Advanced Technology for Human Support in Space (1997)

Human factors focuses on the role of humans in complex systems, the design of equipment and facilities for human use, and the development of environments for comfort and safety. Human factors research is conducted in several technical or academic subject areas, including ergonomics, biomechanics, anthropometrics, workload, and performance. Research on human activities in space is called space human factors (SHF) research. The mission of OLMSA SHF personnel is to understand the impact of SHF on crewed missions, to collect and interpret relevant human factors data in support of space and aerospace missions, to provide operational support for ongoing missions and mission planning, and to make available human factors data, research, and experimental studies to the aviation and aerospace communities at large.

Although human factors work is carried out at many NASA sites, the committee limited its analysis to the two sites where SHF research is funded by OLMSA: JSC and ARC. Standard "terrestrial" human factors concerns were not addressed, although it appears that NASA is generally aware of and responsive to human factors needs.

As with all things related to SHF, when humans participate in long-duration spaceflight, unknowns could affect planning. For example, a truly revolutionary propulsion system that would significantly shorten the time crews were exposed to microgravity, isolation, and radiation would vastly simplify the SHF problems. Likewise, the emergence of dramatically autonomous systems might affect crew size, training, and workload. It appears that the only safe assumptions at this time are (1) that available spaceflight technology will improve incrementally over the next two decades, and (2) that long-duration crewed missions will not be

influenced as much by new technology as by the inherent limitations of the human organism and its ability to survive the concomitant physical rigors, intellectual challenges, and psychosocial interactions in space. Accordingly, the committee's assessment of the present state of SHF research focuses on its application and applicability to future space missions, especially lunar surface habitation and an eventual Mars mission in the years 2010 to 2020.

By definition, the participation of humans in space exploration makes safety and the ability to perform physically and psychologically for prolonged periods integral parts of all planning. Areas where human factors information and expertise are relevant include spacecraft design, life support systems, and extravehicular suits and systems. Previous NRC reports have repeatedly stressed that there is a major difference between "short-term" and "long-term" human spaceflight (NRC, 1993, 1994). Almost all U.S. experience to date has been limited to "short-term" missions and indicates that, for the most part, short-term exposure is reasonably well tolerated. However, on voyages of the duration associated with a mission to Mars using chemical propulsion (about 600 to 1,000 days)¹ physiological and psychological terra incognito will be encountered, and no amount of ''fully informed consent" or "volunteerism'' can vitiate the need for serious scientific study of related problems and the pursuit of realistic solutions in order to manage intelligently the attendant risks. The Russian space program has shown that stays in space of more than 400 days are possible, but missions with a single crew kept together for more than 600 days are well beyond anyone's experience to date.

Based on information in NASA's Long Term Plans in Human Exploration (NASA's "official plans" for the future), the committee assumed that a Mars mission in about 20 years is a realistic goal. This hypothetical future beyond the ISS is divisible into three separate, but intimately related, phases: (1) lunar surface habitation; (2) transfer to and from Mars; and (3) Mars habitation. Based on the requirements for long-duration human missions, numerous topics of research and concern should be addressed. Some of these topics are shown in Table 5-1.

Research areas identified by the SHF program include:

• perception—mathematical models of human perceptual systems: vision, pattern perception, audition, motion perception, spatial understanding, and haptics
• cognition—understanding situational awareness, modeling cognitive workload, and evaluating usability and effectiveness of human-automation interfaces

• communications systems—multimedia, multichannel communication technologies responsive to human perceptual characteristics; compression techniques, lag minimization, and speech perception
• anthropometrics and physical interfaces—evaluations of human-tool interfaces; virtual prototyping to accommodate human variability; and ergonomic analyses of tasks in microgravity
• training procedures and technologies—methods and evaluation metrics for training skills, decision making, coordinated team activities, and routine and unusual tasks; technologies for recognizing the need for and delivering training, as required by the mission

The goals of the approved OLMSA Space Human Factors Program Plan (NASA, 1995) are to:

• "Expand knowledge of human psychological and physical capabilities and limitations in space through basic and applied research, tests and evaluations . . ."
• "Develop cost-effective technologies that support integrating the human and system elements of space flight . . ."
• "Ensure that mission planners use SHF research results and technology developments to increase the probability of mission success and crew safety . . ."
• "Make NASA technology available to the private sector for Earth applications . . . [and] use new technologies developed by private industry where appropriate . . ."

The NASA mission in human factors is currently rather segregated into space and aeronautic components. In general, JSC has the charter to examine SHF issues related to the Space Shuttle, the ISS, and future long-duration space flight but concentrates almost exclusively on the Space Shuttle and ISS. ARC is engaged in work on aviation human factors (especially cockpit issues) and more basic research. There is little overlap or connection between the two centers. The overall impression is that they are targeting very different problem areas. JSC primarily functions as an operational problem-solver, where research questions are raised by experience or known difficulties or are driven by mission requirements. JSC SHF activities are primarily concerned with the "here and now" of space operations. ARC primarily operates as a research community, studying issues of perception, workload, and cognition that have been encountered during aeronautical flight. Occasionally, specific crew-related problems have been catalysts for investigations at ARC, and some interest was expressed in finding applications for research going beyond aeronautics into spaceflight and other fields.

The SHF program has been funded at slightly less than $2 million in FY94, FY95, and FY96. This is enough to fund only a handful of projects (about 10 in 1996).

During calendar year 1996, NASA staff involved in the program from NASA headquarters, JSC, ARC, and KSC were drafting a requirements document for SHF based on projected human lunar and Mars long-duration space flights in the second decade of the twenty-first century. The committee observed some of these discussions and examined a preliminary draft, but the final document was not completed by the end of this study.

In general, SHF research and technology areas are very broad and open-ended, especially as compared to EMC and EVA. It is difficult to establish clear baselines, given the inherent variability of human performance, workload, and personality. Given such breadth, the committee was aware that some of these topics overlapped other NASA codes and divisions, especially with regard to workload, performance, training, and engineering. Nevertheless, the presence of the SHF program within OLMSA as a crucial component of crewed spaceflight is an acknowledgment that a human presence in space will require dedicated, significant, new research, technology development, and resource investments.

Summary Finding. Lunar/Mars crewed missions will require careful consideration of numerous SHF issues. But at the time of this writing, no SHF priorities had been established with regard to NASA's long-term goals. Thus, research should be refocused from generating pure knowledge toward concerted, coordinated efforts to achieve prioritized goals ("goal-oriented" research) for crew safety and the overall success of long-duration missions.

Finding. Currently, there are no established priorities for future human missions, which magnifies the problems associated with the lack of communication and coordination among projects. There is a general awareness that SHF issues and questions related to a mission to Mars or the establishment of a lunar or Mars base must be understood, but there is no apparent programmatic design to answer those questions.

Recommendation 5-1. Programmatic priorities should be based on mission requirements. All parts of NASA with expertise in space human factors should contribute to the development of these priorities and should allocate resources (staff, time, and funding) to facilitate coordination and communication of the program. In a program of this kind, which needs to address many open questions, the need for "goal-directed" research should take precedence over the traditional encouragement of "heart's-desire" research.

Finding. NASA has not dedicated significant resources to long-duration SHF issues. Topics such as life support appear to dominate NASA's thinking in

preparing technology for long-duration missions; but these missions will create unique physiological, psychosocial, performance, and cognitive requirements that must also be understood prior to launch. The emphasis on predictive models, physical and biomechanical models, and passive monitoring is uneven. Some programs are aggressively pursuing them, while others are concentrating on more descriptive models with minimal predictive power. Both predictive and system models will fit very well within the large-scale, integrated, concurrent engineering effort that NASA will have to make for long-duration missions.

Recommendation 5-2. Solving problems specific to NASA's goals for crewed, long-term spaceflight should be the prevailing factor in developing NASA Research Announcements in advance of seeking proposals, in screening proposals prior to peer review, and in the final selection of proposals. Top priorities for long-duration crewed missions should include:

• understanding crew interactions in sustained, isolated, microgravity (vehicle, lunar or Mars) environments
• human performance (both cognitive and physical) and decision making in sustained microgravity environments, including the development of decision support systems
• information management and communication needs, including the role and deployment of virtual environment aids for training, mission rehearsal, maintenance, and emergency or unusual situations
• automation and allocation of functions between humans and computers
• interaction with intelligent systems

Recommendation 5-3. NASA should increase emphasis on the development of predictive models. For example, predictive models can be important with regard to mental workload. Because much of the work in this area so far has been descriptive, the mental workload for a given task can be measured only as the task is actually being performed. This deprives engineers of information that would help in designing new systems in which interactions among humans, equipment, and the environment could optimize mental workload. Predictive models would provide engineers with an analytical tool for evaluating alternative designs in order to study and devise mechanisms to facilitate intellectual performance.

Summary Finding. No discernible work in the SHF program is directed at the long-term needs for the OLMSA program, i.e., no projects are specifically directed at issues unique to lunar or Mars missions. Some work in support of current missions may be indirectly applicable to future missions, but this is fortuitous rather than purposeful.

Finding. No current work at JSC is dedicated to the direct support of lunar/ Mars SHF goals. The committee's investigation revealed that some of the tools (see below) being developed might support far-term, long-duration missions, but they were being developed strictly in support of near-term mission operational requirements. Their applicability to the future would be fortuitous, not planned. Motivated individuals and teams are exploring possible ways they might impact future missions, but their success would be in spite of the system, not because of it. Some examples of promising ongoing SHF efforts that may be applicable to long-duration Mars missions include:

• the development of virtual environment tools and virtual reality displays for training, mission design, and mission rehearsal, especially for long-duration flights on which boredom, skills retention, and emergency planning must be considered
• work on "fatigue and countermeasures," which is significantly applicable to current programs, both in flight and on the ground. Obviously, the role of countermeasures to fatigue will be even more important on flights of long duration

Technologies and systems outside of NASA that might be directly applicable to future plans are not well known or properly appreciated. A consequence of this insularity is that NASA may attempt to apply or modify existing, frequently less than "state-of-the-art" and/or cost-effective technology, when better, perhaps cheaper, tools exist elsewhere.

Recommendation 5-4. Research should be devoted specifically to future long-duration missions. Research on space human factors should always be goal directed, seeking possible applications for far-term missions. Sufficient dedicated funding lines, personnel, and priorities will be needed if objectives are to be achieved.

Recommendation 5-5. Formal programs to increase interaction among projects within NASA space human factors must be established. NASA should encourage a broad view and promote effective and efficient programs between disciplines within the organization, as well as formal, periodic communication with extra-mural organizations to seek out technologies that may be applicable to NASA space human factors programs.

Summary Finding. The SHF Program Plan, which was approved in December 1995, outlines topical areas only in general terms. The Program Plan describes a very broad and ambitious undertaking but lacks a specific, long-term mission to which goals can be tailored. It fails to delineate milestones or dates for specific

achievements or new capabilities. Therefore, the utility and relevance of the plan to current and future NASA programs are not clear.

Finding. The Human Exploration and Development of Space Strategic Plan (NASA, 1996) provides an evolutionary plan that moves from the ISS to the Martian surface, with a possible intermediate phase on the lunar surface. For the most part, this is a thoughtful document, but it contains many assumptions about areas that have not been completely researched. For example, it states, "Human factors research and technology will also ensure . . . that interpersonal interactions are planned to maintain a healthy, constructive attitude, thus enhancing productivity and mission success among an international, culturally-diverse crew (NASA, 1996)." This statement expresses assumptions about the psychosocial dynamics of small groups sequestered for prolonged periods of time that are not justified by current knowledge.

Recommendation 5-6. Crew time and the assignment of individuals to perform various space human factors experiments (psychological and physiological) aboard the ISS will require detailed advanced planning. Crew rotation will present problems for the investigation of the physiological effects of prolonged exposure to microgravity and for the investigation of the psychological effects of prolonged isolation and sequestration in a very limited living area. It will also be essential to study aspects of habitability on the ISS that must be incorporated into the design of a Mars transfer vehicle and other habitats. Thus, space human factors experiment time and crew participation must be integrated with the crew's other scientific and operational chores. This is a daunting task, which will require milestones and coordination between researchers in space human factors and related topics in human behavior and performance.

Summary Finding. At the time of this study, the SHF program consisted of mission support, external contracts, and individual projects selected from proposals submitted in response to NRAs. It was the committee's judgment, based on documentation and briefings, that the quality of these projects varies widely. Some are of outstanding scientific quality, while some others do not meet the minimum standards of scientific and professional research.

Finding. Mission-oriented research is performed at both JSC and ARC, and there are some excellent projects at both centers. The work at JSC is primarily driven by the need to resolve issues related to operating Space Shuttle missions and for planning other near-term programs, such as the ISS. Virtually all the work at JSC is sponsored by NASA. In general, the researchers at ARC seem to be motivated by fundamental scientific questions, as well as by issues related to

aviation safety, airframe design, or enhancing pilot performance. Many of the projects at ARC appear to be supported by, or in cooperation with, specific industrial partners (such as the augmented reality system for wiring-buck cabling supported by Boeing Aircraft) or with other government agencies (such as the FAA for the aviation safety reporting system, and the U.S. Army for the MIDAS pilot simulation). Underlying "cultural" differences between the two centers have given rise to different evaluation metrics. At ARC, the dominant criteria are related to peer recognition; at JSC, they are related to solving near-term operational problems. The lack of an overarching, agency-wide mission and supporting SHF management has led to a lack of focus in the efforts of individual researchers and research teams. The quality of R&D at both JSC and ARC varies significantly.

Recommendation 5-7. Management should establish specific research goals relative to short-term NASA operational support as well as for long-duration, far-term missions. Prioritizing research goals can help focus resources, identify programmatic weaknesses, establish incentives, and establish a competitive, but positive, working atmosphere. Synergy between projects directed toward immediate, short-term missions and projects focused on far-term missions should be sought and encouraged.

Recommendation 5-8. Management should establish evaluation metrics that encourage quality research. They should further ensure that the characteristics that constitute a successful, high-quality project are applied across all programs. Periodic external reviews will also help ensure that all research projects are in line with stated space human factors program priorities.

Summary Finding. Although some work has been done to determine the requirements for the human exploration of space and relevant issues related to SHF, currently there is no official NASA document that establishes the priority of the key research areas. The current NASA structure is not adequately aware of current technologies that may be applicable to long-duration SHF issues.

Finding. NASA is currently at work on a requirements document for SHF research, but no priorities exist at this time. Because there is no official program requirements document, there is no focused effort toward achieving goals consistent with NASA's long range plans for lunar/Mars missions.

Recommendation 5-9. NASA should complete and release an official document spelling out the requirements for space human factors research and technology. The document should be open to review, and once accepted by the agency, it should be used to focus sharply on the critical research that NASA will need to support long-term missions.

Finding. A fundamental problem within NASA relates to a research philosophy that has persisted since the Mercury program in the early 1960s. The unique characteristics of space flight (e.g., microgravity, EVA, life support, and isolation) dictated that NASA was solely dependent on the virtuosity of its own scientists and engineers to create its own tools. Since then, this situation has changed. Academia, industry, and other organizations have evolved technological capabilities in areas that can be helpful to NASA, and in certain disciplines, may even have outstripped NASA.

An example of this "insular mentality" is in work on advanced displays at JSC. Existing, off-the-shelf prototyping systems could have been of considerable help. Although it may be easier to write specific in-house software to integrate existing systems (such as integrating the AD software with the flight simulator), cost-benefit analyses comparing in-house and external software products should be used. Another example of insularity involves the long-term development (about 20 years) of the multimedia-media browser for PC display of the NASA STD-3000 human factors data. NASA STD-3000 has been an extraordinary and useful compilation of data on human factors. JSC has provided a valuable data organization and collection service and has promoted the idea of human factors standards, both within and outside the NASA community. However, the computer access aspect of the document project has faltered because the specialized on-line document viewer is clearly inferior to current hypertext markup language (HTML) browsers based on Internet technology. These HTML browsers can deliver a document to any web browser at any computer work station. By identifying and using or modifying off-the-shelf systems, NASA can focus on the content, rather than the medium (software delivery), which may be available elsewhere.

A good example of an SHF project that is working well at JSC is the Graphics Research and Analysis Facility (GRAF) laboratory. While solving real problems in day-to-day or mission-to-mission operations, the project also maintains a view of software tools that would be needed to help plan and manage future missions, EVA suits, and even human factors in microgravity. GRAF has attempted to use outside software rather than build it all in-house, and GRAF has used internal resources to augment algorithms (developed elsewhere in JSC) for EVA suit modeling and suit sizing, to collect strength data, and to improve engineering-accurate illumination models.

Recommendation 5-10. The NASA space human factors program should focus on issues unique to the crewed exploration of space, which is the prime driver of the program. NASA should not assume that all software and hardware systems must be built by NASA from scratch; many products on the market can assist NASA's mission. Good examples of these are Internet software browsers for documentation and even training, design and visualization software for display mockups and training, and 3D graphics software. Thus, the continuing search for "space-unique" tools should be expanded beyond NASA. Work by an outside

entity, even though it may not be directly applicable to space travel, could be modified or adapted to meet specific NASA requirements. NASA should establish a formal mechanism to identify work being done outside NASA that may be applicable to its purposes.

Summary Finding. The recent establishment of JSC as the lead center for SHF provides an opportunity to consolidate management and invigorate NASA SHF-related programs and projects.

Finding. Understanding human behavior and performance is a high priority for crewed missions. But this area has been arbitrarily separated from SHF in the OLMSA organization. This separation appears to be drawn along the lines of scientific disciplines rather than with respect to functional problems or issues. The area of human behavior and performance includes many of the issues critical to the success of a human mission to Mars. Examples include crew selection and interaction, workload, training, etc. Traditionally and functionally, these programs belong together.

Recommendation 5-11. The OLMSA behavior and performance projects and space human factors projects should be brought under a single management structure and should be working toward the same set of goals.

Finding. If and when long-duration mission requirements are determined, it is unlikely that SHF staffing will be adequate to address the broad range of problems a crewed mission (e.g., to Mars) would encounter. It is also unlikely that the current funding level for SHF would be sufficient to support the needed SHF research for the safe and effective human exploration of the solar system.

Recommendation 5-12. The space human factors program requires strong leadership and advocacy with a long-term view of the entire space human factors area. The individual in charge of this program must have sufficient budgetary and other resources to ensure that the long-term problems of operational space flight and a mission to Mars can be addressed by appropriate, forward-looking research. This individual must have the experience and authority to coordinate disparate disciplines and entities. This can only be accomplished with a space human factors advocate at a high administrative level. Space human factors funding should be a line item in each program/project. This would foster better communication and allow resources to be applied more appropriately. Line item funding would also provide some flexibility for the timely pursuit of emerging issues rather than having to wait for a NASA Research Announcement cycle. Increasing the focus of the program while broadening the research base will require a well orchestrated team effort.

Recommendation 5-13. NASA should direct its limited resources for space human factors research to areas where advances are not likely to be made by others, e.g., issues related to long-term isolation and habitability, etc. Few others besides NASA will be working on the space human factors issues unique to going to Mars or living on the Moon, but others will be working on displays and controls, etc. Many of these technologies are likely to be ready for operational evaluation by the time NASA will begin its development of these missions.

Finding. The NRA process is appropriate for projects addressing long-term needs. However, the process of selecting projects for long-term research should be sharpened in order to foster research that addresses important SHF issues. Unless the NRA process is carefully implemented, it may produce excellent scientific studies on the wrong subjects.

The NRA funding mechanism with peer review puts the more operational SHF projects at JSC at a disadvantage compared to projects at ARC. Because of the inconsistent level of available research funding, JSC has focused on operational requirements but with a view toward the reusability of both data and software for future missions.

In general, the SHF work at JSC focuses on near-term problems (e.g., the Space Shuttle, ISS, Shuttle-Mir, ISS Human Research Facility, and issues related to the ALS tests). It is mission-to-mission oriented, iterative, in response mode, and stimulates little fundamental research. Overarching issues have not been clearly defined, and hence are rarely addressed because the program focuses on near-term ''fixes.'' This may be an appropriate operational mode in an environment of need-to-solve, immediate problems with limited funding, but it will not suffice for addressing long-duration SHF issues.

An SHF research program made up of proposals predominantly selected from NASA NRAs and SBIRs limits the range and focus of research. But the delineation of scientific and technical areas to be funded is not clear. The present NRA process is not structured to foster research directed at answering the critical questions that NASA must address before beginning human missions beyond LEO.

Although there is little duplication of effort among the OLMSA-funded SHF projects under way at JSC and ARC, no incentive or organizational structure to coordinate SHF disciplines currently exists. Work related to SHF at other NASA centers and not funded by OLMSA was not reviewed by the committee.

Recommendation 5-14. A serious effort to design long-duration space flight missions will require a more specific, technology-directed focus than the present NASA Research Announcement system allows. This focus should result in announcements that request proposals in critical areas, thus enabling the space human factors program to focus on the most pressing needs identified by NASA and its advisory groups. A technology-directed focus would simplify the selection process by making it easier for NASA to select among proposals that may be

excellent from a purely scientific point of view but are less relevant to solving pressing space human factors problems. This would also mean that prospective principal investigators (both inside and outside NASA) would not spend significant amounts of time and energy on proposals that are bound to be rejected because they are not relevant to current agency needs, exclusive of their scientific merit.

Summary Finding. The potential for synergy among projects funded by the SHF program and other NASA programs is high. But synergy must be nurtured, and not everyone appreciates that NASA' s long-term goals can be advanced by building upon the work of others, e.g., in computer technology and human-computer interaction. SHF is an integral component of activities such as EVA, ALS, and EMC. All are designed to ensure the safety, survivability, and productivity of human beings in space environments.

Finding. SHF is an intrinsic component of other NASA activities, such as training, behavior and performance, aeronautics, safety, robotics, and tests of new life support technologies. Collaborations at ARC are satisfactory and frequently include scientists from outside NASA. There are also some international collaborations. Some of the research fields covered include, but are not limited to, cognitive science, virtual reality, perceptual limitation, medical imaging, team training and problem solving. With some exceptions, collaborations are less well developed at JSC.

Because the ISS is the acknowledged vehicle wherein critical SHF research related to long-term missions will be conducted, it was disappointing to realize that there is no formal plan for integrating SHF research into all aspects of ISS operations.

The lack of communication between the research and operational SHF communities, combined with the lack of a unified programmatic mission, goal, or priorities, creates an organization that, in large part, is pursuing projects that do not capitalize on potential intramural or external synergism. The lack of communication among overlapping and/or complementary NASA activities precludes the efficient use of resources and undermines technical and programmatic synergy. None of the SHF work at JSC is specifically connected with work on human factors at ARC. The work on virtual reality training at JSC is not part of SHF because it is considered mission planning and training. Also, somewhat arbitrary "turf" demarcations (e.g., separating aviation from space flight) have resulted in poor communication, which makes coordination even more difficult.

Recommendation 5-15. Space human factors personnel should be formally included in the concurrent engineering loop associated with the design, development, and construction of all space systems, such as extravehicular activities, advanced life support systems, habitations, and control and communication systems.

Recommendation 5-16. NASA should establish a formal method for sharing information about current or anticipated operational space human factors problems. NASA should also establish a method for sharing information concerning planned space human factors projects, including all work at NASA centers, so that limited resources can be optimized and leveraged for maximum gain. Regular (semiannual or annual) space human factors meetings should be scheduled to ensure that researchers and others are aware of each other's work and areas of expertise.

NASA should establish a system for keeping appropriate staff up to date on the technical activities of external organizations involved in potentially applicable work. NASA should encourage and provide resources for researchers to participate in technical and professional conferences to foster an exchange of information and ideas with external organizations and individuals.

Recommendation 5-17. To maximize the probability of success of SHF programs for prolonged crewed space flight, NASA should call not only on the talents and capabilities of in-house scientists, but should also capitalize on the knowledge of the best scientists and professionals available, regardless of their location or affiliation. Some examples of areas where synergy should be encouraged include:

• Space human factors researchers could participate in the development of integrated system simulations and virtual environment technologies with humans in the loop, whether for piloting, mission specialist activities, or other training and performance evaluation studies.
• Better connections between the advanced displays group at JSC and the man-machine integration design and analysis (MIDAS) group at ARC would be helpful.

Summary Finding. Spin-off technologies should not be considered primary drivers for space human factors, although they are splendid fringe benefits. The focus of space human factors work must be to identify the problems and discover solutions that will make prolonged, crewed spaceflight as safe and productive as possible. The primary, abiding philosophy must be to seek out and solve these problems. Spin-offs should be viewed as dividends, never goals.

Finding. Several potential dual-use technologies have been developed within the NASA space or aeronautics human factors community, including the NASA-STD-3000, MIDAS, spatial auditory displays, and fatigue countermeasures.

Recommendation 5-18. The space human factors program should primarily allocate its resources on research, analysis, and designs that contribute to mission objectives. Spin-offs should always remain a desirable fringe benefit but should never be considered a primary driver of NASA research.

NASA (National Aeronautics and Space Administration). 1995. Space Human Factors Program Plan. Life and Biological Sciences and Applications Division, Office of Life and Microgravity Sciences and Applications. Washington, D.C.: NASA.

NASA. 1996. NASA's Enterprise for the Human Exploration and Development of Space: The Strategic Plan. Washington, D.C.: NASA.

NRC (National Research Council). 1993. Scientific Prerequisites for the Human Exploration of Space. Committee on Human Exploration, Space Studies Board. Washington, D.C.: National Academy Press.

NRC. 1994. Scientific Opportunities in the Human Exploration of Space. Committee on Human Exploration, Space Studies Board. Washington, D.C.: National Academy Press.

Stafford, Thomas P., et al. 1991. America at the Threshold: America's Space Exploration Initiative. Washington, D.C.: White House Office of Science and Technology Policy.