TECHNOLOGIES FOR HUMAN SPACE EXPLORATION: EARTH S NEIGHBORHOOD AND BEYOND

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1 TECHNOLOGIES FOR HUMAN SPACE EXPLORATION: EARTH S NEIGHBORHOOD AND BEYOND Bret G. Drake, James Geffre, Brian Derkowski, Abhishek Tripathi National Aeronautics and Space Administration Lyndon B. Johnson Space Center Houston, TX ABSTRACT NASA is defining strategic architectures and toplevel requirements for human exploration that radically reduce the cost and risk of such missions through the use of advanced technologies and innovative systems strategies. These architectures are directed toward developing common capabilities and core technologies to be used in human expeditions within Earth s neighborhood and beyond. This paper identifies key exploration technologies in the areas of human support, in-space transportation, power, and miscellaneous cross-cutting technologies. The various technologies are assessed to determine their applicability and benefits to future human exploration endeavors. EXPLORATION STRATEGIES Human exploration beyond low-earth orbit is an endeavor that will confirm the potential for humans to leave our home planet and make our way outward into the cosmos. Though just a small step on a cosmic scale, it will be a significant one for humans, because it will require leaving Earth on long missions with limited return capabilities. For example, the decision to go to Mars is a commitment to several years away from Earth, during which there is a very narrow window when return is possible. Human exploration missions beyond low-earth orbit have been an integral part of NASA s strategic vision. 1,2,3 During the past several years, personnel representing various NASA field centers have formulated Reference Missions addressing human exploration missions beyond low-earth orbit. 4,5,6,7,8 Each of these mission studies, referred to as an architecture, Copyright 2001 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royaltyfree license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. 1 provides descriptive information of the overall exploration theme and its derivation from, and links to, driving national needs. Each of these architectures identifies governing objectives, ground rules and constraints, the mission strategy to be used in developing scientific implementation approaches, implementation and technology options, and important programmatic decision points. Architectures for human exploration consist of the integrated set of functional building blocks that describe the method and style by which humans leave Earth, travel to destinations beyond low-earth orbit, carry out a set of activities to accomplish specified goals, and subsequently return to Earth. Common Capabilities and Core Technologies Previous NASA architecture studies have included destinations such as the Moon, near-earth asteroids, Mars, and the moons of Mars. In each of these studies, the destinations were considered on their own basis, and comparisons between destinations were rarely made. The focus of the recent architecture studies by the Exploration Team was to perform comprehensive studies of missions to the various destinations with emphasis on determining the existence of common capabilities and core technologies between destinations. In this context, a common capability is a specific system or set of systems that meet mission requirements of at least two different destinations. An example of a common capability could be an advanced habitation system that can support a mission crew to the Moon or to the Sun-Earth libration point. Each common capability is comprised of a set of core technologies needed to meet the system requirements. For example, advanced closed-loop life support systems or advanced radiation protection techniques are core technologies within a common habitation system capability. The recent mission architecture and technology studies were conducted to derive these common capabilities and core technologies including their applicability to various mission concepts.

2 Normalized Mass Savings AIAA MEASURING TECHNOLOGY BENEFITS Techniques used for measuring the benefits of various technologies are not straightforward due to the inherent complexity of human exploration missions. Traditionally, total mission mass, specifically the mass savings from the application of a specific technology, has been used as the first order measure of benefit. Total mission mass does indeed provide the mission designer some degree of understanding of the resulting size of the mission architecture, but it also provides insight into other related figures of merit including mission complexity, cost, and to a certain level, mission risk. Although each of these other criteria is related to mass, specific comparisons of the merit of each technology toward these other criteria must be made. Technology Bundling Perhaps an equally important aspect of measuring the value of a technology is the mission model toward which it is applied, specifically the combined technologies that are included in that model. The measured value of a specific technology is highly dependent on the interrelationship of it in concert with other applied technologies, which we term technology bundling. For instance, the value of an advanced technology as applied to a mission model comprised of today s technologies will provide more mass reduction than applying that same technology to a mission model comprised of tomorrow s technologies. In addition, the order in which the technologies are applied can change the measured savings. This technology bundling effect can be seen in Figure 1, which demonstrates the mass savings for a human Mars mission. As can be seen from this figure, the total Today's Technologies Original Technology 3 Bundle Order Understanding Ancillary Benefits Another important criterion to consider when evaluating specific technologies is the ancillary benefit that a technology can provide to the total system or mission architecture. This measure is often overlooked, but it can have a profound impact on the overall mission. One example of this is the incorporation of microminiaturized electronic technologies. Many electronic components are currently small and represent a small fraction of the overall mission mass, and thus further miniaturization will not have a significant impact on the resulting total mission mass. However, one must consider other important mission criteria, such as risk, when evaluating the value of the technology. Since the size of these components continues to decrease, the mission designer can begin to evaluate the risk reduction benefits of incorporating multiple redundancies in the systems designs. One can even begin to investigate the safety benefits of not just dual or triple redundancy, but redundancy strategies of 10 s or even 100 s of similar components. Another example is the inclusion of in-situ resource utilization techniques in the mission approach. Insitu resource utilization is commonly thought of in terms of propellant production, that is producing the crew s return propellant from local resources (oxygen from the lunar regolith or oxygen and methane from the martian atmosphere). These systems can not only provide mass savings, but can also provide functional redundancy of critical systems they can serve as a completely redundant, yet separate, means of providing vital oxygen for the crew. MISSION DESCRIPTIONS The current Human Exploration and Development of Space Strategic Plan 3 provides an overarching strategy for human exploration missions. Missions beyond low-earth orbit include mid-term 100-day missions (near-earth space) evolving to far-term 1000-day missions (missions to Mars) Reversed Technology Bundle Order Tomorrow's Technologies 6 1 Figure 1. The Effect of Technology Bundling savings from start to end is the same, but the relative savings between incremental steps in the process is different. 2

3 Near-Earth Space Mission Model Previous mission studies in near-earth space have focused primarily on missions to the Moon. 9, 10, 11 Objectives of these missions have varied, but all have generally included science of the moon, science from the Moon, and serving as an operational and technology proving ground for longer duration missions such as to Mars. A new set of near-earth mission objectives are emerging, namely the emplacement and maintenance of an array of large scientific telescopes. These new objectives have forced mission designers to re-look at the missions in order to find a better, more integrated approach. During the past few years, a new near-earth mission strategy has been developed. The key to this mission approach is the emplacement of an operational facility, termed a Gateway, in the vicinity of the Moon, specifically at the L 1 libration point. This gateway serves as the primary operational staging location for a variety of missions both in near-earth space and beyond as shown in Figure 2. Missions in near-earth space include cargo missions to and from their intended destinations (lunar orbit, lunar surface, or other libration points) as well as human missions. Requirements for near-earth missions are provided in Figure 3. Mars Mission Classes Human Mars mission concepts under study are categorized as either short-stay tactical missions with surface mission durations of 30 days, or long-stay outposts with surface mission times of approximately 500 days. Traditionally, these mission classes have been treated as options with first order parameters such as round-trip mission time and required propellant mass used as figures of merit. Such analyses must also include considerations that are critical in formulating operationally sound mission strategies. These considerations include crew health effects relative to times spent by the flight crew on the martian surface and in transit, science return, and overall mission operational approaches. Round-trip human missions to Mars can be characterized by the length of time spent on the surface, short-stay and long-stay, as discussed below. Short-Stay Mars Missions: The first Mars mission class consists of short stay-times (typically 40 days in the vicinity of Mars with 30 days on the surface) and round-trip mission times ranging from days. The short-stay missions focus on local exploration of pre-determined sites. Diversity of exploration coverage with the short-stay mission concepts is achieved through visiting separate locations within three or four separate flight opportunities. This class of mission has high propulsive requirements even when employing a gravity-assisted swing-by of Venus or performing a deep space maneuver to reduce the total mission energy. Distinguishing characteristics of the short-stay mission are: 1) short-stay at Mars, 2) short to medium total mission duration, 3) perihelion passage inside the orbit of Venus on either the outbound or inbound legs, and 4) large and variable total energy (propulsion) requirements. Long-Stay Mars Missions: The second Mars mission class is typified by long-duration stay times (as much as 600 days) and long total round-trip times (approx- Cargo Earth-Sun L 2 Cargo Crew Lunar Surface Crew Earth-Moon L 1 Gateway Cargo Crew Mars Figure 2. Near-Earth Mission Scenario Overview 3

4 Support multiple destinations Moon, L 2, Mars Staging location ISS, L 1 Gateway Crew size 4 crew Payload size mt Transit mode Zero-g Transit time < 10 days Launch vehicle EELV-H: 35 mt Mission dates: Lunar: 2010 L 2 : 2012 Pre-deploy Yes (cargo elements) Figure 3. Near-Earth Architecture Requirements imately 900 days). The long-stay outpost missions focus on regional scientific exploration at ranges greater than 100 km. Crew will explore a scientifically compelling region with adequate time to conduct activities and adapt to observations. An outpost will be left behind at mission completion, which may be revisited. These missions represent the global minimum-energy solutions for a given launch opportunity. Unlike the short-stay mission approach, instead of spending a short period on the surface and departing Mars on a non-optimal return trajectory, extensive time is spent on Mars exploring with the crew returning when a more optimal alignment between Mars and Earth occurs. Distinguishing characteristics of the long-stay mission include: 1) long total mission duration with a long stay at Mars, 2) relatively little energy change between opportunities, 3) bounding of both transfer arcs by the orbits of Earth and Mars (closest perihelion passage of 1 AU), and 4) relatively short transits to and from Mars (less than 200 days). Mars Mission Model As can be seen from the discussion above, the shortstay mission is more demanding and pushes the transportation technologies harder than the long-stay mission. When comparing a wide-range of technologies it is desirable to develop a mission model that does not overemphasize one mission aspect over another, and thus the long-stay mission was used as the mission for this study. The philosophy of the long-stay mission architecture approach is to minimize the exposure of the crew to the deep space radiation and zero gravity environment while at the same time maximizing scientific return from the mission. 12 This is accomplished by taking advantage of optimum alignment of the planets for both the outbound and return trajectories by varying the stay time on Mars, rather than forcing the mission through non-optimal trajectories as in the case of the short-stay missions. This approach allows the crew to transfer to and from Mars on relatively fast trajectories, on the order of six months, while allowing them to stay on the surface of Mars for a majority of the mission, approximately eighteen months. The surface exploration capability is implemented through a split mission concept in which cargo is transported in manageable units to the surface or Mars orbit and checked out in advance of committing the crews to their mission (Figure 4). The split mission approach also allows the crew to be transported on faster, more energetic trajectories, minimizing their exposure to the deep-space environment, while the vast majority of the material sent to Mars is sent on minimum energy trajectories. The trajectory analysis discussed earlier was used to insure that the design of the space transportation systems could be flown in any opportunity. This is vital in order to minimize the programmatic risks associated with funding profiles, technology development, and system design and verification programs. Predeployment of the return propellant is not necessary for the long-duration mission option. ASSESSMENT CRITERIA Previous studies have identified a wide variety of technologies needed to support the design, development, and ultimate implementation of human expeditions beyond low-earth orbit. These technologies span a wide range from current state-of-the-art to revolutionary breakthrough concepts. Measuring the effectiveness of the hundreds of technological options as applied to multiple complex human mission architectures is an extremely difficult task. In order to aid in the technology assessment process, a set of evaluation criteria, or figures of merit, were established. These criteria, as shown in Figure 5, were chosen to address many of the important systems design areas including crew safety, performance, programmatic, and cost. When developing these criteria, care was taken to establish those figures of merit that provide the best insight into both the benefits and detriments of the technologies. The set of evaluation criteria used for assessment of human support, in-space transportation, power, and miscellaneous technologies was defined as follows: Safety & Reliability Criteria within this figure of merit were established to derive understanding into the relative benefits of the specific technologies as they pertain to crew safety and overall system reliability. This includes both the flight crew as well as ground operations personnel and includes information on: 4

5 AIAA Hazards that the technology poses to the environment and ground personnel Hazards that the technology poses to the in-space and flight crew (this includes all assembly and checkout crews) Understanding of how well the technology supports in-space aborts and safe return of the crew to Earth (as applicable) Top-level assessments of the relative reliability expected from the candidate technology Technical Performance Criteria within this figure of merit provide insight into the technical performance of the specific technology including such criteria as mission mass and complexity and includes information on: A measure of how well the technology 1. Descent / Ascent Vehicle (DAV) delivered to Mars orbit two years before crew departs Earth. Launch, assembly, and checkout. Minimum energy transfer. Vehicle captures into Mars orbit and waits for crew. 2. Surface Habitat (SHAB) delivered to Mars surface two years before crew departs Earth. Launch, assembly, and checkout. Minimum energy transfer. Vehicle captures into Mars orbit, descends to surface. Vehicle checkout. 3. Mars vehicles verified Go before crew departure. Crew travels to Mars on fast 180-day transfer. Launch, assembly, and checkout. Crew delivered via Shuttle. Fast 180-day transfer. Vehicle captures into Mars orbit and rendezvous with DAV. Go/No Go for landing. 4. Crew conducts short-stay mission. Transition to longstay habitat once acclimated to Mars environment. Regional science. Crew descends to surface. and conducts exploratory mission. Go/No Go for long-stay mission. 5. Crew ascends to Mars orbit. Rendezvous with Transit Vehicle and prepares for return to Earth. After 500-day stay, crew ascends from surface and rendezvous with waiting transit vehicle. 6. Crew returns to Earth with direct Earth entry. Total mission duration days. Direct Earth entry. Fast 180-day transfer. Surface habitat remains for potential re-use. Figure 4. Mars Mission Scenario Overview 5

6 reduces crew exposure to the in-space environment Understanding of the hazards the technology itself poses to the crew A measure of total mission mass and expected mass savings from the technology An understanding of the operational complexity of the technology and how that impacts crew productivity Cost SAFETY - RELIABILITY Ground Operations Environmental Hazards In-Space Environmental Hazards Enables In-Space Abort Scenarios Relative Reliability TECHNICAL Crew Exposure to In-Space Environments Total Launch Mass Mass Savings Operational Complexity Crew Productivity Malfunction Sensitivity COST Technology Advancement Cost Mission Non-Recurring Cost Mission Recurring Cost Operational Cost PROGRAMMATIC Applicability to Other Destinations Science Return Scalability Robotic to Humans SCHEDULE Total Development Time Special Facility Requirements Maturity Level (TRL) These criteria focus specifically on the expected cost of the technology including Expected costs to advance the technology to flight readiness Expected design and development of the first flight unit Expected cost of each additional flight unit Cost of operating the technology Programmatic Figure 5. Figures of Merit Criteria within this figure of merit were established to provide understanding into the relative benefits of the technology in meeting overall driving national needs including The applicability of the technology to multiple exploration destinations (near- Earth, Moon, Mars, etc.) Enhancement of science return from the technology (as applicable) A measure of the ability to scale the technology from small robotic missions to larger human missions Schedule Criteria within this figure of merit were established for understanding the schedule and technology development requirements for the technology including Time to advance the technology from its current state to flight readiness An understanding of special facility needs An overall understanding of the maturity of the technology TECHNOLOGY ASSESSMENT Human Support As humans extend their reach beyond low-earth orbit, they will be exposed to the hazardous environment of deep space for lengthy periods, consequently, protective measures must be devised to ensure crew health and maximize mission success. At NASA s Johnson Space Center, a critical path roadmap has been developed to serve as a focus for technology development efforts of human support technologies. 13 This plan includes various human support technologies including radiation protection, zero-g countermeasures, medical care, and advanced life support systems. Applicability of various human support technology options relative to human exploration missions is shown in Figure 6. Perhaps one of the most important issues to the human crew is radiation protection. As the crew ventures beyond the protective environment of Earth, they are exposed to both Galactic Cosmic Radiation (remnants from the formation of the universe) and Solar Particle Events (solar flares from the Sun). The most effective passive shielding materials are those with the lowest atomic mass and highest concentration of hydrogen. The effectiveness of several forms of advanced hydrocarbon materials is presently being investigated both in the laboratory and on the International Space Station. Estimating the biological damage or dose-equivalent of the interplanetary radiation environment is a difficult process currently. While the composition and effluence of GCR is relatively well understood, there are large uncertainties involving the radiation interaction with biological systems 6

7 and the effectiveness of various shielding materials. 14,15 Radiation protection is important, but predicting when solar particle events or other heliometeorological phenomena occur could be equally vital to the crew. Currently, solar flares can be predicted within a few days up to a week. This is done via instruments that constantly monitor solar conditions and transmit this information to Earth or crews in low-earth orbit. Of course, the ultimate astronaut would be naturally resistant to a certain radiation dosage level. By controlling the biological or chemical processes within the body, perhaps the dangers of space can be countered. This concept is lacking in maturity and would require significant effort to develop to fruition, however by attacking the problem from within the body, valuable spacecraft resources could be saved. An additional threat to exploration crews is the extended period required in the zero-gravity environment. This represents another discipline in which our understanding of long-term exposure is rudimentary at best. It is known, however, that significant physiological changes occur when zero gravity time begins to be measured in months. Bone decalcification, immune and cardiovascular system degradation, and muscular atrophy are a few of the more unpleasant effects. Several potential solutions to the physiological problems associated with zero-gravity missions include: countermeasures (exercise, body fluid management, lower body negative pressure), artificialgravity spacecraft, and reduced mission times. With regular exercise, muscle and bone loss is drastically reduced. Resistive exercise is currently used aboard the International Space Station and was used aboard the Russian Space Station Mir. Unfortunately, resistive exercise takes time out of a busy mission schedule and the needed equipment increases total spacecraft mass. A potential alternative or supplement to exercise is to create an artificial gravity environment by constantly spinning the spacecraft en-route to, and at its destination. At present, though, these methods are not mature, require extensive research, and as a result do not satisfy the criteria of cost or schedule. Since the beginning of human spaceflight, providing medical care to in-space crews has been a major concern. Great care is exercised when selecting crew as to pick those not susceptible to contracting an illness while on a mission. For future human exploration, it is recommended that a medical doctor be included with the crew. This is being practiced somewhat today in that people with medical training are chosen for the flight crews. In the absence of a doctor, an alternative is to have a doctor on the ground monitoring the crew s status. For past missions, bio-medical devices sent vital information to the ground. In the future, advanced data such as metabolic rates or calcium loss as well as CAT scans or MRI could be used to treat an injured crewmember. Although these devices are well known on Earth, adapting them to space would require advanced funding. An alternative to having doctors involved is to have the astronauts treat themselves using pharmacological aids or non-invasive systems. By producing the medicine that a crewmember needs onboard, illness can be treated quickly and effectively. This requires the expertise and material needed to be on the spacecraft. This technology has not been developed, however, and marginally satisfies most of the criteria in the HUMAN SUPPORT Safety Technical Cost Programmatic Schedule Radiation Protection Passive Shielding Techniques Active Shielding Techniques º º º Environmental Monitoring Chemical/Biological º º Zero-g Countermeasures Resistive Exercise Artificial-gravity º º º Medical Care Crew Selection Monitoring Systems º Pharmacological Aids º º º Non-invasive Systems º º Life Support Physical / Chemical Water Recovery Biological Water Recovery º Plant/Food Growth º Exceeds criteria Meets criteria º Does not meet criteria Figure 6. Human Support Technologies 7

8 five areas under consideration. Non-invasive systems such as microsurgery can be very useful to a crewmember, requiring a minimal recovery period. As with pharmacological aids, the concepts are advancing for Earth-based applications, however significant funding and development time would be needed in order to use these ideas on future exploration missions. Developing technologies, which can significantly reduce the consumables required to support the crew during, long-duration is also critical for human exploration. Technologies include air and water loop closure, environment monitoring, solid waste processing, thermal control, and food production. Advanced sensor technologies to monitor and intelligent systems to control the environmental health of the advanced life support system, including air and water, are also needed. Studies have shown that incorporation of advanced biological water recovery systems can save as much as 50% as compared to physical / chemical recovery approaches, such as those used on the International Space Station. 16 In-Space Transportation The efficient and cost-effective delivery of both cargo and humans to and from various exploration destinations is critical for human exploration missions. Given the total mass involved in many exploration architectures, this area is of prime importance and thus has been the focus of many studies and technology development efforts. Historically, propulsion technologies include high and low thrust propulsion systems involving chemical, nuclear, solar, and aeroassist forms of energy exchange. Examples of these systems are the well-known and highly developed chemical propulsion and the less welldeveloped systems based on nuclear thermal propulsion, nuclear electric propulsion, solar electric propulsion, and aerocapture. Applicability of various inspace transportation technology options for human exploration missions beyond low-earth orbit is shown in Figure 7. Chemical transportation systems have been utilized for decades. They are well understood, have substantial heritage, and existing test facilities. For human exploration missions beyond low-earth orbit, though, this method of propulsion has drawbacks even considering the advances in fuel types and efficiencies that could come with more research funding. The biggest is mass. Carrying a large volume of fuel to distant destinations, such as Mars, and back propagates all the way back to the initial mass to low-earth orbit requirements. Advanced chemical propulsion technologies are best suited for more near-earth mission concepts. Early nuclear thermal rocket propulsion systems were demonstrated in the late 1960 s as part of the NER- VA program. Advanced nuclear thermal propulsion concepts promise higher specific impulse than chemical systems (twice the efficiency) thus providing the capability of reducing total mission mass and triptime thereby lowering crew risk of exposure to the hazards of space. 17 However, restarting the technical design, including development and test facilities, is a challenge for this propulsion concept. Another option, bi-modal nuclear propulsion, offers the same advantages of nuclear thermal while offering a source for power generation. This concept, unfortunately, is less mature and thus has a lower technology maturity level and increased complexity. Electric propulsion is another class of in-space transportation that has benefits for human exploration. Electric propulsion concepts utilize solar or nuclear power to accelerate propellant to higher exit velocities than those from a chemical reaction. Such sys- IN-SPACE TRANSPORTATION Safety Technical Cost Programmatic Schedule Crew & Cargo Chemical º º Nuclear Thermal Propulsion º º Bi-Modal Nuclear Thermal º º º Solar Electric Nuclear Electric (Ion/MPD) º º Nuclear Electric (VASIMR) º Hybrid (NTR/NEP) º Aeroassist Cargo Only Plasma Sails º º Solar Sails º Electrodynamic/Momentum Tethers º Exceeds criteria Meets criteria º Does not meet criteria Figure 7. In-Space Transportation Technologies 8

9 tems have the advantage of a high specific impulse system that maximizes engine efficiency usually at the expense of thrust. 18 Since electric propulsion is safer and more scalable, it has a near-term advantage over nuclear thermal systems. A specific application of nuclear electric propulsion known as the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) combines the advantages of each with one simple thruster core. 19 The thrust can be heightened sacrificing efficiency for those mission phases when high thrust is more important, and the specific impulse can be increased for the phases when high thrust is less important. Another approach to combining a high thrust and high specific impulse is the NTR/NEP hybrid engine. This works by combining the thrust and power generation capabilities of the bi-modal NTR with the high efficiency of electric propulsion concepts. Although providing additional mass savings, this technology is at a lower stage of technical and operational maturity. One common requirement for all electric propulsion systems is a substantial supply of power generated either from solar or nuclear energy. Solar power has the advantages of safety and technology readiness, as nuclear systems require extensive shielding measures and most nuclear technology development programs have been significantly scaled back. Large photovoltaic arrays must be built and deployed to power the systems for a trip to Mars or near-earth locations, while none of the shielding requirements that all nuclear options carry would be of concern. Research in how to deploy and orient these structures and how to get maximum efficiency are of prime importance. However, solar arrays must be made larger for missions away from Earth s neighborhood as the Sun s energy density decreases with distance. For the category of in-space transportation, though, the figures of merit clearly show the importance of developing solar electric propulsion. The final three means of in-space transportation were designated for cargo-only missions. Tethers, solar sails, and plasma sails are all promising technologies for cargo missions if implemented correctly, yet all are technologically immature and unproven. All three means could be cheap alternatives to getting cargo to future locations of interest. Power Technology advances for power systems are focused on efficiently providing continuous high power at low cost across all phases of human exploration missions. These areas include in-space and stationary surface power generation, mobile power for rovers, energy storage, and power distribution systems. 20 To enable robust exploration in near-earth space and beyond, major advances in each phase will be required. Applicability of various power system technologies relative to human exploration missions beyond low- Earth orbit is shown in Figure 8. Key technologies in power generation include advanced photovoltaic systems, efficient thermal-toelectric conversion systems for solar and nuclear energy, and mechanical power generation from chemical reactions. Several options from this field have been examined, and from these, three leading candidates have emerged: photovoltaic, solar dynamic and nuclear fission. Advanced photovoltaic arrays and solar dynamic systems can provide power at very low cost and specific mass, however are subject to eclipse periods and day-night cycles. During such periods without sunlight, these technologies require substantial energy storage systems to provide continuous power for the crew. Nuclear fission systems can continuously provide high power for long periods with a low total system mass, yet suffer from technology development and crew safety concerns. Solar-based systems appear better suited for power generation during in-space mission phases, however nuclear fission offers the potential for coupling with an integral propulsion system, thereby reducing total system mass. In addition, nuclear power generation for surface habitats is not affected by the environmental impacts facing solar energy systems, thus it emerges as the leading candidate in this area. Both nuclear fusion and tethers for power generation were deemed too costly and technologically immature to meet the criteria required for mid-term and far-term human exploration. Mobile power for roving vehicles enables scientific expeditions stretching beyond the limits of a stationary surface habitat. To achieve this goal, rovers require high power generation or energy storage capacity while maintaining volumetric compactness. The technologies examined for mobile power included radioisotope thermal generation (RTG), advanced batteries, fuel cells, and internal combustion. While each technology met the criteria laid out, a final assessment is highly dependent upon the mission s roving requirement. A long distance, long duration rover favors continuous power systems which do not consume fuel quickly, such as a RTG. However, the associated mass of providing appropriate crew safety leads to non-nuclear systems such as batteries and fuel cells for less ambitious roving expeditions. Advanced energy storage systems are enabling technologies for non-continuous power generation systems and can provide emergency power during sys- 9

10 tem failures. Such systems viable for human exploration must feature low cost and mass per unit energy stored while remaining reliable and operationally simple. Several storage options were considered in this area, including chemical energy with advanced batteries, regenerative fuel cells, or solar dynamic systems, mechanical energy via flywheels, and magnetic energy using superconducting inductors. Flywheels and chemical batteries are both low cost, low risk solutions, and therefore leading individual technologies in this field. However, a single solar dynamic system can potentially offer both energy storage and power generation, thus being less massive and complex than a combined photovoltaic and battery/flywheel system. The needs for human exploration also include efficient high power distribution technologies for surface transmission and electric propulsion systems. High voltage power transmission can reduce the total mass of power systems through increased efficiency, although it requires additional crew safety measures over similar low voltage operation. By advancing to superconducting materials, a system could potentially deliver the same power without the resistive losses associated with high temperature materials, however operational costs and schedule concerns currently plague this technology. Miscellaneous Technologies A number of cross-cutting technologies, including sensor miniaturization, in-situ resource utilization, advanced habitation, and advanced EVA, may offer important ancillary benefits such as crew risk reduction and enhanced science return to a mission architecture. These various technologies, as applied to human exploration missions beyond low-earth orbit, are shown in Figure 9. Sensors and Instruments: As human expeditions venture beyond low-earth orbit, more durable and reliable instrumentation will be needed. This includes a general category from computers and communications to bio-medical sensors. State-of-the-art components are rapidly improving in reliability and speed on the ground, however this push needs to be directed to space-based applications. Current electronics still require large packaging volumes and high cooling requirement that is levied upon other spacecraft systems, thus adding further complexity. Micro- and nano-size sensor technologies are viable options for reducing the size and mass of electronics, making them attractive to the designer, however this technology has just recently seen an increase in devel- POWER Safety Technical Cost Programmatic Schedule In-Space Photovoltaic/Fuel Cells º º Solar Dynamic º º º º Nuclear Fission º º Nuclear Fusion Tether º Surface Photovoltaic/Fuel Cells º º Nuclear Fission Nuclear Fusion º Mobile Battery º Radioisotope º º º Fuel Cell º º Internal Combustion º º º Storage Chemical Battery Flywheel º Regenerative Fuel Cell º º Solar Dynamic º º º Magnetic (Superconducting Inductors) º Conditioning / Distribution Low Voltage Operation º High Voltage Operation º º Superconducting Transmission/Dist. º Exceeds criteria Meets criteria º Does not meet criteria Figure 8. Power Technologies 10

11 opment. MISCELLANEOUS In-Situ Resource Utilization: Technologies for living off the land are needed to support a long-term strategy for human exploration. 21 Rather than transporting consumables such as oxygen, water, and ascent propellant from Earth, a planet s atmosphere and other natural resources can be transformed into products needed for human exploration. The resources generated on-site can supplement existing consumables to reduce mission risk, or replace them for significant mass savings. While additional assets are required to process the raw materials, resulting products can be shared between separate functions, such as generating pure oxygen for both propellant and breathable air. Advanced Habitation and Life Support: Structural and materials advancements to provide large habitable volumes while minimizing mass, both in-transit to and from planetary destinations as well as during surface explorations, are desired for human exploration missions. Key technology thrusts include habitat concepts and emplacement methods, advanced lightweight structures, and developing integrated radiation protection for crew health and safety. 22 Other technologies can significantly reduce the amount of consumables required for supporting the crew for longduration missions is also critical for human exploration. Such technologies include air and water loop Safety Technical Cost Programmatic Schedule Sensors and Instruments Current State of the Art º º º Micro Sensors º º Nano Sensors º In-Situ Resources Bring All Consumables º Propellant Production º Oxygen Production º º Water Production º º º Exceeds criteria Meets criteria º Does not meet criteria Figure 9. Miscellaneous Cross-Cutting Technologies closure, environment monitoring, solid waste processing, thermal control, and food production. Advanced EVA: Technologies that enable routine surface exploration are critical to exploration activities. This includes advanced extra-vehicular activity suits and short and long-range rovers for surface exploration. Systems that provide routine and continuous surface exploration are key to maximizing mission return. Key technologies include: advanced materials research which provide enhanced mobility and dexterity while maximizing radiation and puncture protection; low-weight, fast recharge batteries; lowweight efficient thermal control; and advanced sensors for environmental monitoring. 23 SUMMARY The exploration community is continuing to refine and advance the technologies and mission approaches needed to support future human exploration missions. The primary goal of these efforts is to develop mission architectures, including technology options, which can significantly reduce the cost and risk of human exploration. During the technology development planning process, emphasis is being placed on those technologies that can provide the most leverage in terms of risk reduction and cost reduction. REFERENCES: NASA, The NASA Strategic Plan, 1992, 1994, 1996, NASA, NASA s Enterprise for the Human Exploration and Development of Space, The Strategic Plan, January NASA, Human Exploration and Development of Space Strategic Plan, NASA, Report of the 90-Day Study on Human Exploration of the Moon and Mars, NASA-JSC,

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