3

Mission Theme Reconsideration

The second question in the statement of task asked whether any of the changes in the scientific, programmatic, or technological context for the Ocean Worlds, Trojan Tour and Rendezvous, Io Observer, and LGN mission themes were sufficient to warrant reconsideration of their inclusion in the announcement of opportunity (AO) for the NF5 mission. Subsequent sections address each of these mission themes in turn.

OCEAN WORLDS

Titan

Dragonfly is a scientifically and programmatically exciting mission, and it is well-aligned with the overall Ocean Worlds mission theme to “search for the signs of extant life and/or characterize the potential habitability of Titan.” Dragonfly very clearly addresses the first of the two science objectives articulated in the NF4 AO (see Box 1.1), and partially addresses the second, as follows:

• Understand the organic and methanogenic cycle on Titan, especially as it relates to prebiotic chemistry; and
• Investigate the subsurface ocean and/or liquid reservoirs, particularly their evolution and possible interaction with the surface.

V&V strongly recommends optimum balance among mission classes and targets, with “optimum” in this context meaning “the appropriate balance across the solar system … found by selecting the set of missions that best addresses the highest priorities among the overarching [scientific] questions.”¹ Selecting another Titan mission, in NF5, based on the science objectives in the NF4 AO, would not be in keeping with optimum program balance, in that New Frontiers missions once selected for implementation are removed from the candidate list for the next AO.

Nevertheless, a mission of some class focused on the second scientific objective for Titan would address many high-priority scientific questions that Dragonfly cannot address. Moreover, Titan and Enceladus’s co-location in the Saturn system also permits mission concepts that would explore both moons, as is implied by the NF4 call for missions to “Titan and/or Enceladus.” Such a mission could scientifically align with the large-scale mission to the Saturn system studied in V&V. In this context, the committee notes that selection of two New Frontiers missions targeted at Titan, each fundamentally focused on one of the science objectives above, might resemble, and could address the preponderance of science objectives of, the deferred, large-scale mission to Titan and the Saturn system studied in depth in V&V. No endorsement by the committee of this concept is expressed or implied, but the concept does illustrate the kind of programmatic flexibility that could be studied as part of the next planetary science

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¹ National Research Council (NRC), 2011, Vision and Voyages for Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., pp. 9-10.

and astrobiology decadal survey. No single New Frontiers class mission can accomplish the science objectives of a large-class mission to Titan similar to that described in V&V.

Enceladus

The scientific objectives of the NF4 AO for Enceladus (see Box 1.1) were to “Assess the habitability of Enceladus’s ocean” and “Search for signs of biosignatures and/or evidence of extant life.” These are compelling objectives, especially with regard to biosignature and life detection, and two Enceladus missions were proposed to the NF4 call. Enceladus remains a prime destination for understanding the origins of subsurface oceans, their habitability, and life detection in the solar system. Indeed, it is the only ocean world where confirmed cryovolcanic plumes and plume materials enable sampling of ocean materials for astrobiological study using current technology. The selection of Dragonfly for Titan under the Ocean Worlds mission theme obviously does not relieve these particular scientific objectives because of the significant differences between the bodies and potential for major scientific advances.

Since the NF4 call, scientific discoveries have increased the likelihood that Enceladus’s ocean is habitable and have enhanced the position of Enceladus as a high-priority target in the search for life in the solar system.² Extensive technological development efforts are under way to mature flight instruments for biosignature and life detection on icy bodies and in plumes (see the discussion of ICEE and COLDTech in the “Ocean Worlds” section of Chapter 2). Accordingly, the committee views the case for continued exploration of Enceladus to be compelling on scientific and technological grounds. The committee notes that the implication of the second part of the statement of task is that the Ocean Worlds mission theme will appear in the NF5 mission theme (target) list unless it is reconsidered.

TROJAN TOUR AND RENDEZVOUS

As noted above, NASA has selected for flight through its Discovery program the Lucy multiple-Trojan-asteroid flyby mission. Lucy takes advantage of significant advancement in the application of solar-power for deep-space missions. Moreover, the Lucy team also discovered a unique and powerful mission design that allows the spacecraft to tour the leading L4 Trojan cloud starting in 2027 and then fall back through the inner solar system to undergo an Earth flyby, thus enabling it to reach the trailing L5 Trojan cloud in 2033. There, it will fly by the giant binary asteroid Patroclus-Menoetius. Lucy will encounter one main belt asteroid and seven Trojan asteroids in its nominal tour (including a satellite of one Trojan and the Patroclus-Menoetius binary as two). All of the major taxonomic types among the Trojan asteroids (and outer asteroid belt asteroids in general)—so-called C, P, and D types—will be encountered. Lucy will carry visible, near-infrared, and thermal-infrared imagers and spectrometers, as well as radio science (using the high-gain antenna and associated telecommunications systems to determine the masses of the target asteroids by Doppler tracking).

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² See, for example, A.R. Hendrix, T.A. Hurford, L.M. Barge, M.T. Bland, J.S. Bowman, W. Brinckerhoff, B.J. Buratti, et al., 2019, The NASA Roadmap to Ocean Worlds, Astrobiology 19:1-27.

V&V recommended a New Frontiers class mission that would encounter at least two Trojan asteroids and rendezvous with one for an extended exploration. Possible instrumentation could include visible and near-infrared spectrometers to measure spectral reflectance and infer composition; gamma-ray and neutron spectroscopy to elucidate elemental composition; multispectral imaging; an ultraviolet spectrometer to search for outgassing; a thermal mapper; and possibly a lidar system. Information on the interior structure of the rendezvous Trojan would be obtained from shape determination and radiometric tracking.

The Lucy mission aims to accomplish the preponderance of the objectives of the Trojan Tour and Rendezvous mission theme. The science objective of the Trojan Tour and Rendezvous, as described in the NF4 AO, was to “visit, observe, and characterize multiple Trojan asteroids.” While Lucy will meet this science objective, determination of elemental abundances in surface materials will not be accomplished. This relates to a major scientific rationale for the Trojan Tour and Rendezvous mission, to use composition to discriminate between planet formation hypotheses: either Trojans were captured during a solar-system-wide cataclysm during giant planet migration³ or they are leftovers of Jupiter’s local formation-swarm. Recent advances concerning the formation of Jupiter and its effect on planetesimal populations make this discrimination less clear.^4,5 Results from the Lucy mission should nevertheless shed great light on planet formation models.

With current technology, elemental abundances of surface materials can be determined by gamma-ray and neutron spectroscopy, which require station-keeping with a Trojan asteroid for a considerable length of time to build up the necessary signal to noise. Complete high-resolution imaging of the rendezvous targets will also not be possible, and precise mass determination from radio tracking is more difficult from a single flyby. Lucy’s advantage, through clever tour design, is that a large number of Trojan asteroids of diverse spectral types will be visited, and on a Discovery budget. A future New Frontiers–class mission to the Trojan asteroids could potentially take advantage of technology development, such as advanced solar-electric propulsion, which could permit extended rendezvous at multiple targets, in the manner of the Dawn mission to Vesta and Ceres. However, selecting a Trojan tour mission in NF5 would not be in keeping with the optimum program balance among mission targets and types recommended strongly in V&V. Further exploration of the Trojan asteroids is justifiable on scientific grounds, but the next steps, beyond Lucy, would be best left for determination by the forthcoming decadal survey.

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³ A. Morbidelli, H.F. Levison, K. Tsiganis, and R. Gomes, 2005, Chaotic capture of Jupiter’s Trojan asteroids in the early solar system, Nature 435:462-465.

⁴ S.N. Raymond and A. Izidoro, 2017, Origin of water in the inner solar system: Planetesimals scattered inward during Jupiter and Saturn’s rapid gas accretion, Icarus 297:134-148.

⁵ T.S. Kruijer, C. Burkhardt, G. Budde, and T. Kleine, 2017, Age of Jupiter inferred from the distinct genetics and formation times of meteorites, Proceedings of the National Academy of Sciences 114:6712-6716.

IO OBSERVER

V&V determined that “Io provides the ideal target to study tidal dissipation and the resulting variety of volcanic and tectonic processes in action, with fundamental implications for the thermal co-evolution of the Io-Europa-Ganymede system as well as for habitable zones around other stars.”⁶ As such, V&V determined that an Io mission was of high scientific priority and was found to be a plausible candidate for the New Frontiers program. Called Io Observer, the science goals of the mission were stated to include the following:

• Study Io’s active volcanic processes;
• Determine the melt fraction of Io’s mantle;
• Constrain tidal heating mechanisms;
• Study tectonic processes;
• Investigate interrelated volcanic, atmospheric, plasma-torus, and magnetospheric mass- and energy-exchange processes;
• Constrain the state of Io’s core via improved constraints on whether Io generates a magnetic field; and
• Investigate endogenic and exogenic processes controlling surface composition.

Io Observer was envisioned in V&V as a radio-isotope-powered⁷ Jupiter orbiter mission, carrying a narrow-angle camera, ion-neutral mass spectrometer, thermal mapper, and magnetometers, and performing 10 high-inclination Io flybys; an enhanced payload included a plasma instrument. In terms of programmatic developments, the Discovery 15/16 Step-1 selection, IVO, would also take this approach. The proposed IVO payload matches that of the enhanced payload of Io Observer and would perform a similar 10 high-inclination flybys of Io.⁸ The significant difference between Io Observer and IVO is that the latter is solar powered and proposed on a Discovery budget. The final selections for flight for Discovery 15/16 are anticipated in 2021, prior to the release of the NF5 AO (see Table 1.1).

Whether the inclusion of Io Observer in the NF5 target list warrants reconsideration thus depends on the outcome of Step 2 of the Discovery 15/16 selection process in 2021. It is entirely possible that IVO will not be selected for flight. However, Discovery proposals have been a useful step in maturing mission concepts. For example, the missions that became OSIRIS-REx and Juno were proposed to the Discovery program twice and three times, respectively, before being revised, reproposed, and selected as New Frontiers missions.

In addition, Juno is still operating at Jupiter and may make further observations of Io, perhaps in an extended mission. The Juno scientific payload is optimized for observations of Jupiter and thus is not

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⁶ NRC, 2011, Vision and Voyages for Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., p. 246.

⁷ The Satellites Panel of the decadal survey committee looked at two possible versions of the Io Observer, solar- and radioisotope-powered. The version of the mission prioritized in Chapter 9 of V&V (p. 266) and described in detail in Box C.9 (p. 346) is the radioisotope-powered version.

⁸ See, for example, “IVO Mission Fact Sheet,” http://ivo.lpl.arizona.edu.

expected to be able to address all of the science goals of the Io Observer mission. Regardless, the scientific objectives at Io remain highly compelling.

LUNAR GEOPHYSICAL NETWORK

The LGN would consist of several identical, long-lived landers distributed across the lunar surface, each carrying instrumentation for geophysical studies.⁹ The stated primary science objectives are to characterize the Moon’s internal structure, seismic activity, global heat flow budget, bulk composition, and magnetic field. According to V&V,

Since V&V, no geophysical stations have been placed on the Moon. The Chinese Change’3 and Change’4 missions each successfully placed a lander and rover on the lunar surface, on Mare Imbrium and in the South Pole-Aitken basin, respectively, but were instrumented for astronomical and local geological studies only. NASA’s new CLPS program is designed to have NASA-provided or other payloads transported to the lunar surface and operated by private companies. As such, the goals of the CLPS program are directly relevant to the goals of the LGN. In the short term, CLPS missions could help enable LGN by retiring key risks such as deployment strategy, lander noise characterization, and thermal and power performance. Indeed, the Lunar Instrumentation for Subsurface Thermal Exploration with Rapidity (LISTER) heat flow probe, LMS magnetotelluric sounder, and the NGLR are scheduled to fly in 2022 on a CLPS lander to Mare Crisium. However, the CLPS model is presently unproven.

Importantly, initial CLPS landers are not designed to survive the lunar night, although future landers may have that capability. An inability to survive the lunar night will preclude the ability to make some key measurements. But, for example, the thermal sensors for the new heat flow instrument have much less heat capacity than their massive Apollo-era predecessors. Thus, they need only a few hours to thermally equilibrate with the surrounding regolith, not weeks like the Apollo probes did. The CLPS mission is expected to demonstrate that a compact heat flow probe can be deployed from a robotic lander and that its quick thermal response time would enable us to determine the heat flow with a reasonable accuracy (±10 percent) based on the thermal measurements obtained in less than a lunar day. The long-term observation planned for the LGN would yield even higher quality data.

In addition, the structure of the CLPS program does not provide for an integrated mission or mission architecture (i.e., multiple identical spacecraft) to accomplish the scientific objectives set out for the LGN in V&V. The single exception would be in regard to lunar laser reflectors, which require no onsite power or communications. Additional laser ranging nodes could be provided by CLPS landers, which would

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⁹ Committee member Clive Neal recused himself from discussion of, or contributions to, the findings in this section of the report.

¹⁰ NRC, 2011, Vision and Voyages for Planetary Science in the Decade 2013-2022, The National Academies Press, Washington, D.C., p. 130.

expand the existing Apollo and Lunokhod network and provide significant scientific return.¹¹ The impact of future NASA programmatic elements on the LGN is less certain. The planned 2023 VIPER mission, for example, is not focused on geophysical investigations. LSII has just been initiated, so it is unclear how this could help/enable LGN. Artemis is also being formulated, but crewed landings on the Moon are some years off at best. It is, however, conceivable that humans arriving at the lunar surface sometime in the future could deploy a geophysical station to be included in the LGN, thus augmenting and enhancing what could be achieved with a New Frontiers–class mission.

Recent mission and other data reaffirm the need for LGN. For example, understanding the complex history of the magnetic field recorded in Apollo samples needs the global context of an LGN-type mission with measurements to address paleomagnetism. A globally distributed LGN is required to build on the investments of GRAIL and LRO missions in crustal and interior structure. The existing low-resolution Apollo seismic data that have been used to constrain the GRAIL gravity data are inadequate. Additional laser ranging nodes would provide substantial scientific return that would complement seismic, magnetic, and heat flow data. Moreover, additional nodes would also yield fundamental astrophysical data that could further test the general theory of relativity, the inverse square law, and the equivalence principle.¹²

Although there is some flexibility achieving the decadal-level science within a given mission architecture (e.g., replacing point atmospheric probes with a global microwave radiometer and sounder on Juno), the LGN requires a global network of long-lived (many years) lunar surface stations to achieve its scientific goals. A single station, with a single seismometer and heat flow probe, as with InSIGHT, can be of great scientific value, especially if it is the first such station. But the Moon, given its lateral variability and much deeper existing knowledge base, requires a long-lived, globally distributed surface network to answer the scientific objectives set out in V&V.¹³ Further study of LGN has been funded by NASA through its Planetary Mission Concept Study Program; this 2020 study is intended to further refine the mission and to reduce risk in its implementation as part of the NF5 competition. In addition, advances have been made in solar power (panels and batteries) and in radioisotope thermoelectric generators over the past decade, such that power for a 6-year mission can be sustained both with a solar power approach and with the new Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs).¹⁴

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¹¹ See, for example, J.G. Williams and J.O. Dickey, 2003, “Lunar Geophysics, Geodesy, and Dynamics,” 13^th International Workshop on Laser Ranging, Proceedings from the Science Session; and J.G. Williams, A.G. Turyshev, D.H. Boggs, and J.T. Ratcliff, 2006, Lunar laser ranging science: Gravitational physics and lunar interior and geodesy, Advances in Space Research 37:67-71.

¹² J.G. Williams, S.G. Turyshev, D.H. Boggs, and J.T Ratcliff, 2006, Lunar laser ranging science: Gravitational physics and lunar interior and geodesy, Advances in Space Research 37:67-71.

¹³ See, for example, M. Grott, J. Knollenberg, and C. Krause, 2010, Apollo lunar heat flow experiment revisited: A critical reassessment of the in situ thermal conductivity determination, Journal of Geophysical Research 115:E11005.

¹⁴ T. Hammel, R. Bennett, and B. Sievers, 2016, “Evolutionary Upgrade for the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG),” IEEE Aerospace Conference, https://ieeexplore.ieee.org/document/7500748/citations?tabFilter=papers#citations.