3

Integrated Ranking of Top Technical
Challenges and High-Priority Technologies

As explained in Chapter 2, the panels’ assessment of the level 3 technologies in each individual roadmap considered a wide variety of factors.1 In prioritizing the 83 technologies evaluated as high-priority by the panels across all 14 draft roadmaps, the steering committee established an organizing framework that addressed balance across NASA mission areas; relevance in meeting the highest-priority technical challenges; and expectations that significant progress could be made in the next 5 years of the 30-year window of the roadmaps. Furthermore, the steering committee constrained the number of highest-priority technologies recommended in the final list in the belief that in the face of probable scarce resources, focusing initially on a small number of the highest-priority technologies offers the best chance to make the greatest impact, especially while agency mission areas, particularly in exploration, are being refined and can be shaped by technology options. Within this organizing framework, technology objectives were defined by the steering committee to address the breadth of NASA missions and group related technologies.

TECHNOLOGY OBJECTIVES

The 2011 NASA Strategic Plan (NASA, 2011, p. 4) states:

New in this 2011 Strategic Plan is a strategic goal that emphasizes the importance of supporting the underlying capabilities that enable NASA’s missions.

The steering committee interpreted this formulation of NASA’s strategic vision as the need to assess the technologies by the measure of how well they supported NASA’s various missions.

The question became one of identifying the totality of NASA’s missions that were all-inclusive of the agency’s responsibilities and yet easily distinguished by the type of technologies needed to support them. The steering committee defined the following technology objectives to serve as an organizing framework for prioritization of technical challenges and roadmap technologies.

image

1The draft space technology roadmaps are available at http://www.nasa.gov/offices/oct/strategic_integration/technology_roadmap.html.



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3 Integrated Ranking of Top Technical Challenges and High-Priority Technologies As explained in Chapter 2, the panels’ assessment of the level 3 technologies in each individual roadmap con - sidered a wide variety of factors.1 In prioritizing the 83 technologies evaluated as high-priority by the panels across all 14 draft roadmaps, the steering committee established an organizing framework that addressed balance across NASA mission areas; relevance in meeting the highest-priority technical challenges; and expectations that significant progress could be made in the next 5 years of the 30-year window of the roadmaps. Furthermore, the steering committee constrained the number of highest-priority technologies recommended in the final list in the belief that in the face of probable scarce resources, focusing initially on a small number of the highest- priority technologies offers the best chance to make the greatest impact, especially while agency mission areas, particularly in exploration, are being refined and can be shaped by technology options. Within this organizing framework, technology objectives were defined by the steering committee to address the breadth of NASA missions and group related technologies. TECHNOLOGY OBJECTIVES The 2011 NASA Strategic Plan (NASA, 2011, p. 4) states: New in this 2011 Strategic Plan is a strategic goal that emphasizes the importance of supporting the underlying capabilities that enable NASA’s missions. The steering committee interpreted this formulation of NASA’s strategic vision as the need to assess the technolo - gies by the measure of how well they supported NASA’s various missions. The question became one of identifying the totality of NASA’s missions that were all-inclusive of the agency’s responsibilities and yet easily distinguished by the type of technologies needed to support them. The steering committee defined the following technology objectives to serve as an organizing framework for prioritization of technical challenges and roadmap technologies. 1 The draft space technology roadmaps are available at http://www.nasa.gov/offices/oct/strategic_integration/technology_roadmap.html. 59

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60 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES Technology Objective A: Extend and sustain human activities beyond low Earth orbit. Supporting technologies would enable humans to survive long voyages throughout the solar system, get to their chosen destination, work effectively, and return safely. This objective includes a major part of NASA’s mission to send humans beyond the protection of the Van Allen belts, mitigate the effects of space radiation and long exposure to the microgravity environment, enable the crew to accomplish the goals of the mission (contained in Technology Objective B), and then return to Earth safely. This objective includes using the International Space Station (ISS) for technology advancement to support future human space exploration, providing opportunities for commercial companies to offer services to low Earth orbit and beyond, and developing the launch capability required for safe access to locations beyond low Earth orbit. Technology Objective B: Explore the evolution of the solar system and the potential for life elsewhere. Supporting technologies would enable humans and robots to perform in situ measurements on Earth (astrobiol - ogy) and on other planetary bodies. This objective is concerned with the in situ analysis of planetary bodies in the solar system. It includes the detailed analysis of the physical and chemical properties and processes that shape planetary environments and the study of the geologic and biological processes that explain how life evolved on Earth and whether it exists elsewhere. It involves development of instruments for in situ measurements and the associated data analysis. This objective includes all the in situ aspects of planetary science; measurement of interior properties, atmospheres, particles, and fields of planets, moons, and small bodies; and methods of planetary protection. Technology Objective C: Expand our understanding of Earth and the universe in which we live. Supporting technologies would enable remote measurements from platforms that orbit or fly by Earth and other planetary bodies, and from other in-space and ground-based observatories. This objective includes astrophysics research; stellar, planetary, galactic, and extra-galactic astronomy; par- astronomy; par- ticle astrophysics and fundamental physics related to astronomical objects; solar and heliospheric physics; and magnetospheric physics and solar-planetary interactions. This objective also includes space-based observational Earth-system science and applications aimed at improving our understanding of Earth and its responses to natural and human-induced changes. This objective includes all space science activities that rely on measurements obtained remotely from various observational platforms. These objectives are not independent and are often shared by a single mission (e.g., humans to explore planetary bodies or to service observatories, as was the case with the Hubble Space Telescope), and there are technologies that support more than one of these objectives (e.g., multifunctional structures, electric propulsion, GN&C). Yet this taxonomy is a useful way to categorize NASA’s responsibilities as described in its strategic plan and serves to prioritize the various technologies and technical challenges identified in this study. Balance One of the steering committee’s basic assumptions was that NASA would continue to pursue a balanced space program across its mission areas of human exploration, space science, space operations, space technology, and aeronautics. Indeed, this balance is emphasized in the 2011 NASA Strategic Plan (NASA, 2011) and addressed in the NRC report America’s Future in Space: Aligning the Civil Space Program with National Needs (NRC, 2009), where breadth across NASA’s mission areas contributes to making the U.S. a leader in space. Therefore, since the technology program of the Office of the Chief Technologist (OCT) should broadly support the breadth of the agency’s missions and serve to open up options for future missions, the steering committee established priorities

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61 INTEGRATED RANKING OF TOP TECHNICAL CHALLENGES AND HIGH-PRIORITY TECHNOLOGIES TABLE 3.1 Relationships Among NASA’s Mission Areas and the Three Technology Objectives Technology Objective B: Technology Objective A: Explore the Evolution of the Technology Objective C: Extend and Sustain Human Solar System and the Potential Expand Understanding of Activities Beyond Low for Life Elsewhere (In Situ Earth and the Universe NASA Mission Areas Earth Orbit Measurements) (Remote Measurements) Planetary Science X X X Astrophysics X Earth Science X X Heliophysics X Xa Human Exploration X X Operations X X X a If telescopes and observatories are serviceable by astronauts. for each of the three technology objectives independently. No one technology objective area was given priority over another. Table 3.1 relates the three technology objectives with NASA’s mission areas and illustrates the balance of using the adopted organizing framework. As mentioned previously, aeronautics was not part of the roadmap study. TECHNICAL CHALLENGES With the three technology objectives defined, the steering committee evaluated the top technical challenges from the panels’ prioritized rankings for each roadmap TA01-14.2 In some cases, the steering committee combined similar challenges, particularly across roadmaps. The steering committee members began the process by voting on the highest-priority technical challenges in multiple iterations, first using a 1-10 ranking to rate their top 10 chal - lenges for each objective, and then using a weighted scale: 0 = Not relevant; 1 = Minor importance; 3 = Significant; 9 = Essential. The steering committee then discussed any significant scoring variations by different members and the relative priority of each challenge implied by the average and mean scores of the members’ scores. This discussion continued until a final group consensus was reached on a prioritized list of the final ten technical challenges for each objective. The top 10 technical challenges for each of the three technology objectives are described below. Top Technical Challenges for Technology Objective A: Extend and sustain human activities beyond low Earth orbit. A1. Improved Access to Space: Dramatically reduce the total cost and increase the reliability and safety of access to space. A2. Space Radiation Health Effects: Improve understanding of space radiation effects on humans and develop radiation protection technologies to enable long-duration space missions. A3. Long-Duration Health Effects: Minimize the crew health effects of long-duration space missions (other than space radiation). A4. Long-Duration ECLSS: Achieve reliable, closed-loop environmental control and life support systems (ECLSS) to enable long-duration human missions beyond low Earth orbit. 2 See the sections titled “Top Technical Challenges” in Appendixes D-Q for the panels’ prioritized technical challenge rankings.

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62 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES A5. Rapid Crew Transit: Establish propulsion capability for rapid crew transit to and from Mars or other distant targets. A6. Lightweight Space Structures: Develop innovative lightweight materials and structures to reduce the mass and improve the performance of space systems such as (1) launch vehicle and payload systems; (2) space and surface habitats that protect the crew, including multifunctional structures that enable lightweight radiation shielding, implement self-monitoring capability, and require minimum crew maintenance time; and (3) lightweight, deployable synthetic aperture radar antennas, including reliable mechanisms and structures for large-aperture space systems that can be stowed compactly for launch and yet achieve high-precision final shapes. A7. Increase Available Power: Eliminate the constraint of power availability for space missions by improv- ing energy generation and storage with reliable power systems that can survive the wide range of environments unique to NASA missions. A8. Mass to Surface: Deliver more payload to destinations in the solar system. A9. Precision Landing: Increase the ability to land more safely and precisely at a variety of planetary locales and at a variety of times. A10. Autonomous Rendezvous and Dock: Achieve highly reliable, autonomous rendezvous, proximity operations, and capture of free-flying space objects. Top Technical Challenges for Technology Objective B: Explore the evolution of the solar system and the potential for life elsewhere (in situ measurements). B1. Improved Access to Space: Dramatically reduce the total cost and increase the reliability and safety of access to space. B2. Precision Landing: Increase the ability to land more safely and precisely at a variety of planetary locales and at a variety of times. B3. Robotic Maneuvering: Enable mobile robotic systems to autonomously and verifiably navigate and avoid hazards and increase the robustness of landing systems to surface hazards. B4. Life Detection: Improve sensors for in situ analysis to determine if synthesis of organic matter may exist today, whether there is evidence that life ever emerged, and whether there are habitats with the necessary condi - tions to sustain life on other planetary bodies. B5. High-Power Electric Propulsion: Develop high-power electric propulsion systems along with the enabling power system technology. B6. Autonomous Rendezvous and Dock: Achieve highly reliable, autonomous rendezvous, proximity opera- tions, and capture of free-flying space objects. B7. Increase Available Power: Eliminate the constraint of power availability for space missions by improv- ing energy generation and storage with reliable power systems that can survive the wide range of environments unique to NASA missions. B8. Mass to Surface: Deliver more payload to destinations in the solar system.

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63 INTEGRATED RANKING OF TOP TECHNICAL CHALLENGES AND HIGH-PRIORITY TECHNOLOGIES B9. Lightweight Space Structures: Develop innovative lightweight materials and structures to reduce the mass and improve the performance of space systems such as (1) launch vehicle and payload systems; (2) space and surface habitats that protect the crew, including multifunctional structures that enable lightweight radiation shielding, implement self-monitoring capability, and require minimum crew maintenance time; and (3) lightweight, deployable synthetic aperture radar antennas, including reliable mechanisms and structures for large-aperture space systems that can be stowed compactly for launch and yet achieve high-precision final shapes. B10. Higher Data Rates: Minimize constraints imposed by communication data rate and range. Top Technical Challenges for Technology Objective C: Expand understanding of Earth and the universe in which we live (remote measurements). C1. Improved Access to Space: Dramatically reduce the total cost and increase the reliability and safety of access to space. C2. New Astronomical Telescopes: Develop a new generation of astronomical telescopes that enable discov - ery of habitable planets, facilitate advances in solar physics, and enable the study of faint structures around bright objects by developing high-contrast imaging and spectroscopic technologies to provide unprecedented sensitivity, field of view, and spectroscopy of faint objects. C3. Lightweight Space Structures: Develop innovative lightweight materials and structures to reduce the mass and improve the performance of space systems such as (1) launch vehicle and payload systems; (2) space and surface habitats that protect the crew, including multifunctional structures that enable lightweight radiation shielding, implement self-monitoring capability, and require minimum crew maintenance time; and (3) lightweight, deployable synthetic aperture radar antennas, including reliable mechanisms and structures for large-aperture space systems that can be stowed compactly for launch and yet achieve high-precision final shapes. C4. Increase Available Power: Eliminate the constraint of power availability for space missions by improv- ing energy generation and storage with reliable power systems that can survive the wide range of environments unique to NASA missions. C5. Higher Data Rates: Minimize constraints imposed by communication data rate and range. C6. High-Power Electric Propulsion: Develop high-power electric propulsion systems along with the enabling power system technology. C7. Design Software: Advance new validated computational design, analysis, and simulation methods for design, certification, and reliability of materials, structures, and thermal, EDL, and other systems. C8. Structural Monitoring: Develop means for monitoring structural health and sustainability for long dura - tion missions, including integration of unobtrusive sensors and responsive on-board systems. C9. Improved Flight Computers: Develop advanced flight-capable devices and system software for real-time flight computing with low-power, radiation-hard, and fault-tolerant hardware. C10. Cryogenic Storage and Transfer: Develop long-term storage and transfer of cryogens in space using systems that approach near-zero boil-off.

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64 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES HIGHEST-PRIORITY LEVEL 3 TECHNOLOGIES ACROSS ALL ROADMAPS Process for Prioritizing Technologies Across Roadmaps Utilizing the panel results, which established a high degree of correlation between high-priority level 3 tech - nologies and the respective technical challenges for each roadmap (see correlation matrices in Appendixes D-Q), the steering committee was able to relate high-priority technologies that aligned with each of the three technol - ogy objectives. This organizing principle in turn helped categorize similar technologies with similar drivers (i.e., technologies driven by keeping humans alive, able to be productive, and transported; in situ measurements; and remote measurements) and enabled prioritization among them on a meaningful basis. The process followed by the steering committee was as follows: First, the steering committee considered only the 83 high-priority level 3 technology as selected by the panels. These 83 technologies are listed in Table 3.2. Next, following the correlation procedure used by the panels, the steering committee mapped those technologies against the top technical challenges for each of the three objectives. In many cases the correlation matrix was sparsely filled; for example, technologies from roadmaps relating to human exploration or life support would have little correlation with Technology Objective C, which is focused primarily on remote measurements from observational platforms, except if servicing is done by astronauts. The correlation information was then used by the steering committee as it voted on the priority of technologies against the three objectives. Each steering committee member voted on the importance of each technology to each objective using a weighted scale: 0 = Not relevant; 1 = Minor importance; 3 = Significant; 9 = Essential. The total of the members’ scores assigned to each technology was then summed to create a rank-ordered list of technologies for each technology objective. There were several iterations of voting and discussion first to develop an interim list of 11 to 15 technologies per objective, followed by another iteration of voting and discussion to obtain a consensus on the final list of 7 or 8 technologies per objective. The robustness of the final results was tested by the steering committee in numerous ways. The steering committee used other weighting schemes (such as voting on top five technologies rather than using a 0-1-3-9 weighting factor) and other voting schemes (such as voting to remove technologies rather than voting to include them). Initially the steering committee had removed from the voting any technologies that were uncorrelated to any technical challenge; to make certain all technologies were properly considered, that constraint was relaxed and all 83 technologies were voted upon. In all cases, however, the changes to the methods had little or no impact on the final outcome. Tables 3.3, 3.4, and 3.5 show the correlation matrices for the high-priority technologies and the top technical challenges for Objectives A, B, and C, respectively. All empty rows (i.e., all technologies that do not correlate to any challenges for that objective) have been removed from the matrix. The steering committee determined that, in several instances, technologies on the original list of 83 high- priority technologies were highly coupled or addressed the same technology pedigree. During the prioritization process, these highly coupled technologies were grouped together and considered as one unit. Table 3.6 shows the mapping of the technology area breakdown structure (TABS) technologies to each unified technology that was considered during the final prioritization process. Results and Recommendations for Prioritized Technologies Across Roadmaps Table 3.7 represents the steering committee’s consensus viewpoint following the first iteration of voting, discussion, and prioritization, with the technologies listed by objective in ranked order. To obtain as short a list as is reasonable in the face of anticipated constrained budgets, a second iteration of prioritization was conducted

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65 INTEGRATED RANKING OF TOP TECHNICAL CHALLENGES AND HIGH-PRIORITY TECHNOLOGIES TABLE 3.2 The 83 High-Priority Level 3 Technologies Selected by the Panels TA01 Launch Propulsion Systems TA06 Human Health, Life Support, and TA09 Entry, Descent, and Landing (EDL) Systems 1.3.1 Turbine Based Combined Habitation Systems 9.4.7 GN&C Sensors and Systems (EDL) Cycle (TBCC) 6.5.5 Radiation Monitoring Technology 9.1.1 Rigid Thermal Protection Systems 1.3.2 Rocket Based Combined 6.5.3 Radiation Protection Systems 9.1.2 Flexible Thermal Protection Systems Cycle (RBCC) 6.5.1 Radiation Risk Assessment Modeling 9.1.4 Deployment Hypersonic Decelerators 6.1.4 Habitation 9.4.5 EDL Modeling and Simulation TA02 In-Space Propulsion 6.1.3 Environmental Control and Life 9.4.6 EDL Instrumentation and Health Technologies Support System (ECLSS) Waste Monitoring 2.2.1 Electric Propulsion Management 9.4.4 Atmospheric and Surface Characterization 2.4.2 Propellant Storage and 6.3.2 Long-Duration Crew Health 9.4.3 EDL System Integration and Analysis Transfer 6.1.2 ECLSS Water Recovery and 2.2.3 (Nuclear) Thermal Propulsion TA10 Nanotechnology Management 2.1.7 Micro-Propulsion 10.1.1 (Nano) Lightweight Materials and 6.2.1 Extravehicular Activity (EVA) Structures Pressure Garment TA03 Space Power and Energy Storage 10.2.1 (Nano) Energy Generation 6.5.4 Radiation Prediction 3.1.3 Solar Power Generation 10.3.1 Nanopropellants 6.5.2 Radiation Mitigation (Photovoltaic and Thermal) 10.4.1 (Nano) Sensors and Actuators 6.4.2 Fire Detection and Suppression 3.1.5 Fission Power Generation 6.1.1 Air Revitalization 3.3.3 Power Distribution and TA11 Modeling, Simulation, and Information 6.2.2 EVA Portable Life Support System Transmission Technology and Processing 6.4.4 Fire Remediation 3.3.5 Power Conversion and 11.1.1 Flight Computing Regulation 11.1.2 Ground Computing TA07 Human Exploration Destination Systems 3.2.1 Batteries 11.2.4a Science Modeling and Simulation 7.1.3 In Situ Resource Utilization (ISRU) 3.1.4 Radioisotope Power 11.3.1 Distributed Simulation Products/Production Generation 7.2.1 Autonomous Logistics Management TA12 Materials, Structures, Mechanical Systems, 7.6.2 Construction and Assembly TA04 Robotics, TeleRobotics, and and Manufacturing 7.6.3 Dust Prevention and Mitigation Autonomous Systems 12.2.5 Structures: Innovative, Multifunctional 7.1.4 ISRU Manufacturing/Infrastructure 4.6.2 Relative Guidance Algorithms Concepts etc. 4.6.3 Docking and Capture 12.2.1 Structures: Lightweight Concepts 7.1.2 ISRU Resource Acquisition Mechanisms/Interfaces 12.1.1 Materials: Lightweight Structure 7.3.2 Surface Mobility 4.5.1 Vehicle System Management 12.2.2 Structures: Design and Certification 7.2.4 Food Production, Processing, and and FDIR Methods Preservation 4.3.2 Dexterous Manipulation 12.5.1 Nondestructive Evaluation and Sensors 7.4.2 Habitation Evolution 4.4.2 Supervisory Control 12.3.4 Mechanisms: Design and Analysis Tools 7.4.3 Smart Habitats 4.2.1 Extreme Terrain Mobility and Methods 7.2.2 Maintenance Systems 4.3.6 Robotic Drilling and Sample 12.3.1 Deployables, Docking, and Interfaces Processing 12.3.5 Mechanisms: Reliability/Life Assessment/ TA08 Science Instruments, Observatories, and 4.2.4 Small Body/Microgravity Health Monitoring Sensor Systems Mobility 12.4.2 Intelligent Integrated Manufacturing and 8.2.4 High-Contrast Imaging and Cyber Physical Systems Spectroscopy Technologies TA05 Communication and Navigation 8.1.3 Optical Systems (Instruments and 5.4.3 Onboard Autonomous TA14 Thermal Management Systems Sensors) Navigation and Maneuvering 14.3.1 Ascent/Entry Thermal Protection Systems 8.1.1 Detectors and Focal Planes 5.4.1 Timekeeping and Time 14.1.2 Active Thermal Control of Cryogenic 8.3.3 In Situ Instruments and Sensors Distribution Systems 8.2.5 Wireless Spacecraft Technology 5.3.2 Adaptive Network Topology 8.1.5 Lasers for Instruments and Sensors 5.5.1 Radio Systems 8.1.2 Electronics for Instruments and Sensors NOTE: Technologies are listed by roadmap/technology area (TA01 through TA14; there are no high-priority technologies in TA13). Within each technology area, technologies are listed by the QFD score assigned by the panels, in descending order. This sequencing may be considered a rough approximation of the relative priority of the technologies within a given technology area.

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66 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES TABLE 3.3 Correlation Matrix Between High-Priority Technologies Selected by Panels and Top Technical Challenges for Technology Objective A. Rows with all blanks are not shown. Rendezvous and Dock Increase Available Precision Landing Improved Access Space Structures Space Radiation Mass to Surface Long Duration Long Duration Health Effects Health Effects Autonomous Lightweight Rapid Crew to Space Technology Objective A: ECLSS Transit Power Extend and sustain human activities beyond low Earth orbit ● 1.3.1 TBCC ● 1.3.2 RBCC ● 2.2.3 (Nuclear) Thermal Propulsion ● 3.1.3 Solar Power Generation (Photovoltaic and Thermal) ● 3.1.5 Fission Power Generation ● 3.2.1 Batteries ● 3.3.3 Power Distribution and Transmission ● 3.3.5 Power Conversion and Regulation ● 4.6.2 Relative Guidance Algorithms ● 4.6.3 Docking and Capture Mechanisms/Interfaces ● 5.4.1 Timekeeping and Time Distribution ● ● 5.4.3 Onboard Autonomous Navigation & Maneuvering ● 6.1.1 Air Revitalization ● 6.1.2 ECLSS Water Recovery and Management ● 6.1.3 ECLSS Waste Management ● 6.1.4 Habitation ● 6.2.2 EVA Portable Life Support System ● 6.3.2 Long-Duration Crew Health ● 6.5.1 Radiation Risk Assessment Modeling ● 6.5.2 Radiation Mitigation ● 6.5.3 Radiation Protection Systems ● 6.5.4 Radiation Prediction ● 6.5.5 Radiation Monitoring Technology ● ● 9.1.1 Rigid Thermal Protection Systems ● ● 9.1.2 Flexible Thermal Protection Systems ● ● 9.1.4 Deployment Hypersonic Decelerators ● 9.4.7 GN&C Sensors and Systems (EDL) ● 10.1.1 (Nano) Lightweight Materials and Structures ● ● 12.1.1 Materials: Lightweight Structure ● ● ● 12.2.1 Structures: Lightweight Concepts ● 12.2.2 Structures: Design and Certification Methods ● 12.2.5 Structures: Innovative, Multifunctional Concepts ● 12.3.1 Deployables, Docking and Interfaces ● ● 14.3.1 Ascent/Entry TPS

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67 INTEGRATED RANKING OF TOP TECHNICAL CHALLENGES AND HIGH-PRIORITY TECHNOLOGIES TABLE 3.4 Correlation Matrix Between High-Priority Technologies Selected by Panels and Top Technical Challenges for Technology Objective B. Rows with all blanks are not shown. Autonomous Rendezvous and Dock High-Power Electric Propulsion Robotic Surface Maneuvering Lightweight Space Structures Improved Access to Space Increase Available Power Higher Data Rates Precision Landing Mass to Surface Life Detection Technology Objective B: Explore the evolution of the solar system and the potential for life elsewhere (in situ measurements) ● 1.3.1 TBCC ● 1.3.2 RBCC ● 2.2.1 Electric Propulsion 3.1.3 Solar Power Generation (Photovoltaic and ● Thermal) ● 3.1.5 Fission Power Generation ● 3.2.1 Batteries ● 3.3.3 Power Distribution and Transmission ● 3.3.5 Power Conversion and Regulation ● 4.2.1 Extreme Terrain Mobility ● ● 4.6.2 Relative Guidance Algorithms ● 4.6.3 Docking & Capture Mechanisms/Interfaces ● 5.3.2 Adaptive Network Topology ● ● 5.4.1 Timekeeping and Time Distribution ● ● ● 5.4.3 Onboard Autonomous Navigation and Maneuvering ● 5.5.1 Radio Systems ● 8.3.3 In Situ Instruments ● ● 9.1.1 Rigid Thermal Protection Systems ● ● 9.1.2 Flexible Thermal Protection Systems ● ● 9.1.4 Deployment Hypersonic Decelerators ● 9.4.7 EDL GN&C Sensors and Systems ● 10.1.1 (Nano) Lightweight Materials and Structures ● ● 12.1.1 Lightweight Structure ● ● ● 12.2.1 Structures: Lightweight Concepts ● 12.2.2 Structures: Design and Certification Methods ● 12.2.5 Structures: Innovative, Multifunctional Concepts ● 12.3.1 Deployables, Docking and Interfaces ● ● 14.3.1 Ascent/Entry TPS

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68 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES TABLE 3.5 Correlation Matrix Between High-Priority Technologies Selected by Panels and Top Technical Challenges for Technology Objective C. Rows with all blanks are not shown. High-Power Electric Propulsion New Astronomical Telescopes Lightweight Space Structures Cryogenic Propellant Storage Improved Flight Computers Improved Access to Space Increase Available Power Structural Monitoring Higher Data Rates Design Software Technology Objective C: Expand understanding of Earth and the universe (remote measurements) ● 1.3.1 TBCC ● 1.3.2 RBCC ● 2.2.1 Electric Propulsion ● 2.4.2 Propellant Storage and Transfer 3.1.3 Solar Power Generation (Photovoltaic and ● Thermal) ● 3.1.5 Fission Power Generation ● 3.2.1 Batteries ● 3.3.3 Power Distribution and Transmission ● 3.3.5 Power Conversion and Regulation ● 5.3.2 Adaptive Network Topology ● 5.4.1 Timekeeping and Time Distribution ● 5.5.1 Radio Systems ● 8.1.1 Detectors & Focal Planes ● 8.1.3 Optical Systems ● 8.2.4 High-Contrast Imaging and Spectroscopy ● 9.4.5 EDL Modeling and Simulation ● 10.1.1 (Nano) Lightweight Materials and Structures ● 11.1.1 Flight Computing ● 12.1.1 Lightweight Structure ● ● 12.2.1 Structures: Lightweight Concepts ● ● 12.2.2 Structures: Design and Certification Methods ● 12.2.5 Structure Innovative, Multifunctional Concepts ● 12.3.4 Design and Analysis Tools and Methods 12.3.5 Mechanisms Reliability / Life Assessment / ● Health Monitoring ● 12.5.1 Nondestructive Evaluation & Sensors ● 14.1.2 Active Thermal Control of Cryogenic Systems

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69 INTEGRATED RANKING OF TOP TECHNICAL CHALLENGES AND HIGH-PRIORITY TECHNOLOGIES TABLE 3.6 Technologies That Represent Multiple Highly Coupled Technologies from the Technology Area Breakdown Structure Unified Technology Technology Area Breakdown Structure Technologies X.1 Radiation Mitigation for Human 6.5.1 Radiation Risk Assessment Modeling Spaceflight 6.5.2 Radiation Mitigation 6.5.3 Radiation Protection Systems 6.5.4 Radiation Prediction 6.5.5 Monitoring Technology X.2 Lightweight and Multifunctional 10.1.1 (Nano) Lightweight Materials and Structures Materials and Structures 12.1.1 Lightweight Structures 12.2.1 Structures: Lightweight Concepts 12.2.2 Structures: Design and Certification Methods 12.2.5 Structures: Innovative, Multifunctional Concepts X.3 ECLSS 6.1.1 Air Revitalization 6.1.2 Water Recovery and Management 6.1.3 Waste Management 6.1.4 Habitation X.4 GN&C 4.6.2 Relative Guidance Algorithms 5.4.3 Onboard Autonomous Navigation and Maneuvering 9.4.7 EDL GN&C Sensors and Systems X.5 EDL TPS 9.1.1 Rigid Thermal Protection Systems 9.1.2 Flexible Thermal Protection Systems 14.3.1 Ascent/Entry TPS TABLE 3.7 Initial Prioritization of Top Technologies, Categorized by Technology Objective Technology Objective B: Technology Objective C: Technology Objective A: Explore the evolution of the solar system Expand understanding of Extend and sustain human activities and the potential for life elsewhere (in situ Earth and the universe beyond low Earth orbit measurements) (remote measurements) Radiation Mitigation for Human GN&C (X.4) (Instrument and Sensor) Optical Systems Spaceflight (X.1) (8.1.3) Long-Duration (Crew) Health (6.3.2) Electric Propulsion (2.2.1) High-Contrast Imaging and Spectroscopy Technologies (8.2.4) ECLSS (X.3) Solar Power Generation (Photovoltaic Detectors & Focal Planes (8.1.1) and Thermal) (3.1.3) GN&C (X.4) In Situ (Instruments and Sensor) (8.3.3) Lightweight and Multifunctional Materials and Structures (X.2) Thermal Propulsion (2.2.3) Fission (Power) (3.1.5) Radioisotope (Power) (3.1.4) Fission (Power) (3.1.5) Extreme Terrain Mobility (4.2.1) Electric Propulsion (2.2.1) Lightweight and Multifunctional Materials Lightweight and Multifunctional Materials Solar Power Generation (Photovoltaic and and Structures (X.2) and Structures (X.2) Thermal) (3.1.3) EDL TPS (X.5) Radioisotope (Power) (3.1.4) Science Modeling and Simulation (11.2.4a) Atmosphere and Surface Characterization Robotic Drilling and Sample Handling Batteries (3.2.1) (9.4.4) (4.3.6) Propellant Storage and Transfer (2.4.2) EDL TPS (X.5) (Instrument and Sensor) Electronics (8.1.2) Pressure Garment (6.2.1) Docking and Capture Mechanisms/ Active Thermal Control of Cryogenic Interfaces (4.6.3) Systems (14.1.2) (Mechanisms) Reliability / Life Assessment / Health Monitoring (12.3.5) Vehicle System Management and FDIR (4.5.1)

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70 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES TABLE 3.8 Final Prioritization of Top Technologies, Categorized by Technology Objective Technology Objective B: Technology Objective C: Technology Objective A: Explore the evolution of the solar system Expand understanding of Extend and sustain human activities and the potential for life elsewhere (in situ Earth and the universe beyond low Earth orbit measurements) (remote measurements) Radiation Mitigation for Human GN&C (X.4) (Instrument and Sensor) Optical Systems Spaceflight (X.1) (8.1.3) Long-Duration (Crew) Health (6.3.2) Solar Power Generation (Photovoltaic High-Contrast Imaging and Spectroscopy and Thermal) (3.1.3) Technologies (8.2.4) ECLSS (X.3) Electric Propulsion (2.2.1) Detectors and Focal Planes (8.1.1) GN&C (X.4) Fission (Power)(3.1.5) Lightweight and Multifunctional Materials and Structures (X.2) Thermal Propulsion (2.2.3) EDL TPS (X.5) Active Thermal Control of Cryogenic Systems (14.1.2) Lightweight and Multifunctional Materials In Situ (Instruments and Sensor) (8.3.3) Electric Propulsion (2.2.1) and Structures (X.2) Fission (Power) (3.1.5) Lightweight and Multifunctional Materials Solar Power Generation (Photovoltaic and and Structures (X.2) Thermal) (3.1.3) EDL TPS (X.5) Extreme Terrain Mobility (4.2.1) to determine the highest-priority technologies to emphasize over the next 5 years. It is not that other technology development is unimportant, but rather that some technology development can wait, some depends on obtaining prior results before progress can be made, and some is best served by low-level funding of exploratory concept development before proceeding. Alternatively, some technologies are so game-changing that early results are needed to define and shape possible paths to future missions (e.g., radiation protection). Table 3.8 shows the final technology prioritization for each technology objective, listed in ranked order. It should be noted that the prioritizations in Tables 3.7 and 3.8 may differ from the prioritizations determined by the panels in Appendixes D through Q for two principal reasons. First, the steering committee organized and prioritized its technologies against the three different technology objectives, which the panels did not do; a technol - ogy’s priority can change significantly depending upon the objective. Second, the steering committee emphasized technology development in the next 5 years, a specific timing constraint that was not given to the panels. The steering committee’s consensus viewpoint on a short list of the highest-priority technologies to emphasize over the next 5 years is shown in Table 3.8 (three columns with 16 different technologies). Table 3.9 provides a single list of the 16 technologies and shows which technology objectives each one supports. The relationship of these technologies to the top technical challenges is shown in Tables 3.10, 3.11, and 3.12. The steering committee assumes NASA will pursue all three objectives in a balanced approach, each according to the approved resources and mission plans allocated. The steering committee does not recommend or advocate support for one objective over the others. The steering committee noted that Technology Objective B has many common technology needs with Objectives A and C. Objectives A and C each have dominant technologies to enable NASA’s strategic goals; i.e., radiation protection, long-duration-mission crew health, and ECLS are mostly unique to Objective A, while optical systems, high-contrast imaging and spectrometry, and detectors are mostly unique to Objective C. However, GN&C, lightweight and multifunctional materials and structures, and solar power are primary to Objective B but are common to all three objectives. The steering committee reasoned that this intentionally limited set of recommended high-priority technologies constituted a scope that could reasonably be accommodated within the most likely expected funding level available for technology development by OCT (in the range of $500 million to $1 billion annually). Also considered within the scope of a balanced technology development program is the importance of low technology readiness level (TRL; 1 and 2) exploratory concept development and high-TRL flight demonstrations. The steering committee

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71 INTEGRATED RANKING OF TOP TECHNICAL CHALLENGES AND HIGH-PRIORITY TECHNOLOGIES TABLE 3.9 The 16 Technologies That Appear in the Final Prioritization, Showing the Priority Assigned for Each Technology Objective Technologies included in the final prioritization, Technology Technology Technology listed by TABS number Objective A Objective B Objective C 2.2.1 Electric Propulsion 3 6 2.2.3 Thermal Propulsion 5 3.1.3 Solar Power Generation (Photovoltaic and Thermal) 2 7 3.1.5 Fission (Power) 7 4 4.2.1 Extreme Terrain Mobility 8 6.3.2 Long-Duration (Crew) Health 2 8.1.1 Detectors & Focal Planes 3 8.1.3 (Instrument and Sensor) Optical Systems 1 8.2.4 High-Contrast Imaging and Spectroscopy Technologies 2 8.3.3 In Situ (Instruments and Sensors) 6 14.1.2 Active Thermal Control of Cryogenic Systems 5 X.1 Radiation Mitigation for Human Spaceflight 1 X.2 Lightweight and Multifunctional Materials and Structures 6 7 4 X.3 ECLSS 3 X.4 GN&C 4 1 X.5 EDL TPS 8 5 NOTE: The content of technologies X.1 through X.5 is shown in Table 3.6. TABLE 3.10 Linkages Between Top Technologies and Technical Challenges for Technology Objective A Lightweight and Multifunctional Radiation Mitigation for Human Materials and Structures (X.2) Long-Duration (Crew) Health Thermal Propulsion (2.2.3) Fission (Power) (3.1.5) Spaceflight (X.1) EDL TPS (X.5) ECLSS (X. 3) GN&C (X.4) Technology Objective A: (6.3.2) Extend and sustain human activities beyond low Earth orbit ● 1 Improved Access to Space ● 2 Space Radiation Health Effects ● 3 Long Duration Health Effects ● 4 Long Duration ECLSS ● 5 Rapid Crew Transit ● ● 6 Lightweight Space Structures ● 7 Increase Available Power ● ● 8 Mass to Surface ● ● 9 Precision Landing ● 10 Autonomous Rendezvous and Dock

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72 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES TABLE 3.11 Linkages Between Top Technologies and Technical Challenges for Technology Objective B Lightweight and Multifunctional Materials In Situ (Instruments & Sensor) (8.3.3) (Photovoltaic and Thermal) (3.1.3) Extreme Terrain Mobility (4.2.1) Electric Propulsion (2.2.1) Solar Power Generation Fission (Power) (3.1.5) and Structures (X.2) EDL TPS (X.5) GN&C (X.4) Technology Objective B: Explore the evolution of the solar system and the potential for life elsewhere (in situ measurements) ● 1 Improved Access to Space ● ● 2 Precision Landing ● ● 3 Robotic Surface Maneuvering ● 4 Life Detection ● 5 High-Power Electric Propulsion ● 6 Autonomous Rendezvous and Dock ● ● 7 Increase Available Power ● 8 Mass to Surface ● 9 Lightweight Space Structures ● 10 Higher Data Rates consensus is that low-TRL, NASA Innovative Advanced Concepts-like funding should be on the order of 10 per- cent of the total, and that the research should quickly weed out the least competitive concepts, focusing on those that show the greatest promise in addressing the top technical challenges. At the high-TRL end of the spectrum, flight demonstrations, while expensive, are sometimes essential to reach a readiness level required for transition of a technology to an operational system. Such technology flight demonstrations are considered on a case-by-case basis when there is ample “pull” from the user organization, including a reasonable level of cost sharing. At some point, the scale of technology development for nuclear thermal propulsion and fission power tech - nologies in Table 3.8 will grow to a level where large-scale efforts may need to be deferred if the OCT space technology research budget is substantially below the expected level. Even in such a case, technology development should still proceed at a low level in these high-priority areas because the technology will take years to advance and they represent game-changing approaches to NASA’s mission.3 Recommendation. Technology Development Priorities. During the next 5 years, NASA technology devel- opment efforts should focus on (1) the 16 identified high-priority technologies and associated top technical challenges, (2) a modest but significant investment in low-TRL technology (on the order of 10 percent of NASA’s technology development budget), and (3) flight demonstrations for technologies that are at a high- TRL when there is sufficient interest and shared cost by the intended user. 3 By statute, DOE must take the lead in the development of reactor components for a NASA fission power or nuclear thermal rocket pro - pulsion system.

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73 INTEGRATED RANKING OF TOP TECHNICAL CHALLENGES AND HIGH-PRIORITY TECHNOLOGIES TABLE 3.12 Linkages Between Top Technologies and Technical Challenges for Technology Objective C Solar Power Generation (Photovoltaic Active Thermal Control of Cryogenic Detectors and Focal Planes (8.1.1) Lightweight and Multifunctional (Instrument and Sensor) Optical Materials and Structures (X.2) High-Contrast Imaging and Electric Propulsion (2.2.1) and Thermal) (3.1.3) Spectroscopy (8.2.4) Systems (14.1.2) Systems (8.1.3) Technology Objective C: Expand understanding of Earth and the universe (remote measurements) ● 1 Improved Access to Space ● ● ● 2 New Astronomical Telescopes ● 3 Lightweight Space Structures ● 4 Increase Available Power ● 5 Higher Data Rates ● 6 High-Power Electric Propulsion 7 Design Software ● 8 Structural Monitoring 9 Improved Flight Computers ● ● 10 Cryogenic Storage and Transfer The Importance of Improved Access to Space In most cases, the steering committee and the panels have identified technologies that will make substantial progress in achieving the top technical challenges at the steering committee level and at the panel level. However, the importance of a challenge is not diminished simply because technologies to achieve the challenge are not read - ily available. For example, improving access to space (by dramatically reducing the total cost and increasing the reliability and safety of access to space) was the number one technical challenge for each of the three technology objectives cited. Despite this, only one of the top 16 technologies selected by the steering committee that was relevant to this challenge, Lightweight and Multifunctional Materials and Structures (X.2), made it to the final short list of technologies for emphasis over the next 5 years. While low-cost access to space is critically important, technologies to achieve that particular challenge are few, and some of the high-leverage factors affecting launch cost tend to be operational rather than technological. Some non-technological approaches to solving this problem may exist, such as on-orbit assembly or ground operations. In addition, for any given set of launch vehicles, advanced technologies that reduce payload mass and volume could reduce launch costs on a per mission basis if they allow missions to be launched with smaller, less expensive launch vehicles. In Appendix D, the Propulsion and Power Panel describes possible architecture changes that would increase launch rates and potentially reduce costs. In Appendix P, this panel addresses operational efficiencies associated with ground operations (TA13, Ground and Launch Systems Processing) that are not technology issues. The panel also identified two high-priority technologies to align with this challenge: Turbine Based Combined Cycle (TBCC) and Rocket Based Combined Cycle (RBCC) engines. RBCC and TBCC would provide a revolutionary new, next-generation reusable launch system. Although the steering committee acknowledges the potential benefits of TBCC/RBCC technologies toward the challenge of low-cost access to space, it did not recommend these for

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74 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES highest emphasis for the following reasons. The development of either an RBCC or a TBCC propulsion system would require a national-level effort that includes partnerships with the DOD and other organizations. Before a national-level program could be started, vehicle system design trades would need to clearly show the benefits of chosen configurations and designs. Because combined cycle propulsion is so integral to the design of the airframe, configuration and design are critical. To date, one of the main deterrents to RBCC and TBCC is their development cost. Also, since these systems are targeted for reusable configurations, high flight rates are required to attain promised cost savings. Technologies Near a Tipping Point A “tipping point” is defined as a point in the research process such that a small increase in the research effort could produce a large advance in its technology readiness. The steering committee identified two such technolo - gies nearing a tipping point: ASRGs and cryogenic storage and transfer. Both of these technologies are ready for near-term flight demonstrations. Advanced Stirling Radioisotope Generator Radioisotope power systems provide electrical power for spacecraft and systems that are unable to use solar power. Radioisotope power systems, in the form of Radioisotope Thermoelectric Generators (RTGs), have been used reliably for 50 years to enable NASA’s solar system exploration missions. Plutonium-238 (Pu-238) is the only isotope suitable as the heat source for RPSs, but no Pu-238 has been produced in the United States since the late 1980s. Currently, Pu-238 is not being produced anywhere in the world, and the stockpile of Pu-238 available to NASA is almost depleted (NRC, 2006, 2010). Because of the limited supply of Plutonium-238, NASA and the Department of Energy have begun research and development in higher-efficiency technologies. The Advanced Stirling Radioisotope Generator (ASRG) is a new type of radioisotope power system still in development. An ASRG uses a Stirling engine coupled to linear alternators to convert heat to electricity. ASRG Stirling converters have efficiencies several times greater than the thermoelectric converters of traditional RTGs, and thus they require much less Pu-238 for the same electric power output (NRC, 2010). The demonstration of the long-duration reliability and flight readiness of ASRGs is still to be achieved, however. The planetary science decadal survey committee determined that the completion and validation of the Advanced Stirling Radioisotope Generator is its highest priority for near-term technology investment (NRC, 2011b, p. 307). In 2011, NASA selected three candidate Discovery missions for potential downselect for launch in 2016. Two of the candidates would utilize ASRG flight units and would demonstrate their utility on long-duration, deep-space missions. NASA is on a good course to bring this critical technology at a “tipping point” to full demonstration. Recommendation. Advanced Stirling Radioisotope Generators. The NASA Office of the Chief Technolo- gist should work with the Science Mission Directorate and the Department of Energy to help bring Advanced Stirling Radioisotope Generator technology hardware to flight demonstration on a suitable space mission beyond low Earth orbit. Finding: Plutonium-238. Consistent with findings of previous NRC reports on the subject of plutonium-238 (NRC, 2010, 2011b), restarting the fuel supply is urgently needed. Even with the successful development of Advanced Stirling Radioisotope Generators, if the funds to restart the fuel supply are not authorized and appropriated, it will be impossible for the United States to conduct certain planned critical deep-space mis - sions after this decade.

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75 INTEGRATED RANKING OF TOP TECHNICAL CHALLENGES AND HIGH-PRIORITY TECHNOLOGIES Reduced Gravity Cryogenic Storage and Transfer The storage and handling of cryogenic fluids will be needed to support missions beyond low Earth orbit. Technology to effectively store, manage, and transfer propellants over long periods in space would improve mis - sion feasibility and affordability. Flight experiments are needed to test and validate key capabilities and technologies required for the storage and transfer of cryogenic propellants to and from advanced propulsion stages and propellant depots. The ISS could play an important role in validating long-term storage and handling of cryogenic propellants. Technologies to be demonstrated include: • Cryogenic fluid instrumentation and sensors • Passive thermal control • Active thermal control Instrumentation and sensors are needed to ascertain and monitor fluid mass and location in reduced gravity tanks. Cryogenic systems include passive techniques (such as multilayer insulation and vapor-cooled shields), as well as active thermal control techniques (such as cryocoolers) to manage remaining heat leaks after passive techniques are applied. In addition to supporting cryogenic propellant storage and transfer, active thermal control technology can enable long-term storage of consumables such as LOX for human missions and support scientific instruments that require cryogenic conditions. The 2011 NRC decadal survey Recapturing a Future for Space Exploration: Life and Physical Sciences Research for a New Era recommended near-term research and technol- ogy development in zero-boil-off propellant storage (both passive and active techniques) and cryogenic handling and gauging (NRC, 2011a). These technologies are approaching a high level of technical maturity but remain to be tested and demonstrated in a reduced-gravity environment. Recommendation. Cryogenic Storage and Handling. Reduced-gravity cryogenic storage and handling tech- nology is close to a “tipping point,” and NASA should perform on-orbit flight testing and flight demonstrations to establish technology readiness. Relevance of High-Priority Technologies to National and Commercial Space Needs When pursuing the 16 technologies recommended by the steering committee as high-priority efforts in the next 5 years, it is useful to simultaneously consider the value of those technologies to the interests of others outside NASA, specifically those that address broader national needs as well as the needs of the commercial space industry. Alignment with national and commercial needs outside NASA (both aerospace needs and non-aerospace needs) was a scoring category used by the panels as they made their initial assessments of all the level 3 technologies, although the weighting factor given to this category was not high relative to other categories such as benefit and risk. The steering committee identified those technologies that would either be essential or could make a signifi - cant contribution to national and commercial space interests outside NASA (shown in Table 3.13). In the case of national needs—for example, dual-use technology of interest to the Department of Defense—this information shows those technologies that offer the best chance to partner with other government institutions through sharing of information and resources. The technologies shown in Table 3.13 were selected based on NASA’s most critical needs and highest priorities. NASA is the first and primary user, although there is relevance to other national interests. The strong importance of commercial space activities to NASA was recognized by the steering committee, and this relationship is discussed further in Chapter 4.

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76 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES TABLE 3.13 Relevance of High-Priority Technologies to National and Commercial Space Needs National Commercial Technologies included in the final prioritization, listed by TABS number Needs Needs   2.2.1 Electric Propulsion ○  2.2.3 Thermal Propulsion ● ● 3.1.3 Solar Power Generation (Photovoltaic and Thermal) ○ ○ 3.1.5 Fission (Power) ○ ○ 4.2.1 Extreme Terrain Mobility ○  6.3.2 Long-Duration (Crew) Health   8.1.1 Detectors & Focal Planes   8.1.3 (Instrument and Sensor) Optical Systems   8.2.4 High-Contrast Imaging and Spectroscopy Technologies ○ ○ 8.3.3 In Situ (Instruments and Sensors)   14.1.2 Active Thermal Control of Cryogenic Systems ○ ○ X.1 Radiation Mitigation for Human Spaceflight   X.2 Lightweight and Multifunctional Materials and Structures ○  X.3 ECLSS   X.4 GN&C   X.5 EDL TPS ● Essential Key  Significant ○ Minor REFERENCES NASA (National Aeronautics and Space Administration). 2011. 2011 Strategic Plan. NASA, Washington, D.C. NRC (National Research Council). 2006. Priorities in Space Science Enabled by Nuclear Power and Propulsion. The National Academies Press, Washington, D.C. NRC. 2009. America’s Future in Space: Aligning the Civil Space Program with National Needs. The National Academies Press, Washington, D.C. NRC. 2010. Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration. The National Academies Press, Washington, D.C. NRC. 2011a. Recapturing a Future for Space Exploration: Life and Physical Sciences Research for a New Era. The National Academies Press, Washington, D.C. NRC. 2011b. Vision and Voyages for Planetary Science in the Decade 2013-2022. The National Academies Press, Washington, D.C.