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4 Technical Analysis and Affordability Assessment of Human Exploration Pathways
Pages 109-176

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From page 109...
... Given the complexity of human space exploration -- that is, human spaceflight beyond LEO -- and the fact that U.S. goals in human spaceflight have changed on timescales much shorter than the time it would take to accomplish the goals, it makes sense to decompose a human spaceflight program into smaller building blocks.
From page 110...
... Depending on practical factors, an actual human spaceflight program might have to take an off-ramp to an intermediate destination before the final destination is reached. Each pathway to Mars includes three to six different DRMs, as follows: • ARM-to-Mars pathway -- ARM -- Martian moons -- Mars surface • Moon-to-Mars pathway -- Lunar surface sortie -- Lunar surface outpost -- Mars surface • Enhanced Exploration pathway -- Earth-Moon L22 -- Asteroid in native orbit -- Lunar surface sortie -- Lunar surface outpost -- Martian moons -- Mars surface Completing any of the above pathways would require a variety of mission elements.
From page 111...
... • An operationally viable affordability scenario, in which the pace of progress reflects a compromise between the human spaceflight budget and operational tempo. This scenario would require a human spaceflight budget that increases faster than inflation but not as fast as for the schedule-driven scenario.
From page 112...
... human spaceflight beyond the Earth-Moon system would be exploring a near-Earth asteroid, which would lead to human 3  Presidential Directive on National Space Policy, February 11, 1988, available in NASA Historical Reference Collection, History Office, NASA, Washington, D.C., http://www.hq.nasa.gov/office/pao/History/policy88.html. 4  NASA, Report of the 90-Day Study on Human Exploration of the Moon and Mars, November 1989, available in NASA Historical Refer ence Collection, History Office, NASA, Washington, D.C., http://history.nasa.gov/90_day_study.pdf.
From page 113...
... The appetite for ambitious, Apollo-style goals beyond LEO and the attendant budgets has been notably lacking. Even so, the National Aeronautics and Space Administration Authorization Act of 2010, 12 which mandated this report, called explicitly for the development of a heavy-lift launch vehicle capable of supporting human spaceflight beyond the ISS and LEO, with a focus on cislunar space in the near term.
From page 114...
... This study used a set of representative DRMs to define the three pathways and to assist in evaluating the challenges of expanding human spaceflight beyond LEO as far as the "horizon goal" of a human mission to the Mars surface. DRMs to other feasible destinations, such as the Earth-Moon L1 point and the Earth-Sun L1 and L2 points, 23 could also have been used, but pathways that use the selected DRMs are sufficient for assessing the full scope of the technical and affordability challenges faced by human space exploration.
From page 115...
... The asteroid and Mars Moons missions would allow demonstration of spacecraft vehicles and systems and validate the ability to sustain human health during long-duration missions that are similar in scale to a Mars surface mission. 4.2.2.1  The Space Launch System and the Design Reference Missions The SLS is a heavy-lift launch vehicle that is being developed by NASA to support human space exploration beyond LEO.
From page 116...
... of $12 billion through first flight in late 2017 and an additional $6 billion for development of the Orion Multi-Purpose Crew Vehicle. 33,34 Orion is the crew capsule being developed in concert with the SLS to support human space exploration beyond LEO.
From page 117...
... These assets would be delivered using a similar architecture of SLS launch vehicles, reusable lunar ascent-descent vehicles, a staging orbital facility, and disposable propulsion stages. The additional assets would extend the surface mission duration from 28 days to as much as 6 months.
From page 118...
... 4.2.2.8  Mars Surface The horizon goal for human spaceflight is the human exploration of the Mars surface. Numerous concepts for surface exploration missions have been described in various documents; the analysis in this report is based on NASA's Mars DRA 5.0.
From page 119...
... is essentially the current administration's proposed U.S. human spaceflight program.
From page 120...
... Alternatively, if the actual crewed exploration of Mars became infeasible for financial, technical, or crew health reasons, the Moon-to-Mars pathway would constitute a natural off-ramp, leaving the United States to lead global exploration and exploitation of the Moon. The third pathway, Enhanced Exploration, presents a potentially lower risk than the other pathways, but it is also a longer-duration pathway, exploring several destinations while slowly increasing the capability of key mission elements needed for a Mars surface mission.
From page 121...
... 4.2.4.1 Launch Launch system requirements for human space exploration are driven by the total payload mass required in a s ­pecific orbit (typically LEO) , payload diameter and volume, and reliability requirements.
From page 122...
... 4.2.4.2  In-Space Transportation Human spaceflight missions would require in-space transportation systems to perform major propulsion burns that are not provided by the launch vehicle, often at multiple points during a mission. These transportation systems would consist of high-performance, multiple-restart engines with large fuel tanks that are capable of long-term propellant storage and management.
From page 123...
... The in-space habitat will also need to operate in a dormant state with no crew during the 500-day surface mission. A mission to Mars that does not include a stay on the surface of Mars, such as the Mars Moons DRM, will need an in-space habitation system that can maintain crew health continuously for 2 years or longer.
From page 124...
... Surface exploration missions will require specialized suits, tools, and vehicles that need to be developed for the destinations of interest. Deep-space space suits and surface spacesuits would be necessary for the EVAs during space and surface operations.
From page 125...
... AEROASSIST SYSTEM ADVANCED PROPULSION CREW COMMAND & SERVICE MODULE (ORION) DEEP SPACE HABITATION TELE-ROBOTIC ROVERS SURFACE NUCLEAR POWER LONG DURATION SURFACE HABITAT ADVANCED EVA MARS ASCENT VEHICLE TRANSITIONAL MISSION ELEMENTS DEAD-END MISSION ELEMENTS CRYOGENIC PROPULSION SYSTEM ASTEROID RETRIEVAL VEHICLE LUNAR MODULE MULTI-YEAR DEEP SPACE HABITAT LUNAR ORBITAL OUTPOST LARGE STORABLE STAGE MARS ORBIT TRANSFER VEHICLE SPACE EXPLORATION VEHICLE FIGURE 4.3  Primary mission elements for a DRA 5.0 human mission to the Mars surface along with transitional mission elements and dead-end mission elements and their associated icons.
From page 126...
... PRESSURIZED SURFACE MOBILITY LUNAR MODULE SURFACE NUCLEAR POWER CRYOGENIC PROPULSION SYSTEM TELE-ROBOTIC ROVERS DEEP SPACE HABITATION HEAVY LIFT LAUNCH VEHICLE (SLS 100+ MT) ADVANCED EVA AEROASSIST SYSTEM MARS ASCENT VEHICLE LONG DURATION SURFACE HABITAT LUNAR ORBITAL OUTPOST ASTEROID RETRIEVAL VEHICLE MULTI-YEAR DEEP SPACE HABITAT ADVANCED PROPULSION MARS ORBIT TRANSFER VEHICLE LARGE STORABLE STAGE SPACE EXPLORATION VEHICLE FIGURE 4.4  Buildup of mission elements for the ARM-to-Mars pathway.
From page 127...
... PRESSURIZED SURFACE MOBILITY LUNAR MODULE SURFACE NUCLEAR POWER CRYOGENIC PROPULSION SYSTEM TELE-ROBOTIC ROVERS DEEP SPACE HABITATION HEAVY LIFT LAUNCH VEHICLE (SLS 100+ MT) ADVANCED EVA AEROASSIST SYSTEM MARS ASCENT VEHICLE LONG DURATION SURFACE HABITAT LUNAR ORBITAL OUTPOST ASTEROID RETRIEVAL VEHICLE MULTI-YEAR DEEP SPACE HABITAT ADVANCED PROPULSION MARS ORBIT TRANSFER VEHICLE LARGE STORABLE STAGE SPACE EXPLORATION VEHICLE FIGURE 4.5  Buildup of mission elements for the Moon-to-Mars pathway.
From page 128...
... PRESSURIZED SURFACE MOBILITY LUNAR MODULE SURFACE NUCLEAR POWER CRYOGENIC PROPULSION SYSTEM TELE-ROBOTIC ROVERS DEEP SPACE HABITATION HEAVY LIFT LAUNCH VEHICLE (SLS 100+ MT) ADVANCED EVA AEROASSIST SYSTEM MARS ASCENT VEHICLE LONG DURATION SURFACE HABITAT LUNAR ORBITAL OUTPOST ASTEROID RETRIEVAL VEHICLE MULTI-YEAR DEEP SPACE HABITAT ADVANCED PROPULSION MARS ORBIT TRANSFER VEHICLE LARGE STORABLE STAGE SPACE EXPLORATION VEHICLE FIGURE 4.6  Buildup of mission elements for the Enhanced Exploration pathway.
From page 129...
... The surface exploration capabilities are matured on the lunar missions, leaving the advanced in-space propulsion system as the only significant development for the Mars Moons mission. The only completely new development for the Mars Surface mission is the one capability that cannot be demonstrated anywhere else -- Mars EDL.
From page 130...
... 4.2.6  Challenges in Developing Key Capabilities Developing the capabilities needed for a human mission to the Mars surface will require considerable resources and technological innovation in many disciplines to accommodate the environments to be encountered in space and during surface operations.47,48,49 Technology has made huge leaps since the early days of human spaceflight, as has understanding of the risks and challenges posed by the space environment. The ISS has proved to be an essential platform for investigating and enhancing the ability of humans to survive most of the hardships of space exploration.
From page 131...
... The capability assessment ranked the following capabilities as a high priority: • Mars EDL • Radiation safety • In-space propulsion and power50 -- Fission power -- In-space cryogenic propulsion -- NEP -- NTP -- SEP • Heavy-lift launch vehicles • Planetary ascent propulsion • ECLSS • Habitats • EVA suits • Crew health • ISRU (Mars atmosphere) Advances in many other capabilities will be essential for human exploration beyond LEO.
From page 132...
... ADVANCED PROPULSION TELE-ROBOTIC ROVERS SURFACE NUCLEAR POWER ADVANCED EVA PRESSURIZED SURFACE MOBILITY AEROASSIST SYSTEM CREW COMMAND & SERVICE MODULE (ORION) DEEP SPACE HABITATION LONG DURATION SURFACE HABITAT MARS ASCENT VEHICLE TRANSITIONAL MISSION ELEMENTS CRYOGENIC PROPULSION SYSTEM LUNAR MODULE LUNAR ORBITAL OUTPOST DEAD-END MISSION ELEMENTS ASTEROID RETRIEVAL VEHICLE MULTI-YEAR DEEP SPACE HABITAT LARGE STORABLE STAGE MARS ORBIT TRANSFER VEHICLE SPACE EXPLORATION VEHICLE FIGURE 4.9  Relationship of mission elements to high-priority capabilities.
From page 133...
... ¡  For a given capability, a substantial development effort would be required to execute a particular mission, at which point the development effort would need to continue essentially unabated to prepare for a Mars surface mission. l  For a given capability, a substantial development effort would be required to execute a particular mission, at which point minimal additional development would be needed to prepare for a Mars surface mission.
From page 134...
... A human mission to the Mars surface could require landing payloads of 40-80 MT in close proximity (tens of meters) to pre-positioned assets.53 Because existing EDL technologies do not scale up to payloads of this size, the EDL systems required for a Mars surface mission would have little resemblance to those in use today.
From page 135...
... With longer ISS tours planned and because radiation effects are cumulative, crew radiation exposure on the ISS is becoming a matter of greater concern. Because the ISS does not provide an environment typical of deep space, NASA's Human Research Program 55  NASA, Human Exploration Destination Systems Roadmap, Technology Area 07, NASA, Washington, D.C., 2012, http://www.nasa.gov/ offices/oct/home/roadmaps/, p.
From page 136...
... Technical challenges are ranked high because a suitable approach for providing adequate radiation safety has yet to be identified.60 The capability gap is ranked high because the ability to provide the level of radiation safety required for a human mission to the Mars surface is so far beyond the state of the art. Regulatory challenges are ranked medium because part of the solution may be to relax current radiation exposure limits (based on greater knowledge of the human health effects of the radiation environment in space and on the Mars surface and/or a reconsideration of the level of acceptable risk)
From page 137...
... NTP delivers Isp that is double that of cryogenic systems but comes with a high development risk due in large part to difficulties of safely ground testing an open-cycle nuclear fission system. Cryogenic propulsion is a more mature technology than the other options, but it offers lower performance and poses additional technical challenges, primarily in connection with low-loss, long-term storage and in-space transfer of cryogenic fuels and oxidizers.
From page 138...
... The assessment of fission power systems needed for a human mission to the Mars surface is summarized in Figure 4.13. Technical challenges are ranked medium because of extensive experience with reactor technologies although some new technologies would be needed to provide reliable, long-term operation in space and on the surface of Mars.
From page 139...
... The propulsion modules used for a human Mars mission would need to be stored in LEO for perhaps 4-6 months during vehicle assembly, and the Mars orbit insertion module and trans-Earth injection module would need to operate reliably after being exposed to the space environment for years. Currently, chemical propulsion systems are the only option available for human exploration missions, but chemical propulsion has a lower I sp (450 seconds in vacuum for the shuttle main engine, which uses liquid hydrogen and liquid oxygen)
From page 140...
... The cost and schedule challenges, regulatory challenges, and technical challenges are driven largely by the challenges associated with the fission power system (see Figure 4.13) that lies at the heart of the NEP system.
From page 141...
... The assessment of megawatt-class SEP systems that could be used for a human mission to the Mars surface is summarized in Figure 4.17. Technical challenges are ranked low because SEP systems are well developed and have a long history of operation in space.
From page 142...
... 4.2.6.1.4  Heavy-lift Launch Vehicles Heavy-lift launch systems (that is, launch systems with a payload capability of about 50 MT or more to LEO) would reduce the number of launches required for human exploration missions beyond LEO.
From page 143...
... As a result, the currently planned time between SLS launches is much greater than in past human spaceflight programs: the first two SLS flights (EM-1 and EM-2) will be launched 4 years apart, in 2017 and 2021.
From page 144...
... Human Space Flight Plans Committee, Seeking a Human Spaceflight Program Worthy of a Great Nation, 2009.
From page 145...
... 4.2.6.1.6  Environmental Control and Life Support System A reliable closed-loop ECLSS is needed for spacecraft, surface habitats, and EVA suits to enable long-duration human missions beyond LEO. For missions to Mars and other missions without an early-return abort option, the ECLSS must be highly reliable and easily repairable.
From page 146...
... 4.2.6.1.7  Habitats All human missions to space require a pressurized and safe environment in which crews can live and work productively. Habitats of interest include short-term in-space habitats, such as the Orion Multipurpose Crew Vehicle; long-term in-space habitats, such as the ISS and the transit habitats for long-duration missions; and surface habitats for missions to the surface of the Moon or Mars.
From page 147...
... Cost and schedule challenges are ranked medium because substantial resources and time would be needed to close the capability gap. 4.2.6.1.9  Crew Health The ability to maintain crew health during long-duration exposure to the space environment is critical for the success of human missions to Mars and other distant destinations.
From page 148...
... would be needed to counteract the effects of weightlessness during the full extent of a human mission to the Mars surface, including the transit times to and from Mars. Managing the effects of weightlessness for a human mission to the Mars surface would be further eased if the partial-gravity environment on the surface of Mars allowed astronauts to recover from at least some of the effects of weightlessness encountered during the transit to Mars.
From page 149...
... Regulatory challenges are ranked medium because new standards may be needed as research into physiological and psychosocial issues continues, particularly given the results of a recent report on ethical issues associated with human spaceflight.80 Cost and schedule challenges are ranked medium because substantial resources and time would be needed to overcome the technical and regulatory challenges and to close the capability gap. 72  Institute of Medicine, Health Standards for Long Duration and Exploration Spaceflight: Ethics Principles, Responsibilities, and Decision Framework, The National Academies Press, Washington, D.C., 2014.
From page 150...
... 81   NASA, "Human Exploration of Mars Design Reference Architecture 5.0," 2009, http://www.nasa.gov/pdf/373665main_NASASP-2009-566.pdf, p.
From page 151...
... Without belaboring the point, the relative paucity of green in this summary highlights the difficulty and cumulative scale of technology development required to achieve the horizon goal of a human mission to the Mars surface, whatever the intermediate destinations along the pathway. This technology development challenge bears directly on the next major section of this chapter, which addresses the affordability of a human spaceflight program over the decades required to extend human presence beyond LEO and make meaningful progress in addressing the enduring questions (see Chapters 1 and 2)
From page 152...
... and lays the foundation for future developments by advancing technologies and reducing knowledge gaps. Research funding is spread across many competing technologies with the goal of developing enhanced capabilities that are relevant to a variety of potential missions but without a generally accepted guiding roadmap for what is specifically required for future human spaceflight beyond LEO.
From page 153...
... SOURCE: NASA FY 2014 President's Budget Request Summary, http://www.nasa.gov/pdf/750614main_NASA_ FY_2014_Budget_­ stimates-508.pdf, E accessed January 24, 2014; NASA Historical Data Books, SP-4012, Volumes 2-7, http://history.nasa.gov/SP-4012/cover.html. RT % PPO 15 SU OPE RATI ARCH 41% ONS RESE 11% DE VEL OP ME 33 NT % FIGURE 4.28  Approximate distribution of NASA's FY 2018 proposed human spaceflight budget, which is used as the basis for projecting the cost of current human spaceflight programs beyond 2018.
From page 154...
... For budgets beyond FY 2018, this analysis assumes a lower bound of a flat budget for human spaceflight beyond FY 2018 (with no increase for inflation) and an upper bound of a human spaceflight budget increasing with inflation, projected to be approximately 2.5 percent per year in NASA's 2013 new start inflation index.84 Figure 4.29 is a projection of the current human spaceflight program of record relative to these upper and lower bounds (see also Box 4.1)
From page 155...
... $12B NEW PROJECTS: INFLATION ADJUSTED BUDGET Flat Budget $8B NEW PROJECTS: ISS ISS 2028 FLAT BUDGET $4B SLS 2020 2025 2030 2035 HEOMD Support Exploration Technology Orion & Research Development FIGURE 4.29  Projected available budget and costs of the currently planned human spaceflight program. BOX 4.1 Sand Charts The figures in this report that show notional projections of annual costs and available funding for ­human spaceflight as a function of time are commonly referred to as sand charts.
From page 156...
... 156 PATHWAYS TO EXPLORATION CUMULATIVE THEN-YEAR $ FOR NEW PROJECTS $400B $300B $200B $100B 0 2020 2030 2040 2050 2060 FLAT BUDGET / ISS 2020 $40B INFLATED BUDGET / ISS 2020 $20B FLAT BUDGET / ISS 2028 0 INFLATED BUDGET / ISS 2028 2020 2030 FIGURE 4.30  Projected cumulative then-year dollars available for new projects for human spaceflight beyond LEO, with inset showing detail through 2030. • For any given cost estimate for a pathway to a Mars surface mission, Figure 4.30 indicates when, for a budget increasing with inflation, a landing could be achieved.
From page 157...
... Thus, the committee treads cautiously in noting the difficulties associated with human space exploration beyond LEO, based primarily on the reference architectures developed by NASA. 4.2.7.2  Pathway Cost Range Methodology Now that the range of available resources, or budget, for human spaceflight has been established, affordability can be assessed.
From page 158...
... If large enough, international contributions could overcome the cost increases associated with management complexity, but that was not the case with the ISS. To make the Mars pathways affordable to the United States, international contributions would need to be large enough to cover the costs of increased complexity and additional cost elements introduced by the partnership with enough excess to cover the gap between the projected human spaceflight budget and the projected costs of each pathway, which are described in the sections that follow.
From page 159...
... Figure 4.31a shows that a large increase in the human spaceflight budget would be required to use the Enhanced Exploration pathway to land on Mars prior to 2040. The schedule-driven results for all three pathways with and without the ISS extension to 2028 are shown in Figure 4.31b.
From page 160...
... Flat Budget 2020 2030 2040 2050 Program of Record & Fixed Costs ISS 2028 Lunar Sortie Lunar Outpost B, SCHEDULE-DRIVEN PATHWAYS ANNUAL COST (THEN-YEAR $) 4x Inflation 2x Inflation Human Spaceflight Budget Increasing with Inflation 2039 Enhanced Exploration Mars Landing (2.5% per year)
From page 161...
... The lower bound of the budget uncertainty, or flat budget, was not considered, because this condition cannot sustain any pathway to land humans on Mars. To achieve pathway cost scenarios constrained to trend with the human spaceflight budget increasing with inflation, the year in which humans first land on Mars must slip to the right, flight gaps between DRMs must increase, and crewed flight rates will have to be lowered to below historical rates.
From page 162...
... Flat Budget 2020 2030 2040 2050 Program of Record & Fixed Costs ISS 2028 Lunar Sortie Lunar Outpost B, BUDGET-DRIVEN PATHWAYS ANNUAL COST (THEN-YEAR $) 4x Inflation 2x Inflation 2051 Enhanced Exploration Mars Landing 2043 Moon to Mars Landing Human Spaceflight Budget Increasing with Inflation 2037 ARM to Mars Landing (2.5% per year)
From page 163...
... Assuming that the ISS is extended to 2028 and that the human spaceflight budget is increased by 5 percent per year (twice the rate of inflation) , the earliest that a crewed surface mission to Mars is likely to occur is about 2040-2050.
From page 164...
... Flat Budget 2020 2030 2040 2050 Program of Record & Fixed Costs ISS 2028 Lunar Sortie Lunar Outpost B, OPERATIONALLY VIABLE PATHWAYS ANNUAL COST (THEN-YEAR $) 4x Inflation 2x Inflation 2048 Enhanced Exploration Mars Landing 2043 Moon to Mars Landing Human Spaceflight Budget Increasing with Inflation (2.5% per year)
From page 165...
... Thus, without a considerable increase in human spaceflight funding for NASA, the ARM-to-Mars pathway presents the prospect of a long period of technology development during which NASA's stakeholders do not see human exploration missions taking place. This problem poses one of the most serious challenges to program sustainability that the study's Technical Panel identified.
From page 166...
... The operationally viable scenarios are all marginal both in affordability (they would require the human spaceflight budget to increase at twice the rate of inflation) and in operational tempo (the mission rate would still be well below historical precedents, although not as low as in the budget-driven scenarios)
From page 167...
... This incremental growth in capabilities implies that the Enhanced Exploration pathway has lower development risk than either the ARM-to-Mars or Moon-to-Mars pathway. However, given the level of technological advances required to develop the 11 primary mission elements, supporting systems, and associated capabilities, the Enhanced Exploration pathway still has high development risk.
From page 168...
... Important new capabilities may also be derived from commercial activities that are pursuing analogous or related goals for their own purposes, including traditional aerospace activities, such as launch vehicle or satellite development, as well as nascent industries, such as space tourism. Other government agencies, principally DOD, also produce new technologies that can be leveraged into the human spaceflight program.
From page 169...
... 4.3.2  Human Exploration and Operations Mission Directorate 4.3.2.1  Exploration System Development The SLS heavy-lift launch vehicle, Orion MPCV, and related ground systems are being developed with the goal of restoring the ability of the United States to conduct human spaceflight beyond LEO. The development of these systems is not directed at any particular destination.
From page 170...
... It is well to note, however, that establishment of a commercial space-based economy with human spaceflight as a major component is highly speculative. 4.3.2.4  Space Technology Mission Directorate The Exploration Technology Development Program is developing technologies to support human exploration beyond Earth orbit with a focus on advanced technologies that have a long development time.
From page 171...
... , the Government Accountability Office, and the National Research Council have each reported on NASA infrastructure and offered worrisome observations.95 NASA's OIG determined that about 80 percent of NASA facilities are more than 40 years old, that maintenance costs for these facilities amount to more than $24 million a year, and that continuing shortfalls in maintenance are adding to an already substantial backlog in deferred maintenance. 96 Carrying such costs constitute latent threats to the development of newer infrastructure that will be needed to support human exploration beyond LEO.
From page 172...
... This is not surprising given the small market for such capabilities and the fact that NASA is developing its own launch vehicle, the SLS, for human spaceflight beyond LEO. 4.3.4  Department of Defense Research and development of new technology and capabilities in government agencies other than NASA could benefit future human spaceflight programs.
From page 173...
... The roadmap demonstrates how initial capabilities can enable a variety of missions in the lunar vicinity, responding to individual and common goals and objectives, while contributing to building the partnerships required for sustainable human space exploration." 100 The 12 space agencies and countries that created the ISECG The Global Exploration Roadmap continue to support human exploration beyond LEO. 4.3.6  Robotic Systems 4.3.6.1  Robotic Science and Exploration Robotic space exploration, which is driven by science objectives, has greatly expanded knowledge of the solar system, including its origin and evolution.
From page 174...
... The most common response to the criticism that human spaceflight is extremely expensive is that humans' capacity for contextual reasoning and problem-solving is indispensable in exploration scenarios. Nonetheless, much more has been learned about our solar system by relatively primitive robots than by human explorers.
From page 175...
... Increasing NASA's budget to allow increasing the human spaceflight budget by 5 percent per year would enable pathways with potentially viable mission rates, greatly reducing technical, cost, and schedule risk.
From page 176...
... , and the horizons of human existence will not be expanded -- at least not by the United States. With such a consensus, however, and with strict adherence to the pathways approach and principles outlined in this report, the United States could maintain its historical position of leadership in space exploration and embark on a program of human spaceflight beyond LEO that, perhaps for the first time in the more than half-century of human spaceflight, would be sustainable.


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