Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity (2008)

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3 Summary of Mission-Specific Recommendations Three top-level recommendations that the committee considers to be relevant to the New Frontiers Program are offered in Chapter 1. Eight candidate missions and an innovative mission option for the next New Frontiers announcement of opportunity are considered in Chapter 2. The committee expects that NASA and the scientific community will use this report in slightly different ways. NASA will be more interested in the science goals for each mission, which it will use to formulate the science goals of the missions included in the next announcement of opportunity. Potential proposers from the science community will be interested primarily in mission-specific options rather than the science goals of all the options and will therefore focus on the mission sections individually. Thus, the mission-specific recommendations for each candidate mission are repeated below for easy reference. These eight missions are not prioritized. South Pole-Aitken Basin Sample Return The committee identified no changes to recommend for the scientific objectives or engineering implementation of the South Pole-Aitken Basin Sample Return mission as presented in the decadal survey. However, the committee recommends that NASA not be overly prescriptive about specific approaches to address the scientific objectives. Instead, NASA should allow proposers to develop their own innovative approaches. The committee believes that the following science goals, which are not listed in priority order, should be established for this mission: • Elucidate the nature of the Moon’s lower crust and/or mantle by direct measurement of its composition and of sample ages; • Determine the chronology of basin-forming impacts and constrain the period of late, heavy bombardment in the inner solar system, and thus address fundamental questions of inner-solar-system impact processes and chronology; • Characterize a large lunar impact basin through “ground truth” validation of global, regional, and local remotely sensed data on the sampled site; • Elucidate the sources of thorium and other heat-producing elements in order to understand lunar differen- tiation and thermal evolution; and • Determine ages and compositions of farside basalts to determine how mantle source regions on the Moon’s farside differ from regions with basalts sampled by Apollo and Luna. 58 

SUMMARY OF MISSION-SPECIFIC RECOMMENDATIONS 59 Venus In Situ Explorer The committee concluded that several of the goals of NASA’s Venus Exploration Analysis Group (VEXAG) should be included with the goals established in the decadal survey, particularly those concerning understanding the thermal balance of the atmosphere of Venus and gathering global mineralogic data. The New Frontiers announcement of opportunity should not exclude a Venus In Situ Explorer (VISE) mission that addresses the major goals for chemical sampling of Venus’s mid- to lower atmosphere and for characterizing atmospheric dynamics but that lacks a surface sampling component. The science goals for a VISE mission, which are not in priority order, should be as follows: • Understand the physics and chemistry of Venus’s atmosphere through measurement of its composition, especially the abundances of sulfur, trace gases, light stable isotopes, and noble-gas isotopes; • Constrain the coupling of thermochemical, photochemical, and dynamical processes in Venus’s atmo- sphere and between the surface and atmosphere to understand radiative balance, climate, dynamics, and chemical cycles; • Understand the physics and chemistry of Venus’s crust, for example through analysis of near-infrared descent images from below the clouds to the surface and through measurements of elemental abundances and mineralogy from a surface sample; • Understand the properties of Venus’s atmosphere down to the surface through meteorological measurements and improve understanding of Venus’s zonal cloud-level winds through temporal measurements over several Earth days; • Understand the weathering environment of the crust of Venus in the context of the dynamics of the atmo- sphere of Venus and the composition and texture of its surface materials; and • Map the mineralogy and chemical composition of Venus’s surface on the planetary scale for evidence of past hydrological cycles, oceans, and life and constraints on the evolution of Venus’s atmosphere. Comet Surface Sample Return The Comet Surface Sample Return mission candidate should seek to answer the following science questions as they were originally stated in the decadal survey (not in priority order), although not all of them must be answered with a single mission. • What is the elemental, isotopic, organic, and mineralogical composition of cometary materials? • How is cometary activity driven? • How do small bodies accrete? • What are the scales of physical and compositional heterogeneity? • How are the particles on a cometary nucleus bound together? • What are the macroscopic mineralogical and crystalline structure and isotopic ratios in cometary solids? The committee further recommends that the New Frontiers announcement of opportunity should leave the choice of target comet to the proposer; the choice of target should be a major factor for evaluation. Network Science The committee recommends that a Network Science mission be included in the forthcoming NASA New Fron- tiers announcement of opportunity. The decadal survey identified a network science mission’s primary objective as geophysics. For Mars, atmospheric measurements near the surface are a valuable supplement to the geophysics measurements but cannot be a substitute for them. This text is taken from several sections in the decadal survey. See New Frontiers in the Solar System, pp. 25, 180, 182-183, and 195. 

60 OPENING NEW FRONTIERS IN SPACE In light of the decadal survey’s recognition of the importance of network science on all the terrestrial planets and the Moon, the committee recommends that network science missions to the Moon, Venus, and Mercury also be considered as candidate missions for the New Frontiers announcement of opportunity in addition to a Mars mission. The science objectives of a Network Science mission should be drawn from a subset of the objectives described in the decadal survey (not in priority order). For the Interior • Determine the internal structure, including horizontal and vertical variations in the properties of the crust and mantle, and evaluate implications for how the core, mantle, and crust evolved. • Determine the characteristics of the metallic core (e.g., size, density, and presence and distribution of liquid) and explain the strength or absence of a present-day magnetic field. • Determine the heat flow and the distribution of heat-producing elements in the crust and mantle. • Determine interior composition and compositional variations to elucidate differentiation, crust-mantle evolution (plate tectonics, basin formation by impacts, conditions for life), and how the bulk composition relates to that of Earth and other terrestrial planets and how planetary compositions are related to nebular condensation and accretion processes. For the Surface/Atmosphere • Measure the surface winds and their time variability and the near-surface global circulation. • Measure the temperature, pressure, humidity, and radiative flux. • Measure the atmospheric, elemental, and isotopic compositions. • Understand the relationship between the near-surface general circulation and the physical processes that force it. • Determine how the near-surface general circulation controls the exchange of dust, water, CO2, etc., between the atmosphere and the surface. • Begin to establish a weather-monitoring infrastructure to support future robotic and manned missions. • Provide an enhanced assessment of year-to-year atmospheric mass exchange between the atmosphere and polar caps and regolith. • Determine the mineralogic composition of the surface and its thermophysical properties. Trojan/Centaur Reconnaissance The Trojan/Centaur Reconnaissance mission originally described in the decadal survey should be modified so that NASA informs potential proposers of the kind of science questions that should be answered and does not prescribe how the mission should be accomplished. The mission requirements should also permit orbital encounters and state that a main-belt asteroid flyby is not considered critical to this mission. Such a mission should have the following science objectives (not in priority order): • Determine the physical properties (e.g., mass, size, density) of a Trojan and a Centaur. • Map the color, albedo, and surface geology of a Trojan and a Centaur at a resolution sufficient to distinguish features important for deciphering the history of the object (e.g., craters, fractures, lithologic units). Each of these objectives can be addressed by appropriate imagers, spectrometers, or spectrographs that resolve the target. For example, spacecraft tracking could obtain data on mass, so that a density can be determined. If a This list is culled from several places in the decadal survey. See New Frontiers in the Solar System, pp. 7, 42, and 62. 

SUMMARY OF MISSION-SPECIFIC RECOMMENDATIONS 61 target Centaur has suspected cometary or quasi-cometary activity, then in situ instrumentation capable of address- ing such activity and its constituents should be considered. Asteroid Rover/Sample Return The committee recommends that although the Asteroid Rover/Sample Return mission should be included as a possible mission for the New Frontiers Program, the mission objectives should be changed to reflect new scien- tific information acquired since the decadal survey. Specifically, the unique scientific value of organic-rich targets may elevate them for consideration in comparison with the type of asteroid visited by the Near Earth Asteroid Rendezvous mission as emphasized by the decadal survey. Such a mission should have the following science objectives, which are not prioritized: • Map the surface texture, spectral properties (e.g., color, albedo), and geochemistry of the surface of an asteroid at sufficient spatial resolution to resolve geologic features (e.g., craters, fractures, lithologic units) neces- sary to decipher the geologic history of the asteroid and provide context for returned samples. • Document the regolith at the sampling site in situ with emphasis on, e.g., lateral and vertical textural, mineralogical, and geochemical heterogeneity at scales down to the submillimeter. • Return a sample to Earth in an amount sufficient for molecular (or organic) and mineralogical analyses, including documentation of possible sources of contamination throughout the collection, return, and curation phases of the mission. The committee considers sample return an essential component of this mission, and the inclusion of global mineralogical, geochemical, and textural and in situ imaging/analyses of the regolith differentiates this mission from Discovery-class missions. However, the committee acknowledges that it may not be possible to accomplish both global mapping and in situ regolith characterization within the New Frontiers cost cap. Io Observer An Io Observer mission that addresses fundamental goals for solar system exploration may be possible and consequently should be included in the suite of possible missions included in the next New Frontiers announce- ment of opportunity. The science objectives that could be addressed by an Io Observer mission can include the following (not in priority order): • Determine the magnitude, spatial distribution, temporal variability, and dissipation mechanisms of Io’s tidal heating. • Determine Io’s interior structure, e.g., whether it has a magma ocean. • Determine whether Io has a magnetic field. • Understand the eruption mechanisms for Io’s lavas and plumes and their implications for volcanic processes on Earth, especially early in Earth’s history when its heat flow was similar to Io’s, and elsewhere in the solar system. • Investigate the processes that form Io’s mountains and the implications for tectonics under high-heat-flow conditions that may have existed early in the history of other planets. • Understand Io’s surface chemistry, including volatiles and silicates, and derive magma compositions (and ranges thereof), crustal and mantle compositions and implications for the extent of differentiation, and contribu- tions to the atmosphere, magnetosphere, and torus. • Understand the composition, structure, and thermal structure of Io’s atmosphere and ionosphere, the domi- nant mechanisms of mass loss, and the connection to Io’s volcanism. 

62 OPENING NEW FRONTIERS IN SPACE Ganymede Observer Because the Ganymede Observer was not described in significant detail in the decadal survey, the committee chose to list science questions that such a mission could address, but stresses that this list should not be regarded as exclusive, and other science questions can also be considered for Ganymede. In no case should these science objectives be considered to be mission requirements; they are merely options. • Understand Ganymede’s intrinsic and induced magnetic fields and how they are generated, and characterize their interaction with Jupiter’s magnetic field. • Determine Ganymede’s internal structure, especially the depths to, and the sizes or thicknesses of, the probable metallic core and deep liquid water ocean, and the implications for current and past tidal heating and the evolution of the Galilean satellite system as well as ocean chemistry. • Understand Ganymede’s endogenic geologic processes, e.g., the extent and role(s) of cryovolcanism, the driving mechanism for the formation of the younger, grooved terrain, and the extent to which Ganymede’s tectonic processes are analogs for tectonics on other planetary bodies (both icy and silicate). • Document the non-ice materials on Ganymede’s surface, and characterize in detail the connection between Ganymede’s magnetosphere and aspects of its surface composition (e.g., polar caps). • Document the composition and structure of the atmosphere, identifying the sources and sinks of the atmo- spheric components and the extent of variability (spatial and/or temporal). Innovative Mission Options Scientific understanding of the solar system has continued to advance since the decadal survey was produced; thus, there may be new science to be explored that was not included in the decadal survey but might be viable as the basis for a New Frontiers mission. The committee concluded that NASA’s next New Frontiers announcement of opportunity should not be strictly limited to the eight mission options discussed above, but should also be open to proposals with extraordinary justification and inventiveness. This conclusion was the foundation for the committee’s third recommendation (see in Chapter 2 the section titled “Innovative Mission Options”). The committee stresses, however, its third recommendation’s emphasis that any such mission option should “offer the potential to dramatically advance fundamental scientific goals of the decadal survey, and should accom- plish scientific investigations well beyond the scope of the smaller Discovery Program.” CLOSING COMMENTS As the committee affirms at the beginning of this report, the New Frontiers Program is valuable and is a vital part of NASA’s solar system exploration program. It combines the strengths of both flagship and Discovery-class missions—the strategic emphasis in the flagship missions, which take their direction from the decadal survey, with the competition and innovation fostered by the Discovery missions. The committee’s ultimate goal is to provide NASA with sufficient options, and also to provide potential proposers with sufficient flexibility in their propos- als, to enable NASA to select a mission that can be done within the constraints of the New Frontiers Program, particularly the cost cap. The committee believes that as long as NASA provides the scientific community with the flexibility it requires, the next round of New Frontiers competition can produce the world-class science that has so far typified this program.