This decade has been one of the most important and scientifically productive periods ever in the history of planetary science. New worlds have been explored and new discoveries have been made. Only 5 years ago Pluto was no more than a blurry smudge in the view of the most powerful telescopes, but today Pluto is known as a highly complex world. The atmosphere and magnetic forces of Jupiter have been explored in greater detail than ever before. The deep oceans of Enceladus and Europa have been identified, and the rings, moons, and atmosphere of Saturn have been revealed in new ways. A fleet of spacecraft is examining the atmosphere and the surface of Mars, uncovering mysteries like the possibility of water flowing just beneath the surface. Meanwhile, the most sophisticated rover ever built is currently sampling the rocks and soils of Mount Sharp, accompanied half a world away by the Opportunity rover, currently hunkered down on Mars amid a global dust storm, but hopefully still active 14 years after it made a bouncing landing on the red planet. Soon a new spacecraft will set down on Mars and begin listening for seismic rumbles deep underground. Because of the efforts of the National Aeronautics and Space Administration (NASA) and the United States’ international partners, Mercury, Venus, Ceres, Vesta, comet 67P/Churyumov-Gerasimenko, asteroid Itokawa, and the Moon have all served up answers and new mysteries about their origins and compositions, and provided hints about the formation of the solar system.
These developments in many cases result from the strategic guidance and scientific prioritization provided by the two decadal surveys that NASA has asked the planetary science community to perform via the National Academies of Sciences, Engineering, and Medicine. The most recent planetary science decadal survey, Vision and Voyages for Planetary Science in the Decade 2013-2022 (NRC, 2011), was released in March 2011. The decadal surveys, particularly Vision and Voyages, have served the planetary science community well in justifying a plan for planetary science that has been successful both in obtaining funding for missions and supporting research, and in protecting that plan from proposals and objectives that have not been vetted by the community-based decadal process.
NASA is required to conduct decadal surveys by law, and is also required to conduct midterm assessments of their progress toward meeting the goals of the decadal survey. In late 2016 NASA asked the National Academies to undertake a planetary midterm review. In spring 2017, after the passage of the NASA Authorization Act, the agency asked the National Academies to expand the scope of its midterm review to also include an assessment of the Mars Exploration Program (MEP), which is managed within the Planetary Science Division (PSD). This report assesses NASA’s performance at achieving the goals of the decadal survey at its midterm, as well as specifically assessing the Mars Exploration Program (the statement of task is included in Appendix A). The report offers
recommendations to NASA for achieving the goals of the current decadal survey until the next decadal survey is released, and makes recommendations for preparing for the next decadal survey effort, currently planned to begin holding meetings in spring 2020.
Vision and Voyages fully recognized the possibility that both NASA budgets and development challenges could impact execution of their recommended program. To help address this possibility they provided three decision rules to be used for planetary science program descopes:
- Descope or delay a large strategic (flagship) mission,
- Slip New Frontiers or Discovery missions only if large strategic (flagship) adjustments cannot solve the problem, and
- Place high priority on preserving research and analysis (R&A) and technology funding.
In response to the decadal survey’s recommendations, NASA descoped both the Mars Astrobiology Explorer-Cacher (MAX-C) mission (which became Mars 2020) and the Jupiter Europa Orbiter (JEO; which became Europa Clipper). This was done because neither would have been affordable within the planned budget. NASA did not descope or delay a large strategic (flagship) mission in response to budget cuts.1 It did slip both New Frontiers and Discovery missions. NASA did place high priority on preserving R&A and technology funding.
The decadal survey also recommended the following priorities if more funding than expected was made available for the planetary science program during the next decade (and assuming that the Uranus orbiter and probe mission was funded in addition to MAX-C and Europa orbiter):
- An increase in funding for the Discovery program,
- Another New Frontiers mission, and
- Either the Enceladus Orbiter or the Venus Climate Mission.
The committee concluded that despite significant cuts to the PSD’s budget early in this decade, NASA has made impressive progress at meeting the decadal survey’s goals. The agency has begun development of two of the decadal survey’s top recommended large strategic (i.e., flagship) class missions, the Mars 2020 rover and the Europa Clipper. It has also met or exceeded the decadal survey recommendations for funding both R&A and technology research and development (R&D) programs. (See Figure S.1.)
At the same time, the committee also concluded that budgetary and policy decisions limited NASA’s ability to achieve the recommended cadence for Discovery and New Frontiers mission announcements and NASA has conducted or begun planning for fewer announcements of opportunity for these missions than the recommended pace. NASA has indicated that it is planning on initiating a second New Frontiers announcement of opportunity within the decadal survey period, and the committee endorses this plan, but believes that it will be challenging to meet this schedule. Similarly, NASA will have to conduct several further Discovery mission selections to achieve the recommended cadence before the decadal period is finished.
In addition, for several years since the release of Vision and Voyages, NASA was precluded from beginning recommended technology development for realizing an eventual Mars sample return mission. The Mars 2020 rover will collect samples for eventual return to Earth, but the return portion of that effort will be technologically difficult, and Vision and Voyages recommended a technology development effort that did not begin until late 2017. The committee concluded that the PSD’s Mars sample return technology development plan is on the right track, and endorsed its proposed “focused Mars sample return” strategy. However, the committee is also concerned about the aging orbital infrastructure at Mars, which conducts valuable science, but is vital for communicating with rovers on the surface. The committee cautions that the loss of one or more of these spacecraft could make it difficult for NASA to communicate with its surface rovers, and reduce their science return.
The committee’s findings and recommendations are summarized by category.
1 The initial de-scoping of both MAX-C and the Jupiter Europa Orbiter were originally directed by the decadal survey.
LARGE STRATEGIC MISSIONS (FLAGSHIPS)
Vision and Voyages devoted considerable attention to the large strategic missions (often referred to as “flagships”). It recommended the MAX-C rover and the Jupiter Europa Orbiter missions based on their high science value, but also recommended that both missions be descoped in order to fit within available budgets. These missions have been implemented by NASA as the Mars 2020 rover and Europa Clipper.
The Europa Clipper concept currently in Phase B is reduced in cost and scope from the Jupiter Europa Orbiter mission that was proposed to Vision and Voyages and its cost appears to be within the guidelines established by the decadal survey. New funding has been allocated by Congress for this mission. This committee finds that the Europa Clipper mission addresses most of the recommendations laid out by Vision and Voyages. (This is further discussed in Chapter 3.) The committee also cautions that large strategic missions pose the greatest potential danger to the overall planetary program if they experience significant cost overruns. Thus, they require careful monitoring and management.
Recommendation: NASA should continue to closely monitor the cost and schedule associated with the Europa Clipper to ensure that it remains executable within the approved life cycle cost (LCC) range approved at Key Decision Point-B (KDP-B) without impacting other missions and priorities as defined
by the decision rules in Vision and Voyages (p. 36). If the LCC exceeds this range, NASA should descope the mission in order to remain consistent with the Vision and Voyages decision rules. (Chapter 3)2
Recommendation: NASA’s Planetary Science Division should implement an Independent Cost and Risk Review Process at Mission Definition/System Definition Review (Key Decision Point-B, or KDP-B) specifically for large planetary large strategic (flagship) missions to ensure that potential mission costs and cost risks are understood. (Chapter 3)
NASA is currently working to define the scientific goals, assess the feasibility of implementation, define the mission concept, and estimate the cost of a Europa lander. A lander was not prioritized or discussed in detail in Vision and Voyages, where it was referred to as a “far term” mission. It also did not undergo a cost and technical evaluation like other large missions prioritized in the decadal survey.3 Given its cost and its potential impact on the rest of the planetary science program, the committee concluded that the mission should be vetted within the decadal survey process.
Recommendation: As a prospective large strategic (flagship) mission, the results of the NASA Europa lander studies should be evaluated and prioritized within the overall Planetary Science Division program balance in the next decadal survey. (Chapter 3)
The third prioritized large strategic (flagship)-class mission in Vision and Voyages was an ice giants mission.4 Such a mission to either Uranus or Neptune has not yet been initiated by NASA. Exoplanet discoveries further enhance the importance of an ice giants mission. The notional ice giants mission described in Vision and Voyages would address a broad range of ice giant science objectives using mature instrumentation. The objectives of the mission concept described in a NASA-sponsored 2017 ice giants study have been changed significantly from the original Vision and Voyages science objectives. The committee found that the scientific payload proposed in the study carries significant risk of failing to make the measurements proposed in Vision and Voyages. New objectives were proposed in the ice giants study. A Doppler imager, not mentioned in Vision and Voyages for this mission and not yet flown on a spacecraft, has been added to the payload to make measurements of planetary oscillations that may not be detectable. If this component of the mission were not successful scientifically, a large part of the revised science objectives would be degraded or lost.
Recommendation: NASA should perform a new mission study based on the original ice giants science objectives identified in Vision and Voyages to determine if a more broad-based set of science objectives can be met within a $2 billion cost cap. (Chapter 3)
DISCOVERY AND NEW FRONTIERS
The committee found that NASA’s decision to eliminate Phase E (operations) funding and launch vehicle cost from the Discovery announcement of opportunity (AO) has been enabling for missions to the outer solar system. This was recommended in Vision and Voyages and is another example of NASA successfully following the decadal survey.
Although two Discovery missions were selected from the 2014 AO, the next AO will not be issued until 2019.
2 KDP-B refers to Key Decision Point-B, when a mission has reached an advanced design stage but prior to KDP-C, when a mission begins full-scale development.
3 All space science decadal surveys follow a cost and technical evaluation (CATE) process, using an independent contractor in response to a legally mandated requirement. The CATE process varies slightly for each decadal survey but is used to determine the technical maturity of a mission concept and its approximate cost. One value of the CATE process is to enable a decadal survey to determine the overall affordability and viability of its recommended program. The five flagship-class missions prioritized in Vision and Voyages all underwent a CATE.
4 A Uranus orbiter and probe mission was recommended by Vision and Voyages due primarily to trajectory concerns, but both ice giants were considered equally scientifically interesting by the decadal survey.
NASA will not have met the Vision and Voyages goal of a Discovery AO release every 24 months unless three missions are selected from the two potential future AOs.
Recommendation: NASA should issue Discovery announcements of opportunity (AOs) at the Vision and Voyages recommended cadence of ≤24 months, recognizing that an AO that selects two missions would count as two AOs for the purpose of meeting the Vision and Voyages recommendation. To approach meeting the Vision and Voyages recommendation, NASA should select three missions from AOs issued in 2019 and 2021. (Chapter 3)
New Ocean Worlds targets were introduced into the New Frontiers 4 call. This addition to the list of allowed New Frontiers missions was made outside the decadal survey process. While the Outer Planets Assessment Group (OPAG) supported the addition, the Lunar Exploration Analysis Group (LEAG), Small Bodies Assessment Group (SBAG), Venus Exploration Analysis Group (VEXAG), and Mars Exploration Program Analysis Group (MEPAG) did not support this change (as per presentations to this committee). Such a process could undermine the scientific priorities of the decadal survey and community support for them. The Space Studies Board’s Committee on Astrobiology and Planetary Science (CAPS) was not authorized to express a formal position on this change to the New Frontiers 4 call at the time, but is now able to produce letter reports regarding issues relevant to the decadal survey at NASA’s request and provides a method for evaluating proposed changes to the decadal survey.
Recommendation: If scientific discoveries or external factors compel NASA to reassess decadal survey priorities, such as the list of New Frontiers missions, NASA should vet these changes via the Committee on Astrobiology and Planetary Science and allow for input from the community via assessment and analysis groups as time permits. (Chapter 3)
The committee also found that the pace of New Frontiers class missions is significantly behind the recommended cadence of two per decade, with only one mission likely this decade. Given the current cadence for New Frontiers, the New Frontiers 5 call may occur while the next decadal survey is under way, but both Lunar Geophysical Network and Io Observer were recommended by Vision and Voyages for New Frontiers 5, and the committee believes they still remain valid missions for New Frontiers 5.
Recommendation: NASA should issue the New Frontiers 5 announcement of opportunity as soon as possible, but at a minimum release the announcement of opportunity no later than 5 years after the issuance of the New Frontiers 4 announcement of opportunity (i.e., December 2021). (Chapter 3)
THE NASA MARS EXPLORATION PROGRAM (MEP)
Vision and Voyages recommended that NASA begin technology development to enable the next steps toward sample return from Mars. During the first several years of the period covered by the decadal survey, NASA did not do this. However, by fall 2017 the PSD began technology demonstration tests and had developed a “focused and rapid” architecture for returning samples from Mars. Although NASA has considerable work to do to make this a reality, the committee was impressed and encouraged by these new developments.
Recommendation: NASA should continue planning and begin implementation of its proposed “focused and rapid” architecture to return samples from the Mars 2020 mission to achieve the highest-priority decadal survey large strategic (flagship)-class science for consideration by the next decadal survey. (Chapter 5)
NASA currently operates Mars Odyssey, Mars Reconnaissance Orbiter (MRO), and Mars Atmosphere and Volatile Evolution mission (MAVEN) around Mars, all of which have exceeded their design lifetimes. In addition
to performing science, these missions also provide vital telecommunications support to surface assets.5 There is a risk that ongoing and soon-to-be landed assets on Mars will be left without telecommunications support because of the fragile and aging orbiters. The loss of even one of the three U.S. orbiters capable of relay communications would create tactical challenges for continued operation of current and planned landed missions beyond 2021, and compromise the ability of the Mars Exploration Program to continue its science return.
Recommendation: NASA should ensure the longevity of the telecommunications infrastructure at Mars to support the science return from current and planned landed assets (e.g., Mars Science Laboratory, InSight, ExoMars, Mars 2020) to mitigate the risks associated with the existing aging assets. This should not be accomplished by sacrificing the science being conducted by existing orbiters. (Chapter 5)
Missions to Mars being led by non-U.S. entities (including ExoMars, Trace Gas Orbiter, Mars HOPE, and Mars Moon Explorer) benefit and significantly augment the U.S. Mars Exploration Program and lead to a broader scientific exploration of Mars.
Recommendation: NASA should immediately work to reinvigorate international cooperation to help implement Mars exploration more effectively and affordably. This could involve international contributions of instruments, other hardware, or whole missions that complement what the United States is providing or leading, as suggested in Vision and Voyages and as proposed in the “focused and rapid” concept for Mars sample return. (Chapter 5)
There are strong arguments for continuing Mars exploration through a program rather than as a series of independent, unconnected missions. Although the current MEP has a broad focus across most areas of Mars as a system, the program going forward beyond Mars 2020 is focused entirely on sample return. There is currently no vision for a program beyond sample return, either for scientific investigation or to prepare for future human exploration.
There are no plans at present to replace the site characterization and monitoring capabilities of MRO that have proven so important for landing-site certification and strategic planning of landed science. The MEP has not yet put forward a complete architecture and attendant strategic plan that addresses the long-term goals of Mars exploration and optimizes science return across the spectrum of past, current, and future missions.
Recommendation: NASA should develop a comprehensive Mars Exploration Program (MEP) architecture, strategic plan, management structure, partnerships (including commercial partnerships), and budget that address the science goals for Mars exploration outlined in Vision and Voyages. The architecture and strategic plan should maximize synergy among existing and future domestic and international missions, ensure a healthy and comprehensive technology pipeline at the architectural (versus individual mission) level, and ensure sustenance of foundational infrastructure (telecommunications, imaging for site certification, etc.). This approach of managing the MEP as a program, rather than just as a series of missions, enables science optimization at the architectural level. This activity should include assurance that appropriate NASA/MEP management structure and international partnerships are in place to enable Mars sample return. (Chapter 5)
TELESCOPES AND PLANETARY SCIENCE
Not all planetary science is or can be conducted at planetary bodies. Significant discoveries have been made by space and ground-based telescopes. For example, the Arecibo observatory is uniquely important for radar studies of asteroids, including characterization of potentially hazardous asteroids. The loss of the unique capabilities of the Hubble Space Telescope (HST) in the next decade will leave fewer opportunities for space-based telescope time
5 The European Space Agency (ESA) also operates two Mars orbiters that have additional telecommunications capabilities. One of these spacecraft is also significantly beyond its planned lifetime.
allocated to solar system targets in the visible and ultraviolet (UV) wavelengths. The James Webb Space Telescope (JWST) will obtain limited observations of solar system targets but will not have the spectral coverage of HST.
Recommendation: NASA should conduct an assessment of the role and value of space-based astronomy, including newly emerging facilities, for planetary science. This assessment should be finished before the next decadal survey is significantly under way. (Chapter 3)
RESEARCH AND ANALYSIS
Vision and Voyages stressed the importance of a strong R&A program in planetary science and recommended an increase to the overall budget devoted to it. The committee determined that R&A spending levels have risen 32 percent relative to fiscal year (FY) 2011 spending levels, the year for which Vision and Voyages had budget information. This well exceeds the Vision and Voyages recommendation. The committee found that analyzing R&A budget levels was difficult because the PSD does not track spending on R&A and technology in the way the decadal survey defined them. This can create misunderstandings within the science community.
Recommendation: NASA is largely following or exceeding the Vision and Voyages-recommended levels of research and analysis and technology spending. It should continue to make these critical investments. (Chapter 3)
Recommendation: The next decadal survey committee should work with NASA to better understand the categorization and tracking of the budget for each of the research and analysis program elements, specifically providing insight into the budget for (1) principal investigator (PI)-led, competed, basic research and data analysis; (2) ground-based observations; (3) infrastructure and management; and (4) institutional or field center support. Also, the next decadal survey should be unambiguous when stipulating programs and recommended levels of spending. (Chapter 3)
A subject that is repeatedly raised within the community is the selection rates for R&A programs, and this will logically be part of the future discussion.
TECHNOLOGY RESEARCH AND DEVELOPMENT
The committee found that the PSD has to-date met and is expected to continue to fully meet the decadal survey’s technology investment recommendation. Since the decadal survey, the Department of Energy has restarted production of Pu-238, which the committee considers a welcome development.
Recommendation: NASA should continue to work closely with the Department of Energy to ensure that the schedules for Pu-238 and clad production and the development of the Multi-Mission Radioisotopic Generators are maintained. It is also important that NASA continue the longer-term developments of advanced energy conversion techniques. (Chapter 4)
NASA created the PICASSO and MatISSE programs to provide sustained, broad-based science instrument technology development through technology readiness level (TRL) 6 as recommended by Vision and Voyages. The high number of proposals submitted, relative to the funding available, shows a strong community demand for these programs.
The PSD has embraced the decadal survey’s technology recommendations, and they have constructed a rational and comprehensive technology portfolio that can enable new and more challenging planetary science missions in the future. Vision and Voyages recommended investing 6 to 8 percent of the PSD budget in technology R&D, and NASA has essentially done that, which the committee applauds.
Recommendation: NASA should continue investment in development of the mission-enabling technologies at the 6 to 8 percent level. (Chapter 4)
INFRASTRUCTURE AND LABORATORY SUPPORT TO PLANETARY SCIENCE
The 2014 Discovery AO and 2017 New Frontiers AO require early planning and coordination for sample return missions. The actual costs for all aspects of curation, from planning through distribution and storage, including all required laboratory construction or modification, are required to be borne by the mission from inception to 2 years following sample return. Therefore, curation activities (and their associated costs) during phases A-D fall under the AO cost cap and activities during phase E fall under the PI-managed mission cost (but not the AO cost cap). Whereas long cruise missions can defer such costs to phase E, this situation penalizes short missions that have to include curation and laboratory costs in phases B-D.
Recommendation: NASA should consider the budget for curation by sample return missions, as developed in the announcement of opportunity-required curation planning documents, a phase E cost, regardless of the phase in which the costs are actually incurred. This would ensure that sample return missions are on equal footing with other mission proposals and discourage unrealistically low budgets for sample curation. (Chapter 3)
Recommendation: NASA should ensure that all constituencies relating to sample return missions, both competed and directed, be coordinated through the Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM) to optimize communication, avoid duplication of effort, and maximize existing expertise. (Chapter 3)
The committee also notes that the Deep Space Network (DSN) is vital to the success of planetary missions and is concerned that changes made to the DSN could impact current and future missions. The recommendation for Ka-band uplink and downlink at all stations has not yet been met. Ka-band downlink is available at all three stations (U.S., Spain, Australia), but Ka-band uplink is available only at Goldstone. The plan is to incorporate Ka-band uplink at all stations in the next few years, but this is not yet a committed capability that missions in development can count on, as it is not listed in the Space Communications and Navigation (SCAN) catalog.
Recommendation: The committee endorses the Vision and Voyages recommendation that all three Deep Space Network complexes should maintain high-power uplink capability in the X- and Ka-bands, and downlink capability in the S-, X-, and Ka-bands. (Chapter 3)
EDUCATION AND PUBLIC OUTREACH
The intent of the Vision and Voyages endorsement of 1 percent of mission budgets going toward education and public outreach activities was to have scientists who are involved in NASA’s missions directing and participating in public education and outreach activities. Currently, the STEM Activation program is not uniformly engaging NASA missions; some missions are not being engaged at all. Furthermore, the STEM Activation program is not utilizing the mission scientists to define or provide science content; therefore, the critically important connections between the mission scientists and these education programs have been greatly reduced. While NASA center-managed public engagement efforts are connecting with some missions, in other cases there is no direct tie between missions that are producing results for the programs and the work of the NASA education program.
Recommendation: In order to enable the excitement of space exploration to be fully communicated to the broader public, the STEM Activation program should work with all NASA planetary missions to define science content and program implementation. NASA’s Planetary Science Division should link
education and outreach activities directly to the missions that are providing the science content for them, interfacing through the principal investigators for competed missions, and through the project scientists for directed missions. Education experts within the STEM Activation program should work directly with the mission scientists and engineers (subject matter experts, or SMEs) to ensure a strong connection to NASA’s mission results. NASA had previously provided funds equal to 1 percent of the overall project budget to support these activities. New funding at this level would provide robust support for project engagement in these education and outreach activities. (Chapter 3)
PREPARING FOR THE NEXT DECADAL SURVEY
With less than 2 years left before the next decadal survey begins, there is a very limited amount of time to conduct new mission concept studies to assist the next decadal survey committee. During the last decadal survey mission concept studies were being conducted during the survey itself, placing great strain on the committee members and staff, as well as the NASA budget that supported the studies. This committee concluded that it is important to avoid such a rushed process during the next decadal survey. To date, NASA has conducted only the ice giants study (which this committee has recommended be redone), and started Ceres lander and Venus studies. The committee believes that numerous additional studies are still required.
Recommendation: NASA should sponsor 8 to 10 mission concept studies based on the list produced by the Committee on Astrobiology and Planetary Sciences, prioritized with input from the assessment and analysis groups, prior to the next decadal survey. Mission concept studies for large strategic (flagship)class missions should include options as described in the 2017 report Powering Science: NASA’s Large Strategic Science Missions (NASEM, 2017). (Chapter 6)
The recently launched InSight mission to Mars includes two planetary CubeSats, a technology capability that did not exist when Vision and Voyages was written. NASA is undertaking numerous studies of additional CubeSat and SmallSat missions in order to determine their viability for planetary science. Aside from requirements derived from the competitively selected SIMPLEx missions and PSDS3 mission concepts, there is not a clear pathway for prioritizing development of the key CubeSat and SmallSat technologies or planetary deployment and operational architectures that would enable operations beyond the Earth-Moon environment. These include, but are not limited to, destination delivery approaches, propulsion, telecommunications, and deployable elements to provide power generation or instrument aperture.
Recommendation: In preparation for the next decadal survey, NASA should consider priorities and pathways for advancing the state of the art of CubeSats and SmallSat technology, and how science-driven planetary small mission concepts that leverage emerging capabilities are identified and possibly implemented for flight. (Chapter 6)
NASA operates two “virtual institutes” for supporting planetary science research. The committee was briefed about both of them, but notes that the last evaluation of either institute was conducted a decade ago and the virtual institutes are not well addressed in Vision and Voyages.
Recommendation: A formal assessment by NASA of how well the program structure and funding of the virtual institutes are aligned with the Planetary Science Division’s science goals should be conducted on a regular basis, appropriately phased to the cycle of decadal surveys and midterm reviews. (Chapter 6)
The committee notes that there have been substantial developments in communications and computer technology, such as the emergence of “cloud computing.” These are impacting many areas of science, including planetary science.
Recommendation: The next decadal survey committee should assess NASA’s ability to respond to new needs for data archiving and interoperability from spacecraft, laboratories, and publications. (Chapter 6)
Chapter 1 of this report explains the background of both the decadal survey and this report. Chapter 2 outlines some—but by no means all—of the substantial planetary science discoveries made in the past few years. Chapter 3 evaluates NASA’s progress at meeting the goals of Vision and Voyages, and provides recommendations for further progress. Chapter 4 addresses NASA’s technology development program. Chapter 5 assesses the Mars Exploration Program. Last, Chapter 6 addresses preparations for the next planetary science decadal survey.
NASEM (National Academies of Sciences, Engineering, and Medicine). 2017. Powering Science: NASA’s Large Strategic Science Missions. The National Academies Press, Washington, DC.
NRC (National Research Council). 2011. Vision and Voyages for Planetary Science in the Decade 2013-2022. The National Academies Press. Washington, DC.