survey New Frontiers in the Solar System: An Integrated Exploration Strategy.2 The solar system roadmap appropriately recognizes the importance of timely technology development to support cost-capped missions and emphasizes the contributions to NASA’s education and outreach program and the need to continue this vital effort. SCIENCE GOALS AND OBJECTIVES One major area of concern this panel has with the solar system roadmap is that the concept of “habitability” is central to the roadmap but is not well developed in regard to scientific lines of enquiry and appropriate missions to respond to those scientific questions. A second, overarching concern is that habitability is the basic premise for scientific exploration in this roadmap. Although habitability is clearly of great importance, the panel thinks that it is inadequate as the sole theme for solar system exploration and thus it undermines the role of fundamental discovery science. The roadmap addresses the issue of habitability across two broad themes: (1) habitability in planetary environments and (2) habitability associated with planetary system architecture. Within that context five objectives are identified: (1) learn how the Sun’s family of planets and minor bodies originated, (2) determine how the solar system evolved to its current diverse state including the origin and evolution of Earth’s biosphere, (3) explore the space environment to discover potential hazards and search for resources that would enable permanent human presence, (4) understand the processes that determine the fate of the solar system and life within it, and (5) determine whether there is or ever has been life elsewhere in the solar system. The fundamental issue of how planetary systems become habitable is addressed in this roadmap from two complementary perspectives—comparative exploration of worlds and exploration of planetary architecture. Both threads connect this roadmap to other strategic roadmaps through the exploration of Mars as a once habitable world, the exploration of the Moon as a preserved record of the earliest evolution of Earth and its impact environment, and the potential variety and habitability of planetary systems around other stars. The panel agrees with this breakdown of goals and notes that it is consistent with, and supports, the goals outlined in the NRC solar system decadal survey. However, the panel cautions that the question of whether the basic requirements for the origin and persistence of life were present at some time during the history of a planet is complex. It is generally accepted that the requirements for life are reduced to three basic components: (1) liquid water, (2) sources of basic building blocks (carbon, hydrogen, nitrogen, oxygen, phosphorus, plus minor elements), and (3) sources of energy. Initial steps to evaluating the habitability of a planetary environment require a determination of whether or not this combination of requirements was present long enough to allow life to originate, evolve, and persist. The roadmap uses the concept of habitability as its basic premise for scientific exploration, yet in its current form it does not clearly articulate how the planned investigations will address these requirements for life and how each mission will build on previous mission results. In addition, nowhere is objective 3, which is concerned

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Review of Goals and Plans for NASA’s Space and Earth Sciences with the search for potential hazards and resources, addressed with respect to habitability or in any other context. The panel recommends that where habitability is determined to be the main focus for exploration, a proper hierarchy of scientific goals and objectives be developed and stronger pathways between the concept of habitability and proposed missions be articulated and maintained. The panel also recommends that basic discovery science should not be ignored in any future promulgation of the solar system roadmap science and implementation program. MISSION CLASS SIZE AND MIX In contrast to the NRC solar system decadal survey, which spanned from 2003 to 2013, this roadmap extends from 2005 to 2035. Phase 1 of this roadmap adheres to the criteria that were established in the NRC solar system decadal survey. Missions that were deferred in that report required technology development that would not be available in that decade. This roadmap is clearly influenced by the current success of the Cassini-Huygens mission, and the roadmap includes a flagship mission to Titan. The panel supports the roadmap phase 1 plan. However, for phases 2 and 3 the plan suffers from incomplete articulation and unclear science flowdown. Each theme addressed in the solar system roadmap addresses previous decadal survey and NASA roadmap goals through a mix of missions of different classes: small Discovery-class missions ($300 million to $500 million), medium-class New Frontiers missions ($500 million to $800 million), and flagship-class missions ($800 million to $1,400 million or $1,400 million to $2,800 million). The panel endorses this breakdown, which follows the lines of the NRC solar system decadal survey in classifying different missions into classes dependent on budget and timescale objectives. Such a classification enables this roadmap to be directly compared with the NRC decadal survey document for ease of use and implementation by the planetary science community, which is becoming accustomed to thinking about space exploration within these discrete class boundaries. The roadmap appropriately recognizes the importance of the medium-class New Frontiers missions as a vital resource for the planetary community, not only because they have the potential to return unprecedented scientific data, but also because they sustain capabilities and train new generations of planetary scientists. The roadmap describes New Frontiers-class mission priorities as they are addressed in the NRC solar system decadal survey, and the highest-priority missions from that report are reiterated in the roadmap (Venus In Situ Explorer, Comet Surface Sample Return, Jupiter Orbiter with Probes, Lunar Aitken Basin Sample Return). The roadmap recognizes that the current program is based on implementing those missions. The recent selection of the Juno polar orbiting Jupiter spacecraft (announced after the roadmaps were completed) has altered the context of the roadmap, resulting in the definition of a new track since the construction of the roadmap. However, the competitive nature of the New Frontiers mission line forestalls the planning of a specific mission queue; as indicated in this roadmap, the initial list recommended by the decadal survey should be expanded using community-wide input. The decadal constraint of the decadal survey report is reflected in phases 2 and 3 of the roadmaps. The panel recommends that roadmap activity be continued in order to optimize science flowdown as new scientific results and technology become available. As with the New Frontiers program, no recommendations are made in the roadmap with respect to the scope of science that is to be done in the context of the Discovery program. Although openly competed, this program is discussed in the roadmap only as a vehicle for studying small bodies. However, the recent increase in the cost-cap and launch vehicle capabilities (see, e.g., NASA Research Opportunities in Space and Earth Sciences—20053) may enable missions to be proposed that were previously outside the Discovery scope, as well as those outside the scope of the roadmap. The panel recommends that the Discovery program be considered an integral part of any future plans for solar system exploration.

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Review of Goals and Plans for NASA’s Space and Earth Sciences The five flagship missions chosen by the roadmap committee have the potential to produce science that is paradigm altering. The targets of flagship missions include the surface of Venus, the lower atmosphere and surface of Titan, the surface and subsurface of Europa, the deep atmosphere of Neptune and the surface of its moon Triton, and cryogenically preserved samples on the surface of a comet nucleus. The roadmap appropriately recognizes that basic technology is required to implement both mid-sized and flagship missions; however, the panel strongly believes that the roadmap seriously underestimates the size of some of the missions. A Neptune mission that includes a Triton lander and deep probes into Neptune is a major mission. It is not convincing that either a Titan Explorer with a retractable surface probe or a Venus Explorer with an extended stay on the surface could possibly be a “small” flagship. Related to this point is the fact that the solar system roadmap suffers from a lack of discussion or clarity about how technology paths for outer solar system exploration will be achieved. These missions have significant lead times, and technology requirements must be clearly laid out if they are to succeed. Some of the required technology development may also overlap with that required by other roadmaps. The panel recommends that any future solar system roadmaps include realistic cost analyses to a level of detail consistent with the class of mission. In addition, the panel recommends that technology paths and decision points be clearly specified. CROSSCUTTING OPPORTUNITIES This roadmap does not address overlaps with the Robotic and Human Exploration of Mars roadmap or the Sun-Solar System Connection roadmap. Missions to the Moon, Mars, or solar environment that are designed to support the vision for space exploration will return science data, and these data will likely be highly significant for understanding of the origin and evolution of the solar system and therefore for the goals of this roadmap. A shortcoming of the roadmap is that it does not provide a summary of the extent to which the selected missions in this roadmap will fulfill the five primary science objectives, nor does it integrate contributions from the other roadmaps. The panel recommends that scientific and technical goals common among this roadmap and other roadmaps be highlighted and linked in future planning efforts. The panel endorses the roadmap’s recommendation of a Europa Geophysical Orbiter mission, which is also strongly supported by the NRC solar system decadal survey. A Venus flagship mission is promoted in the roadmap in the context of habitability; however, the roadmap fails to address the need for improved understanding of Venus to support comparative terrestrial planetology. The panel notes that Venus mission planning would have benefited by linking with the Sun-Solar System Connection roadmap that recognizes Venus relative to global warming on Earth. The panel recommends articulation of such crosscutting themes in future planning efforts and Venus mission development. CAPABILITY ISSUES Conspicuous by its absence in the roadmap is a significant discussion of the necessity for upgrading NASA’s Deep Space Network (DSN), developing enhanced telecommunications capabilities, or providing facilities to handle the data. With the increased number of missions active at any one time (including the ever-rising number of extended missions) and the DSN already pushing the limits of its capability, the need for enhanced telecommunications is becoming ever more urgent. The roadmap devotes a short paragraph to the need for DSN upgrades and acknowledges that the Mars program is leading that development. It does not describe the scope of expected data rates or address issues related to data access, processing, and archiving. The requirements of solar system exploration in this regard are of the utmost importance and must be addressed. The panel recommends that any future solar system

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Review of Goals and Plans for NASA’s Space and Earth Sciences exploration planning consider the necessity of adequate support for mission telemetry and that new telecommunications technologies be considered for development. REFERENCES 1. National Aeronautics and Space Administration (NASA), Advanced Planning and Integration Office. 2005. SRM 3—The Solar System Exploration Strategic Roadmap. NASA, Washington, D.C. Available at <www.hq.nasa.gov/office/apio/pdf/solar/solar_roadmap.pdf>. 2. National Research Council (NRC). 2003. New Frontiers in Solar System Exploration: An Integrated Exploration Strategy. The National Academies Press, Washington, D.C. 3. NASA. 2005. NASA Research Opportunities in Space and Earth Sciences—2005. NASA, Washington, D.C. Available at <nspires.nasaprs.com>.