The study reports listed in Table 1 contain varying degrees of quantitative information relating to important mission parameters (e.g., mass, power requirements, and technical readiness of various spacecraft systems, subsystems, and instruments). Due to lack of time, the committee was not able to determine if all of the study reports contain all of the necessary quantitative data to subject the respective missions to the type of cost and technical evaluation (CATE) used to assess the concepts discussed in Vision and Voyages.14
Overall, CAPS concludes that the organization and execution of the mission studies listed in Table 1 represent an excellent start to the preparatory activities for the next planetary science decadal survey as recommended by the authors of Vision and Voyages.
Priority Science Areas Requiring Studies
What are the priority areas, as defined in Vision and Voyages, where publicly available mission studies have not been undertaken? After reviewing and assessing the detailed mission studies listed in Table 1, CAPS was able to identify a number of additional priority science areas where investment ahead of the next decadal survey would be beneficial. These concepts are outlined in Table 2, and their scientific rationales are described in more detail below. No priority is implied by the ordering of the concepts. Because this study is specifically concerned with the identification of large- and medium-class missions, those activities best served by small missions (e.g., studies of comets, asteroids, and other primitive bodies15) are not emphasized.
Exploration of Venus is fundamentally important to our understanding of the solar system and terrestrial planets in particular. Why is it so different from Earth (not merely in its atmosphere but in its geophysical state—lack of plate tectonics, slow rotation, lack of magnetic field, and uncertain level of volcanic activity)? In many respects, Venus is now the least well understood of all the planets known in ancient times. Past activities have been limited, in part because of the technical difficulties presented by the extreme conditions at the surface. Some of the scientific approaches to Venus, such as seismology, were previously thought to be possible only on the surface but might be possible from space or a balloon-hosted instrument.16 Global geophysical mapping that is greatly improved over what was achieved by the Magellan synthetic-aperture radar orbiter is also possible using different orbital instruments. There may also be more innovative geochemical approaches than those previously considered for key data (e.g., measurement of oxygen isotopes of surface rocks). The kinds of Venus missions that would host these technologies are not among those considered at the time of the last decadal survey (Venus Climate Mission, Venus Mobile Explorer, Venus Intrepid Tessera Lander).17 They are also different from the proposed Russian mission (Venera-D).18
14 For a detailed description of the CATE process, see NRC, Vision and Voyages, 2011, pp. 331-353.
15 For details as to why small-class missions are ideally suited to the exploration of primitive bodies see, for example, NRC, Vision and Voyages, 2011, pp. 105-107.
16 See, for example, Keck Institute for Space Studies, Venus Seismology Study Team, Probing the Interior Structure of Venus, Keck Institute for Space Studies, Pasadena, Calif., April 2015, http://resolver.caltech.edu/CaltechAUTHORS:20150727-150921873.
17 See NRC, Vision and Voyages, 2011, pp. 338 and 357-358.
18 Space Research Institute, Venera-D: Expanding our Horizon of Terrestrial Planet Climate and Geology through the Comprehensive Exploration of Venus—Report of the Venera-D Joint Science Definition Team, Russian Academy of Sciences, Moscow, 2017, http://iki.rssi.ru/events/2017/venera_d.pdf.
TABLE 2 Priority Areas That Are Candidates for Large- or Medium-Class Mission Studies (Unprioritized)
|Area To Be Studied||Notes|
|Venus exploration missions||Additional concepts beyond the Venera-D orbiter and lander concept referenced in Table 1|
|Lunar science missions||Understanding interior processes and polar volatiles|
|Mars sample-return next-step missions||Mission elements beyond Mars 2020 (e.g., Mars ascent vehicle, sample containment, and Earth return) necessary for the second and third phases of a Mars sample-return campaign|
|Mars medium-class missions||Multiple mobile explorers, polar explorers, and life-detection investigations, responsive to new discoveries (e.g., the diversity of intact stratigraphies from ancient environments, the detail of the polar record, and the modernity of some liquid water-related deposits)|
|Dwarf planet missions||Large- and medium-class mission concepts designed to exploit recent discoveries concerning Ceres and Pluto (and the Pluto-like world, Triton)|
|Io science||Reexamine the science and technology case for a dedicated mission to determine internal structure and mechanisms driving Io’s extreme volcanism|
|Saturn system missions||Affordable, large strategic missions that visit multiple targets and/or contain multiple elements are now feasible and are worthy of additional consideration|
|Dedicated space telescope for solar system science||Consider scientific return of a space telescope designed to monitor dynamic phenomena on planetary bodies|
NOTE: See Table A.1 in the Appendix for the alignment between these missions and the crosscutting science themes from Vision and Voyages for Planetary Science in the Decade 2013-2022 (National Research Council, The National Academies Press, Washington, D.C., 2011).
LUNAR SCIENCE MISSIONS
The exciting scientific possibilities offered by a South Pole Aitken Sample Return and a Lunar Geophysical Network caused both to be listed in Vision and Voyages as targets for the New Frontiers program. Since that time, new lunar mission concepts and novel means by which they might be executed have emerged that are of interest to the science, private sector, and human exploration communities. For example, public-private partnerships (e.g., Moon Express19) have emerged as opportunities since the last decadal survey. Moreover, work summarized and synthesized by the Lunar Exploration Analysis Group (LEAG) has increased our understanding of interior/origin volatiles.20 The authors of Vision and Voyages did commission a series of brief studies of a Lunar Polar Volatiles Explorer Mission—a lander and rover combination designed to conduct in situ measurements of ices deposited in a permanently shadowed
20 Lunar Exploration Analysis Group, Volatiles Specific Action Team: Final Report, December 31, 2014, http://www.lpi.usra.edu/leag/reports/vsat_report_123114x.pdf.
crater near one of the Moon’s poles.21Vision and Voyages concluded that such a mission “represents an important opportunity to study the nature, composition, and dynamics of volatiles trapped in the frigid interiors of lunar polar impact craters.” But also noted is the fact that “the polar crater environment presents a number of technical challenges, including rover survivability, sample collection and characterization, and navigation. Although some technical maturation is required, there remain no major impediments to such a mission within this decade.”22 A reexamination of the technical approaches and challenges posed by such a mission is warranted.
MARS SAMPLE-RETURN NEXT-STEP MISSIONS
Vision and Voyages recommended initiation of a three-step Mars sample-return campaign as its highest-priority, large-class mission. The Mars 2020 rover, the current incarnation of the survey report’s descoped Mars Astrobiology Explorer-Cacher (MAX-C), will begin those steps with selection, collection, and caching of well-characterized and diverse samples that are compelling to science and suitable for return. Immediate, systematic forward planning, coupled with an appropriate budget for mission starts and technology, is crucial for the cost-effective implementation of the return of scientifically valuable samples from Mars. This work will also require that attention be payed to the overall architecture of the Mars Exploration Program—for example, the need for maintenance and renewal of telecommunication-capable orbiters to facilitate all steps in a sample-return campaign. A new study and subsequent cost and technical evaluation of the next steps in the sample-return campaign—that is, the sample-retrieval and Earth-return missions—is necessary to update the concepts evaluated in Vision and Voyages.
A new evaluation is particularly needed in light of technical developments since 2011. These developments include Mars 2020’s precision-landing capability and the adoption of a so-called adaptive caching strategy in place of the single sample canister discussed in Vision and Voyages. In addition, investments and studies made in key technologies for sample return over the last decade have been made. These include the following:
- Investments by the Canadian Space Agency in the development of a prototype fetch rover and sample manipulator arm for the sample-retrieval mission;23
- A study by JPL of technologies required for the Mars Ascent Vehicle (MAV) required to place collected samples into orbit about Mars and follow-on plans for the development of hybrid propulsion system for a MAV;24,25 and
- A study by NASA’s Johnson Space Center of approaches to sample containment.26
In light of these investments, the changed scientific landscape, various potential collaborative partnerships (including public-private activities), and burgeoning interest in human exploration of Mars, a
21 See NRC, Vision and Voyages, 2011, pp. 359-360 and 381.
22 See NRC, Vision and Voyages, 2011, pp. 359-360.
23 See, for example, Canadian Space Agency, “Mars Sample Return Simulation: Preparing Our Country for the Future,” date modified November 2, 2016, http://www.asc-csa.gc.ca/eng/rovers/analogue-msrad.asp.
24 See, for example, A.C. Karp, “Hybrid Rocket Propulsion for a Low Temperature Mars Ascent Vehicle,” Lecture, Keck Institute for Space Studies, Pasadena, Calif., March 13, 2017, http://kiss.caltech.edu/new_website/lectures/2017_Karp.html.
25 A.C. Karp, B. Nakazono, R. Shotwell, J. Benito, D. Vaughan, and G.T. Story, “Technology Development Plan and Preliminary Results for a Low Temperature Hybrid Mars Ascent Vehicle Concept,” 53rd AIAA/SAE/ASEE Joint Propulsion Conference, AIAA Propulsion and Energy Forum, AIAA 2017-4900, https://doi.org/10.2514/6.2017-4900.
26 See, for example, M.J. Calaway, F.M. McCubbin, J.H. Allton, R.A. Zeigler, and L.F. Pace, “Mobile/Modular BSL-4 Facilities for Meeting Restricted Earth Return Containment Requirements,” 48th Lunar and Planetary Science Conference, March 20, 2017, https://www.hou.usra.edu/meetings/lpsc2017/pdf/1221.pdf.
new examination of architectures for the next steps of a sample-return campaign, including required sample-receiving and curation facilities, will provide important input to the next planetary science decadal survey. Whether or not a robotic sample return mission is a prerequisite to the human exploration of Mars, as some have suggested,27 is beyond the scope of the current report.
MARS MEDIUM-CLASS CANDIDATES
Studies conducted by Mars rovers and orbiters since the release of Vision and Voyages have shown the great diversity of environments preserved in the martian rock record. Two particular types of environments have been identified as being of particular interest. First, at least a dozen distinct aqueous environments have been identified in the martian geological record between some 4.2 billion and 3.0 billion years ago.28,29,30 Second, multiple locales with potentially recent liquid water have been identified as a result of studies of climate-related deposits.31,32 These two types of environments have been cited as compelling locations to search for potential biomarkers and for the mechanisms that drive climate change.
Community interest has increasingly focused on these types of locales—diverse ancient stratigraphies and sites with evidence of modern aqueous fluids/ices—as targets for further science and exploration. Missions specifically designed to explore these environments were not studied for cost and feasibility in the context of Vision and Voyages. Nor is it clear whether missions to explore such ancient and modern environments will require the resources of a medium-class mission. Some might be executable as a small mission, while others might require flagship-class capabilities. Examples of potential approaches to the exploration of these environments that could inform Mars science priority planning for the next decade include the following:
- Multiple rovers or other mobile platforms built simultaneously with defined payload interfaces;
- Mars polar climate missions akin to those mentioned in Vision and Voyages;33 and
- Landed spacecraft, appropriately prepared in light of planetary protection considerations, to search for potential biomarker at sites with possible recent surface or near-surface water.
DWARF PLANET MISSIONS
Recent exploration of the Pluto system by New Horizons and the ongoing Dawn mission at Ceres have revealed the scientific richness of these worlds, their unexpected complexity, and ongoing geologic
27 See, for example, NRC, Scientific Prerequisites for the Human Exploration of Space, National Academy Press, Washington, D.C., 1993, p. 11; and NRC, Safe on Mars: Precursor Measurements to Support Human Operations on the Martian Surface, National Academy Press, Washington, D.C., 2002, pp. 37-40.
28 B.F. Ehlmann, J.F. Mustard, S.L. Murchie, J.-P. Bibring, A. Meunier, A.A. Fraeman, and Y. Langevin, Subsurface water and clay mineral formation during the early history of Mars, Nature 479:53-60, 2011.
29 J.P. Grotzinger, D.Y. Sumner, L.C. Kah, K. Stack, S. Gupta, L. Edgar, D. Rubin, et al., A habitable fluviolacustrine environment at Yellowknife Bay, Gale Crater, Mars, Science 343:124277, 2014.
30 J.P. Grotzinger, S. Gupta, M.C. Malin, D.M. Rubin, J. Schieber, K. Siebach, D.Y. Sumner, et al., Deposition, exhumation, and paleoclimate of an ancient lake deposit, Gale Crater, Mars, Science 350:acc7575, 2015.
31 C.I. Fassett, J.L. Dickson, J.W. Head, J.S. Levy, and D.R. Marchant, Supraglacial and proglacial valleys on Amazonian Mars, Icarus 208:86-100, 2010.
32 D.K. Weiss, J.W. Head, A.M. Palumbo, and J.P. Cassanelli, Extensive Amazonian-aged fluvial channels on Mars: Evaluating the role of Lyot Crater in their formation, Geophysical Research Letters 44:5336–5344, 2017.
33 See, for example, NRC, Vision and Voyages, 2011, pp. 167 and 362.
activity.34,35,36 The implication for our understanding of Kuiper belt objects, and the trans-Neptunian region and dwarf planets generally, is that there is much that is not understood and much that could be learned about the structure, origin, and evolution of the solar system by further investigation of dwarf planets. Several dwarf planets and likely dwarf planets in the Kuiper belt remain unexplored. Moreover, results from the New Horizons and Dawn missions have put the limited information gleaned about Triton (likely a captured Kuiper belt dwarf planet) during Voyager 2’s 1989 flyby in sharp relief. Logical follow-on missions that could be usefully studied in advance of the next planetary science decadal survey include a Ceres lander, a Pluto system orbiter, and Centaur and/or Kuiper belt object flybys. Synergistic combinations with ice giant flybys could also be considered in this context, especially as regards Triton. A medium-size spacecraft is likely optimal for some, but not necessarily all, of these possible missions.
NEW FRONTIERS FIVE
Jupiter and its icy moons are well served by current and planned missions (Juno, Europa Clipper, and the European Space Agency’s [ESA’s] Jupiter Icy Moons Explorer). Io, the solar system’s most volcanically active body, was recommended by Vision and Voyages as a possible target for the fifth New Frontiers (NF5) mission opportunity.37 Given that the announcement of opportunity for NF5 will likely come in the early 2020s, the revalidation of the Io Explorer as having appropriate scientific merit for inclusion in the New Frontiers program will fall to the next planetary science decadal survey (currently scheduled for release during the first quarter of 2022). As such, the technical feasibility of the Io Explorer concept, and the other NF5 candidate in Vision and Voyages, the Lunar Geophysical Network (LGN), could be fruitfully reexamined.
SATURN SYSTEM MISSIONS
Vision and Voyages identified the Jupiter Europa Orbiter and the Titan Saturn System Mission (TSSM) as having comparable scientific value, but TSSM was deferred to the next decade because of a lack of technical maturity that drove the concept to a prohibitively high cost. Since publication of Vision and Voyages, the Cassini mission has continued to make discoveries that only increase the compelling nature of the reason to return to the Saturn system. For example, measurements from Cassini indicate the presence of organic material and water-rock reactions within a global subsurface salty ocean on Enceladus.38,39,40 The ocean of Enceladus possesses many, if not all, of the requirements for life. Similarly, results from Cassini have also demonstrated that Titan is an accessible location for studying
34 S.A. Stern, F. Bagenal, K. Ennico, G.R. Gladstone, W.M. Grundy, W.B. McKinnon, J.M. Moore, et al., The Pluto system: Initial results from its exploration by New Horizons, Science 350:aad1815, 2015.
35 J.M. Moore, W.B. McKinnon, J.R. Spencer, A.D. Howard, P.M. Schenk, R.A. Beyer, F. Nimmo, et al., The geology of Pluto and Charon through the eyes of New Horizons, Science 351:1284-1293, 2016.
36 C.T. Russell, C.A. Raymond, E. Ammannito, D.L. Buczkowski, M.C. De Sanctis, H. Hiesinger, R. Jaumann, et al., Dawn arrives at Ceres: Exploration of a small, volatile-rich world, Science 353:1008-1010, 2016.
37Vision and Voyages specifically recommended (and NASA has accepted) that the Io Explorer and Lunar Geophysical Network missions be specifically excluded for consideration as candidates for the fourth New Frontiers launch opportunity. See NRC, Vision and Voyages, 2011, pp. 266-268.
38 H.-W. Hsu, F. Postberg, Y. Sekine, T. Shibuya, S. Kempf, M. Horányi, A. Juhász, et al., Ongoing hydrothermal activities within Enceladus, Nature 519:207-210, 2015.
39 C.R. Glein, J.A. Baross, and J.H. Waite, Jr., The pH of Enceladus’ ocean, Geochimica et Cosmochimica Acta 162:202-219, 2015.
40 J.H. Waite, C.R. Glein, R.S. Perryman, B.D. Teolis, B.A. Magee, G. Miller, J. Grimes, et al. Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes, Science 356: 155-159, 2017.
planetary organic chemistry—including prebiotic and potentially exotic biochemistries.41 Because abiotic synthesis of organic compounds must have occurred prior to life’s emergence on Earth, understanding where and how those processes occur is critical to the origins of life on other worlds.
In parallel with the above scientific advances, substantial work has been done to mature the technologies required for a comprehensive exploration of the Saturn system. Accordingly, pieces of TSSM have been proposed to NASA’s Discovery program (e.g., Titan Mare Explorer, Journey to Enceladus and Titan, and Enceladus Life Finder) and ESA’s medium-class programs (Exploration of Enceladus and Titan). In addition, multiple Titan and Enceladus mission concepts are currently under consideration to be selected as the fourth New Frontier mission. In an effort to optimize the science value of outer solar system exploration in the next decade, the feasibility of lower-cost flagship missions that, for example, combine elements of proposed Discovery and New Frontiers concepts, are worthy of study. The co-location of three high-priority targets—Saturn, Titan, and Enceladus—in the Saturn system provides a potential opportunity to accomplish significant science for multiple targets using a common spacecraft.
DEDICATED SPACE TELESCOPE FOR SOLAR SYSTEM SCIENCE
The advent of space telescopes, primarily for astronomical and astrophysical investigations, has revolutionized studies of aqueous and dynamic phenomena in the solar system. For example, understanding and discovery of possible plumes on Ceres and Europa were enabled by space-based observation. Synoptic monitoring of Titan’s weather and surveys of water, ices, and composition of asteroids are presently mostly conducted from the ground. Access to critical wavelength regions in the ultraviolet and near- and mid-wavelength infrared, precluded by Earth’s atmosphere, could also be greatly maximized due to lack of interfering absorptions. Vision and Voyages noted that “a highly capable planetary space telescope in Earth orbit could be accomplished as a Discovery mission. Such a mission could be particularly valuable for observations of the giant planets and their satellites.”42 In addition, Vision and Voyages included a recommendation that the scope of the Discovery program be expanded to allow proposals for space-based telescopes.43
Synoptic observations of solar system bodies are limited by two factors, the availability of telescope time and resolution. First, while current (e.g., Hubble Space Telescope and Spitzer Space Telescope) and future (e.g., James Webb Space Telescope and Wide-Field Infrared Space Telescope) space observatories are available to the planetary astronomy community and are not resolution constrained, such assets are in great demand for other astronomical studies. Therefore, the availability of telescope time for long-term monitoring of, for example, Titan, Europa, and Io or for surveys is highly limited. Second, the resolution of such observations is primarily dictated by telescope aperture (the larger the aperture the greater the cost of the mission). Hence, studies to determine the potential scientific return of a space telescope dedicated to the monitoring and studies of solar system bodies that can be achieved within the scope of either the Discovery or the New Frontiers programs would benefit the next planetary science decadal survey.
41 F. Raulin, C. Brassé, and P. Coll, Prebiotic-like chemistry on Titan, Chemical Society Reviews 41:5380-5393, 2012.
42 NRC, Vision and Voyages, 2011, p. 264. Note that NEOCam, proposed to the most recent Discovery round, is a space-based infrared telescope for surveying potentially hazardous asteroids, and is in an extended phase A.
43 NRC, Vision and Voyages, 2011, p. 264.