National Academies Press: OpenBook
« Previous: Front Matter
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×

Summary

Historically, the United States has been a world leader in aerospace endeavors in both the government and commercial sectors. A key factor in aerospace leadership is continuous development of advanced technology, which is particularly critical to U.S. ambitions in space, including a human mission to Mars. NASA is executing a series of aeronautics and space technology programs using a roadmapping process to identify technology needs and improve the management of its technology development portfolio. In 2010 NASA created a set of 14 draft technology roadmaps to guide the development of space technologies. These roadmaps were the subject of a comprehensive external review by the National Academies of Sciences, Engineering, and Medicine.1 That review was documented in the 2012 National Research Council (NRC) report NASA Space Technology Roadmaps and Priorities: Restoring NASA’s Technological Edge and Paving the Way for a New Era in Space.2 As noted in that report, “As the breadth of the country’s space mission has expanded, the necessary technological developments have become less clear, and more effort is required to evaluate the best path for a forward-looking technology development program.”3

In 2015, NASA issued a revised set of roadmaps. A significant new aspect of the update has been the effort to assess the relevance of the technologies by listing the enabling and enhancing technologies for specific design reference missions (DRMs) from the Human Exploration and Operations Mission Directorate and the Science Mission Directorate.4 Also in 2015, the Academies were asked to assess the priority of space technologies in the 2015 roadmaps that were not assessed in the 2012 NRC report.5 The Committee on NASA Technology Roadmaps, which was organized to undertake these assessments, was also tasked with recommending a methodology for conducting independent reviews of future updates to NASA’s technology roadmaps, which are expected to occur every 4 years.

___________________

1 Effective July 1, 2015, the institution is called the National Academies of Sciences, Engineering, and Medicine. References in this report to the National Research Council (NRC) are used in a historical context to refer to activities before that date.

2 NRC, 2012, NASA Space Technology Roadmaps and Priorities: Restoring NASA’s Technological Edge and Paving the Way for a New Era in Space, The National Academies Press, Washington, D.C.

3 NRC, 2012, NASA Space Technology Roadmaps and Priorities, p. 11.

4 NASA, 2015, Technology Roadmaps, Introduction, Crosscutting Technologies, and Index, Washington, D.C., July, pp. i-61 to i-67.

5 This study is not reviewing aeronautics technologies. They appeared for the first time in the 2015 roadmaps, so the 2012 NRC report provides no baseline for comparison.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×

TECHNOLOGY AREA BREAKDOWN STRUCTURE

The content of the 2015 NASA roadmaps is organized using a four-level technology area breakdown structure (TABS). Level 1 represents the technology area (TA), which is the title of the roadmap:

  • TA 1, Launch Propulsion Systems
  • TA 2, In-Space Propulsion Technologies
  • TA 3, Space Power and Energy Storage
  • TA 4, Robotics and Autonomous Systems
  • TA 5, Communications, Navigation, and Orbital Debris Tracking and Characterization Systems
  • TA 6, Human Health, Life Support, and Habitation Systems
  • TA 7, Human Exploration Destination Systems
  • TA 8, Science Instruments, Observatories, and Sensor Systems
  • TA 9, Entry, Descent, and Landing Systems
  • TA 10, Nanotechnology
  • TA 11, Modeling, Simulation, Information Technology, and Processing
  • TA 12, Materials, Structures, Mechanical Systems, and Manufacturing
  • TA 13, Ground and Launch Systems
  • TA 14, Thermal Management Systems
  • TA 15, Aeronautics

Each roadmap describes level 2 technology subareas, level 3 technologies, and level 4 research tasks. The 2012 NRC report focused its review on the level 3 technologies. The TABS for the 2010 draft NASA roadmaps contained 320 level 3 technologies. The modified TABS recommended in the 2012 NRC report contained 295 level 3 technologies. The TABS for the 2015 NASA roadmaps now contains 340 level 3 technologies. The net increase in the number of technologies in the various TABS is due to many factors: Technologies have been added, deleted, revised, merged, and so on. A detailed comparison of the technologies in the 2010, 2012, and 2015 TABS (see Appendix B) revealed that 42 technologies met the criteria for review in this report as “new” technologies. The distribution of these new technologies by TA is as follows:

  • TA 1, Launch Propulsion Systems (11 new technologies)
  • TA 4, Robotics and Autonomous Systems (11 new technologies)
  • TA 5, Communications, Navigation, and Orbital Debris Tracking and Characterization Systems (4 new technologies)
  • TA 7, Human Exploration Destination Systems (1 new technology)
  • TA 9, Entry, Descent, and Landing Systems (3 new technologies)
  • TA 11, Modeling, Simulation, Information Technology, and Processing (8 new technologies)
  • TA 13, Ground and Launch Systems (3 new technologies)
  • TA 14, Thermal Management Systems (1 new technology)

HIGH-PRIORITY TECHNOLOGIES

Based on the committee’s review of the new technologies, which used the prioritization process documented in the 2012 NRC report, five of the new technologies have been ranked as a high priority.

Finding 1. Based on the review and analysis of the 42 new level 3 technologies that appear in the 2015 NASA roadmaps, 5 of those 42 new technologies have been added to the list of 83 high-priority technologies from the 2012 NRC report (listed in numerical order):

  • 4.3.7, Grappling
  • 4.4.8, Remote Interaction
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×
  • 9.2.7, Terrain-Relative Sensing and Characterization
  • 9.2.8, Autonomous Targeting
  • 14.3.2, Thermal Protection System Modeling and Simulation

Technology 4.3.7, Grappling

Grappling systems are ranked as a high priority because they enable the physical capture of small asteroids and asteroid-sourced boulders, the attachment of said objects to robotic spacecraft, and the capture of free-flying spacecraft. Grappling technology would thereby support the transport of asteroids from their natural orbit to a lunar orbit, the human collection and return of samples from a boulder in lunar orbit, orbital debris mitigation, the protection of Earth from small planetary bodies, and the assembly of large spacecraft in orbit for future exploration missions. Potential commercial uses include securing boulder-sized asteroid samples for detailed sampling or processing in commercial space resources operations and securing dead satellites for return, disposal, salvage, or repair. The recent signing of the U.S. Commercial Space Launch Competitiveness Act, which entitles U.S. citizens to any asteroid or space resource obtained (or grappled and returned) from an asteroid may spur interest in commercial asteroid mining. Even so, NASA’s development of grappling technology is a high priority because related work by other government organizations and industry is unlikely to meet NASA-specific needs, especially in light of the Asteroid Retrieval Mission schedule.

The content of technology 4.3.7, Grappling, overlaps somewhat with 4.6.3, Docking and Capture Mechanisms and Interfaces. Technology 4.6.3, Docking, however, focuses on the docking of one spacecraft with another, whereas 4.3.7, Grappling, also includes interactions with natural objects, such as asteroids and boulders from asteroids. Asteroids are massive tumbling targets with unstructured physical properties, and new grappling technologies will be needed to capture either a small asteroid or a boulder from a larger asteroid.

The capture, preloaded manipulation, and retrieval of samples from a boulder transported from the surface of an asteroid represent an unprecedented set of tasks for a NASA robotic or human mission. There is not much to borrow from with respect to developments by the Department of Defense or other organizations involved in aerospace research and development. Development of grappling technologies to enable the robust physical capture and preload of a boulder, other natural bodies, and spacecraft would greatly simplify the robotic control demands of an overall grappling system. The lack of detail in the TA 4 roadmap for this technology is a concern. Only a single level 4 research task was proposed, and its description provides little additional detail compared to the level 3 description. Another level 4 research task could be nonrigid approaches to grappling these large, spinning objects (e.g., looking at grapples attached to adjustable tethers) for de-spinning and securing objects to the spacecraft (or securing the spacecraft and its engines to the object).

Technology 4.4.8, Remote Interaction

Remote Interaction is assigned a high priority because it is defined as providing control and communication methods that enable humans to remotely operate otherwise autonomous systems and robots. Supervisory control incorporates techniques necessary for controlling robotic behaviors using higher-level goals instead of low-level commands, thus requiring robots to have semiautonomous or autonomous behaviors. This technology will support the design of game-changing science and exploration missions, such as new robotic missions at remote locations and simultaneous robotic missions with reduced human oversight. Remote Interaction also includes technology for enabling manual control of remote systems and for enabling operators to monitor system status, assess task progress, perceive the remote environment, and make informed operational decisions, such as tactical plans.

Technology 9.2.7, Terrain-Relative Sensing and Characterization

NASA successfully completed the survey of our solar system with the recent New Horizons mission to Pluto. NASA is continuing planetary exploration with a new era of increased surface exploration. This technology would produce “high-rate, high-accuracy measurements for algorithms that enable safe precision landing near areas of

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×

high scientific interest or predeployed assets.”6 As a result, 9.2.7 would help enable many critical missions in this new era and would likely lead to many surprising new discoveries. Terrain-Relative Sensing and Characterization is the most promising of the TA 9 level 3 technologies reviewed. It is a game-changing technology that could enable important new missions not currently feasible for the next 20 years. It impacts multiple missions in multiple mission areas, both human and robotic. It also has a broad impact across the aerospace community and is already influencing commercial and military autonomous vehicles, such as the rapid advancement of unmanned air vehicles.

Technology 9.2.8, Autonomous Targeting

Autonomous Targeting, which is highly coupled to 9.2.7, Terrain-Relative Sensing and Characterization, is also ranked as a high priority because it is a potentially game-changing technology that would enable important new missions, such as several of the New Frontier missions. By improving the ability of vehicles to assess and characterize the terrain they are facing for landing and exploration, this technology would enable the next step of autonomous targeting, which could be critical when interplanetary distances make remote guidance difficult or impossible. Even if a vehicle is piloted for a human mission, this technology could be critical to help assure a safe landing. Like technology 9.2.7, this technology will have a moderate impact across the aerospace community but mostly on commercial and military autonomous vehicles.

Technology 14.3.2, Thermal Protection Systems Modeling and Simulation

Thermal Protection Systems (TPS) Modeling and Simulation is ranked as a high priority because uncertainties in the modeling of strong radiative shocks are a major limitation in the design of effective heat shields for high-speed entry into the atmospheres of Earth, Mars, and other bodies. Early TPS design was largely empirical, based on extensive direct (and expensive) testing in Earth’s atmosphere. Testing in ground test facilities is also difficult and expensive because of the extreme environments associated with atmospheric entry. Computational methods employing physics-based models, including modeling of materials, are improving to the point that with validation via laboratory and flight testing and verification of TPS, they can more reliably predict TPS performance. However, further development is required to build confidence that design margins can be substantially reduced and that weight savings will be realized. Major challenges remain in increasing the accuracy and precision of physics-based modeling of entry shocks, thermal radiation, and their interaction with an ablating heat shield, challenges that are addressed by this technology. Currently, uncertainties are +80 percent to −50 percent for Mars return missions; missions to other destinations have different uncertainty ranges.7 The goal of proposed research for technology 14.3.2 is to reduce uncertainty below 25 percent for all planetary missions. This reduction in uncertainty would enable the use of heat shields that weigh less, thereby reducing spacecraft weight and/or increasing allowable payload weight. This technology couples closely with the 2012 highly ranked crosscutting technology of X.5, Entry, Descent, and Landing TPS, which includes both rigid and flexible systems. For that technology to advance and realize its potential, the modeling must improve. As noted in the roadmap for TA 14, “a significant challenge facing the development of this technology is the limitations in the available flight and ground test data” (p. TA 14-93).

HIGHEST-PRIORITY TECHNOLOGIES

The 2012 NRC report defines the highest-priority technologies in terms of their ability to support three technology objectives:

  • Technology Objective A, Human Space Exploration: Extend and sustain human activities beyond low Earth orbit. This objective is focused on human missions.

___________________

6 NASA, 2015, NASA Technology Roadmaps: TA 9 Entry, Descent, and Landing Systems, Washington, D.C., p. TA 9-25.

7 NASA, 2015, Technology Roadmaps, TA 14: Thermal Management Systems, p. TA 14-32.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×
  • Technology Objective B, In Situ Measurements: Explore the evolution of the solar system and the potential for life elsewhere. This objective includes both robotic and human missions.
  • Technology Objective C, Remote Measurements: Expand our understanding of Earth and the universe in which we live. This objective is focused on robotic missions.

These three objectives encompass the full breadth of NASA’s endeavors in space science, Earth science, and exploration. The 2012 NRC report does not assess or comment on the relative priority of these technology objectives.

The 2012 report includes a list of the 16 highest-priority technologies. However, 5 of the 16 were groups of related technologies, designated X.1 through X.5. Altogether, the top 16 (individual and grouped) technologies comprised 31 individual technologies.8

The committee added three of the five new technologies ranked as high priority to the list of highest-priority technologies from the 2012 NRC report. The new list of grouped technologies, which includes two additional technologies from the TABS in the 2012 NRC report, appears below, and the new list of the highest-priority technologies appears in Table S.1. In both the list and the table, new items are shaded.

X.1, Radiation Mitigation for Human Spaceflight

6.5.1, Radiation Risk Assessment Modeling

6.5.2, Radiation Mitigation9

6.5.3, Radiation Protection Systems

6.5.4, Radiation Prediction

6.5.5, Radiation Monitoring Technology

X.2, Lightweight and Multifunctional Materials and Structures

10.1.1, (Nano) Lightweight Materials and Structures

12.1.1, Materials: 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, Environmental Control and Life Support System (ECLSS)

6.1.1, ECLSS: Air Revitalization

6.1.2, ECLSS: Water Recovery and Management

6.1.3, ECLSS: Waste Management

6.1.4, ECLSS: Habitation

X.4, Guidance, Navigation, and Control (GN&C)10

4.6.2, Relative Guidance Algorithms (for Automation Rendezvous and Docking)11

5.4.3, Onboard Autonomous Navigation and Maneuvering (for Position, Navigation, and Timing)

9.2.7, Terrain-Relative Sensing and Characterization (for Descent and Targeting)

9.2.8, Autonomous Targeting (for Descent and Targeting)

X.5, Entry, Descent, and Landing (EDL) Thermal Protection Systems (TPS)

9.1.1, Rigid Thermal Protection Systems

9.1.2, Flexible Thermal Protection Systems

14.3.1, Ascent/Entry TPS

X.6, Grappling, Docking, and Handling

4.3.6, Sample Acquisition and Handling (formerly Robotic Drilling and Sample Handling)

4.3.7, Grappling

4.6.3, Docking and Capture Mechanisms and Interfaces

___________________

8 The relative priority of the individual and grouped technologies varies from one technology objective to another, as shown in Table S.1.

9 Renamed Radiation Mitigation and Biological Countermeasures in the 2015 TABS.

10 Technology 9.4.7, GN&C Sensors and Systems (for entry, descent, and landing), which was an element of group X.4 in the 2012 NRC report, has been deleted because it has no technical content in the 2015 roadmap for TA 9.

11 Renamed GN&C Algorithms in the 2015 TABS.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×

TABLE S.1 The Committee’s Final 2016 List of Highest-Priority Technologies, Ranked by Technology Objective, Comprising 17 Individual and Grouped Technologies, with Up to 9 per Technology Objective

Highest-Priority Technologies for Technology Objective A, Human Space Exploration Highest-Priority Technologies for Technology Objective B, In Situ Measurements Highest-Priority Technologies for Technology Objective C, Remote Measurements
Radiation Mitigation for Human Spaceflight (X.1) GN&C (X.4) Optical Systems (Instruments and Sensors) (8.1.3)
Long-Duration Crew Health (6.3.2) Solar Power Generation (Photovoltaic and Thermal) (3.1.3) High-Contrast Imaging and Spectroscopy 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 Generation (3.1.5) Lightweight and Multifunctional Materials and Structures (X.2)
(Nuclear) Thermal Propulsion (2.2.3) EDL TPS (X.5) Active Thermal Control of Cryogenic Systems (14.1.2)
Lightweight and Multifunctional Materials and Structures (X.2) In Situ Instruments and Sensors (8.3.3) Electric Propulsion (2.2.1)
Fission Power Generation (3.1.5) Lightweight and Multifunctional Materials and Structures (X.2) Solar Power Generation (Photo-voltaic and Thermal) (3.1.3)
EDL TPS (X.5) Extreme Terrain Mobility (4.2.1)
Grappling, Docking, and Handling (X.6) Grappling, Docking, and Handling (X.6)

Finding 2. Based on the review and analysis of the five new level 3 technologies that have been added to the list of high-priority technologies, three of the technologies (4.3.7, 9.2.7, and 9.2.8), along with two other technologies (4.3.6 and 4.6.3) that previously appeared in the interim list of highest-priority technologies in the 2012 NRC report, have been added to the list of the 16 highest-priority technologies, as follows:

  • Technology group X.4, Guidance, Navigation, and Control, has been expanded to include 9.2.7, Terrain-Relative Sensing and Characterization (for Descent and Targeting), and 9.2.8, Autonomous Targeting (for Descent and Targeting). Technology 9.4.7, GN&C Sensors and Systems (for Entry, Descent, and Landing), which has no technical content in the 2015 roadmap for TA 9, has been deleted.
  • A new technology group has been created: X.6, Grappling, Docking, and Handling. This group consists of 4.3.6, Sample Acquisition and Handling (formerly Robotic Drilling and Sample Handling); 4.3.7, Grappling; and 4.6.3, Docking and Capture Mechanisms and Interfaces. Group X.6 has been added to the list of highest-priority technologies for Technology Objective A, Human Space Exploration, and Technology Objective B, In Situ Measurements.

FUTURE INDEPENDENT REVIEWS

This report recommends a methodology for conducting independent reviews of future updates to NASA’s space technology roadmaps. This methodology takes into account the extent of changes expected to be implemented in the roadmap from one generation to the next and the time elapsed since the most recent comprehensive independent review of the roadmaps. This methodology is summarized in the following four recommendations.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×

PROPOSED METHODOLOGY FOR FUTURE INDEPENDENT REVIEWS

Recommendation 1. Independent reviews of the roadmaps should be conducted whenever there is a significant change to them. NASA’s technology roadmap revision cycle is expected to be performed every 4 years, but significant changes in NASA direction may necessitate more frequent reviews. The reviews should be one of two types: either a comprehensive review of the complete set of roadmaps (including TA 15), such as the one performed in 2012, or a focused review, such as the one in this report. Focused reviews can be conducted using more limited resources because they address only a subset of the total technology portfolio. In making recommendations about the review methodology, each future independent review should focus on the methodology to be used for the subsequent review rather than on a long-range plan covering multiple reviews.

Recommendation 2. Before the next independent review, the NASA Technology Executive Council and the Center Technology Council (NTEC/CTC), in accordance with their charters, should prioritize the technologies that will be examined in the review. The NTEC/CTC should present the results and rationale for the priorities to the next independent review committee. The prioritization process should take into account the factors included in the prioritization process described in Appendix C. It should also be supported by additional factors such as linkage of technologies to a concise list of design reference missions (DRMs), including an assessment of the technologies as enabling or enhancing; the use of systems analysis to establish the technology’s benefit to the mission relative to the benefit of alternative technologies; and correlation of technology priorities with both expected funding and required development schedule.

Recommendation 3. As part of its prioritization process, NTEC/CTC should classify each technology to be examined by the next independent review (at TABS level 3 or level 4) as Lead, Collaborate, Watch, or Park. In addition, the Office of the Chief Technologist (OCT) should update NASA’s electronic technology database, TechPort, so that it, too, indicates for each technology whether NASA is pursuing it as Lead, Collaborate, Watch, or Park. For collaborative efforts, OCT should include in TechPort details on the nature of the collaboration, including facilities, flight testing, and the development of crosscutting technologies.

Recommendation 4. The next independent review should be a comprehensive review if there have been major changes to the roadmaps and/or the DRMs, or it should be a focused review and cover only new technologies if the number of new technologies in the next version of the roadmaps once again constitutes a small percentage of the total number of technologies. The scope of the review should include the following:

  • The prioritization of technologies previously completed by the NTEC/CTC and the process used to conduct the prioritization.
  • Roadmap for TA 15 Aeronautics.
  • The first volume of the technology roadmaps, TA 0 Introduction, Crosscutting Technologies, and Index.
  • The relevance of technologies to the DRMs as either enabling or enhancing.
  • Recommendation for the methodology to be used for the review that in turn follows it.
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×
Page 1
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×
Page 2
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×
Page 3
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×
Page 4
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×
Page 5
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×
Page 6
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. NASA Space Technology Roadmaps and Priorities Revisited. Washington, DC: The National Academies Press. doi: 10.17226/23582.
×
Page 7
Next: 1 Introduction »
NASA Space Technology Roadmaps and Priorities Revisited Get This Book
×
Buy Paperback | $55.00 Buy Ebook | $43.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Historically, the United States has been a world leader in aerospace endeavors in both the government and commercial sectors. A key factor in aerospace leadership is continuous development of advanced technology, which is critical to U.S. ambitions in space, including a human mission to Mars. To continue to achieve progress, NASA is currently executing a series of aeronautics and space technology programs using a roadmapping process to identify technology needs and improve the management of its technology development portfolio.

NASA created a set of 14 draft technology roadmaps in 2010 to guide the development of space technologies. In 2015, NASA issued a revised set of roadmaps. A significant new aspect of the update has been the effort to assess the relevance of the technologies by listing the enabling and enhancing technologies for specific design reference missions (DRMs) from the Human Exploration and Operations Mission Directorate and the Science Mission Directorate. NASA Space Technology Roadmaps and Priorities Revisited prioritizes new technologies in the 2015 roadmaps and recommends a methodology for conducting independent reviews of future updates to NASA’s space technology roadmaps, which are expected to occur every 4 years.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!