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1 The Constellation System and Opportunities for Science NASA asked the National Research Council (NRC), through the Space Studies Board and the Aeronautics and Space Engineering Board, to establish an ad hoc committee to assess potential space and Earth science research concepts that could take advantage of the capabilities of the Constellation System of launch vehicles and spacecraft and could launch in the period 2020-2035. The Constellation System is being developed by NASA to implement the initial phases of the Vision for Space Exploration and consists of the Orion spacecraft and the Ares I and Ares V launch vehicles. The Committee on Opportunities Enabled by NASA’s Constellation System is conducting its assessment in two phases; this interim report represents the conclusion of the first phase. This interim report is not intended as a review or endorsement of the Constellation System as a whole or of any of its elements, nor is this interim report intended to make any suggestions regarding potential changes to the Constellation System. Rather, this interim report accepts the Constellation System as it was defined for the committee in February 2008 and seeks to assess a set of proposed science missions and identify those that could benefit from Constellation’s capabilities and would potentially fly sometime after the next decade.1 In addition, this report does not prioritize science goals for NASA. That responsibility properly lies with the decadal survey process.2 The kinds of science missions that the committee evaluated for this interim report⎯and the kind that it envisions will be proposed in response to its request for information⎯are relatively large and ambitious. They all fall into a category that is generally referred to as flagship-class missions. The committee grouped the missions into three “cost bins” for the purpose of this study: less than $1 billion, $1 billion to $5 billion, and more than $5 billion. Of the mission concepts evaluated, only one was considered to be marginally in the less than $1 billion category, and seven were considered to be in the greater than $5 billion category, which would make them larger than any other space science mission developed by NASA to date (with the possible exception of the James Webb Space Telescope, currently estimated to cost $4.5 billion). The committee notes that adding a heavy-lift launch vehicle option could lead to larger science missions and even higher costs. There is a direct relationship between the size of a spacecraft and its cost. Expensive space science programs will place a great strain on the space science budget, which has been essentially flat for several years and is already under strain from an ambitious slate of 85 flight missions.3 To estimate the costs of potential large space science missions, the committee used NASA’s Advanced Missions Cost Model and estimated the costs of three new-design planetary science missions 1 Marc Timm, “Constellation Overview” presented to National Research Council Committee on Science Opportunities Enabled by NASA’s Constellation System, February 2008. The NASA presentation on the Constellation System is available at 2 The decadal survey process is the method by which NASA and its relevant scientific communities establish the scientific goals in each particular discipline at roughly one-decade intervals. The astronomy and astrophysics decadal survey has been conducted since the 1960s; decadal surveys in solar system exploration, heliophysics, and Earth science have been started only within the last decade. 3 NASA’s current science program consists of 94 flight missions⎯53 in operation plus 41 in development (Alan Stern, NASA Science Mission Directorate, presentation to Space Studies Board, March 10, 2008). 5

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representing the three levels of technological difficulty used in NASA’s model.4 Illustrated in Figure 1.1, the results indicate the correlation between rising payload mass and higher cost. The committee was not provided cost estimates of either the Ares I or the Ares V launch vehicles, but the latter can be expected to be substantially more expensive than even the largest Evolved Expendable Launch Vehicle (EELV) currently in the U.S. inventory⎯the Delta IV heavy with a launch cost of about $250 million.5 The combined effect of expensive payloads and expensive launchers would distort the balance of the space science program. Finding 1. The greatly increased payload capability promised by Ares V could lead to much more costly science payloads. Finding 2. The committee determined that the Ares I capabilities are not sufficiently distinct from those of Atlas V and Delta IV to enable different types or a higher quality of space science missions. FIGURE 1.1 Estimated costs as a function of payload mass for three classes of difficulty for solar system exploration missions utilizing NASA’s Advanced Mission Cost Model. NOTE: For comparison, the Cassini-Huygens payload mass was approximately 4.6 metric tons. Note that the Ares V capabilities would actually span a number of payload masses in this figure. For example, Ares V could launch 10 metric tons to Uranus or Neptune, and significantly more to Jupiter and Mars. 4 The Advanced Mission Cost Model is available at 5 The committee bases the conclusion that the Ares V will be significantly more expensive than the Delta IV heavy on several factors. But one of the most straightforward is counting the number of engines: the Delta IV heavy has three RS-68 main engines, whereas the Ares V is expected to have five to six RS-68s, plus two five-segment solid rocket boosters. 6

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BACKGROUND In 2004, to extend analyses of potential future space science missions and to identify precursor technology requirements, NASA funded studies for a variety of advanced missions referred to as the space science “Vision Missions.” These missions were inspired by a series of NASA roadmap activities conducted in 2003 and were not connected to the Vision for Space Exploration announced by President George W. Bush in January 2004. The Vision Mission concepts were predicated on the launch vehicles that were available at the time: the EELVs such as the Atlas V and the Delta IV in their various configurations. Studies of 14 potential missions were funded, and final reports on 11 of these studies were prepared and delivered to NASA in 2006-2007. These studies fell into three broad categories: astronomy and astrophysics, heliophysics, and solar system exploration (i.e., planetary exploration), with some overlap between them. The mission concepts studied were: • Advanced Compton Telescope (ACT), • Generation-X (Gen-X), • Single Aperture Far Infrared Telescope (SAFIR), • Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, • Modern Universe Space Telescope (MUST), • The Big Bang Observer, • Stellar Imager (SI), • Interstellar Probe, • Innovative Interstellar Explorer, • Solar Polar Imager, • Neptune Orbiter with Probes (2 studies), • Titan Explorer, • Titan Organics Exploration Study, and • Palmer Quest. Of the studies listed above, three did not result in final reports: the Big Bang Observer, the Innovative Interstellar Explorer, and the Titan Organics Exploration Study. However, for the uncompleted Innovative Interstellar Explorer mission study, the committee received a briefing that focused primarily on the changes to the mission profile that could result from use of the Constellation System. The committee addressed the two Neptune mission studies in a single assessment. Starting in 2005 NASA began development of the Constellation System to enable the implementation of the United States Space Exploration Policy. Constellation currently consists of three main elements: an Ares I launch vehicle capable of launching 25.6 metric tons into low Earth orbit; a heavy-lift launch vehicle called the Ares V, reminiscent of the Saturn V, with a 10-meter-diameter payload fairing and the capacity to launch about 143.4 metric tons into low Earth orbit (64 metric tons to translunar injection); and the Orion human-carrying spacecraft, capable of carrying up to six crew members to low Earth orbit or four to lunar orbit.6 The committee was briefed on the Ares I capabilities, which are generally similar to those of the EELV family of launch vehicles. Although the committee was informed of the possible availability of a “science shroud” for the Ares I (i.e., a launch vehicle shroud adapted for the Ares I to enable it to carry payloads other than the Orion spacecraft), the committee determined that the Ares I capabilities are not sufficiently distinct from EELV capabilities that they will enable different types or quality of space science missions. The lift capability and payload volume of the Ares I, with a science shroud, are roughly equivalent to the Delta IV heavy launch vehicle configuration for missions to a low Earth orbit (LEO) at 6 For comparison, the space shuttle is capable of launching 25 metric tons to low Earth orbit, and the Saturn V was capable of launching 119 metric tons into low Earth orbit and 47 metric tons into translunar injection. 7

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200 km. Compared to the performance of the Delta IV, the performance of the Ares I drops dramatically at higher altitudes because the Ares I design is optimized to deliver its payload, the Orion spacecraft, to LEO, whereas the Delta IV has been designed for a broad range of mission profiles.7 Orion was not a significant factor in the committee’s deliberations because of the types of science missions the committee was asked to evaluate; therefore the committee did not receive much information on the Orion capabilities for this first phase of its study. The committee does note, however, that Orion or its future derivative might be needed as an option for assembly, servicing, repair, and modernization for some of the mission concepts the committee analyzed. The committee plans to discuss Orion in greater detail in its final report. The committee received a substantial briefing on the Ares V launch vehicle and believes that Ares V offers the greatest potential for an impact on science. The availability of a launcher capable of placing large-diameter, large-volume, heavy spacecraft into orbit seemingly removes the physical, although not the financial, constraints on missions that would benefit from being able to fly large, heavy payloads to their destinations. The committee was informed that the current baseline Ares V shroud has a usable volume of ~860 cubic meters, with an 8.8-meter diameter and 17.2-meter length (the maximum external diameter is 10 meters). The committee was also informed of a notional Ares V shroud with a volume of ~1,410 cubic meters, an 8.8-meter diameter, and a 26.2-meter length. During its deliberations, the committee heard statements from several presenters that for their missions, volume is more important than mass to orbit. They stated that Ares V already has the capability to lift significantly more mass than they could realistically require, but that added volume might dramatically simplify some design requirements. (For example, see the discussion of Generation-X in Chapter 2.) The committee concluded that a larger shroud for the Ares V may have important potential science utility. However, shortly after the committee’s meeting, NASA publicly acknowledged that the 143.4-metric ton payload capability of the Ares V is insufficient for the planned lunar mission and that the baseline vehicle will have to be improved to launch approximately 10 metric tons more mass to low Earth orbit. These enhancements may increase the length of the vehicle and therefore make it impossible to increase the length of the Ares V payload shroud due to height limitations in the Vehicle Assembly Building.8 According to NASA, the first flight of Ares V is not expected until 2018 at the earliest. Lunar missions would begin in 2019 or 2020, and for at least the first several years of operations the Ares V would be firmly committed to supporting a buildup of the human lunar outpost. Ares V could therefore not be available to support science missions until the early or mid-2020s at the earliest. APPROACH AND EVALUATION CRITERIA The committee chose a two-phase approach to assessing potential space and Earth science mission concepts in view of the new Constellation architecture. First, the committee analyzed the 11 Vision Mission concepts already in hand from NASA’s 2004 solicitation; this analysis is the subject of this interim report. Second, the committee will review additional concepts received from the scientific and technical community in response to its request for information (see Appendix A). 7 The Constellation System has identified a notional Ares I configuration that includes an additional upper stage, the dual engine Centaur, for interplanetary missions. The preliminary performance estimates indicate that it provides slightly less payload capability at escape velocity (C3 = 0 km2/s2) than does the Delta IV heavy-lift vehicle, but slightly greater capability for missions with high C3 requirements. Since this is only a notional configuration, there is significant uncertainty about the performance estimates driven by the uncertainty in the design of the interface between the Ares I and the Centaur. Although the performance benefit of the Ares I is not significant, there may be value in utilizing the Ares I if the launch costs can be shown to be lower than those for the existing fleet of EELV vehicles. 8 F. Morring, “Heavier Still: NASA Needs Bigger Ares V to Meet Lunar Requirements,” Aviation Week & Space Technology, March 3, 2008, pp. 34-35. 8

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The committee was impressed by the quality of the studies of the 11 Vision Mission concepts that it reviewed. Even a cursory reading of the studies leaves the reader with an overwhelming impression of great scientific challenges and opportunities in astronomy, astrophysics, heliophysics, solar system exploration, and astrobiology. The Vision Mission studies, some of them more than 250 pages in length and replete with appendixes, contained penetrating and extensive scientific and technical analyses and represent an enormous investment of intellectual energy by large teams of highly experienced scientists, engineers, and program managers. These missions were developed using the relatively limited capabilities of EELVs, although in several instances the missions envisioned required new and innovative techniques such as solar sails, solar-electric propulsion, or aerocapture to meet their objectives.9 In their briefings to the committee, representatives for many of these missions indicated that their studies could have benefitted from a larger vehicle with greater mass, volume, and propulsion capabilities. In the committee’s opinion, the representatives had not had enough time to fully develop and document the full scope of scientific opportunities provided by the capability of the Ares V vehicle in their presentations and supplementary material. In addition, in the opinion of the committee, there are missions for which risk or possibly even spacecraft cost might be decreased by using a launch vehicle more capable than a member of the EELV family. The committee evaluated the 11 Vision Mission concepts using two criteria: 1. Does the concept offer a significant advance in a scientific field? (“Significant” is defined as providing an order-of-magnitude or greater improvement over existing or planned missions, and enabling a qualitative new approach to the important scientific questions in the field.) 2. Does the concept have a unique requirement for Constellation System capabilities, e.g., ⎯Does use of the Constellation System’s elements make a previously impossible mission technically feasible? ⎯Does use of the Constellation System’s elements reduce mission risk or enhance mission success for a previously complicated mission? ⎯Does use of the Constellation System’s capabilities offer a significant cost reduction (i.e., 50 percent or more) in the cost of accomplishing the mission? The committee was tasked with assessing the relative technical feasibility of mission concepts compared to each other. Relative technical feasibility was judged on the basis of material submitted to the committee without any detailed independent analysis. The committee was not asked to assess the relative scientific merit of the Vision Missions. Such prioritization of missions is more appropriately the work of decadal surveys looking at proposed missions in the context of others in each scientific field. However, the committee did take note of Vision Mission proposals that had been mentioned in previous NRC reports. The committee had only a limited set of proposals/subjects to work with, and the evaluations presented in this interim report do not imply any restriction on future areas of interest that may be addressed in the final report. In particular, it is possible that Earth science missions could take advantage of Constellation’s capabilities, although none of the Vision Mission studies addressed in this interim report proposed Earth science missions. The committee also notes that whereas the Vision Mission studies did not assume the presence of astronauts, astronauts may be included in the concepts described in responses to the committee’s request for information. The request for information states that broad areas of space science could potentially utilize the capabilities being developed for Constellation. Of the 11 mission concepts reviewed for this interim report, the committee determined that all could potentially offer a significant advance in their scientific fields. However, of the 11 mission 9 Several space missions have employed the technique of aerobraking, which involves using a planetary atmosphere to slowly lower a spacecraft’s orbit. In contrast, aerocapture is the use of a planetary atmosphere to enable the spacecraft to be captured into an initial orbit. 9

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concepts, the committee determined that four (Advanced Compton Telescope, SAFIR, Kilometer- Baseline Far-Infrared/Submillimeter Interferometer, and Palmer Quest) did not directly benefit from Constellation. Nevertheless, the committee was impressed by the scientific objectives and ambition of all of these missions, including the ones that would not benefit from Constellation. The missions will undoubtedly be evaluated in the decadal surveys to which they are relevant and may perform well in those. That the committee did not identify several proposed missions for further study as Constellation- enabled missions should not imply anything about their future viability. The reasons for the committee’s assessment that some of the Vision Missions would or would not benefit from the Constellation System generally fall into two categories. The first concerns the mass, volume, and complexity of the proposed missions and whether they fit into the capabilities of the existing family of EELV rockets. The second category concerns the propulsion requirements of the missions and whether they could benefit from a powerful launch vehicle that could place them in their required mission orbit (propulsion required to either accelerate them or decelerate them into their proper orbits). The committee did not conduct detailed cost assessments of any of the Vision Missions. Instead, it used the cost estimates provided by their proposers (not adjusted for inflation) and relied on the experience of the committee members and comparisons with similar missions. In most cases, the proposers were basing their cost estimates on an EELV launch vehicle and not on the Constellation launch vehicles or architecture, particularly Ares V. Transitioning of payloads to a new, more capable vehicle would require reconsideration of several items: complexity of instrument packaging and deployment, need for auxiliary propulsion, and so on, all of which would affect mission costs. The committee was informed by NASA that no reliable cost estimates of the Ares V are available. 10