Appendix D

Missions Studied But Not Sent for TRACE

As explained in Appendix C, the science mission concepts considered in this report came from three main sources: (1) missions studied by NASA science definition teams (SDTs); (2) missions selected and studied via NASA’s predecadal mission concept study (PMCS) process, and (3) missions identified by the decadal survey’s panels and prioritized by the steering group. The latter group of nine concepts were studied at the committee’s request by leading design centers (including the Jet Propulsion Laboratory, the Goddard Space Flight Center, and the Johns Hopkins University Applied Physics Laboratory). The complete set of SDTs, PMCS, and decadal mission studies were further prioritized by the steering group and 17 of them were selected for technical risk and cost evaluation (TRACE) by the Aerospace Corporation.

This appendix describes the eight missions that were fully studied to Concept Maturity Level (CML) ~4 to 5 via one of the three mechanisms outlined above but were not selected for TRACE. These missions are the following:

• Venera-D;
• ADVENTS—Assessment and Discovery of Venus’s Past Evolution and Near-Term Climatic and Geophysical State;
• Lunar In Situ Geochronology;
• Vesta In Situ Geochronology;
• MORIE—Mars Orbiter for Resources, Ices, and Environment;
• MOSAIC—Mars Orbiter for Surface-Atmosphere-Ionosphere Characterization;
• CROCODILE—Cryogenic Return of Cometary Organics, Dust, and Ice for Laboratory Exploration; and
• Persephone—Pluto System Orbiter and KBO Flyby.

These missions were not subjected to TRACE analysis because the committee considered them to have lower science merit and/or to be less technically ready than the missions discussed in Appendix C. Each of the above eight missions is described in more detail in the sections below.

VENERA-D

Origin and Rationale

This mission concept study was performed by the Venera-D Joint Science Definition Team at the request of the Russian Space Agency, the Institute for Space Research Institute of the Russian Academy of Sciences, and NASA (Venera-D 2019). The purpose of this study was to define a comprehensive mission to investigate the atmosphere and surface of Venus using an orbiter, a lander, and the NASA-contributed Long-Lived In Situ Solar System Explorer (LLISSE).

Goals

Science goals of Venera-D address key outstanding questions related to Venus’s atmosphere and surface. The goals of Venera-D’s orbiter are, in priority order, to:

• Study the dynamics and nature of superrotation, radiative balance, and nature of the greenhouse effect;
• Characterize the thermal structure of the atmosphere, winds, thermal tides, and solar-locked structures;
• Measure the composition of the atmosphere; study the clouds, their structure, composition, microphysics, and chemistry;
• Study the composition of the low atmosphere and low clouds, study surface emissivity, and search for volcanic events on the night side; and
• Investigate the upper atmosphere, ionosphere, electrical activity, magnetosphere, the atmospheric escape rate, and solar wind interaction.

The goals of the Venera-D’s lander and LLISSE are to:

• Measure elemental and mineralogical abundances of the surface materials and near subsurface (~a few cm), including radiogenic elements;
• Study the interaction between the surface and atmosphere;
• Investigate the structure and chemical composition of the atmosphere down to the surface, including abundances and isotopic ratios of the trace and noble gases;
• Perform direct chemical analysis of cloud aerosols;
• Characterize the geology of local landforms at different scales;
• Study variation of near-surface wind speed and direction, temperatures, and pressure (LLISSE).
• Measure incident and reflected solar radiation (LLISSE);
• Measure near-surface atmospheric chemical composition (LLISSE); and
• Detect seismic activity, volcanic activity, and volcanic lightning.

Implementation

The mission consists of an orbiter and a 1980s-vintage Soviet VEGA-type lander equipped with modern instrumentation and LLISSE, an independent NASA-provided meteorological, radiation, and compositional instrument package designed to survive on the surface for approximately 3 months. Possible augmentations include subsatellites, additional small long-lived landers, and various types of airborne platforms.

The Venera-D spacecraft (orbiter and landers) is designed for launch on an Angara-5 rocket. After entering a 24-hour, near-polar orbit around Venus and deploying the landers, the orbiter conducts its own investigations and acts as a communications relay with Earth. The operating lifetime of the orbiter is about 3 years.

Following release from the orbiter, the lander will sample the atmosphere and image the surface during descent to a landing site in a high-latitude region of the northern hemisphere. The lander includes a thermally insulated titanium pressure vessel with thermal storage batteries and is designed to operate for about 3 hours after landing.

Mission Challenges

This mission has not yet received full support from the Russian Space Agency to begin development. The comprehensive nature of the mission and the multiple, as yet undefined, enhancements and international contributions will require substantial coordination.

Conclusions

Venera-D is designed to meet the highest-priority science objectives for Venus. This mission goes far beyond the currently selected VERITAS, DAVINCI, and Envision missions by landing and conducting in situ science on the Venus plains. Continuing support for its definition and development is required if this joint U.S.–Russian activity is to be implemented.

ADVENTS

Origin and Rationale

This concept was proposed by the decadal survey’s Panel on Venus, and the study (CML ~4–5) was conducted at NASA’s Goddard Space Flight Center in the early months of 2021 (NASA 2021a). This mission explores whether a medium-class mission addressing aspects of the Venus Sub-Cloud Aerobot, Venus Life Potential, and Venus Investigations of Dynamics from an Equatorial Orbit proposals (see Appendix E) could be formulated by descoping the large lander, radar, and SmallSats from the Venus Flagship mission (see Appendix C).

Goals

The goals of this concept are to:

• Understand how Venus formed and evolved for comparison to other rocky planets and exoplanets;
• Study the potential past habitability of the Venus surface; and
• Determine the composition, dynamics, and potential habitability of the present-day atmosphere of Venus.

Implementation

ADVENTS will deliver an orbiter and a variable-altitude aerobot to Venus. Synergistic instruments on both platforms collect in situ and remote measurements of the atmosphere, along with remote sensing of the surface, for at least 60 Earth-days. A single dropsonde is also deployed by the aerobot to sample the chemistry of the atmosphere from the clouds to the surface. The orbiter is placed in a 12-hour period equatorial orbit and conducts its own observations and acts as a communications relay between the aerobot and Earth. Instruments on the orbiter include near-infrared surface and cloud imagers, a magnetometer, an extreme ultraviolet monitor, and a radio occultation package. Instruments on the aerobot include an aerosol mass spectrometer with nephelometer, a tunable laser spectrometer, a magnetometer, a meteorological package, and the dropsonde.

Mission Challenges

The ADVENTS mission is complex, and faces mass and cost challenges associated with delivering to Venus and deploying an aerobot and a dropsonde within the medium-class mission cost cap. The need for a dedicated orbital relay element also represents a considerable mission cost.

Conclusions

ADVENTS can address many of the high-priority science objectives for Venus not covered by the VERITAS, DAVINCI, or Envision missions. However, ADVENTS overlaps with some objectives of the VISE mission, resulting in it being given a low priority for a TRACE study. Continued development of numerous Venus-relevant technologies—including atmospheric entry, balloon altitude control, and instrument miniaturization—would help enable the ADVENTS concept to meet the constraints of a medium-class mission.

LUNAR IN SITU GEOCHRONOLOGY

Origin and Rationale

This concept was proposed in response to the PMCS announcement of opportunity, and the study (CML ~4–5) was conducted at NASA’s Goddard Space Flight Center (NASA 2020a). The purpose of the study was to determine the absolute age of a selected location on the Moon. This concept was developed as a part of a larger effort to study the absolute dating of features on various planetary bodies including Mars (see Appendix C), Vesta (see below), and the Moon. The relative dating of features on a planetary body can be determined by applying the basic principle that younger features overlay those that are older. Similarly, a more heavily cratered surface is likely to be older than an adjacent surface with fewer craters. Turning these relative ages into absolute dates requires the isotopic analysis in terrestrial laboratories of carefully selected samples from key geological features. While the Apollo and former Soviet Union’s Luna missions returned samples to Earth, as will planned human exploration missions, the number of scientifically significant terrains far exceed any currently conceivable ability to collect and return samples to terrestrial laboratories for geochronological analysis. This concept explores the idea that technology may have advanced sufficiently to perform geochronological analysis in situ using a robotic spacecraft.

Goals

The goals of the lunar version of the in situ geochronology mission concept are to:

• Establish the chronology of basin-forming impacts by measuring the radiometric age of samples directly sourced from the impact melt sheet of a pre-Imbrian lunar basin. In situ dating of an impact-melt sheet of a lunar basin thought to be significantly older than the Imbrium basin would place it either within the canonical cataclysm (3.9 Ga) or as part of a declining bombardment in which most impacts are 4.2 Ga or older.
• Establish the age of a very young lunar basalt to correlate crater size–frequency distributions with crystallization ages. In situ dating would reduce the uncertainty in absolute model ages derived from crater size–frequency distribution measurements to no more than 20 percent of the current uncertainty associated with different lunar chronology functions.

Implementation

This proposed medium-class mission relies on a static lunar lander designed to launch on a Falcon 9 Heavy Recoverable. Once placed in lunar transfer trajectory, the lander would enter polar lunar orbit, overfly the selected landing site, deorbit, and land. The lander relies on solar power, limiting its operations to daytime and relying on batteries to survive the lunar night. The design life of the mission is 12 months. Two different on-board instruments—one examining rubidium and strontium isotopes and the other potassium and argon isotopes—are used to determine independent age estimates. Sampling is accomplished by a pneumatic acquisition system and associated sample-handling and sample-preparation subsystems. Additional instruments carried include an imaging spectrometer and cameras to document the geological context of the landing site and to characterize the samples themselves and a trace-element analyzer to augment sample contextualization.

Mission Challenges

A lack of mobility in this mission concept leads to uncertainty in the ability to obtain the needed samples to provide accurate dating of key events, particularly in geologically complex terrains. Top science objectives associated with dating ancient impact basins may be particularly challenging. Lunar rock samples may display a complex collection of mineral phases, which even in the laboratory require careful separation before being analyzed for their isotopic composition. As such, it is not clear that the in situ capabilities for sample selection and phase separation would produce accurate dates for basin-forming impacts, and the return of samples via the Endurance-A mission was considered a scientifically superior strategy to address this highest-priority science. Dating young lunar basaltic terrains may be more favorable for in situ analysis, but this was judged to have less scientific importance than other medium-class missions.

Conclusions

In situ geochronology is a promising but still relatively new technique, and demonstration of its ability to produce meaningful ages from a potentially limited choice of geologically complex lunar samples is needed. A static lander exacerbates such challenges.

VESTA IN SITU GEOCHRONOLOGY

Origin and Rationale

This concept was proposed in response to the PMCS announcement of opportunity, and the study (CML ~4–5) was conducted at NASA’s Goddard Space Flight Center. The purpose of the study was to determine the absolute age of two selected locations on the main belt asteroid Vesta. This concept was developed as a part of a larger effort to study the absolute dating of features on various planetary bodies including Mars (see Appendix C), the Moon (see above), and Vesta (NASA 2020a). While the relative dating of features on a planetary body can be determined by techniques such as crater counting, determining the actual age of specific features requires radiometric studies of samples of known geological context. While the so-called HED meteorites originated on Vesta, their context is unknown and thus, of no use to dating specific features on this large asteroid. Unfortunately, there are no current plans to return samples from Vesta via robotic or other means.

Goals

The goal of the vestan version of the in situ geochronology mission concept is to determine the radiometric ages of vestan samples from locations whose geological context is known. In situ dating would constrain Vesta’s geologic timescale by dating key stratigraphic craters and adjacent geologic terrains. Given the large disagreement in ages derived by various indirect methods, a few absolute dates would not only reveal the ages of key basins but would also set firm constraints on impactor flux estimates, used throughout the asteroid belt to establish relative dates via the crater counting technique.

Implementation

This proposed medium-class mission is conceptually similar to, and carries the same instrumentation, as the lunar in situ geochronology concept (see above). The principal difference is that here the lander is not static: Vesta’s low gravity enables the spacecraft to hop once so that a second landing site can be examined.

Following launch on a Falcon 9 Heavy Expendable rocket, and a 49-month journey, the lander enters a 250 km altitude circular orbit. After 6 months of orbital mapping, the lander descends to the first of its two landing sites. Following 142 days of surface operations, the lander retracts its large solar arrays and hops to a second site several hundred km distant. Once at the new site, the lander redeploys its solar panels and begins a second set of surface operations for another 142 days.

Similar to the lunar dating mission (see above), two different on-board instruments—one examining rubidium and strontium isotopes and the other potassium and argon isotopes—are used to determine independent age estimates. Sampling is accomplished by a pneumatic acquisition system and associated sample-handling and sample-preparation subsystems. Additional instruments carried include an imaging spectrometer and cameras to document the geological context of landing sites and to characterize the samples themselves, and a trace-element analyzer to augment sample contextualization.

Mission Challenges

The PMCS report identified four major challenges. First, mission-enabling payload and lander technologies have not yet reached TRL 6. Second, the best highest-resolution images of Vesta are too coarse to confirm that a suitable landing site exists prior to the spacecraft’s arrival. Challenges associated with the addition of landing algorithms, terrain-relative navigation, and other related capabilities could drive the mission concept design and approach. Third, landing and operating at multiple sites is a complexity that could drive the design and operations. Fourth, a reliable sample acquisition and distribution system is necessary to achieve the science goals and drives the mission design. In addition, there remain uncertainties associated with whether a set of in situ isotopic measurements of a limited suite of samples can be confidently interpreted to represent an age.

Conclusions

The concept is notable for being a versatile medium-class mission that, with technology maturation, would advance in situ instrumentation. However, the science addressed by returning to Vesta was deemed to be narrower and more focused in comparison to other missions considered that would visit unexplored targets or return samples. Thus, other medium-class missions under consideration were evaluated as higher science priorities for the limited TRACE opportunities.

MORIE

Origin and Rationale

This concept was proposed in response to the PMCS announcement of opportunity, and the study (CML ~4–5) was conducted at the Jet Propulsion Laboratory. The purpose of this mission is to address when, where, and how water has modified the martian surface through time.

Goals

The goals of the mission are to (NASA 2020b):

• Determine when elements of the cryosphere formed and how ice deposits are linked to the planet’s ancient, recent, and current climate;
• Explore the evolution of surface environments and their transition through time; and
• Prospect for in situ resources necessary to support future human activities on the surface.

Implementation

This medium-class mission is designed to conduct the first synthetic aperture radar imaging from Mars orbit, fine-scale radar sounding of the depth to buried ice, compositional mapping of outcrops, monitoring ice and dust aerosols, and surface mapping at 1 m resolution. The orbiter would use solar-electric propulsion (SEP) to enable a 2-year journey to Mars and allow it to enter a entered a Sun-synchronous orbit crossing the equator at 3:00 p.m. local solar time. After one martian year, the orbit’s inclination would be changed from 92.7 degrees to 90 degrees to enable radar sounding of previously unobserved portions of Mars’s polar caps.

The spacecraft’s instrument complement includes the following: full polarization, ultra-high frequency synthetic aperture radar; a dual-band ice-sounding radar; a high-resolution, multiband imager; shortwave and thermal infrared imaging spectrometers, dual stereo cameras, and a wide-angle multispectral imager.

Mission Challenges

The mission had no novel risks or technical challenges; accommodation of the large radar imager and sounder and their power and data needs drove the SEP implementation choice. The full mission design achieves the optimal science at a cost (per the PMCS estimate) commensurate with implementation as a medium-class mission.

Conclusions

MORIE science was deemed high priority. Given the initiation of iMIM by NASA as a directed mission (with its objective of ice mapping for in situ resources), which was announced after the PMCS was completed, MORIE may be better positioned as a follow-on to iMIM or elements of MORIE may be incorporated into iMIM to achieve Mars cryospheric science objectives.

MOSAIC

Origin and Rationale

This concept was proposed in response to the PMCS announcement of opportunity, and the study (CML ~4–5) was conducted at the Jet Propulsion Laboratory (NASA 2020c). The purpose of this mission is twofold: first, to understand the processes determining Mars’s contemporary climate and, in particular, to delineate the interconnections between Mars’s surface, upper and lower atmospheres, and ionized environment; and second, to identify hazards, characterize resources, and demonstrate technologies that might enable future human exploration activities on Mars.

Goals

The scientific questions this mission is designed to address are as follows:

• How do volatiles move between the subsurface, surface, and atmosphere?
• How does the martian lower-middle atmosphere respond diurnally, on meso- and global scales, to the seasonal cycle of insolation?
• How does coupling from the lower atmosphere combine with the influence of space weather (i.e., solar wind, solar energetic particles and solar extreme ultraviolet) to control the upper atmospheric system and drive atmospheric escape?

In addition, this concept is designed to address the following questions relating to the human exploration of Mars:

• How, where, and when can future astronauts access extractable water ice resources?
• With what degree of accuracy can martian weather be forecast, for operational purposes?
• How will mesospheric and thermospheric winds affect aerobraking spacecraft?
• How will space weather effects on the martian ionosphere affect surface–surface and surface–orbit communications?
• How will energetic particle radiation affect astronauts in Mars orbit?
• Can reliable high-bandwidth Earth–Mars communication be maintained?

Implementation

This large-class mission is designed to conduct simultaneous and systematic observations of the martian climate via eight science investigations carried out by 22 unique science instruments, hosted on 10 individual spacecraft. All are launched on a single Falcon Heavy Recoverable launch vehicle. The 10 spacecraft are deployed into three different orbits about Mars: low, near-polar Sun-synchronous; inclined elliptical; and areostationary. The Sun-synchronous orbits enable vertical profiling of wind, aerosols, water, and temperature, as well as mapping of surface and subsurface ice. The elliptical orbits sampling all of Mars’s plasma regions enable multi-point in situ measurements necessary to understand mass/energy transport and ion-driven escape. The areostationary orbits enable synoptic views of the lower atmosphere, global views of the hydrogen and oxygen exospheres, and upstream measurements of space weather conditions.

Mission Challenges

MOSAIC is very clearly a large-class mission addressing multiple questions with a complex array of instruments deployed on 10 orbiters. Portions that might be flown as descoped constellations or stand-alone missions still may be too costly for consideration as medium-class missions. Moreover, such descope would lose the advantage of simultaneous, multi-point measurements at/from what was prioritized by the concept study team.

Conclusions

Given that the Mars Sample Return campaign is now under way, programmatic balance considerations argue against prioritization of another large-class Mars mission at this time. Considerable portions of the science may also be achievable within lower cost implementation choices, albeit with loss of synchronicity.

CROCODILE

Origin

This concept was proposed by the decadal survey’s Panel on Small Solar System Bodies, and the study (CML ~4–5) was conducted at NASA’s Goddard Space Flight Center in the early months of 2021 (NASA 2021b). The rational for the mission is to determine if the return of cryogenic samples of cometary material is feasible within the scope of a medium-class mission in the next decade. The concept explores the possibility of rendezvousing with a Jupiter-family comet, mapping its nucleus, selecting an optimal sampling site, sampling the nucleus below the surface, and returning cryogenically preserved material to Earth for laboratory analysis.

Goals

The overall science goal of the mission is to assess the elemental, isotopic, and structural composition of the volatile, organic, and inorganic components of a comet nucleus to address the following issues:

• The compositional reservoirs present in the early solar system;
• The role of comets in the delivery of water and organic molecules to the early Earth, terrestrial planets, and satellites; and
• The evolutionary processes spanning from the protoplanetary disk to current cometary activity.

Implementation

The study focused on sampling Comet 67P/Churyumov-Gerasimenko because of its suitable distance from Earth and its previous characterization by ESA’s Rosetta spacecraft. Following launch on a Falcon Heavy Recoverable, the baseline mission envisages a solar-electric powered spacecraft undertaking a 5.7-year cruise to 67P.

Following arrival, 4.1 years is devoted to the selection of sampling sites and sample-collection campaign. The return cruise to Earth takes another 5.7 years.

The spacecraft’s payload consists of a suite of infrared imaging spectrometers and cameras, a sample collection system, cryogenic sample Dewars, and the Earth Entry Vehicle. The Dewars containing solid argon are maintained at the desired temperatures by two-stage Stirling cryocoolers. A sample return temperature of 120 K was selected to preserve amorphous water ice and entrained volatiles, as well as water-soluble organics and salt. Two ~1–100 g samples are collected from ~25 cm below the surface of 67P’s nucleus via the so-called shoot-and-go technique using a harpoon while the spacecraft remains 5–10 m above the surface.

Mission Challenges

The mission study report identified four major technological challenges: (1) operation of the cryocooler over the full 15.5-year mission duration; (2) confirmation of the successful collection of a small sample, possibly only ~1 g; (3) multiple aspects of the cryogenic sample acquisition and storage systems: for example, heat transfer associated with sampling and retrieval systems, the design and testing of long-term cryogenic storage assemblies and mechanisms, and long-life automated cryogenic seals; and (4) the high power requirements of the cryocooler and the solar-electric propulsion systems.

Conclusions

While the concept can address key science questions, the mission concept study suggested that overcoming the identified technical challenges pushed the estimated cost of the spacecraft beyond that for a medium-class mission.

PERSEPHONE

Origin and Rationale

This mission concept was proposed in response to the PMCS announcement of opportunity, and its study (CML ~4–5) was conducted at the Applied Physics Laboratory (NASA 2020d). Persephone was designed to address several key questions arising from the results returned by the New Horizons mission to Pluto and the Kuiper belt.

Goals

Persephone is designed to address the following primary questions:

• What are the internal structures of Pluto and Charon, and what is the evidence for a subsurface ocean on Pluto?
• How have surfaces and atmospheres in the Pluto system evolved?
• How has the KBO population evolved?

In addition, the mission could address the following secondary questions:

• What is Pluto’s internal heat budget?
• What is Charon’s magnetic field environment?
• How do KBOs and the heliosphere interact?

Implementation

This large-class mission is designed to launch on an SLS Block 2 with a high-energy upper stage. A combination of a Jupiter gravity assist (in the early 2030s or early 2040s) and a radioisotope electric propulsion system enables Persephone to put itself on a 28-year-long trajectory to the Pluto system. Some 19 years after launch,

the spacecraft will encounter a 50–100 km diameter KBO. Once at Pluto, the spacecraft conducts a 3-year tour of the dwarf planet and its five known satellites. An optional extended mission, following the completion of the Pluto-system tour, could enable Persephone to encounter another 50–100 km diameter KBO some 36 years after launch. The spacecraft is equipped with 11 instruments including the following: plasma and spectrometers; panchromatic and color narrow-angle, panchromatic wide-angle, and thermal infrared cameras; near-infrared and ultraviolet spectrometers; a subsurface radar sounder; a laser altimeter; and a radio science system.

Mission Challenges

The 30+-year prime mission duration and the requirement for five next-generation radioisotope power systems strongly suggest that this concept’s requirements are beyond the likely acceptable parameter range for implementation in the near to mid-term. Reliance on an SLS Block 2 also added significant risk. Moreover, detection of an internal ocean at Pluto is not straightforward because of the absence of a strong time-varying background magnetic field that would be certain to produce an inductive field in a conductive ocean, and the lack of time-varying tidal forcing owing to Pluto and Charon’s dual synchronous state.

Conclusions

A Pluto orbiter mission is technically much more challenging than a flyby. Although determining whether Pluto has an ocean would be of great geophysical and astrobiological interest, the associated technical challenges and extremely long mission lifetime led to it being rated less highly than other large-class missions considered.

REFERENCES