4

Prioritizations and Rankings to Optimize and Enable the Expansion of Deep Space Human Exploration

NASA’s goal of moving beyond low Earth orbit (LEO), when considered against the current timeline and plans for the International Space Station (ISS), creates a sense of urgency for science accomplishment in support of deep space exploration. As generally acknowledged by NASA and within the broader microgravity community, and as described in examples given in Chapter 3, there is not sufficient research time on the ISS to fully explore all aspects of the 2011 space life and physical sciences decadal survey¹ priorities within the station’s planned lifespan. To meet the task of identifying the highest-priority recommendations as targeted research priorities, the committee considered the decadal survey recommendations within the context of ISS implementation plans, while utilizing the ranking criteria from the decadal survey. In addition to those criteria, the committee also considered the progress in microgravity research and programmatic considerations, together with available microgravity platforms, to produce a specific set of targeted research priorities to meet the committee’s statement of task. Once ranked, those targeted priorities were then considered within the context of the broader and general advances in relevant scientific disciplines. The committee recognized that budget limitations play a key role in the need for identifying top priorities for exploration.

4.1 TARGETING DECADAL SURVEY EXPLORATION RESEARCH PRIORITIES

4.1.1 The Decadal Survey Approach to Organizing Priorities

The 2011 decadal survey was carried out by the Committee for the Decadal Survey on Biological and Physical Sciences in Space (survey committee), with prioritized disciplinary recommendations determined by seven supporting disciplinary panels, with one member of the survey committee also serving as a member of each panel.

Each panel identified and prioritized program elements that were especially important to their area of coverage and reported this prioritized list to the survey committee. The survey committee took the highest priorities of all seven panels and characterized each individual panel recommendation according to whether it enabled space exploration and/or was enabled by space exploration. Given that there were several NASA exploration policy options that were undecided, the survey committee further ranked each of these high-priority program elements

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¹ National Research Council, 2011, Recapturing a Future for Space Exploration: Life and Physical Sciences Research for a New Era, The National Academies Press, Washington, D.C.

as “high,” “medium,” or “low,” according to its degree of relevance to each of eight possible prioritization criteria. Those results were tabulated in tables within Chapter 13 of the 2011 decadal survey report. Among the eight criteria, several were particularly relevant to exploration needs.

The statement of task for this midterm assessment study specifically requested further prioritization only for those decadal survey recommendations that were determined to be the highest priority for exploration, and therefore, it was important that the current committee consider the current landscape of exploration options being discussed within NASA.

4.1.2 Overarching Space Exploration Strategy

The overall space exploration strategy continues to evolve with the recently announced Deep Space Gateway. Figures 4.1 and 4.2 lay out the current top-level NASA Exploration Strategy and Reference Phase 1 plan. NASA’s space exploration strategy is intended to be flexible and account for technical lessons learned from expanding into deep space and findings from microgravity research investigations. As NASA progresses along its planned strategy, it might find it helpful to implement a periodic review process to address known and potential exploration strategy changes and to develop options to react to such changes. Results of the ISS post-2024 strategy and the exploration progress over time could be incorporated into these periodic reviews.

The 2011 decadal survey developed “Bounding Policy Options” to assess the research priorities. Also, the NASA exploration strategy has evolved over time from the “Vision for Space Exploration,” including the Constellation Program, to the “Journey to Mars,” with the Asteroid Redirect Mission and the Space Launch System and Orion, to the current “Deep Space Gateway” strategy. It is clear from this history that NASA’s exploration strategy can be a changing and moving target. NASA works diligently to align the resources and priorities based on its strategy at the time. It is also clear to the committee that such changes can, and will, impact the criteria and priorities for research. For each strategy, the detailed requirements found in a Design Reference Mission are a useful way to guide research priorities and to assess potential design options based on what is learned from the research.

Finding 4-1: Several exploration pathways have been proposed by NASA since the 2011 decadal survey, involving the Moon, asteroid missions, Mars, and cislunar space. NASA has also used a separate set of Design Reference Missions to develop its Technology Roadmaps. These frequent exploration strategy changes result in unclear traceability between research investigations and exploration needs.

4.2 CRITERIA FOR MIDTERM RANKING OF THE HIGH-PRIORITY RECOMMENDATIONS FOR EXPLORATION

As noted above, Chapter 13 of Recapturing a Future for Space Exploration laid out a specific set of criteria that were used to categorize research along the continuum of basic, applied, or translational science within that document. Those decadal survey criteria are reprinted in Appendix E. In order to address the charge for this report, the committee made extensive use of these criteria to assess how relevant the high-priority recommendations were to exploration—specifically, by giving weight to those criteria that most prominently support the exploration agenda. As such, the committee considered Criteria 1, 2, 3, and 5 (listed below) to be the most directly relevant to exploration needs. These criteria were used in the decadal survey to rank those recommendations that were a high priority for enabling exploration. The midterm committee respected the rankings in the decadal survey and determined that these criteria still accurately capture the bulk of the principles that most enable the exploration agenda. These criteria best distinguish the science that is needed for exploration from the science that is enabled by microgravity.

The following prioritization criteria from the 2011 decadal survey guided the top-level identification of targeted research priorities for exploration for this midterm assessment:

• Prioritization Criterion 1: The extent to which the results of the research will reduce uncertainty about both the benefits and the risks of space exploration (positive impact on exploration efforts, improved access to data or to samples, risk reduction).
• Prioritization Criterion 2: The extent to which the results of the research will reduce the costs of space exploration (potential to enhance mission options or to reduce mission costs).
• Prioritization Criterion 3: The extent to which the results of the research may lead to entirely new options for exploration missions (positive impact on exploration efforts, improved access to data or to samples).
• Prioritization Criterion 5: The extent to which the results of the research are uniquely needed by NASA, as opposed to any other agencies (needs unique to NASA exploration programs).

The midterm committee also recognized the need to overlay the following additional and overlapping considerations that most directly affected the current feasibility or utility for exploration of the decadal survey recommendations:

• Consideration A: The extent to which the science is needed to support technology development for deep space exploration.
• Consideration B: The extent to which a long development lead time is required for the science to enable a needed technology.
• Consideration C: The extent to which the research question can only be adequately studied on the ISS or takes best advantage of remaining ISS time.
• Consideration D: The extent of urgency or criticality to a specific exploration goal, technology, or biomedical countermeasure.
• Consideration E: The extent to which the science may now be complete.

The midterm committee further recognized that it is not possible to quantify or objectively measure these considerations. Therefore the commmitee comprehensively discussed the science outputs from the discipline areas and consulted the literature and various NASA reports to apply these considerations in a deliberative process of additional prioritization. Some level of informed subjectivity was expected and even encouraged, to allow for specific science and program expertise to play a role in the prioritization within and across disciplines. A summary of the selection process and outcomes is presented in Figure 4.3.

4.3 HIGHEST-PRIORITY RECOMMENDATIONS FOR EXPLORATION

As directed by the statement of task, the committee drew strictly from recommendations that were listed in Table 13.1 the 2011 decadal survey. The committee used the decadal survey “Bounding Policy Option” of Mars Exploration to most directly address the statement of task, and to stay within the priorities identified in the decadal survey. All recommendations that were listed in Table 13.1 of the decadal survey report were further discussed in the context of their placements in Tables 13.2 and 13.3. Tables 13.1, 13.2, and 13.3 are reprinted in Appendix E. Then, based on the committee’s decadal survey criteria and additional considerations as described above, the committee assigned a relative ranking of each of the recommendations, noting that these rankings considered the most important science needs in the pathway to exploration of deep space. The committee also considered whether it was necessary that the science of each recommendation be conducted in LEO or beyond LEO, and if the research could be conducted in ground-based or suborbital platforms. The results are summarized in Table 4.1, which lists

TABLE 4.1 Ranking and Performance Environment of Research Priorities for Deep Space Exploration in the 2011 Decadal Survey^a

^a See National Research Council, 2011, Recapturing a Future for Space Exploration: Life and Physical Sciences Research for a New Era, The National Academies Press, Washington, D.C.

^b These brief descriptions cannot adequately capture the sense of the decadal survey recommendation, and readers are referred to the more complete descriptions of the recommendations in the decadal survey and in Chapter 3 of this report.

^c An S indicates that the location would be supporting for addressing some or all of the experiments needed to address the research priority. An N indicates that the location is likely to be necessary for completing some or all of the experiments that would be involved in addressing the priority.

^d Beyond low Earth orbit includes both open space and the surfaces of other planetary bodies and asteroids.

^e Translational priorities represent areas of physical sciences research identified in the decadal survey that support the TSES technology category. Thus TSES14 includes such research areas as liquid-metal cooling in reduced gravity.

all of the high-priority recommendations of the 2011 decadal survey, along with the committee ranking identifying the higher- and highest-priority recommendations for exploration. In addition to this ranking, Table 4.1 summarizes the committee’s assessment of the locations of the research activities to accomplish each of the science recommendations. The details of the committee’s assessments within each individual discipline area, which informed the rankings in this table, can be found in the following sections of this chapter.

4.4 SCIENCE CONTEXT FOR THE RANKINGS

Each of the targeted priority rankings in Table 4.1 of this chapter arises from the context of current science accomplishments in areas that include, but reach beyond, NASA exploration. Each priority also stems directly from science needed to enable and enhance exploration of deep space. The committee therefore examined the progress in major discipline areas in order to establish a scientific context for the stated NASA plans for the ISS and the development of deep space exploration. This broader scientific context, further informed by the roles of the discipline in human exploration, serves as the basis for identifying and ranking the highest priorities for remaining years of the current decadal survey. This context also provides an overview of the progress of microgravity science compared to related fields outside of exploration-related science.

4.4.1 Plant Biology and Microbiology

Since the decadal survey was published in 2011, there has been rapid progress in several areas of plant and microbial biology, in particular great decreases in expense, and increases in power, of genomic and metagenomic sequencing, and in transcriptomic analysis by arrays and RNASeq for gene expression, genomic sequencing and assembly, and ChIP-seq for understanding regulatory processes. In the microbial area, this same set of methods has empowered metagenomic sequencing, which ties metabolic function to genomes and could be of great value in assessment of the bacterial and viral environment of the space environment, including the water, air, and surface growth of bacteria, and the microbiome of the astronauts, and of the plants grown for life support. One additional area that is under rapid development is genome modification (of bacteria, plants, and animals) using novel, bacterially based systems such as the CRISPR-Cas9 system. This allows large-scale and accurate engineering of genomes, a capability which could be of importance not only in experimental design and controls, but also in engineering organisms adapted to spaceflight conditions for bioregenerative and nutritional support of flight crews. An example of this would be the engineering of vegetables such as lettuce and tomatoes to reduce their response to environmental stress signals such as ethylene, a plant stress and fruit ripening/spoilage hormone that emanates from plastics and is generated by microorganisms present in spaceflight environments.

4.4.1.1 Role in Human Exploration

Plant and microbial biology are key components of the NASA exploration strategy due to the integral roles of microbes and plants within almost any human ecosystem. Plants provide the connection between humans and terrestrial nature, while also completing the life-support cycle by recycling air, water, and nutrients. And while NASA relies heavily on physio-chemical life support systems, plants continue to be envisioned as part of the regenerative life-support systems while potentially providing psychological support to the crew. A consistent body of research has demonstrated the positive psychological benefits of interacting with plants;² further, self-reports by astronauts have borne out the salutogenic, health-enhancing effects of the presence and maintenance of plants.³

Microbes inhabit all plant and human ecosystems, including exploration vehicles and habitats. The relevance of microbial life to long-term space missions such as those to Mars is two-fold: first, organisms that cause disease

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² J. Stefan, N. Gueguen, and S. Meineri, 2015, Influence of the interior and outdoor plants on health: Synthesis of research, Canadian Psychology 56(4): 405-425.

³ J. Bennington-Castro, 2014, “Up on the Farm? Five Reasons NASA Needs Space Greenhouses,” National Geographic, January 21, http://news.nationalgeographic.com/news/2014/01/140121-space-greenhouses-plants-astronaut-mars.

in plants or humans need to be monitored and controlled during the flight as a preventive contribution to astronaut health and for diagnosis of disease. Secondly, the nondisease-causing microbes that constitute important microbiomes of plants, humans, and potentially mutual microbiomes, are both contributors to, and indicators of, the state of the health of the system. Therefore these microbes will need to be monitored and understood in order to encourage beneficial microbiomes.

The decadal survey called out three major and specific areas for prioritized study applicable to the exploration goals of NASA: P1—multigenerational studies of microbial populations, P2—analysis in space of plant and microbial growth and of the physiological responses of plants and microbes to the spaceflight environment, and P3—a research program to demonstrate the roles of microbial-plant systems in long-term life support. As previously described in Chapter 3, NASA and its research partners have achieved much science in these three areas in the first 5 years since the decadal survey.

4.4.1.2 Plant and Microbial Biology Research Platforms and Approaches Relevant to Exploration

NASA exploration plans are reliant on transit vehicles without large-scale artificial gravity. The Gateway habitat, for example, will be in a continual microgravity state. Therefore, microgravity phenomena and the associated biological effects of microgravity remain key objectives for study even in the deep space era. Therefore, access to long-term LEO microgravity will be necessary for continued progress. Certain microgravity phenomena will have to be conducted in deep space, particularly any biological phenomena that are impacted by deep space radiation, making deep space research important for areas P2 and P3. In all cases, control experiments and testing of experimental protocols can be done on the ground, using Earth analogs that can inform the deep space experiments. The necessary facilities are available, and such needs are easily met with existing equipment and access to short-term microgravity in drop towers and suborbital flights.

Research associated with radiation and radiation effects on biological systems are expected to be a strong element of research conducted in space beyond LEO. The impacts of deep space radiation on biology have not been well explored in general, and with plant and microbial systems in particular. Deep space science would need to include best practice approaches to analyzing the genomic and physiological effects of deep space radiation on plants and microbes. For plants, the important areas would be the long-term viability and genomic stability of seeds that accompany deep space trips. For microbes, the important areas would include the effects of deep space radiation on the evolution of microbes and microbial systems. Long-term exposures on the Gateway, even during times without human habitation, would be an important part of the deep space biology portfolio.

Plant growth systems are envisioned as providing radiation protection to crew, particularly when the plant growth systems are arrayed on the vehicle walls. As part of the wall system, the plants and their water would provide shielding for the crew. However, the effects of such radiation on plant and plant/microbial interaction physiologies are largely unknown and would need priority assessment.

The decadal survey specifically called out the aggressive use of data-rich omics studies as part of this area of research. Much of the success and achievement in microbial and plant space biology has been through the directed application of omics technologies in alignment with the decadal survey recommendations. Examples are the GeneLab and Microbial Observatory initiatives within NASA and the ISS. The further advancement of omics technologies, processes, and instrumentation has driven plant and microbial space biology toward the capability of on-board omics analyses for rapid real-time evaluations on one hand, while sample storage technologies have also enabled the archiving of samples for detailed study once samples are returned from spaceflight.

A strong example of NASA commitment to supporting microgravity research resides in the facilities at its centers. For example, the NASA Kennedy Space Center (KSC) flight payload support group conducts integrated tests and performs near real time or post-flight ground-control experiments using ground unit hardware within the ISS environmental simulation (ISSES) chambers available at KSC. The ISSES chambers are configured to mimic those environmental conditions of temperature, relative humidity, light exposure, and CO₂ concentrations that the flight hardware experiences on the ISS. Additionally, KSC is in the process of establishing a suite of microgravity simulators that allow researchers the option of performing micro-, partial-, and hypergravity ground controls. The ISSES chambers and microgravity simulation facility are all part of a large and well-equipped infrastructure

within the Space Station Processing Facility (SSPF) at KSC. The SSPF has payload integration support facilities, off line laboratories for principal investigators (PIs), office spaces, ISS experiment-monitoring areas, laboratory support and equipment.

4.4.1.3 Recent Advances Relevant to Plant and Microbial Microgravity Science

Among the DNA advances are development of rapid DNA sequencing methods that use equipment suited to the spaceflight environment. Next-generation sequencing has been developed to the point where sequence analysis in space is practical and recently demonstrated. This provides a path to diagnosis of infectious human and plant disease, and toward real-time, in-site environmental biological monitoring to identify plant and microbial adaptations, as well as future threats in the form of microbial evolution of host ranges or growth strategies. It also provides methods to monitor crew health through analysis of the gut and skin microbiome of the astronauts, the colonization of vehicle systems by microbes, and plant health within plant growth facilities designed provide bioregenerative life support.

Another advance has been in use of RNA sequencing methods to understand the physiological changes experienced by plants during short-term growth in space, which gives indications of the types of stresses that are relevant to long-term growth. These technologies provide data on plant and microbial responses to spaceflight and can be used to monitor plant health and production of plant products within exploration production systems.

One key action that can be taken to optimize the science value of the research program in context of enabling deep space exploration would be to develop robust methods, standards, and equipment for bacterial and virus monitoring in space. These methods are applicable to all decadal survey recommendations in this area, and are of direct use in space missions. The necessary actions consist of assessment of available sequencing technologies, miniaturization of the equipment, and testing equipment on multiple types of samples in spaceflight, with preserved samples returned to Earth for ground-based validation.

Also important for the further implementation of the decadal survey recommendations is the continued development of the facilities for long-term plant growth, with parallel development of microbial monitoring for plant pathogens and of the plant microbiome to assess (and correct) growth conditions for optimal growth. NASA has a long history of developing plant growth systems for spaceflight applications. Most of those systems have been research directed, in that they were designed for managing and controlling environmental conditions specific to science goals. The Veggie system currently on the ISS is one of the few spaceflight systems that has been designed for actual plant production on orbit. This seemingly subtle differentiation between a research plant growth unit and a production plant growth unit has driven some incredible, exploration support science. Within Veggie, plants have been grown for crew consumption, creating the procedures and methods for repeated plant production on orbit. In addition, these plant production experiments have revealed plant microbial pathologies on the ISS, exposing the need to more deeply understand the plant microbiome as it exists and evolves in an exploration habitat.

Finally, plants could provide a solution to the long-term storage of nutrients, such as vitamins and proteins, as called out in recommendation CC5 of the decadal survey, as growing plants from seeds could provide a freshly synthesized source through the term of a deep-space mission. However, plants have yet to be optimized for growth and production in microgravity spaceflight environments.

4.4.1.4 Highest-Priority Plant and Microbial Recommendations for Exploration

The results from that science have illuminated many space biology science effects, strongly suggesting that spaceflight has significant influence on biological systems and that biology in space is not the same as biology on the ground. Moreover, these results require that continued study of space plant and microbial biology be a priority during the era of deep-space exploration precisely because the results that have been obtained to date indicate that deeper understanding is required to truly make use of plants and microbes for supporting exploration.

The committee considered P2 (analysis in space of plant and microbial growth and of the physiological responses of plants and microbes to the spaceflight environment) and P3 (a research program to demonstrate the roles of microbial-plant systems in long-term life support) to be the highest-priority plant and microbial recom-

mendations that are relevant to the exploration of deep space. P2 and P3 have the largest role in determining the biological adaptations to spaceflight and the understanding of how biological life support systems may function in support of exploration. P1, multigenerational studies of microbial populations, is still considered an important activity, yet this activity has been largely established and has entered a role of continued monitoring rather than of discovering principles needed for exploration. P2 and P3, the study of plant adaptations to spaceflight and the interactions of plant/microbial systems in spaceflight, is still largely in the mode of discovering principles that apply to exploration life support.

The patterns of gene regulation that respond to microgravity and the spaceflight environment, and the basic microbiology of plant-microbe interactions in the space environment, are rapidly being elucidated. However, long-term studies are lacking. And the research to apply this gene expression knowledge to improve plant growth for food and nutrition on a deep space mission—to mitigate the effects of weightlessness on plants and on their immediate environment—is in the future, and not clearly on course to be fully translated to life support production by 2024.

4.4.2 Behavior and Mental Health

Developments and progress in the wider nonspace research community related to behavior and mental health include the explosion of interest and knowledge in the neuroscience area to better understand normal as well as psychopathological processes. Omics advances, such as the identification of genes associated with types of depression and other forms of psychopathology and the study of epigenetic markers, are some of the areas in which recent progress has occurred. The research by Feinberg and colleagues⁴ on epigenetic markers and the mechanisms of epigenetic modification in normal development and in mental disorders is particularly notable.

The development of the Research Domain Criteria (RDoC) by the National Institute of Mental Health (NIMH) as a new paradigm for understanding mental disorders focuses on dimensions of psychopathology (assessed by a combination of observable behavior and neurobiological and physiological processes) rather than categorical diagnostic types. This approach and the findings derived from studies using RDoC criteria have the potential to provide a more robust framework for understanding the complex and multifaceted nature of mental disorders and the commonalities across dimensions evident in different diagnostic groups.

There is ongoing research by the military and industrial organization/relations groups to identify individual traits of hardiness and the promotion of resilience, and also to better understand the dynamics of complex team systems. This area of research examines factors contributing to optimal single and multiteam performance in challenging situations/environments. In addition, advancements in virtual reality technologies based on games and other recreational pursuits have enhanced the fidelity of the virtual experience and the possibility to improve the effectiveness of virtual reality training procedures.

4.4.2.1 Role in Human Exploration and Future Implementation

Exploration missions require a small group of astronauts to live and work together for an extended period of time in a confined vehicle and habitat. The effectiveness of their individual and teamwork performance is crucial to the success of the mission. The decadal survey priorities designated for the behavior and mental health area cover both individual and team factors and are addressed below.

Priority B1 was ranked relatively lower than priorities B2, B3, and B4 for the purposes of this study because of the overriding importance of identifying optimal individual and team characteristics to inform crew selection for long-duration exploration missions (LDEMs). Nonetheless, the B1 priority also requires additional research.

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⁴ A.P. Feinberg, M.A. Koldobskiy, and A. Göndör, Epigenetic modifiers and mediators in cancer aetiology and progression, Nature Reviews Genetics 17:284-299.

Assessing the actual work performance of ground- and space-based personnel during a mission is a sensitive topic; however, it is critical to the eventual safe conduct of a deep-space exploration mission. Innovative strategies need to be developed to obtain this information without potential bias by the personnel involved. It should be possible to develop a global picture of the ideal astronaut based on debriefings and other materials collected on past and current astronauts. While specific technical skills are crucial, they are not a substitute for a broader conception in which aspects—such as cooperation, psychosocial skills, empathy, and excellence under stress—are taken into account. In addition, the monitoring measures currently used in space need to be enhanced. Improvement of both subjective and objective (including nonintrusive) measures of performance are needed. Analogs can provide relevant information, but there currently is no analog that can fully take the place of real-time performance of both astronauts and mission control personnel.

Current Human Research Program (HRP) funded projects conducting integrated translational research in the above area need to continue. Often this process consists of projects by different investigators coordinated at an analog site such as HI-SEAS (Hawai’i Space Exploration Analog and Simulation) or HERA (Human Exploration Research Analog). Inclusion of additional projects also is highly important. The challenge is to coordinate the activities and findings of the individual projects into a coherent whole through which overall conclusions can be reached.

During the past decade, there has been a leap forward in research and knowledge in the neuroscience and omics areas. Improvement of various imaging technologies, including technical adaptations to enable the study of ongoing cognitive and emotional processes, is evident. These advances present an important opportunity to better understand the underpinnings of individual differences in adaptation to challenging and stressful environments, and possible changes in brain function and structure as a result of living and working in an isolated, confined, and extreme environment for an extended period. In addition, research on genetic and epigenetic factors, among other omics approaches related to resilience would be beneficial. Targeted research priorities integrating individual factors such as personality trait configuration, psychological and physiological markers of resilience following stressful events and sleep loss, and relationships to work performance are needed. Eventual understanding of the unique aspects of individual differences in the various combinations of genetic, physiological, and psychological underpinnings of resilience should provide the data for an individualized medicine approach.

The entire team performance area requires still greater attention, including additional research on multiteam systems. Research examining the dynamics of conflict resolution in small isolated and confined groups would be beneficial. Ongoing integrated analog studies of team performance are relevant to many aspects of crew functioning. A fine-grained analysis of the emergence of conflicts, effects on other team members, and the process of conflict resolution is important. Furthermore, mission control personnel are a vital part of the mission, and their inclusion in research is necessary. Another interrelated team issue is the different levels of autonomy the crew will experience. The crew will become more independent in functioning as they travel to deep space, and live and explore the Mars surface; however, the crew will need to deal with lesser autonomy as they return to Earth and progressively are under the greater command of mission control.

4.4.2.2 Research Platforms and Approaches Relevant to Exploration

Both Earth analogs and flight experiments are important to furthering the research agendas during the remaining period of the current decadal survey. To ensure validity, as many projects as possible would need to be conducted in space. However, considerations of the total number of astronauts available on flights, as well as the need to obtain individual agreements to cooperate on a study, require that high-fidelity Earth analogs be used as well. The sample size of space flyers participating in research projects will be increased through greater cooperation with other space agencies, particularly Roscosmos. It may be useful to pursue this possibility more actively. Each analog will simulate different aspects of the vehicle or surface environment; increasing the number of analog sites available, including additional polar sites, will be helpful. Also, cooperative use of simulation facilities such as the Russian Mars habitat will add to the analog possibilities available.

A significant opportunity in the Behavior Medicine (BMed) area exists to make use of scientifically robust standardized data on astronaut personality traits (collected as part of the selection process) to predict performance in space and the reintegration back to Earth. These data may help answer a major risk for future Mars missions: the later development of psychopathology in initially psychologically healthy astronauts. Confidentiality could be ensured by having this effort funded and confined to the Johnson Space Center (JSC) operational group; a deidentified technical report could inform future research.

While the Behavioral Health and Performance (BHP) group has moved forward in funding analog projects, a significant challenge is the availability of enough relevant analogs to place funded studies in and to maintain these studies over consecutive years. A better understanding of the memorandum of understanding (MOU) between the NSF and NASA for access to Antarctic sites is needed. The current MOU is extremely general, referring only to “Space, Earth, and Biological Cooperative Activities” with no mention of access or collaboration at analog sites. Greater effort to obtain matching funding from other agencies such as the Defense Advanced Research Projects Agency (DARPA) and the National Institutes of Health (NIH) would be helpful; DARPA for studies of individual and team resilience in the face of intense challenges, NIH for studies of medication effects in extended duration microgravity and epigenetic factors.

4.4.2.3 Recent Advances Relevant to Microgravity Science

Developments and progress in the wider nonspace research community with relevance for space purposes include neuroscience research focused on better understanding normal as well as psychopathological processes. The research by Feinberg and colleagues on epigenetic markers and mechanisms of epigenetic modification⁵ in normal development, mental disorders, and microgravity (the ISS Twins study⁶) is highly relevant for LDEM. This knowledge has application for better understanding of the long-term physical impact of stressors on astronauts during LDEM, with possible effects on psychosocial functioning. These and other findings from the nonspace research community could potentially address aspects of the B3 decadal survey priority regarding genetic, physiological, and psychological underpinnings of individual differences in resilience to stressors.

As mentioned earlier, the development of the Research Domain Criteria (RDoC) by the NIMH as a new paradigm for understanding mental disorders focuses on dimensions of psychopathology rather than categorical diagnostic types. This approach and the findings derived from studies using RDoC criteria have the potential to inform the selection of astronauts for space missions based on genetics, epigenetics, neuroscience, and behavioral science criteria (B3).

The ongoing space-relevant research by the military and industrial organization/relations groups on the dynamics of complex team systems has application for identifying factors contributing to optimal single and multiteam performance in challenging situations/environments (B4). This area is highly relevant because it is possible that team performance may increase in risk factor status as the duration of missions increases. The mission challenge is the need for individual teams to function as part of multiteam systems, although the crew also must function with greater autonomy as the distance from Earth increases. In addition, advancements in virtual reality

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⁵ Ibid.

⁶ See NASA, 2017, “Twins Study: The Research,” last updated August 3, https://www.nasa.gov/twins-study/research.

technologies can enhance the fidelity of the virtual reality programs that are being developed for space purposes to promote positive affect and stimulation and used as countermeasures to alleviate boredom and stress.

4.4.2.4 Further Considerations for Implementing Decadal Survey Recommendations

Relevant areas outside of NASA that influence this exploration research include NIH-sponsored projects on changes in brain structure and functioning under stressful conditions, and epigenetic studies assessing changes under different environmental conditions. Research sponsored by the military and in academic settings has provided relevant information on team functioning in challenging situations, as well as furthering the understanding of the personality characteristic of hardiness, and its influence on resilience under stress. NSF-funded studies in polar environments also have provided informative data on individual and team functioning in isolated, confined, and extreme environments.

4.4.2.5 Highest-Priority Recommendations for Exploration

It is likely that candidates currently being selected for the astronaut corps will be among the pool of astronauts selected for exploration missions. Therefore, it is important that the high-priority research designated below be carried out in an expedited manner both in analog and space environments to inform the necessary individual and team criteria optimal for long duration exploration missions.

Following the charge in the statement of task, the committee looked at the highest priorities from the decadal survey and ranked B2, B3, and B4 as most important in terms of relevance to exploration. These recommendations focus on the interrelationships among individual functioning, cognitive performance, sleep, and group dynamics; the genetic, physiological, and psychological underpinnings of individual differences in resilience to stressors during extended space missions; team performance; and effectiveness of multinational crews. While B1 was rated somewhat lower in terms of priority, clearly, the continued development of measures to assess work performance in space is also a significant priority. Considerable research will need to be continued following the 2024 end of the ISS to mitigate the risks designated by the NASA Human System Risk Board (HSRB). For 2024, HSRB categorized “Cognitive or Behavioral Conditions” as high risk and “Team Performance Decrements” as low risk, although not in the “optimized” category. (Risk factors that are not categorized as “optimized” require more research to bring countermeasure technologies to a level where they can be effectively applied during a Mars mission.)

4.4.3 Animal and Human Biology

Since the decadal survey in 2011, there has been rapid progress in several areas of human and animal biology and physiology and in particular, spectacular progress in genetic sequencing and analyses of gene expression for understanding developmental and regulatory processes. Another new area that is emphasized by NIH is research related to the microbiome and its role in health and disease. The microbiome may be adversely affected by degradation of micro-nutrients and food in general. Moreover, extreme confinement of crews during deep-space missions may also adversely affect the microbiome. Thus, NASA will benefit from past and future progress made in this field. Another area in which rapid progress is being made is drug development for prevention of muscle atrophy and bone loss. However, whether or not such drugs work in site-specific ways to counteract Type 1 muscle fiber atrophy and tibial and spinal bone losses needs further investigation. Finally, the rapidly growing field of virtual environment technology will greatly aid crew member adherence to exercise regimens needed to maintain crew health and well-being.

4.4.3.1 Role in Human Exploration

Animal research is necessary for assessing the effects of the space environment on humans in deep space, and for developing mitigating technologies for those effects that are adverse. Musculoskeletal loss including muscle atrophy and bone mass and strength losses (the subject of much successful past research) has now been largely

mitigated by exercise and bone-building medications (bisphosphonates). However, the ARED (Advanced Resistive Exercise Device) resistance training device that has proven successful in mitigating muscle and bone loss is too heavy and large to fly on an exploration mission as presently configured. Moreover, there is evidence that some crew members experience back injuries with this hardware; they then are not allowed to exercise for variable periods of time after the injury. Also, some flight surgeons disagree about whether bisphosphonates are the best option for long-term treatment of bone loss, especially in healthy males. So even though bone loss is mitigated by ARED combined with bisphosphonates for LEO; better, smaller and more integrative countermeasures are needed for deep-space missions. Thus, there still is a need for development of pharmaceutical treatments to maintain muscle mass and protein balance, and multisystem countermeasures to maintain the cardiovascular system and to prevent the head-ward fluid shifts that probably contribute to vision problems. Human research is also necessary in these areas to demonstrate applicability of animal discoveries, and to assess the extent to which these issues affect astronaut performance and survival.

4.4.3.2 Research Platforms and Approaches Relevant to Exploration

The effects on humans of long-term space travel are largely responses to a microgravity environment and radiation (see below). Facilities for animal growth and culture, especially for rodents, are therefore central to future discovery. While simulated-microgravity environments are possible for plant research (for example, clinostats on the ground), there is no substitute for actual microgravity in animal research. Thus, experiments to determine the effects of microgravity on animals and humans would need to largely be done in LEO on the ISS. Ground-based controls are nonetheless necessary for all space-based experiments so that attribution of effects to the microgravity environment is soundly based. Ground-based controls, requiring duplication of space-based facilities, and careful control of nongravity parameters such as temperature, habitat size, nutrition, lighting, and so on, are therefore critical. With reference to one of the highest priorities (AH6), new prototype exercise devices are needed to optimize physical activity by employing multisystem countermeasures. Such multisystem countermeasure research can be supplemented in Earth analog studies, but progress will greatly depend on LEO and beyond Earth orbit platforms to evaluate new prototype exercise devices during actual microgravity. Also, with reference to the other highest priority (AH8) concerning Starling pressures, ground studies have already supplemented this research. However, actual microgravity is needed on LEO and beyond Earth orbit platforms to understand transcapillary fluid exchange and venous and lymphatic returns as well as possible impacts of loss of tissue weight that is only available during space flight.

The need for a flight centrifuge has been cited in National Academies studies reaching back to the 1980s.^7,8 The decadal survey frequently identified the loss of the 2.5-m-diameter animal and plant centrifuge facility on ISS as a severe impediment for providing an important one-gravity control condition for studying the effects of microgravity on animals and plants. Moreover, with the loss of this facility, other studies of partial-gravities (1/6 and 3/8G) and hyper-gravity are very limited. While there are three small centrifuges operational on ISS, none is sufficient to provide a useful 1-G control or to investigate partial- and hyper-gravities because of their small diameters and associated high-gravity gradients and Coriolis effects. These effects keep the small centrifuges from serving as useful controls for experiments with plants and animals. The absence of a large diameter centrifuge remains a severe limitation for fundamental biology on ISS. The absence of useful large-diameter centrifuges for flight experiments was an item that was raised several times in the community input workshop, and is a serious concern to the scientific users of ISS flight modules for animal and plant experiments. While the committee understands that it is not feasible to add a large diameter centrifuge to ISS at this juncture, the need for such a facility will remain unchanged as NASA looks toward development of LEO and beyond Earth orbit platforms. The life science community of researchers is very interested in partial gravity research in addition to using a large-diameter animal and plant centrifuge as a 1-G control.

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⁷ National Research Council (NRC), 1987, A Strategy for Space Biology and Medical Science for the 1980s and 1990s, National Academy Press, Washington, D.C.

⁸ NRC, 1998, A Strategy for Research in Space Biology and Medicine in the New Century, National Academy Press, Washington, D.C.

4.4.3.3 Recent Advances Relevant to Animal and Human Biology Microgravity Science

As in plant and microbial research (see above) new methods and applications of DNA and RNA sequencing have allowed major progress in methods to monitor crew health, including the state of the human microbiome and the diagnosis of human disease.

Since the decadal survey in 2011, there has been rapid progress in several areas of human and animal biology and physiology and outstanding progress in genetic sequencing and analyses of gene expression. Another new area is the microbiome and its role in health and disease. This is emphasized by NIH in its recent Earth-based research. NASA will benefit from past and future progress made in this field. Rapid progress is available for drug development for prevention of muscle atrophy and bone loss, but such drugs may not be site-specific to counteract Type 1 muscle fiber atrophy and bone loss. Also, the rapidly growing field of virtual environment technology will greatly aid crew member adherence to exercise regimens needed to maintain crew health and well-being.

4.4.3.4 Highest Priority Animal and Human Biology Recommendations for Exploration

Extended missions in microgravity continue to require a better understanding of relationships between exercise and microgravity effects. The effectiveness of crew members’ individual and team work performance is crucial to the success of the mission. The decadal survey priorities focus on exercise devices and deterministic mechanisms that underlie countermeasures. New prototype exercise devices are required for deep-space missions. This is prioritized because of the need to optimize physical activity and develop a multisystem countermeasure that maintains cardiovascular health and musculoskeletal strength and prevents fluid shifts toward the head. These head-ward fluid shifts cause inflight congestion and elevated pressure in internal jugular veins,⁹ a constant but subclinical elevation of intracranial pressure compared to that of upright posture on Earth,¹⁰ increased post-flight, lateral ventricular volume,¹¹ and upward shifting of the brain,¹² all of which may cause or contribute to problems of vision. Determining the basic mechanism of adaptations and clinical significance of regional vascular/interstitial pressures, due to altered Starling pressures, continues to be important. This research is of high priority because an understanding of the potential causes and effects of head-ward fluid shifts on structural remodeling and loss of function of neuro-ocular systems is warranted for development of countermeasure strategies beyond those provided by current exercise devices. Lower-body negative pressure (LBNP) garments and LBNP exercise devices are also possible mitigation strategies whose study could be usefully included in this priority area.

Decadal survey priorities such as bone loss and possible countermeasures for bone loss such as drugs and exercise have already progressed well, so these priorities are ranked lower in importance. However, it is still unknown how the space flight environment affects bone fracture healing in-flight, so this risk is rated as a “red risk” for planetary exploration. Other decadal survey priorities such as muscle protein balance, recruitment of muscles, orthostatic intolerance, screening subclinical heart disease, and transmission of structural changes over generations were deemed less important for success of deep-space missions.

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⁹ M.B. Stenger, A.R. Hargens, S.A. Dulchavsky, P. Arbeille, R.W. Danielson, D.J. Ebert, K.M. Garcia, et al., 2017, “Fluid Shifts,” presentation #17601 at A New Dawn: Enabling Human Space Exploration, NASA Human Research Program Investigators’ Workshop on January 23-26, 2017, https://three.jsc.nasa.gov/iws/SRIW-Cvent-Program-2017.pdf.

¹⁰ J.S. Lawley, L.G. Petersen, E.J. Howden, S. Sarma, W.K. Cornwell, R. Zhang, L.A. Whitworth, M.A. Williams, and B.D. Levine, 2017, Effect of gravity and microgravity on intracranial pressure, J. Physiol. 595(6):2115-2127.

¹¹ N. Alperin, A.M. Bagci, and S.H. Lee, 2017, Spaceflight-induced changes in white matter hyperintensity burden in astronauts, Neurology 89(21):2187-2191.

¹² D.R. Roberts, M.H. Albrecht, H.R. Collins, D. Asemani, A.R. Chatterjee, M.V. Spampinato, X. Zhu, M.I. Chimowitz, and M.U. Antonucci, 2017, Effects of spaceflight on astronaut brain structure as indicated on MRI, N. Engl. J. Med. 377(18):1746-1753.

4.4.4 Cross-Cutting Issues for Humans in the Space Environment

4.4.4.1 Role in Human Exploration

Cross-cutting issues for humans to explore deep space are key components of the NASA exploration strategy to maintain the health and well-being of crew members. The decadal survey called out some major and specific areas for prioritized study applicable to the exploration goals of NASA: CC2 to determine whether continuous, artificial gravity (AG) by a long-arm centrifuge is needed as a multisystem countermeasure or if intermittent LBNP exercise or intermittent short-arm centrifugation is sufficient; and CC4 to determine optimal dietary strategies and food preservation for crews. The radiation components of cross-cutting decadal survey recommendations are described below in Section 4.4.5.

4.4.4.2 Research Platforms and Approaches Relevant to Exploration

Again, the human effects of long-term space exposures are largely responses to a microgravity environment and radiation (see Section 4.4.5 below). While a simulated microgravity environment such as head-down-tilt bed rest is possible for human research with and without artificial gravity, there is no substitute for actual microgravity and countermeasure development related to the highest risk category of vision impairment for deep-space missions. Thus, experiments to determine the effects of microgravity and other adverse environments on vision and development of optimized countermeasures for humans are necessary in LEO and beyond Earth orbit platforms. Although multisystem AG countermeasure research can be supplemented in Earth analog studies, progress will greatly depend on LEO and beyond Earth orbit platforms to evaluate these countermeasures effectively. Although not in the “highest-priority” category in Table 4.1, studies of food systems for over 12 months of storage are required for prolonged, deep-space flights. Food systems research is needed in ground-based analogs, during LEO on the ISS through 2024, as well as for beyond Earth orbit with higher radiation exposures.

The decadal survey indicated that NASA should explore ways to study the effects of partial gravity on the human body, not just microgravity. The biomedical community of researchers is very interested in partial gravity research that can be supplemented by Earth analog studies, but further progress will depend on beyond Earth orbit platforms to evaluate the effects of partial gravity on the human body.

4.4.4.3 Recent Advances Relevant to Cross-Cutting Issues for Humans in the Space Environment

As described in preceding research sections, new methods and applications of DNA and RNA sequencing have allowed significant progress in methods to monitor crew health, including the state of the human health and the diagnosis of human disease. Since the decadal survey in 2011, there has been rapid progress in areas of human biology and omics research and great strides in genetic sequencing and analyses of gene expression for understanding adaptations to extreme environments. A new area that is emphasized by NIH is research related to the microbiome and its role in health and disease. As noted previously, the microbiome may be adversely affected by degradation of micro-nutrients and food in general, as well as by extreme confinement of crews during deep-space missions. Thus, NASA will benefit from past and future progress made in this field. Finally, the rapidly growing field of virtual environment technology will greatly aid multisystem, AG countermeasure research.

4.4.4.4 Highest-Priority Cross-Cutting Recommendations for Exploration

CC2: Artificial gravity as a multisystem countermeasure is needed for deep-space missions. Research is still needed to develop and test integrated AG countermeasures, including exercise, for periods exceeding 180 days in microgravity with and without artificial gravity by centrifugation and LBNP. It is important for integrated countermeasures in space to reproduce normal musculoskeletal loads and fluid shifts that occur on Earth and to which

the body is adapted.^13,14 Such integration of countermeasures may maintain not only the structure and function of muscle and bone, but also cardiovascular and cerebral health. Merely miniaturizing exercise hardware presently on ISS for a smaller deep-space habitat is unlikely to maintain health and well-being of crew members. For example, it is known that head-ward fluid shifts in space have undesirable effects on cerebral venous circulations,¹⁵ lymphatic function¹⁶ and near-sighted vision.¹⁷ This syndrome is now referred to as Spaceflight-Associated Neuro-ocular Syndrome (SANS). Moreover, it is likely that all crew members will be affected by these ocular changes as deep-space missions are lengthened. Presently, there is no hardware in space, except lower body negative pressure and perhaps thigh cuffs, to counteract head-ward fluid shifts and attempt to reproduce daily cycles of fluid shifts common to postural transitions on Earth. An alternate hypothesis is that some crew members may be genetically predisposed to endothelial dysfunction induced by oxidative stress.¹⁸ Separately, hardware is needed to preferentially activate and maintain muscles that provide low-level but prolonged contractions common to upright posture on Earth. Current exercise hardware such as the Advanced Resistive Exercise Device and ISS treadmill do not preferentially maintain these postural, core muscle groups.

The addition of virtual environment (VE) technology is recommended for multisystem exercise hardware to improve the exercise experience for crew members and to foster adherence to exercise regimens. VE technology may also contribute to crew behavioral health and performance in small, confined habitats. These objectives will need to be met in integrative and coordinated research schemes, combining hardware development and protocol optimization. This is of utmost priority and is needed soon on ISS to achieve a reasonable sample of human subjects. Ground-based (e.g., bed rest) studies are an important initial step in developing an integrated, multisystem countermeasure combining exercise and foot-ward fluid shift. However, ground-based studies are not always adequate, especially with respect to simulating head-ward fluid shifts and their role in vision impairment. National and international cooperative studies utilizing existing translational networks and NASA’s new Translational Research Institute may promote greater resource sharing and accelerate research. Such studies are necessary in LEO and beyond Earth orbit.

Decadal survey priorities identified in Table 4.1 include some other research. The decadal survey priority of optimizing dietary strategies and food systems was categorized as “higher,” but was determined to be less important for success of deep-space missions than multisystem, AG countermeasure research. Other research related to post-landing vertigo, decompression sickness, and food and energy intake has already progressed well, so these priorities are ranked lower in importance.

4.4.5 Radiation Component of Cross-Cutting Decadal Survey Recommendations

Recent work with whole genome sequencing, especially in cancer research on the varied genomic effects of mutagenesis, has led to increased throughput and capacity for genome reconstruction approaches not only for sequencing of whole organisms but also for sequencing of single cells. At the time that the decadal survey review was being carried out, these techniques were cumbersome and it was difficult to realize their application to everyday science approaches. With the advent of new sequencing approaches, utility for space radiation studies has become feasible. Thus it could be useful for NASA to examine new approaches to consideration of mechanisms of DNA repair, and to direct measurement of genomic damage. This research could be particularly important, as mechanisms

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¹³ L.G. Petersen, J.C. Petersen, M. Andresen, N.H. Secher, and M. Juhler, 2016, Postural influence on intracranial and cerebral perfusion pressure in ambulatory neurosurgical patients. Am. J. Physiol. Regul. Integr. Comp. Physiol. 310(1):R100-R104.

¹⁴ J.S. Lawley, L.G. Petersen, E.J. Howden, S. Sarma, W.K. Cornwell, R. Zhang, L.A. Whitworth, M.A. Williams, and B.D. Levine, 2017, Effect of gravity and microgravity on intracranial pressure, J. Physiol. 595(6):2115-2127.

¹⁵ M.B. Stenger, A.R. Hargens, S.A. Dulchavsky, P. Arbeille, R.W. Danielson, D.J. Ebert, K.M. Garcia, et al., 2017, “Fluid Shifts,” presentation #17601 at A New Dawn: Enabling Human Space Exploration, NASA Human Research Program Investigators’ Workshop on January 23-26, 2017, https://three.jsc.nasa.gov/iws/SRIW-Cvent-Program-2017.pdf.

¹⁶ A.A. Gashev, M.D. Delp, and D.C. Zawieja, 2006, Inhibition of active lymph pump by simulated microgravity in rats. Am. J. Physiol. Heart Circ. Physiol. 290(6):H2295-H2308.

¹⁷ A.G. Lee, T.H. Mader, C.R. Gibson, and W. Tarver, 2017, Space flight-associated neuro-ocular syndrome, JAMA Ophthalmol. 135(9):992-994.

¹⁸ S.R. Zwart, C.R. Gibson, J.F. Gregory, T.H. Mader, P.J. Stover, S.H. Zeisel, and S.M. Smith, 2017, Astronaut ophthalmic syndrome, FASEB J. 31(9):3746-3756.

of repair of the complex DNA lesions that are more common following HZE (high Z (atomic number) and energy) radiation than low LET (linear energy transfer) radiations may be of relevance to radiation mitigation strategies.

4.4.5.1 Role in Human Exploration

It is well understood that radiation is a very high risk for exploration missions, particularly deep-space missions that are planned for Mars and beyond. Risks from exposure to the HZE radiation that will be encountered in space are poorly understood because most of Earth’s environment is shielded from such particle irradiation by the Van Allen belts. Studies of the effects of these qualities of radiation are largely limited to those done in recent years by the space radiation community.

4.4.5.2 Highest-Priority Recommendations for Exploration

Space radiation risks to humans (CC8) highlights a major research topic that is ranked “highest priority” in Table 4.1, a priority that includes a deeper understanding of deep space radiation effects on biological and physical systems. The decadal survey study has detailed some information about risks from such radiation exposures, but recent work by NASA has listed four predominant risks associated with space radiation exposure in the Human Research Program Roadmap: risk of acute (in-flight) and late central nervous system effects from radiation exposure, risks of acute radiation syndromes due to solar particle events (SPEs), risk of cardiovascular disease and other degenerative tissue effects from radiation exposure, and risk from carcinogenesis. Of these four risks, most radiation biologists argue that the risk of acute toxicities is probably not substantial enough to warrant additional studies in this area. Much is known about acute toxicities and mitigation approaches for doses up to 10 Gy have been developed predominantly for cytokine and growth factor stimulation of bone marrow function and bone marrow transplantation. Expected doses on a round-trip journey to Mars are expected to be a maximum of 1.2 Gy, far below the usual threshold for bone marrow syndrome at 2 Gy and therefore of little relevance to Mars space travel.

Other risks however remain significant and worthy of study. Probably the most important is the risk of CNS (central nervous system) effects since acute toxicities (if they truly exist) could limit the functioning of the crew during the mission. Evidence suggests that there may be some acute CNS toxicity (such as cognitive dysfunction) in animals that have been exposed to HZE radiation even at low doses; this work clearly warrants verification and additional study particularly from a mechanisms perspective to assess whether similar effects might occur in humans. This is the highest-priority work because of the possible threat to the mission and because such effects could be “show stoppers.” The extrapolation from animals to humans for this work may be especially difficult because cognitive effects in animals are so difficult to compare to those in humans and because neurological function of different species is quite different. Unraveling the effects in animals and then extrapolating to humans may be very challenging.

Among the other risks that are listed for radiation exposure are late-effect toxicities that may affect astronauts after their return to Earth. Like acute CNS damage at low HZE doses, late CNS endpoints are also poorly understood, and while they are not likely to impact the mission, understanding them may help in defining some of the mechanisms underlying early tissue toxicities (as is the case for other late tissue effects with low-LET radiation). Thus, late CNS effects would be an important priority after the mission-threatening toxicities. In addition, the impact on astronauts following their service to NASA should also be considered as important; a full awareness of the risks of space travel (as much as is possible) would be important decision considerations on whether space flight is worth its risks both for humanity as a whole and also for individual astronauts. Considerations of such late effects will need to play a role in decision-making. Finally (and perhaps most importantly) knowing that a risk exists and understanding the mechanism by which tissue damage is induced might provide approaches to mitigation.

Risks for cardiovascular disease (CVD) also appear to be (at present) mostly late tissue toxicities of radiation exposure. Even low doses of low LET radiation have been associated with late life induction of CVD in recent studies of the atomic bomb survivors’ data sets. While the radiation qualities are different from those encountered in space, space HZE radiations are more damaging in most cases than low LET radiation and thus could pose an even greater risk than what was experienced by the atomic bomb survivor population. These risks are most likely not life-threatening and thus are similar to the CNS late effects noted above. Nevertheless, as in the CNS effects,

for CVD effects there are no real mechanisms that have been identified and the exact risks are not fully understood. CVD risks from HZE radiation are even less well understood than those for CNS toxicities and thus warrant study for the same reason as other late effects.

The risk for cancer induction is perhaps one of the most well-studied endpoints in the radiation field, although there are still some unknowns about HZE radiation. In particular, many of the studies that have been done revolve around acute radiation exposures given in a few minutes or a few hours, but space radiation encounters are likely to be of a much lower dose rate and longer duration. For low LET radiation, a lower dose-rate usually implies a lower risk of carcinogenesis, but most of the data with high-LET radiation (such as neutrons or alpha particles, for example) suggest that there is no dose-rate effect or even possibly an inverse dose-rate effect making a long duration exposure equivalent to the same exposure given in a few moments. This aspect of HZE radiation exposure is not well characterized and could pose a significant danger from radiation exposure in space, although again this will be a late effect. The latent period for most solid cancers induced by low LET radiations is 15+ years, and data in animal models suggests that this may be similar for high-LET radiation. As such, radiation-induced cancer incidence is also not a risk that is likely to occur during the course of the mission but years after the astronauts have returned. As such, this risk needs to be understood in more detail particularly with regard to dose-rate effects and it will be important to initiate more work on countermeasures. This work continues to be important but NASA has continued to work in carcinogenesis at a somewhat reduced level while elevating the importance of studies related to CNS risks because (1) cancer is better understood and (2) CNS problems during the mission could compromise completion.

4.4.5.3 Research Platforms and Approaches Relevant to Exploration

Most of the space radiation work is currently done at the Brookhaven National Laboratory’s Space Radiation Laboratory. This lab was designed specifically to provide exposure to HZE radiations on Earth while mimicking the types of exposures that might be encountered in space. Single ion and now mixed field types of exposures (similar to what would be encountered in space) are possible in the facility. Most exposures in LEO or other similar platforms will not generate sufficient exposure to HZE radiation because (1) LEO is under the Van Allen belts and thus is protected from the effects of HZE radiation to a large extent and (2) generation of a sufficient dose equivalent to a Mars mission is very difficult in that environment. For these reasons, NASA has put significant effort into developing and maintaining the Brookhaven facility. One current limitation to the facility is the difficulty of doing large animal work there; as CNS effects become more important, use of nonhuman primates and larger animal species with a CNS structure more similar to humans may be necessary. Brookhaven currently has no capacity to handle these large animal studies. It will be important to conduct some aspects of radiation research beyond Earth orbit in order to expose microbes, plants, animals, and people to cosmic radiation similar to that for exploration missions.

4.4.5.4 Recent Advances Relevant to Microgravity Science

Research at DOE and NIAID (National Institute of Allergy and Infectious Diseases) has some overlap with NASA’s radiation research so collaborations may be an option for NASA to advance this field further. It is clear from carcinogenesis studies that use of countermeasures to prevent or reduce mutation induction in space might be of value, but other countermeasures such as those for CNS effects and others could reduce space flight radiation risks. NIAID has a strong countermeasures program in part aimed at identifying new protective compounds. Partnerships between NIAID and NASA and some drug companies working to discover and provide these compounds could be important for risk mitigation. Such collaborations could potentially be important in the NASA portfolio.

4.4.6 Fundamental Physical Sciences in Space

The research on complex fluids and soft matter has progressed considerably in recent years. The researchers know now how to prepare colloids with functionalized interfaces in order to control the forces between them and

to succeed in the elaboration of metamaterials.¹⁹ Elaborate numerical simulations of crystallization/gelation with different types of colloids is throwing light on the protein aggregation process that frequently involve more than two species (cataract for instance).²⁰ Microgravity experiments will now proceed on excellent grounds.

The study of protein crystallization became easier with available powerful synchrotrons that allow using small, easier to grow crystals. Furthermore, it is now possible to visualize the conformation changes during protein function using nuclear magnetic resonance (NMR). Microgravity studies on proteins therefore no longer focus on growing large crystals, but rather on issues such as the role of aggregation prior to crystallization. These studies should allow researchers to better master the crystallization process and possibly improve the crystal quality.

Foams studies have recently clarified the issue of drainage in the case of small liquid fractions (“dry” foams),²¹ but the behavior of “wet” foams remains elusive. Foam coarsening is not well understood. Despite the still limited fundamental knowledge, numerous foam applications have been developed during recent years. Two applications are of interest for space exploration: water purification using flotation and solid foams for light materials.

Quantum physics research gave rise to groundbreaking achievements in recent years. This research was distinguished by a number of Nobel prizes, exemplified by the 1997 cold atoms, 2001 Bose-Einstein condensates, and 2012 quantum particle measurement. Important applications in the fields of computers and telecommunications are foreseen in the coming years

4.4.6.1 Role in Human Exploration

Fundamental physics, by its nature, does not directly “enable” space exploration in the near-term. However, results from the fundamental physics discipline will provide the knowledge and discoveries that will likely “enable” new capabilities (i.e. quantum computing) for space exploration over a longer time horizon.

As described in Section 3.5, in the 2011 decadal survey, the Fundamental Physical Sciences Panel identified four research areas that were especially important in the fundamental physical sciences. The panel prioritized them in order as (FP1) Research on Complex Fluids and Soft Matter, (FP2) Research that Tests and Expands Understanding of the Fundamental Forces and Symmetries of Nature, (FP3) Physics and Applications of Quantum Gases, and (FP4) Investigations of Matter in the Vicinity of Critical Points.

4.4.6.2 Highest-Priority Recommendations for Exploration

The committee, in keeping with the criteria discussed later in the report in Section 5.2, did not identify any of the decadal survey established priorities in Fundamental Physics as a high priority for space exploration. While none of the four research areas apply to the statement of task request to rank exploration relevant research, the midterm committee feels strongly that these FP program elements are quite relevant to the broad interests of NASA, and a discussion of these elements is appropriate for this study. In this context, the program elements are discussed in the priority order FP1, FP3, FP2 and FP4.

For the Soft Matter (FP1) projects, most NASA-financed projects concern crystallization/gelation in colloids. These projects are important, because not only can they shed light on the glass transition but they will help to find ways to produce high-quality metamaterials.

For Expanding Understanding of Fundamental Forces and Symmetries of Nature (FP2), the need is for improved understanding of some of the most fundamental questions in science, many of which are addressed through the lens of precision measurements, both ground- and space-based. While much of the work is ground-based, there is a crucial component of a space-based clock as a component of the ACES (Atomic Clock Ensemble in Space) network of synchronized clocks. The tests of the most fundamental properties of nature almost invariably lead to surprises and critical new understandings of our world. It seems completely appropriate that NASA should

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¹⁹ S. Sacanna, W.T.M. Irvine, P.M. Chaikin, and D.J. Pine, 2010, Lock and key colloids, Nature 464:575-578.

²⁰ A. Stradner, G. Foffi, N. Dorsaz, G. Thurston, and P. Schurtenberger, 2007, New insight into cataract formation: Enhanced stability through mutual attraction, Physical Review Letters 99.

²¹ S.A. Koehler, S. Hilgenfeldt, and H.A. Stone, 2000, A generalized view of foam drainage: Experiment and theory, Langmuir 16:6327-6341.

be a player in this field. It is also likely, as mentioned in FP4, that new developments in this work may have an important effect in quantum information and quantum computing.

For Physics and Applications of Quantum Gases (FP3), it is clear that microgravity studies offer the possibility of unique, groundbreaking scientific results. The pico-Kelvin Bose-Einstein Condensate experiments offer the exciting opportunity to explore a new regime, accessible only in microgravity environment. Development of Cold Atom Laboratory (CAL) and these experiments has been expensive, and will come on-line soon with the expected deployment of CAL. It should be noted that the costly design, construction, evaluation and launch phases of this project have been completed and the CAL payload is scheduled to launch in the Fall of 2017. Research on orbit will not impose demands on the limited ISS crew time.

The priority, Matter in the Vicinity of Critical Points (FP4), is fundamental in nature; it was classified by the committee as “enabled by space exploration” only. The committee noted that the research on critical fluids led to the discovery of the “piston” effect, allowing heat transfer in microgravity through pressure waves (on Earth, heath is transported much more efficiently via convection). This effect allows a much faster heat transfer than diffusion and brings a solution to otherwise difficult technical problems.

4.4.6.3 Research Platforms and Approaches Relevant to Exploration

It is apparent that the ISS will be necessary in order to address the unique microgravity and partial gravity environment posed by space exploration and prior to eventual human flights to Mars or to the Moon. Much of the cold atom science will be carried out in the CAL and specifically the pico Kelvin BEC experiments require the extended microgravity period available on the ISS. By its nature, research in Fundamental Physical Sciences, considered “enabled by” the microgravity environment, cannot be “complete” by the 2024 ISS transition.

Earth facilities such as drop towers, parabolic flights, and sounding rockets are extremely useful for conducting low gravity research. These platforms are accessible and do not demand lengthy and costly development of dedicated infrastructure. The drawback is that the microgravity duration is not long enough to obtain answers for many of the projects directed at the decadal survey priorities. For instance, colloid and protein crystallization/gelation are slow processes, liquid crystal domains and foam bubbles growth take hours, and critical fluids require similarly long temperature equilibration times.

New options may be available through the use of CubeSats²² and other free-flyers. However, the available volume with respect to experiments within CubeSats is limited with respect to the experiments in this area of science. As a consequence, only very small and simple experiments can be performed. Suborbital systems such as Virgin Galactic and Blue Origin have developed initial infrastructure in support of this area of research. Suborbital opportunities (e.g. sounding rockets, commercial) can offer more experiment volume than free-flyers and CubeSats, and provide attractive options for those experiments that can be accomplished within a few minutes.

Most of the types of projects associated with the CAL depend on extensive testing and evaluation if they are to meet their research goals (as opposed to a demonstration of principles). The earliest transition timeline away from ISS in 2024 will not allow completion of the program necessary to finalize the fundamental physics projects that can be conducted on CAL. If, however, the ISS can be continued until 2028, it will be important to make this decision rapidly in order to avoid waste of opportunities. The committee notes also that the CAL may well be continued through European developments, in the form of a CAL2 instrument.

4.4.6.4 Recent Advances Relevant to Microgravity Science

The 2015 NASA technology roadmaps document²³ focuses on development activities that “expand knowledge and capabilities in aeronautics, science and space.” While it is correct to say that fundamental physics in space

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²² National Academies of Sciences, Engineering, and Medicine, 2016, Achieving Science with CubeSats: Thinking Inside the Box, The National Academies Press, Washington, D.C.

²³ See NASA Office of the Chief Technologist, “2015 NASA Technology Roadmaps,” https://www.nasa.gov/offices/oct/home/roadmaps/index.html, accessed October 5, 2017.

contributes to this technology development, the prime focus of fundamental physics in space remains science. As stated in the roadmap introduction, “The roadmaps focus on applied research and development activities.” Still, there is clear overlap between fundamental physical science and roadmaps TA 5 (Communications, Navigation, and Orbital Debris Tracking and Characterization Systems), TA 6 (Human Health, Life Support, and Habitation Systems), TA 8 (Science Instruments, Observatories, and Sensor Systems), and TA 10 Nanotechnology).

In the future, an emerging opportunity presents itself in the form of space-based, global, fully secure quantum telecommunications and, more generally, quantum information and quantum computing. The Chinese have already launched a satellite to test quantum communication technology.²⁴ Within NASA, funding for such science appears quite limited, but the potential for both terrestrial and space benefits are very significant.

4.4.7 Applied Physical Sciences in Space

Applied physical sciences are central to many key exploration technologies. Reliable, efficient, and safe life support systems require the scientific knowledge of fluid physics, combustion and material science in confined microgravity environments with limited resources. Many of the design challenges of new exploration technology systems were listed in the decadal survey report. The general progress made in these fields and the need for further progress are discussed below under each discipline.

4.4.7.1 Fluid Physics

4.4.7.1.1 Role in Human Exploration and Future Implementation

The manipulation of fluids is essential to our everyday terrestrial life. From our bodies to our agriculture to our engineered world, gravity plays a usually helpful, ever-present, ever-passive role to collect heavier liquids, particles, and gases below lighter ones. Liquid fuels provide a pertinent example along with power cycle working fluids, coolants, and myriad operations for processing water. The control of such phenomena broadly referred to here as “multiphase fluids transport.” Multiphase fluids transport is similarly ubiquitous in the fluid systems in space, but pose critical challenges to the designers of nearly all fluid systems for spacecraft due to the effective near absence of gravity. Such systems include liquid propellants, fuels, cryogens, refrigerants, plant and animal habitats, equipment for water recycling and wastes, and others. Many current systems employed aboard spacecraft (i.e., ISS life support systems) are maintained by regular crew maintenance and routine resupply, but systems that are far more robust are essential and mission-enabling for the further human exploration of space. As discussed in Chapter 3, NASA has made progress to address critical Fluid Physics decadal survey items such as forced multiphase flows and transport (AP1), capillary flows and phenomena (AP2), complex fluids (AP5) and continued awareness of granular materials (AP3) and flow phenomena relevant to dust mitigation (AP4).

4.4.7.1.2 Research Platforms and Approaches Relevant to Exploration

Reduced-gravity fluids research is often pursued at small size scales in terrestrial laboratories, but access to the low-g environment is required for thorough investigations at appropriate scales for database collection, benchmarking, and establishment of increased technology readiness level. U.S. and international low-g platforms include government, commercial and academic drop towers, low-g aircraft, commercial sub-orbitals, and potentially other free flyers, with the International Space Station providing the most routine, most versatile option with a highly relevant low-g environment. Fluids experiments developed for ISS usually carry the highest resource commitments, but total cost per data point for certain (i.e., fast-to-flight, automated) ISS experiments can compete with ground-based experimental facilities. It is important to be aware of the capabilities, costs, and availability of the various reduced-gravity platforms for both soliciting and evaluating low-g fluid physics research.

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²⁴ See D. Castelvecchi, 2017, China’s quantum satellite clears major hurdle on way to ultrasecure communications, Nature News, June 15, https://www.nature.com/news/china-s-quantum-satellite-clears-major-hurdle-on-way-to-ultrasecure-communications-1.22142.

4.4.7.1.3 Recent Advances Relevant to Microgravity Science

Terrestrial academic and commercial advances in experimental and numerical methods continue to ease the burden of developing space experiments by providing off-the-shelf tools for more rapid, lower-cost, experiment design toward flight certification. Unfortunately, the low-g environment can also lead to unique fluid behavior requiring designs that require an additional level of research prior to the experiment design process. In order to expand the impact of the experimental and theoretical work to complex system interactions, NASA would need to require that numerical analyses accompany most if not all fluids investigations. Such bench-marked algorithms would serve as foundational design tools for extrapolation to advanced full-scale system development for exploration. Commercial software and specific low-g investigator open-software tools may be exploited for the potentially “big data” archive management.

4.4.7.1.4 Application of Fluid Physics Research

The importance of fluid physics to applications in space exploration is reflected by the large number of translational highest-priority listings in Table 13.2 of the decadal survey (TSES1, TSES2, TSES4-S6, TSES12-16). In a cross-cutting manner, due to long term NASA support in this area of research, low-g Fluid Physics results are beginning to mature to the point of direct application to next generation spacecraft fluid systems design as highlighted in Section 3.6. For example, for the highest Fluid Physics Fields (decadal survey identifier topics AP1 and AP2), specific applications being addressed concern advanced cryogenic storage analysis and design, multiphase heat transfer by way of pool and flow boiling and condensation, fluid phase flow separators and devices, plant and animal habitats, water processing systems including urine collection and drying, condensing heat exchanger design, advanced passive CO₂ scrubbing methods, and others.

4.4.7.1.5 Highest-Priority Recommendations for Exploration

As indicated in Table 4.1, it remains clear that forced multiphase flows and transport (AP1) and capillary flows and phenomena (AP2) are the most practical and thus highest-priority fluids research areas for NASA’s immediate future. These topics might in turn be combined into the single subject of “low-g fluid interfacial transport.” This is due to the fact that either by design or accident (i.e., local boiling, degassification, aerobic activity, etc.) most fluid systems aboard spacecraft in turn become multiphase systems that are strongly and often surprisingly impacted by the absence of a strong gravitational acceleration. In such common situations, the locations of the gases and/or liquids are uncertain, requiring special knowledge and accommodation. These areas directly impact progress in the development of robust life support systems as well as other cross-cutting fields cited specifically in Table 4.1 (i.e., TSES1, 2, 6, 12-16). But it is also clear that the lack of understanding of low-g fluid interfacial transport adds risks to the ability to study nearly all fluids operations relating to most subjects of this report; i.e., Plant and Microbiology, Animal and Human Biology, Fundamental Sciences, and all of the Applied Physical Sciences. It is the committee’s view that NASA has done well to maintain flight experiments with this focus despite limited resources (see previous Section 3.6.2 Fluid Physics for discussion of prior work).

The most significant progress expected for applied science research within the fluid mechanics discipline targeting design needs for exploration will most likely come by way of practical stability limits, regime limits, onsets, transitions, and low-g benchmarked numerical methods. An historic example concerns phase change heat transport processes of sufficient inertia to assure g-independence—in other words, quantification of the limits above which terrestrial and low-g flow processes are essentially identical. It is reasonable to expect significant “component-level” progress along these lines by the 2024 time frame. However, it is not certain whether such phenomena may be adequately studied at the “system level” (i.e., entire systems including evaporators, conduits, condensers, filters, valves, manifolds, parallel paths, reservoirs, etc.) and plans do not appear to be in place to pursue such research in the near term. System level stability and interactions are critical to every fluid system aboard spacecraft with special concerns during start-up, shut-down, safing, and transient response to excursions and off-nominal events. Benchmarked methods to model such processes would have to be developed to reduce risks of advanced system performance at an accelerated pace if NASA hopes to deliver significant design products by 2024. The development of such system level fluid physics experiments will likely require more resources than currently available for this physical sciences subdiscipline.

The behavior of granular materials (AP3) and flow phenomena relevant to dust mitigation (AP4) remain “higher” priorities, but not primarily for low-g environments. Barring crewed-asteroid missions, such knowledge is more critical to In Situ Resource Utilization (ISRU) in lunar and Martian environments. Although Complex Fluids (AP5) continues as an active and fruitful “enabled by” field of research on ISS, it is outside of the central thrust of the low-g fluid interfacial transport fields (AP1 and 2) and is winding down on ISS. Complex Fluids research is not currently considered high-priority research for exploration.

4.4.7.2 Combustion

4.4.7.2.1 Role in Human Exploration and Future Implementation

Combustion processes are central to chemical propulsion which will power space exploration under the current plan. In addition, fire in microgravity is a serious safety concern. Fire behaves very differently without gravity. In human spacecraft the existence of a low and purely-forced flow, and the possibility of using Normoxic atmosphere (higher oxygen percentage and lower total pressure than air), can alter the material flammability responses from what is known on Earth. Since there is no escape from a spacecraft far from Earth in a fire, it is essential that fire behavior in such an environment is understood.

4.4.7.2.2 Highest-Priority Recommendations for Exploration

Overall, the combustion program directed by NASA has done well, as described in Section 3.6. Progress has been made in all the three high-priority areas listed in the decadal survey, but they are far from completion. These areas will continue to be the highest-priority items for space exploration research during the next phase.

AP-6, material flammability and fire suppression in space, is directly related to spacecraft safety. By using a small flow tunnel in glovebox leftover from a previous project, the BASS (Burning and Suppression of Solids) project aboard ISS for solid combustion has been a great success. Compared to other projects in combustion, the turn-around time was short. The interaction between ground-based scientists and astronauts performing the tests was superb. The experiments produced valuable results on solid burning, flame spread, and extinction that are unique in a long-duration microgravity environment. Many of the findings have already been published in top journals. NASA is encouraged to continue the use of glovebox facility when appropriate despite its limitations (e.g., atmospheric pressure and limited diagnostics).

The next project for solid combustion on ISS is Sofie (Solid Fuel Ignition and Extinction) to be carried out in the Combustion Integrated Rack (CIR) that is better instrumented. The test atmosphere is expanded to include different oxygen percentage and pressure levels that have large influence on combustion behavior. The five PIs will share the same flow tunnel and other instruments to save resources. Each PI’s project has different goals, sample configurations and in some cases different solid materials. The current launch date for the Sofie insert is 2020. In order to maintain good progress towards meeting decadal survey priorities that support exploration, it will be important for NASA to meet this date or even to accelerate the schedule if possible.

Saffire (AES Spacecraft Fire Safety Demonstration), a large-scale fire experiment, is not on ISS but related to ISS as Cygnus is a supply vehicle to ISS. As mentioned previously, this is an innovative use of resources and the creation of a new microgravity test platform. Three Saffire tests have been completed. One of the most important results is the experimental verification that flame over solid will reach a limiting size in purely forced flow in microgravity. This is in contrast to fire on Earth where an upward spreading flame normally accelerates. Several Saffire flights are in the planning stage and NASA is encouraged to continue this experimental series. Both Sofie and Saffire will include experiments using the Normoxic atmosphere (same partial oxygen pressure as in air but with higher oxygen percentage and lower total pressure), which is a condition being considered for future space exploration. Related to this is the need to test barrier materials such as Nomex fabric, which has been used in space to prevent ignition and burning. The effectiveness of the barrier material in Normoxic atmosphere need to be determined.

The use of carbon dioxide as a flame suppressant has been studied extensively in FLEX (Flame Extinguishment Experiments) using liquid droplets as the fuel. There is a need to investigate other suppression methods such as water mist on other fire configurations including solid burning in microgravity.

Decadal survey priority AP-7 refers to combustion processes in microgravity, and while it did not receive the highest possible ranking against exploration criteria in the decadal survey, it plays a broad supporting role that is worth citing here. Study of combustion fundamentals is crucial for understanding the effect of gravity on flames and the validation of combustion and kinetic models. Microgravity also can enable deeper insight into terrestrial combustion processes. There have been plenty of unexpected findings in microgravity experiments conducted since 2011. One example is the appearance of cool flames after the radiation quenching of the hot flame in the FLEX experiments. These cool flames provide excellent data to study the low-temperature chemistry of liquid fuels that is crucial to knock control in internal combustion engines and the determination of flammability limits.

ACME (Advanced Combustion via Microgravity Experiments) is scheduled to be in CIR aboard ISS in 2017. The primary goal is to improve efficiency and reduce pollutant emissions in terrestrial combustion. The secondary goal is fire prevention. ACME has five independent experiments to investigate the burning rates and soot formation in laminar, gaseous, non-premixed flames with and without an electric field.

Decadal survey priority AP-8 refers to the numerical model simulation of microgravity flames and fire in spacecraft. Numerical simulation has become a powerful research tool in physical sciences including combustion. It is indispensable in engineering design, including fire spread prediction in buildings. In microgravity combustion research, two notable examples are the predictions of flame quenching by radiation and the reach of limiting length in spreading flames over solids. Theoretical models often provide clue for doing novel experiments, and experiments can provide data to improve the model. Given the limited flight opportunities and the high expense to perform microgravity experiments, modeling is not only helpful but necessary. There are a few public and commercial computational codes that can be used for routine engineering tasks. But for more specialized cases in microgravity combustion, special efforts are necessary to establish the proper model and to perform the experimental comparison.

The recent initiated NASA-PSI (Physical Sciences Informatics) program that fund studies by utilizing data from finished space experiments is an important way to expand the investment on the space experiments. This could be continued and expanded.

The committee does see the potential for collaboration with other government agencies and private industries in this area of research. The Center for the Advancement of Science in Space (CASIS)-initiated collaboration with the National Science Foundation (NSF) on fluid and combustion research that resulted in new funding to new or existing PIs is to be congratulated. Opportunities for collaboration with other federal agencies including the Department of Defense (DoD) could be exploited. The work with Orbital ATK on the Saffire project is one good example of collaboration with private industry.

As in many areas of microgravity research, international collaboration continues to be important for combustion research. In addition to collaboration with international colleagues on an individual basis through the PIs grant, systematic international team collaboration exists in the FLARE project led by the Japan Aerospace Exploration Agency and in the Saffire project between NASA, the European Space Agency, and scientists from Europe, Russia, Japan, and Australia. Such collaboration will continue to be important in addressing decadal survey priorities.

4.4.7.2.3 Research Platforms for Combustion and Approaches Relevant to Exploration

Access to the long-duration microgravity environment, such as on ISS or supply spacecraft, is needed to carry out solid material combustion and fire tests. Preparatory ground-based tests are normally performed for combustion projects before flight experiments. Tests in normal gravity and partial gravity, if appropriate, are often used to highlight the impact of gravity on flames. The use of drop-tower (and sometime airplanes) for short duration (~10 s) of microgravity tests have been useful in providing transient data and in identifying shortcomings of the design hardware. Numerical simulations of combustion and fire are indispensable in combustion and spacecraft fire safety research. In order to support Mars exploration needs, it will be important to have a long duration, high-quality partial gravity experimental platform for combustion and fire safety studies. Because of the increased human flight time for the Mars mission, the complexity of the combustion phenomena, the diversity of materials employed in space applications and the limited opportunity to perform tests in microgravity, sufficient scientific understanding of the gravitational effects on material flammability is unlikely to be achieved in the next several years. Continued scientific research into the post-2024 era will be required to minimize the risk of fire hazard in space exploration.

4.4.7.3 Material Science

4.4.7.3.1 Role in Human Exploration, Recent Advances, and Future Implementation

It should be noted that the decadal survey report indicates that “NASA has relied on existing materials,” and that “very little fundamental research has been conducted on advanced materials for space exploration.” This lack of long-term investment in materials science and technology, which has continued since the decadal survey report was written, may limit progress in future exploration. In addition, there are numerous processes and technologies have been developed or proposed for the support of exploration that require strong focus and emphasis. These include 3D printing and additive manufacturing, smart materials (sensitive to pressure, light temperature, and other external stimuli), smart coatings (nanostructured, super-hydrophilic or hydrophobic), flexible plastic electronics, and self-healing materials. Because these materials have broad applications across many industries, NASA has been able to take advantage of the advances being made by other groups and it will benefit them to continue to do so. At the same time, additional investment by NASA is necessary.

As described in Chapter 3, considering the budget limitations, here has been a strong response by NASA to the materials science priorities in the decadal survey, and several projects have been funded. The findings from this work, combined with the earlier NASA support for microgravity materials studies conducted primarily on space shuttle missions and on the ISS, have established strong foundations on understanding materials behavior and processing in reduced gravity. For example, it has been shown that basic materials processes such as nucleation and growth, phase formation kinetics, interfacial stability and microstructure pattern formation, melt-substrate wetting, phase sedimentation, gas bubble formation, particle pushing, mixing and diffusion, and buoyancy-induced convection, are all significantly altered under reduced gravity conditions. Unfortunately, these findings are still very limited and further emphasis and funding will be necessary to address additional materials processing and behavior in complex systems.

Considerable progress has been also made in modeling materials in recent years. Indeed, materials scientists and engineers have been able to utilize computational methods and numerical techniques to predict materials behavior and/or design new materials. Through these research activities, which have been funded by NASA and other agencies, it has also become obvious that predicting materials behavior even in simple systems is much more complicated compared to their counterparts in structural, mechanical, electrical, and nuclear systems. The modeling and calculation requirements become even more complicated when trying to predict complex materials systems’ behavior under unusual environments encountered in extraterrestrial environments. Eventually, however, sheer computational power and reduced computational costs will enable materials practitioners to address the enormously difficult task of designing new materials.

4.4.7.3.2 Highest-Priority Recommendations for Exploration and Research Platforms

Both AP10 (materials that enable operations in space environments) and AP11 (processing materials on extraterrestrial surfaces) were ranked very high against exploration criteria in the decadal survey. This report also ranked them as having the highest importance to exploration because they can provide critically needed reliable materials performance in harsh and demanding environments, such as better crew shielding or protective coating of surfaces to minimize bacterial or algae growth during long occupancy in crew compartments, or extracting and producing materials, return fuel as well as emergency metallurgical repairs such as welding, joining, melting, or casting.

It is obvious that understanding materials behavior during exploration requires systematic and strategic investigations using ground-based facilities, as well as long-term experiments aboard the ISS.

4.4.7.4 Implementing Decadal Survey Recommendations in Applied Physics

Looking toward the remaining years of the current decadal survey, the committee offers the following guidance for the science implementation within NASA’s Low-g Applied Physical Science Program. This guidance is presented in the context that recognizes that NASA has done well to deliver an applied low-g fluid science program despite a clearly limiting funding level. The committee also acknowledges the quality and relevancy of the current suite of conducted and to-be-conducted flight experiments.

Like most of the science disciplines in the decadal survey, low-g Applied Physical Sciences benefit from both “enabling” and “enabled by” research in order to fully exploit the finite duration of the ideally suited ISS, as well as to continue to draw new investigators and investigations into the field. Applied physical sciences also benefits from connections (intra-agency, government, academia, industry, international and even nonspace-faring nations) to identify priority studies. Fundamental and Applied Physical Science includes projects that are either new or were not necessarily emphasized by the 2011 decadal survey report, such as modern heat pipe research and low-g water transport phenomena relevant to robust long-term plant watering methods and systems. Applied physical sciences also benefits from scientists incentivized to reach outside of the Division of Space Life and Physical Sciences Research and Applications (SLPSRA) to other NASA groups to speed maturation of science and engineering knowledge to the level of applications (i.e., advance ZBOT knowledge to the technology demonstration stage with impacts to current applications as well as advance systems design).

Fluid physics is a prime example of how government, commercial, and academic platforms (U.S./international, terrestrial, ground-based, drop tower, aircraft, commercial suborbitals, ISS, and other free flyers) can be exploited for moving science forward through the fostering of connected, multiuser studies. These studies would be performed using the following approaches: with and without hardware adaption, re-runs, data mining to establish “depth” of science return, broadened research base, maximized investment, and pre-during-post experiment theory using numerical analysis.

4.4.8 Translation to Space Exploration Systems

Looking at the SLPSRA projects identified by NASA as relating to the translational decadal survey recommendations, there has been good progress in the areas of fire safety, cryogenic fluid management, and energy conversion research.

In relation to the technology roadmaps of NASA, new materials such as photonic materials are mentioned. These materials will clearly be an output of some of the NASA projects on colloids. More generally, the research on colloids is important for the elaboration of nanomaterials, nano-enhanced composites (TA10 1), nanostructured (TA10 2), or nano-sized materials (TA10 1.1). Clearly underrepresented is the research on granular materials (AP3), including, addressing dust problems (TA07 2). There are seemingly no projects in plasma research in the works, although this topic could be quite relevant to space exploration.

Other interesting topics are smart materials and materials that are sensitive to light, pressure, etc., or multifunctional (TA10-1) and self-repairing materials (TA7 4.2), which are frequently processed starting from soft materials. Omnipresent is the requirement for lightweight materials. In Europe, there is an ESA project on polymers and metallic foam in microgravity. Indeed, foams might be interesting to produce structures for space and even fabricated in situ in space.

Despite this success, it is important that more projects determine their translational research path to exploration systems as this enabling research receives more attention with NASA’s increased focus on exploration. Thus, in the future it would be useful for NASA to identify far more projects that have a reference to this portion of the decadal survey (anecdotally, the committee knew of some projects in other NASA programs that fit this description) and to make sure these research projects have clear traceability to NASA’s technologies and missions.

As identified in the decadal survey, it is important that NASA continue to focus efforts on transitioning basic research into technology to meet mission needs. Driving toward an improved partnership between scientists, engineers, and program managers who all have important roles in the process is critical. More attention will need to be given to understanding how to accomplish this transition within NASA and systematically track this process. Navigating the gap between research studies and application has been commonly referred to as the “valley of death.” NASA must determine ways to better bridge this valley of death. This is not an uncommon problem, and the committee is aware that other industries are struggling with the same challenges, but having some success. In the medical field, there are several examples of connecting patients to the research as end users and researching and charting that path. NASA could attempt to do the same between mission program managers and research scientists, so that connections are clear and places where the process breaks down is well understood. Simply identifying these pathways by improved traceability would be a good first step.

4.4.8.1 Role in Human Exploration

The 16 TSES priorities are all critical to future human exploration, by definition. But since the NASA Technology Roadmaps work that has been performed since the publication of this decadal survey, it is probably merited that this list of 16 be revisited and tied more closely to the technology prioritization performed by the National Academies and NASA itself.

4.5 SUMMARY OF SCIENCE AND TOP PRIORITIES

In this chapter, the decadal survey science recommendations were examined by the committee within the dual contexts of the 2011 decadal survey and the current NASA exploration strategies. The specific highest-priority recommendations of the committee, as identified in the decadal survey and compared to NASA exploration needs, are presented earlier in this chapter in Table 4.1. After reviewing various issues relevant to the recommendations for each of the discipline areas of space life and physical sciences, the committee assessment produced two additional findings.

Finding 4-2: The current funding levels are insufficient to fully address the significant unknowns and risks of human exploration beyond LEO. Fundamental understanding of human health and behavior risks in microgravity, combined with fundamental microgravity physics and materials, in an integrated manner, is essential to extending the human neighborhood beyond LEO. Significant risks remain, particularly in understanding the radiation environment and its effects, environmental control and life support, human behavior, and protecting long-term crew health with integrated countermeasures. These risks are best addressed in the respective disciplines and in an increasingly integrated fashion.

Finding 4-3: NASA has pursued means to improve internal cross-organizational efforts across the research and technology development landscape, based on the linkages with the Technology Roadmaps.