2
Analysis of Space Science Mission Studies

This study evaluates a total of 17 mission concepts for advanced space science missions in the fields of astronomy and astrophysics, heliophysics, and solar system exploration.1 The study did not receive any proposals for Earth science missions. Although NASA had proposed a workshop to help identify potential Earth science missions that could make use of the Constellation System, the workshop was canceled for lack of interest.

The evaluated mission concepts are examples of the types of science missions that could be conducted using the Constellation System. The committee is not endorsing any specific mission team or approach and recognizes that, because of their preliminary nature, any mission concept that is pursued may require significant revision.

BACKGROUND AND APPROACH

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.2 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 Evolved Expendable Launch Vehicles (EELVs) such as Atlas V and Delta IV in their various configurations. NASA funded studies of 14 potential Vision Missions, and final reports on 11 of these studies were prepared and delivered to NASA in 2006 and 2007.

The Vision Mission studies fell into three broad categories: astronomy and astrophysics, heliophysics, and solar system exploration (i.e., planetary exploration), with some overlap. The mission concepts studied were the following:

  • Advanced Compton Telescope (ACT),

  • The Big Bang Observer,

  • Generation-X (Gen-X),

1

Information describing the scientific objectives and current development status of the characteristics of each mission concept evaluated here is derived from the materials provided to the committee by the teams that developed the respective proposed mission concepts.

2

These studies are summarized in Marc S. Allen, NASA Space Science Vision Missions, Progress in Astronautics and Aeronautics Series, No. 224, American Institute of Aeronautics and Astronautics, Reston, Va., 2008.



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2 Analysis of Space Science Mission Studies This study evaluates a total of 17 mission concepts for advanced space science missions in the fields of astronomy and astrophysics, heliophysics, and solar system exploration. 1 The study did not receive any proposals for Earth science missions. Although NASA had proposed a workshop to help identify potential Earth science missions that could make use of the Constellation System, the workshop was canceled for lack of interest. The evaluated mission concepts are examples of the types of science missions that could be conducted using the Constellation System. The committee is not endorsing any specific mission team or approach and recognizes that, because of their preliminary nature, any mission concept that is pursued may require significant revision. BACKGROUND AND APPROACH 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.2 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 Evolved Expendable Launch Vehicles (EELVs) such as Atlas V and Delta IV in their various configurations. NASA funded studies of 14 potential Vision Missions, and final reports on 11 of these studies were prepared and delivered to NASA in 2006 and 2007. The Vision Mission studies fell into three broad categories: astronomy and astrophysics, heliophysics, and solar system exploration (i.e., planetary exploration), with some overlap. The mission concepts studied were the following: • Advanced Compton Telescope (ACT), • The Big Bang Observer, • Generation-X (Gen-X), 1 Information describing the scientific objectives and current development status of the characteristics of each mission concept evaluated here is derived from the materials provided to the committee by the teams that developed the respective proposed mission concepts. 2These studies are summarized in Marc S. Allen, NASA Space Science Vision Missions, Progress in Astronautics and Aeronautics Series, No. 224, American Institute of Aeronautics and Astronautics, Reston, Va., 2008. 0

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES • Innovative Interstellar Explorer, • Interstellar Probe, • Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, • Modern Universe Space Telescope (MUST), • Neptune Orbiter with Probes (2 studies), • Palmer Quest, • Single Aperture Far Infrared (SAFIR) Telescope, • Solar Polar Imager, • Stellar Imager, • Titan Explorer, and • Titan Organics Exploration Study. Of the studies listed above, the three that did not result in final reports were the Big Bang Observer, the Innova- tive Interstellar Explorer, and the Titan Organics Exploration Study. For the incomplete Innovative Interstellar Explorer mission study, the committee received a briefing that focused primarily on the science and the changes to the mission profile that could result from use of the Constellation System. The committee received briefings on and addressed the two Neptune mission studies in a single assessment. The National Research Council’s (NRC’s) Committee on Science Opportunities Enabled by NASA’s Con- stellation System asked the principal investigators or other representatives of the 14 Vision Mission studies to present their studies to the committee at its first meeting, on February 20-22, 2008. The principal investigators were asked to consider how their existing studies might benefit from the Constellation capabilities—primarily the larger payload capability and shroud dimensions of the Ares V rocket and the possibility of the human servicing of spacecraft. Because of the scheduling requirements of the committee’s work, the presenters had only limited time to assess how the Constellation capabilities might affect their proposals. Some indicated that the Constellation System would have no appreciable effect on their concepts, and others indicated that it would serve as a substitute for required technology development. Some of the proposers also indicated that their science objectives might be enlarged and expanded if they had additional mass and volume available. In its interim report, the committee evaluated the 11 Vision Missions on which final studies had been com- pleted.3,4 The committee also issued a request for information to the scientific community seeking proposals for additional space science missions (see Appendix C in the present report), resulting in 6 additional mission proposals: • Advanced Technology Large-Aperture Space Telescope (ATLAST), • Dark Ages Lunar Interferometer (DALI), • 8-Meter Monolithic Space Telescope, • Exploration of Near Earth Objects via the Crew Exploration Vehicle, • Solar Probe 2, and • Super-EUSO (Extreme Universe Space Observatory). For its final report, the committee evaluated these 6 additional mission concepts using the same criteria used for the Vision Mission studies. 3 National Research Council, Science Opportunities Enabled by NASA’s Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. 4 For the interim report, and for this final report, the committee chose to consider both the Vision Mission final reports and the presentations made to the committee during its February 2008 meeting. In some cases there were no or only minor differences between the reports and the presentations. In others, the differences primarily concerned speculation supported by limited analysis on how the mission would benefit from the Constellation capabilities. For the Interstellar Probe concept, the presentation was made by the team that did not produce a final report. However, the science objectives of both Interstellar Probe studies were identical, and the presenter focused primarily on how the mission concept would benefit from the Constellation System.

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 LAUNCHING SCIENCE EVALUATION CRITERIA AND RESULTS The committee analyzed each of the 17 missions—the 11 Vision Mission concepts in hand from NASA’s 2004 solicitation and the 6 additional concepts received from the scientific and technical community in response to its request for information—according to a standard format based on the committee’s statement of task, which specified analysis of the following information for each mission concept considered: 1. Scientific objectives of the mission concept; 2. A characterization of the mission concept insofar as the maturity of studies to date have developed it; 3. The relative technical feasibility of the mission concepts compared to each other; 4. The general cost category into which each mission concept is likely to fall; 5. Benefits of using the Constellation System’s unique capabilities relative to alternative implementation approaches; and 6. Identification of the mission concept(s) most deserving of future study. In view of the Constellation System architecture, the committee evaluated the mission concepts using two criteria: 1. Does the concept offer a significant advance in a scientific field? “Significant” is defined here 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. The committee judged the significance of the scientific advances of each concept on the basis of material submitted or presented to the committee. 2. Does the concept benefit from (or have a unique requirement for) Constellation System capabilities—for example: —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 elements enhance performance? —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 the material submitted to the committee, without any detailed independent analysis. The committee was not asked to assess the relative scientific merit of the missions. Such prioritization of missions is more appropriately the work of decadal surveys, which look at proposed missions in the context of others in each scientific field. However, the committee did take note of mis- sion proposals that had been mentioned in previous NRC reports, particularly decadal surveys. Of the 17 mission concepts reviewed for this report, the committee determined that all could potentially offer a significant advance in their scientific fields (meeting criterion 1 above). However, of the 17, the committee determined that 5 mission concepts (ACT, SAFIR Telescope, Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, Palmer Quest, and Super-EUSO; see Appendix B of this report) did not directly benefit from Constellation (thus not meeting criterion 2 above). Nevertheless, the committee was impressed by the scientific objectives and ambition of these 5 missions. The missions would 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 be interpreted to imply anything about their future viability, only that they may have other paths to pursue. The reasons for the committee’s assessment that some of the mission concepts would or would not benefit from the Constellation System generally fall into two categories. The first concerns the mass, volume, and complexity of each proposed mission and whether it fits into the capabilities of the existing family of EELV rockets (if so, the mission would not need the greater capabilities of Constellation). The second category concerns the propulsion

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES requirements of each mission and whether it could benefit from a powerful launch vehicle that could place it in its required mission orbit (propulsion required either to accelerate the mission or decelerate it into its proper orbit). The committee did not conduct detailed cost assessments of any of the mission concepts. Instead, it used the cost estimates provided by the proposers of the respective concepts (not adjusted for inflation) and relied on the experience of the committee members and comparisons with similar missions to adjust them. The mission concepts were judged by subject area, that is, astronomy and astrophysics, heliophysics, and solar system exploration. This was done for two reasons. First, NASA generally does not integrate its overall sci- ence mission priorities but treats them separately. Second, one of the committee’s goals was to produce a broadly representative sample of science opportunities offered by the Constellation System. For its final report, the committee focused on the 12 mission concepts it had determined would benefit from the Constellation System (see Chapter 2). Because the committee was charged with determining which concepts are “most deserving” of further study, it divided the list of 12 mission concepts into “more deserving” and “deserv- ing” categories based on two considerations: • Mission Impact on Science in the Field of Study—The mission concept must present well-articulated sci- ence goals that the committee finds compelling and worthy of the investment needed to develop the technology. The committee defined “compelling” in terms of the breadth of impact that the science would have on the field, the relative size of the community for that science, and whether or not it would provide an order-of-magnitude improvement in science return. • Technical Maturity—The mission concept must be sufficiently mature in its overall conception and tech- nology. If the technology for accomplishing the mission does not currently exist at a high technology readiness level, the mission must provide a clear path indicating how it will be developed. The committee defined “suf- ficiently mature” as indicating a technology readiness level (TRL) of 5 or higher (see Appendix D) for the major components and a form of “integrated TRL” that considered the challenges of integrating multiple technologies, as well as the lowest TRL rating of the individual technologies. If a mission concept satisfied both criteria to a moderate or high degree, it was designated “more deserving” of further study. In addition, the committee evaluated whether a mission was “enabled” or “enhanced” by Constel- lation. If a mission was “enabled,” it could not be accomplished without the Constellation System’s capabilities. If a mission was “enhanced,” it could be accomplished without Constellation (for instance, by employing a smaller launch vehicle), but it might better accomplish its goals using Constellation’s capabilities: for instance, the mission could return much greater amounts of data or could return useful data faster if “enhanced” by Constellation. Table 2.1 presents a summary of the committee’s evaluations of the 12 individual mission concepts that it identified as being worthy—that is, either “deserving” or “more deserving”—of further study as Constellation missions. As indicated in the table, the committee identified 5 missions that it determined are “more deserving” of further study. Recommendation: NASA should conduct further study of the following mission concepts, which have the most potential to demonstrate the scientific opportunities provided by the Constellation System: 8-Meter Monolithic Space Telescope, Interstellar Probe, Neptune Orbiter with Probes, Solar Polar Imager, and Solar Probe 2. Several of the above missions, particularly among the heliophysics missions, are well defined scientifically and do not require significant study of instruments or related issues. Further study with respect to those mission concepts should focus primarily on the relationship between the Ares V capabilities and their propulsion require- ments. Additional study of scientific capabilities would primarily be confined to how the science would be affected by the additional Ares V capabilities. Because of these narrow requirements, NASA may have the ability to further study other possible Ares V science missions. The other seven “deserving” missions listed in Table 2.1 are also promising and offer great potential science return but will require more effort in order to be brought to a higher level of maturity.

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 LAUNCHING SCIENCE TABLE 2.1 Overall Assessment of Mission Concepts Deemed Worthy of Further Study as Constellation Missions Impact of Summary with Discipline/Mission Constellation Respect to Concept on Mission Impact of Mission on Science in the Field Technical Maturity Further Study Astronomy and Astrophysics Missions Advanced Enabled Has several disparate ultraviolet (UV)/optical Low for mirror technology Deserving Technology science goals; unclear what the scientific (including mass) Large-Aperture impact will be. Medium for detectors and Space Telescope thermal control (ATLAST) 8-Meter Monolithic Enabled Monolithic mirror allows direct study of High for mirror and More deserving Space Telescope extrasolar terrestrial planets and the search structure for signs of life. Low for coronagraphic observation Dark Ages Lunar Enabled Unique capability for viewing the universe at Medium for rovers and Deserving Interferometer a distinctive period: between about 10 million interferometrics (DALI) and 300 million years after the big bang. Low for reducing mass and for deploying and operating in a remote location Generation-X Enabled Observing the birth and evolution of the Low for mirror Deserving (Gen-X) earliest black holes and galaxies. development and operations Modern Universe Enabled Has disparate UV/optical science goals; High for instruments Deserving Space Telescope unclear what the scientific impact will be. Low for coronagraph and (MUST) mirror assembly Astronomy and Astrophysics/Heliophysics Mission Stellar Imager Enhanced Study of stellar magnetism and activity cycles Low for formation flying Deserving to understand solar weather and its potential human implications. Limited lifetime of the mission may not accomplish the stated goals. Heliophysics Missions Solar Probe 2 Enabled Allows first measurements of the acceleration High More deserving of the solar wind and coronal heating. (focused on Ares V) Interstellar Probe Enabled Accomplishes new science on the interaction High for science, More deserving (ISP) of the Sun and the interstellar medium. instruments, and mission (focused on concept Ares V) Solar Polar Imager Enhanced Accomplishes the first close and repeated High for instruments More deserving imaging of the Sun’s poles, providing Propulsion not studied in (focused on comprehensive information on the structure sufficient detail Ares V) and dynamics of the Sun. Solar System Exploration Missions Exploration of Enabled Allows sample return in context and in situ High for instruments Deserving Near Earth Objects measurements of the near-Earth asteroid, Low for human factors via the Crew providing information on the interior such as radiation Exploration Vehicle properties. Neptune Orbiter Enhanced Permits first detailed exploration of an ice High for instruments More deserving with Probes giant planet, with implications for clarifying Low for propulsion the origins of the solar system. Titan Explorer Enhanced Provides great potential for high-value future High for instruments Deserving science, but did not account for Cassini Medium for blimp results. NOTE: The mission concepts are listed by discipline. All of the concepts are robotic missions, with the exception of the proposal for Exploration of Near Earth Objects via the Crew Exploration Vehicle.

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES (a) (b) FIGURE 2.1 (a) Illustration of the Sun-Earth Lagrangian points and the Earth-Moon Lagrangian points. Sun-Earth L2 is the proposed site of several large astronomical and heliophysics observatories. One possible option for spacecraft servicing is mov- 21. b from Lester.eps ing an observatory from Sun-Earth L2 to an Earth-Moon Lagrangian point for easier access. (b) Change in velocity (delta-v) requirements for different locations in the Earth-Moon system and Lagrangian points. NOTE: E-M L1, Earth-Moon libration point L1; GEO, geostationary orbit; GTO, GEO transfer orbit; LEO, low Earth orbit; LLO, low lunar orbit; LTO, lunar transfer orbit; Low-T, low thrust; High-T, high thrust; S-E L1, Sun-Earth libration point L1; S-E L2, Sun-Earth libration point L2; ΔV, delta-v. SOURCE: Courtesy of NASA.

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 LAUNCHING SCIENCE Recommendation: NASA should consider further study of the following mission concepts: Advanced Technology Large-Aperture Space Telescope, Dark Ages Lunar Interferometer, Exploration of Near Earth Objects via the Crew Exploration Vehicle, Generation-X, Modern Universe Space Telescope, Stellar Imager, and Titan Explorer. After this study began, NASA initiated several workshops to explore the potential benefits of the Ares V launch vehicle for science missions.5 The committee believes that during the brief time of this study, the enormous potential capabilities of the Ares V, for example, were not recognized by the scientific community at large. The committee also believes that the short deadlines associated with the second phase of this study impaired the ability of the scientific community to respond with mission concepts, especially in terms of the weight and volume capabilities planned for Ares V in particular. The Ares V workshops sponsored by NASA were a valuable means for facilitating broader involvement of the scientific community in assessing the potential of Constellation for science. Recommendation: NASA should solicit further mission concepts that are most likely to benefit from the capabilities of the Constellation System in each of the space and Earth science disciplines: astronomy and astrophysics, Earth science, heliophysics, and planetary science. The agency should seek mission concepts that are studied in a uniform manner with regard to design, system engineering, and costing. THE SUN-EARTH LAGRANGIAN POINT Several of the mission concepts evaluated in this report (e.g., ATLAST, 8-Meter Monolithic Space Telescope, MUST, Gen-X, Stellar Imager) would operate at the Sun-Earth Lagrangian point L2. The Wilkinson Microwave Anisotropy Probe is already in orbit around the Sun-Earth L2 point, and this is the location planned for the James Webb Space Telescope (JWST) as well as other space telescopes. The value of this location is that spacecraft remain in a stable orbit, they have near-continuous sunlight, and their view of the full sky is not obstructed by Earth. See Figure 2.1 on page 25. ASTRONOMY AND ASTROPHYSICS MISSIONS Advanced Technology Large-Aperture Space Telescope (ATLAST) Scientific Objectives of the Mission Concept New observational capabilities, such as those needed to find faint traces of life on extrasolar planets, will depend on the use of space-based telescopes with large apertures. The Advanced Technology Large-Aperture Space Telescope will be able to explore the nearest ~1,000 stars capable of harboring life for Earth-size planets and characterize their spectra. ATLAST will have up to 10 times the resolution of JWST and up to 300 times the sensitivity of the Hubble Space Telescope (HST). ATLAST will also be a next-generation ultraviolet (UV)/optical Great Observatory, in the model established by the Hubble Space Telescope, capable of achieving breakthroughs in a broad range of astrophysics and adaptable to addressing scientific investigations yet to be conceived. 5The first workshop, Ares V Astronomy Workshop, sponsored by the NASA Ames Research Center and held April 26-27, 2008, focused on the Ares V and possible astronomy missions, the majority of which are also evaluated in this study. The second workshop, Ares V Solar Sys- tem Science Workshop, also sponsored by Ames and held August 16-17, 2008, focused on the Ares V and potential solar system exploration. See http://event.arc.nasa.gov/ aresv/ and http://event.arc.nasa.gov/aresv-sss/index.php?fuseaction=home.home. The Astronomy Workshop is summarized in S. Langhoff, D. Lester, H. Thronson, and R. Correll, eds., Workshop Report on Astronomy Enabled by Ares V, NASA/CP-2008- 214588, August 2008, available at http://event.arc.nasa.gov/main/home/reports/CP-2008-214588-AresV.pdf. The Solar System Workshop is summarized in S. Langhoff, T. Spilker, G. Martin, and G. Sullivan, eds., Workshop Report on Ares V Solar System Science, NASA/CP-2008- 214592, August 2008, available at http://event.arc.nasa.gov/main/home/reports/CP-2008-214592-AresV-SSS_Print.pdf. 6The first two subsections are based on material in M. Postman, T. Brown, A. Koekemoer, and the ATLAST Team, “An Advanced Technol- ogy Large Aperture UV/Optical Space Telescope: A NASA Astrophysics Strategic Mission Concept and Technology Development Study,” in response to a request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. See footnote 1 above.

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES The goals of the ATLAST mission concept are the following: 1. Search for life on exoplanets. 2. Study star formation histories in the local universe. 3. Probe the environments near supermassive black holes. Characteristics of the Mission Concept as Developed to Date ATLAST is a proposal for a 16-m diameter telescope (see Figure 2.2) that would be unfolded in space in a similar manner to the approach employed by JWST, the latter currently being scheduled for launch in 2013. Like JWST, ATLAST would operate at the Sun-Earth L2 point. The ATLAST mirror would be folded along two chords and stacked vertically within the Ares V payload shroud. This mission follows on some of the technologies developed for HST and JWST, particularly in terms of large-mirror technology and construction and placement at L2. The ATLAST mirror would have a mass of approximately 10,000 kilograms (kg). The delta-v required to transport it from low Earth orbit to Sun-Earth L2 is approximately 4 kilometers per second (km/s). The combination of high angular resolution and high sensitivity is, increasingly, a science driver in astrophys- ics. The impressive capabilities anticipated for ground-based observatories in the coming decade will redefine the existing synergy between ground and space telescopes. Space will, however, remain an optimal environment for optical observations that require any combination of very high angular resolution over fields of view larger than ∼1 arc minute (arcmin) or at wavelengths shorter than ∼1 micron, very high sensitivity, very stable point spread function performance across the field of view, high photometric precision and accuracy in crowded fields, and very 36, 2.4 Meter Hex Mirror Deployable Stray Light Baffle Petals (3 Ring) 2.4 Meter Diameter ISIM Tower 19.4 m Telescope Positioning and Isolation Boom 16.8 m Deployable Solar Array 4.4 Meter Diameter Primary Deployable Solar Sail Central Cylinder Structure 103 m FIGURE 2.2 Illustration of the ATLAST 16-meter telescope. SOURCE: Courtesy of Northrop Grumman Space Technology. Figure 2.2.eps Includes low resolution bitmap images

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 LAUNCHING SCIENCE high stability of all of these performance parameters over tens to hundreds of hours of exposure time. 7 Of course, UV observations can only be accomplished in space. ATLAST would provide an unprecedented combination of reproducible high sensitivity and angular resolution at wavelengths from 0.11 to 1 micron over a field of view of at least 25 square arcmin. Relative Technical Feasibility of the Mission Concept A UV/optical space telescope with an aperture of at least 8 m will accommodate some of the science goals discussed above, while some of the more stringent drivers point to a 16-m segmented-aperture telescope. Both would incorporate the following baseline requirements: a wavelength range of 0.11 to 2.5 microns with angular resolution diffraction limited at 0.5 micron and a field of regard of the entire sky over the course of 1 year. Both would occupy a halo orbit at Sun-Earth L2. The James Webb Space Telescope currently uses a segmented-mirror design. Expanding the aperture size to a 16-m UV/optical segmented space telescope such as that employed by ATLAST presents significant technical challenges. JWST is a 6.5-m telescope diffraction-limited at 2 microns. The technical feasibility of aligning and operating a 16-m diffraction-limited (0.5 micron) space-based telescope is beyond that demonstrated by JWST, and the challenges must be identified and addressed in feasibility studies. General Cost Category in Which This Mission Concept Is Likely to Fall The phase A-D costs (i.e., development costs but not including science operations) associated with the ATLAST point design for the 8-meter-class mission (see the subsection below entitled “8-Meter Monolithic Space Tele- scope”) will be in the multibillion-dollar range, comparable to JWST. Because the 16-m mission is larger and more technically complex than the 8-Meter Monolithic Space Telescope, it is expected to cost significantly more. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Ares V cargo launch vehicle, with its upmass capacity and fairing volume, is one of the key enabling technologies for future large UV/optical space telescopes with apertures of 8 m or more. The vehicle would vastly reduce the risk associated with launching and deploying ATLAST. Ares V would enable a folded, segmented telescope with an aperture up to 24 m to be flown in a single launch, and it would enable the use of high-TRL telescope components. For the segmented primary design, the Ares V would enable the use of JWST (2 petal) chord fold deployment for telescopes with apertures up to 16.8 m. The ATLAST mission could also potentially benefit from either human or robotic servicing. Assessment of the Mission Concept for Further Study The ATLAST mission is enabled by the Constellation System. The Ares V cargo launch vehicle is the key enabling technology for ATLAST. The large telescope requires the large mass and volume capacity of Ares V and its ability to deliver such a payload to Earth-Moon L1 or Sun-Earth L2. A telescope with an aperture up to 24 m can be flown in a single launch on the Ares V cargo launch vehicle. The potential scientific impact of this mission concept was unclear. The proposal listed several broad goals cutting across the ultraviolet and optical spectral regions. A detailed science case for ATLAST and trade-off studies of its various options could better define this concept. For example, the goals of imaging and spectros- copy of exoplanets present challenges in terms of scattered light for a segmented-mirror telescope, whereas other 7 Space is an optimal environment for optical observations for several reasons, including the following: (1) weather—clouds and wind can affect observations on the ground; (2) point spread function stability and the fact that low-order tip-tilt systems do nothing if seeing reaches near 1 degree; (3) day/night cycles; (4) scattered light from the Moon or a nearby star can affect observations; (5) sky brightness and background noise are problems on the ground; (6) thermal environment and instrument and detector stability are degraded for ground observations; and (7) cleanliness—dust and mechanical particulates affect ground-based telescopes.

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES science questions require the larger aperture that ATLAST can provide compared with other telescope designs. The proposal suggested that several mirror aperture sizes are possible, but it does not clearly explain the science advantages and disadvantages of each. Several key technologies for ATLAST are immature at this time. The production, deployment, mass, and stability of a large segmented mirror presents significant challenges. The diffraction limit required for ATLAST is currently beyond the capabilities demonstrated by the Hubble Space Telescope or the James Webb Space Tele- scope. Other items, such as the detectors and thermal control system, appear to be at reasonable TRLs to anticipate development in time for such a mission. In summary, the ATLAST mission has a medium/low rating for the mission’s impact on science in the field and for its technical maturity, and therefore it is placed in the “deserving of further study” category. This program could build on JWST’s technology base and significantly advance UV/optical astronomy. However, in order for this mission to move forward, the science goals for ATLAST must be better defined. 8-Meter Monolithic Space Telescope8 Scientific Objectives of the Mission Concept The 8-meter-class UV/optical space observatory is a concept for a powerful telescope with very high angular resolution and sensitivity, broad spectral coverage, and high performance stability that would afford the opportunity to answer compelling science questions: How did the universe come into existence? What is it made of? What are the components of the formation of today’s galaxies? How does the solar system work? What are the conditions for planet formation and the emergence of life? Are we alone? Periodic robotic servicing would allow for extended mission life of 20 or 30 years. The first 5 years of the mission could be focused on UV science, with a narrow-field-of-view UV spectrometer and a wide-field-of-view UV imager. After a servicing mission, the next 5 years could be dedicated to visible science such as terrestrial planet finding, with either an external occulter or an internal coronagraph. Characteristics of the Mission Concept as Developed to Date The 8-Meter Monolithic Space Telescope mission concept has three main subsystems: telescope, support structure, and spacecraft (see Figure 2.3). The telescope consists of a 6- to 8-m primary mirror, secondary mirror, and forward structure/baffle tube. Active thermal management by means of 14 heat pipes is required to hold the primary mirror temperature at a constant 300 kelvin (K) for all Sun angles with less than 1 K of thermal gradient. The spacecraft provides all normal spacecraft functions and houses the science instruments. The support structure supports the primary mirror and carries the observatory mass (of the primary mirror, telescope forward structure, and spacecraft), providing the interface of this mass to the Ares V for launch. If manufactured using conventional ground-based telescope techniques in order to reduce cost, the mirror mass would be approximately 44,000 kg. The delta-v required to transport the telescope to Sun-Earth L2 from low Earth orbit is only 4 km/s. The Ares V is capable of transporting more than 55,000 kg to Sun-Earth L2. A feasibility study conducted by NASA’s Marshall Space Flight Center (MSFC) considered two different telescope optical systems. An F/15 Ritchey-Chrétien design was examined, with its on- and off-axis image quality, compact size, and ultraviolet throughput. Unfortunately, this optical design has only a relatively narrow, 1-arcmin field of view that is diffraction limited at 500 nanometers (nm). Another option is to use a three-mirror anastigmatic telescope with fine steering mirror design, with a wide, 100-arcmin (8.4 by 12 arcmin) field of view that is diffrac- tion limited at 500 nm. But it also has lower UV throughput because of its two additional reflections. A potential 8The first two subsections are based on material in H. Philip Stahl, “Design Study of 8 Meter Monolithic Mirror UV/Optical Space Tele- scope,” in response to request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. This proposal and the ATLAST proposal were produced by many of the same people, and the 8-m telescope is occasionally referred to as the 8-Meter Monolithic ATLAST. See footnote 1 above.

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0 LAUNCHING SCIENCE FIGURE 2.3 Illustration of the 8-Meter Monolithic Space Telescope inside its payload fairing atop an Ares V rocket. SOURCE: Courtesy of NASA. solution is to implement a dual pupil configuration, where UV and narrow-field-of-view instruments operate at the Cassegrain focus, and wide-field-of-view instruments operate off-axis providing their own tertiary mirror. Due to the large lift ability of the Ares V, the primary mirror in this telescope can be a single piece of glass and does not have to be “space light-weighted.” The mirror can be of standard ground-based technology such as those produced for the Very Large Telescope or the Large Binocular Telescope. Mirror masses of 12,000 kg to 20,000 kg would not be too heavy for this mission and could in fact be considerably heavier. The proposed observatory has two separate spacecraft: a telescope bus that houses the optical telescope ele- ment and a replaceable spacecraft/instrument bus. The spacecraft propulsion system is sized to get the observatory from roughly a geostationary transfer orbit into a halo orbit around the Sun-Earth L2 point and performs all sta- tion-keeping operations. The spacecraft has a dual-mode hydrazine-NTP bipropellant/hydrazine monopropellant propulsion system with 5 years of propellant and redundant thrusters. Relative Technical Feasibility of the Mission Concept The NASA MSFC study indicates that it is feasible to launch a 6- to 8-meter-class monolithic primary mirror UV/visible observatory with a 10-m Ares V launch vehicle, have it survive launch, and place it into a halo orbit around the Sun-Earth L2 point. Specific technical areas studied included structural, thermal, and optical design and analysis; launch vehicle performance and trajectory; spacecraft; operations and servicing; mass and power budgets; and system cost. Structural design and analysis were performed for the spacecraft using standard NASA guidelines. No technical problems were identified. The use of existing technology, such as ground-based mirror production, has the advantages of technical maturity and low risk. It has been demonstrated that one can polish an 8-meter-class ground-based telescope mirror to a surface figure of better than 8 nm root mean square. The Ares V launch environment was analyzed by the MSFC Advanced Concepts Office to determine whether an 8-meter-class ground-based telescope mirror could survive launch. To do so, 66 axial support points would keep the stress level on an 8.2-meter-diameter 175-millimeter (mm) thick meniscus primary mirror below 1,000

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 LAUNCHING SCIENCE sion systems are currently at TRL 6. A number of options were developed by the original Vision Mission team to employ the Constellation System. However, the use of Ares V with Centaur and the additional propulsion needed from solar-electric propulsion required additional study. There are options to launch more than one satellite that could significantly enhance the science return of the mission, but these options also require additional study. In summary, the Solar Polar Imager’s combination of clearly defined, high-priority science goals and mature instrument technology places this mission in the “more deserving of further study” category. Further study should be primarily narrowly focused on evaluating the ability to use the Ares V to obtain the desired orbit and on answering the question of whether additional satellites could be launched simultaneously to provide multipoint observations. SOLAR SYSTEM EXPLORATION MISSIONS Exploration of Near Earth Objects via the Crew Exploration Vehicle 34 Scientific Objectives of the Mission Concept Near-Earth objects (NEOs) are asteroids and comets whose orbits come close to Earth’s orbit around the Sun. There is a chance that on occasion a NEO will impact Earth. NEOs range in size from hundreds of meters to tens of kilometers in diameter. In recognition of this potential threat, NASA started the Spaceguard project and, more recently, Congress directed NASA to detect, track, catalogue, and characterize 90 percent of all NEOs down to 140 meters in diameter by 2020. Planetary scientists have long speculated about sending a spacecraft to the vicinity of an asteroid, particularly one whose orbital path brought it near Earth. A spacecraft can easily approach and escape from an asteroid’s weak gravitational field. In fact, landing equipment on and retrieving samples from an asteroid’s surface has already been accomplished robotically. A study sponsored by the Advanced Projects Office within NASA’s Constellation Program examined the feasibility of sending an Orion spacecraft to a NEO (see Figure 2.11). An ideal mission of 90 to 180 days would take a crew of two or three astronauts for a 7- to 14-day stay in the vicinity of a NEO. Compelling reasons for sending humans in spacecraft to a NEO include being able to directly see and acquire data to characterize surface characteristics, morphology, mineral composition, and so on for the NEO. Human direction of sample collecting would enable quick, informed decisions in acquiring samples of different locations on the NEO’s surface and could allow the geological context of those samples to be determined. Such samples could lead to major advances in the areas of geochemistry, impact history, thermal history, isotope analyses, mineral- ogy, space weathering, formation ages, thermal inertias, volatile content, source regions, solar system formation, and so on. Samples directly returned from a primitive body may provide the same kind of breakthrough that the Apollo samples have done for the understanding of the Moon and its formation history. The main goal of the Exploration of Near Earth Objects via the Crew Exploration Vehicle mission would be to collect macroscopic samples from various terrains on the NEO’s surface by means of astronaut extravehicular activities (EVAs). This would enable sample collection in geological context. Intact samples of the surface would also be used to evaluate space weathering and surface alteration effects in a deep space environment. In addition, supplemental robotic collection of samples from either different or difficult-to-reach sites on the NEO would expand the sample suite. Another primary goal of this mission would be to investigate and determine the interior characteristics of the target NEO. This would place some constraints on the macroporosities that may be found among this popu- lation of objects and help scientists understand the accretion and impact history of the early solar system. Such investigations could be combined with a detailed examination of any features or structures associated with crater formation in microgravity environments to further refine impact-physics models. Active detonation of a kinetic 34The first two subsections are based on material in P.A. Abell, D.J. Korsmeyer, R.R. Landis, T.D. Jones, D.R. Adamo, D.D. Morrison, L.G. Lemke, A.A. Gonzales, R. Gershman, T.H. Sweetser, and L.L. Johnson, “Exploration of Near-Earth Objects via the Crew Exploration Vehicle,” NASA Johnson Space Center, Houston, Tex., in response to request for information by the Committee on Science Opportunities Enabled by NASA’s Constellation System, May 2008. See footnote 1 above.

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES FIGURE 2.11 Illustration of an Orion spacecraft approaching a near-Earth object. SOURCE: Hyabusa image courtesy of JAXA. Orion image courtesy of NASA. Photoillustration by Tim Warchocki. energy experiment after the deployment of a seismic network would also serve to measure the internal properties of the NEO, while gaining insights into the effects of crater excavation. Momentum transferred to the NEO orbit after charge detonation could be measured, and the change in orbital motion of the NEO could be observed. Such information might provide useful insights for future hazard-mitigation scenarios.

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 LAUNCHING SCIENCE Characteristics of the Mission Concept as Developed to Date This study of a mission to an asteroid has not progressed as far as have some of the Vision Mission studies. This study focused on the feasibility of sending a piloted mission to a NEO employing the basic hardware described in the Constellation System. This initial study was constrained to limited modifications to the Orion spacecraft and did not consider additional equipment, particularly a dedicated habitation module, that may be required. The four launch options that were assessed were the following: (1) the “lower-bookend” option, (2) the midvolume Ares IV single-launch option,35 (3) the midvolume Ares V single-launch option, and (4) the “upper-bookend” option. The lower-bookend option consists of a dual-launch of an EELV, such as the Atlas V or Delta IV Heavy, carry- ing an unmanned Centaur upper stage and an Ares I rocket carrying an Orion spacecraft. The midvolume Ares IV single launch is a modified Ares V with an Ares I upper stage carrying a Crew Exploration Vehicle (CEV). Similarly, the midvolume Ares V single launch is an Ares V with an Orion on top. The upper-bookend option is a dual-launch scenario most like the proposed lunar architecture, with spacecraft similar to an Altair Lunar Surface Access Module atop an Ares V vehicle and an Ares I rocket carrying an Orion. Defining studies need to be made to enable the proper choice of a launch vehicle or vehicles for this mission concept. Work needs to be done on understanding the physical characteristics of the target asteroid: determining surface morphology and properties, studying the asteroid’s gravitational field structure, determining the rotation period, estimating the mass and density, and attempting to learn something about its general mineral composition. Such information will aid in the planning process for exploration once the spacecraft arrives at the asteroid. Efforts are needed to maximize mission efficiency for proximity and surface operations and for sample col- lection. A robotic precursor mission would be launched before the human mission to better characterize the target asteroid and reduce the risk for the astronauts traveling there. The Orion spacecraft would require several basic capabilities in order to complete the scientific and technical objectives of the mission, which would involve equipment and techniques supporting remote sensing, deploy- ment and redeployment of surface experiment packages, and surface sampling. The majority of Orion operations should take place during close proximity (from a few to several hundred meters) to the NEO. The crew of the Orion should be able to match the rotation of the NEO and maintain a stable orbit from which they can conduct a detailed scientific exploration of the target’s surface. This would require station-keeping capability. In terms of remote sensing capability, the Orion should have a high-resolution camera for detailed surface characterization and optical navigation. A light detection and ranging (lidar) system would be mandatory for hazard avoidance during close-proximity operations and for detailed topography measurements. In addition, the Orion has in its current design a radar transmitter that could be outfitted to perform radar tomography of the object. This would allow a detailed examination of the interior structure of the NEO. Given that several NEOs appear to have a high degree of porosity, it is important to measure this physical characteristic of the target NEO. Such informa- tion has implications for the understanding of the formation and impact history of the NEO, and may also have implications for future hazard-mitigation techniques of such objects. Relative Technical Feasibility of the Mission Concept Much of the technology needed for this mission is already under development for other missions, by NASA or elsewhere. Therefore, the expected new technology development required for crewed missions to NEOs is minimal in terms of the overall cost of the Orion and Ares infrastructure. In terms of technology development, a relatively small amount of new technology would be required for the mission. Some technologies associated with onboard operations automation, radiation shielding, microgravity EVA equipment, inflatable habitats, and NEO surface science packages would be required. However, most of these technologies are already being considered within NASA and the private sector for other space exploration missions. The expected new technology development required for crewed missions to NEOs is minimal in terms of the overall cost of the Orion and Ares infrastructure. 35The Ares IV is a theoretical hybrid of the Ares V first stage and the Ares I upper stage. It is not under active consideration by NASA.

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES The ideal scenario for the mission would involve launching an Orion spacecraft atop an Ares V rocket.36 NASA has no current plans for this vehicle configuration, although the agency is preserving the option of developing it in the Ares V design. However, the most significant technology challenge may be that of providing radiation shielding for the crew. Methods of accomplishing this depend in part on the mass available for the mission. The Ares launch vehicles and Orion CEV will have undergone rigorous testing and will presumably have flown astronauts multiple times to the ISS and, possibly, to the Moon. Hence, these missions will be using the best-understood space transportation system infrastructure for human scientific and exploration missions outside of low Earth orbit. General Cost Category in Which This Mission Concept Is Likely to Fall Both an Ares I with Orion and an Ares V with an Earth departure stage are assumed for the Exploration of Near Earth Objects via the Crew Exploration Vehicle mission concept. The proposer’s cost estimate is that one should expect to spend funds similar to those spent for an extended lunar sortie—that is, $2 billion to $4 billion. However, given the unique nature of this mission, the risk-mitigation measures likely required, and the develop- ment of proximity operation techniques around the asteroid, the committee places this mission in the more-than- $5 billion category. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches By definition, this mission involves sending humans to a nearby asteroid and therefore requires the Orion spacecraft. Because Constellation hardware already is under design and development for trips to the ISS and the lunar surface, the incremental development costs would be minimized for the Orion and the Ares family of launch- ers that would be needed for science missions to NEOs. Assessment of the Mission Concept for Further Study The Exploration of Near Earth Objects via the Crew Exploration Vehicle mission is enabled by the Constel- lation System. Each of the four mission concept options considered requires the Orion crew vehicle in conjunc- tion with some combination of Ares I and Ares V launch vehicles. No other method of sending humans to such distances currently exists or is planned. The potential mission impact on science in the field is moderate. The main scientific goal of a NEO mission would be to collect macroscopic samples within the context of their surroundings in order to evaluate any space weathering and surface alteration. These samples would be returned to sophisticated terrestrial laboratories for studies in geochemistry, isotopic analysis, mineralogy, and volatile content, as well as impact and thermal history. The examination of these samples would yield significant knowledge about their origin, age, and history, just as with typical meteorites, but now realizing the precise source of the samples. Furthermore, knowledge of the physi- cal properties of objects that may be on a collision course with Earth is essential for the success of any strategies that call for changing the object’s trajectory. The emplacement of an array of suitable instruments would provide an understanding of the object’s internal structure, which would be valuable for understanding the accretion pro- cesses that brought this small solar system body together, but also for recognizing appropriate mitigation strategies to deflect any Earth-threatening NEO. Although the committee found the argument that a crewed mission to a NEO is the best, and possibly the only, way to gather a sample within the context of its surroundings and return a significant amount of material to Earth, the primary focus of scientific research in this field at the present time is devoted to characterizing as many diverse bodies as possible. 36The Ares V and Orion spacecraft launch option remains a potential future development for NASA. See Steve Cook, “Lunar Program Industry Briefing: Ares V Overview,” September 25, 2008, available at http://www.nasa.gov/pdf/278840main_7603_Cook-AresV_Lunar_ Ind_Day_Charts_9-25%20Final%20rev2.pdf.

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0 LAUNCHING SCIENCE The technical maturity for this mission concept is closely tied to the development of lunar mission architecture. For instance, the most capable of the four options considered for the NEO mission requires the same hardware as a lunar mission (an Ares V with an Earth Departure Stage and Altair lunar lander, and Ares I with an Orion crew vehicle), all of which are part of the Constellation System. Once those vehicles are developed, the technical maturity for the asteroid mission would be relatively high. Additional modifications to first-generation Constella- tion hardware would be required for a human mission to a NEO, such as an extended-duration Orion spacecraft, radiation protection, EVA capability, and robotics. These capabilities will also be needed to meet NASA’s long- term Mars exploration goals as well as for human servicing missions of robotic spacecraft in cislunar space (see Chapter 4). In summary, although a human NEO mission would be expected to yield interesting science, it is not as compelling as the other mission concepts that the committee evaluated; therefore, this mission is placed in the “deserving of further study” category. However, the most compelling rationale for a human NEO mission is for its value as an intermediate step between lunar exploration and interplanetary travel—to prove new hardware and to maintain the momentum of the human exploration program by showing sustained progress. Although the com- mittee did not rate this mission as “more deserving of study” because of its scientific limitations, the committee does believe that this mission is an intriguing and exciting mission concept and deserves serious consideration for further study by NASA’s Exploration Systems Mission Directorate. Neptune Orbiter with Probes37 Scientific Objectives of the Mission Concept The planets in our solar system fall into three classes: terrestrial planets, gas giants, and ice giants. Only the last category has not been studied comprehensively, meaning that the Neptune Orbiter (Figure 2.12) would be the first detailed exploration of such a body. The Voyager 2 flyby of the Neptune system nearly two decades ago produced most of the knowledge of this distant planet. It revealed a surprisingly dynamic atmosphere, surrounded by a displaced and highly distorted magnetosphere. Many of the extrasolar planets detected to date are similar in terms of mass and size. A Neptune mission would also survey the almost-planet-sized, geologically active satellite Triton, which—owing to its retrograde orbit—is suspected to be a captured Kuiper Belt Object. Within the context of comparative planetary studies, these objects have significant scientific interest. The Neptune system was ranked as one of the three “other important objects,” after the top four choices, in the NRC’s 1994 report An Integrated Strategy for the Planetary Sciences -00, and was the focus for two of the nine “deferred high-priority flight missions” listed in the 2003 NRC solar system exploration decadal survey. 38 A Neptune mission was also recommended for further science definition in the next solar system decadal survey by the 2007 NRC report Grading NASA’s Solar System Exploration Program: A Midterm Review.39 A comprehensive study of an ice giant would be a logical mission to follow the detailed explorations of Jupiter by Galileo (mid- 1990s) and of Saturn by Cassini-Huygens (ongoing). 37The first two subsections are based on material in B. Bienstock, The Boeing Company, and the Neptune Orbiter with Probes Team, “NASA Vision Mission Neptune Orbiter with Probes,” Contract No. NNH04CC41C, Final Report, Volumes 1 and 2, Revision 1, September 2005; A.P. Ingersoll, California Institute of Technology (Caltech), and T.R. Spilker, Jet Propulsion Laboratory (JPL), “Study of a Neptune Orbiter with Probes Mission, Final Report,” May 2006; T.R. Spilker, JPL, and A.P. Ingersoll, Caltech, “Aerocapture Implementation of NASA’s ‘Neptune Orbiter with Probes’ Vision Mission,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 20, 2008; and B. Bienstock, The Boeing Company, and D. Atkinson, University of Idaho, “Neptune Orbiter with Probes,” presentation to the Committee on Planetary and Lunar Exploration, July 21, 2005 (presented by T. Spilker, JPL, to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 20, 2008). Note that both Neptune reports were presented by T. Spilker, a lead author of one of the studies. See footnote 1 above. 38 National Research Council (NRC), An Integrated Strategy for the Planetary Sciences -00, National Academy Press, Washington, D.C., 1994; NRC, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003. 39 National Research Council, Grading NASA’s Solar System Exploration Program: A Midterm Review, The National Academies Press, Washington, D.C., 2007.

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES FIGURE 2.12 Illustration of a possible Neptune Orbiter mission employing aerocapture. The aerocapture stage is on the left; the coast stage, including two probes, is on the right. SOURCE: Used with permission, courtesy of T.R. Spilker, Jet Propul- sion Laboratory. Figure 2.12.eps Bitmap images - Low resolution Characteristics of the Mission Concept as Developed to Date Neptune is 30 times farther from the Sun than Earth is, so any mission will take many years to reach its target; for example, the most energetically efficient orbit takes 30 years. An EELV-based mission using chemical propul- sion first to reach Neptune and then to decelerate into orbit provides either too little payload mass or too long a flight time. In order to reduce flight times to an acceptable duration, new technology is required to slow down the spacecraft on approach to its target. In the proposed non-nuclear version of this mission, solar-electric propul- sion is employed to speed the interplanetary transit, and then aerocapture reduces the orbital energy, yielding an elliptical orbit about Neptune. In the alternative version, nuclear-electric propulsion is used to accelerate and then decelerate the spacecraft to allow a conventional, powered capture. Both of these proposed Neptune missions, with launch in 2016, would perform a Jupiter flyby before arriving at Neptune in 2029. Early on, just before and just after capture of the mother spacecraft into a Neptune orbit, a probe would be released for insertion into Neptune’s atmosphere (one at the equator and another at high latitude). Four years later the spacecraft would rendezvous with Triton on its retrograde orbit in order to allow deployment of an orbiter, or a lander, on the satellite. The mission would end in 2033. Since the gravity assist by Jupiter makes this mission feasible, similar missions can be accomplished only every dozen years, meaning that another opportunity will occur late in the 2020s with the mission’s end happening about 2045. Any Neptune orbiter mission requires strategic investments in power sources, transportation, communica- tion, and sensor technology. In order to have reasonable trip times while carrying a comprehensive payload, some new technology is required, either a significantly powered flight (via nuclear-electric propulsion) or aerocapture. Maneuvering within the Neptune system is accomplished in one case by solely using gravity assists by Triton, whereas a mission carrying nuclear-electric propulsion would be able to use that capability to supplement gravity assists when switching orbits. Of the two missions reviewed by the committee, only the non-nuclear version can fly on a Delta IV Heavy launch vehicle, according to the current Delta IV Payload Planners Guide.40 Using the performance characteristics given in the 2002 Delta IV Payload Planners Guide,41 the proposers concluded that, if a lander were to be deployed, a Delta IV Heavy would be inadequate. However, the more recent Payload Planners Guide claims an increased payload capability, and this should be sufficient for all stated mission options. This proposed mission uses both 40 Delta IV Payload Planners Guide, 06H0233, United Launch Alliance, Littleton, Colo., September 2007, available at http://www.ulalaunch. com/docs/product_sheet/DeltaIVPayloadPlannersGuide2007.pdf. 41 Delta IV Payload Planners Guide, The Boeing Company, Huntington Beach, Calif., 2002.

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 LAUNCHING SCIENCE solar-electric propulsion and aerocapture. Aerocapture represents a technology challenge. Currently NASA does not plan to develop or demonstrate aerocapture. The Neptune mission would carry at least two entry probes. In order to better define formation models of Neptune, the probes would measure the elemental abundances of He, Ne, Ar, Kr, Xe, C, and S, and the isotopic ratios 15N/14N and D/H. By combining these data with similar available information for Jupiter and possibly Saturn, it should be possible to determine whether the gas giants and the ice giants are born from the same material in the same general locale. One of the probes would carry a gas chromatograph mass spectrometer (GCMS) that is critical to the mission goals. The probes would study temperature and abundance data from Neptune’s stratosphere down to pressure levels of ∼100 to 200 bar. Complementary measurements of winds, structure, composition, cloud- particle size, and lightning would also be obtained. The orbiter would carry the usual broad array of instruments to observe the planet, its satellites, rings, and magnetosphere. These include imaging instruments at ultraviolet, optical, infrared, and radio wavelengths, as well as detectors to measure the magnetosphere, both in situ and remotely. Dust and plasma wave detectors might be included in the payload. Any Neptune orbiter would also have the ability to map Triton’s atmosphere and surface globally in order to establish the composition and age of the surface, the inventory of volatiles, the satellite’s current geologic activ- ity, and the atmosphere’s character. A Triton surface lander would enable direct measurement and sampling of the atmosphere and surface for purposes of the study of surface-atmosphere interactions, and might allow direct seismic probing of Triton’s interior. Relative Technical Feasibility of the Mission Concept The technical maturity of the instrument complement for the Neptune Orbiter with Probes mission concept is high, whereas the necessary propulsion options are relatively immature. Each version of the basic Neptune mission with probes concept (i.e., aerocapture or nuclear-electric propulsion) has an aspect that would require significant further development. General Cost Category into Which This Mission Concept Is Likely to Fall The Cassini-Huygens mission, an ongoing comprehensive exploration of the Saturn system that dropped a single probe into Titan’s atmosphere in early 2005, is an acceptably close analog to this proposed flight. Cassini- Huygens cost $3 billion to $4 billion. Given inflation and the long duration of the Neptune missions and their some- what more complex mission profiles, the proposed Neptune missions likely would cost more than $5 billion. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The nuclear-electric propulsion mission concept significantly exceeds the capabilities of the largest rocket currently in any nation’s inventory. By contrast, Ares V would be able to lift the 36 metric tons identified for this concept. However, the nuclear-electric propulsion required for this mission represents significant technical challenges. In the case of the mission concept using RTGs and no electric propulsion, an Ares V rocket would likely allow such a mission to be flown without the requirement for aerocapture at Neptune. Because there is currently no development program for either aerocapture or nuclear reactors, conventional chemical propulsion may provide the most mature option. However, this would severely limit the amount of payload that could be carried to Neptune and would also eliminate some important scientific objectives, such as a Triton lander.

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Neptune Orbiter with Probes mission concept is worthy of further study as a Constellation-enhanced science mission. The Ares V launch vehicle could eliminate the need for aerocapture in Neptune’s atmosphere. The study of an ice giant planet is pivotal to understanding the origins of the solar system. Neptune and Triton are also important for comparative planetary studies, in particular for understanding what drives the activity of these bodies in the frigid outer solar system. Finally, understanding an ice giant would be useful for calibrating models of extrasolar planets. Assessment of the Mission Concept for Further Study The Neptune Orbiter with Probes mission is enhanced by the Constellation System. This mission’s science goals can only be achieved with a very capable spacecraft that, in most cases, requires more mass and volume than can be accommodated on conventional launch vehicles. However, such carrying capacity will be readily available with the Ares V. Ares V would allow the mission to carry more instruments, possibly both atmospheric probes and a Triton lander (a mission employing a smaller launch vehicle would be limited to only one option). In addi- tion, because Neptune is so far from Earth, conventional missions would only be completed in several decades, a duration that many consider unacceptable. An Ares V could launch a mission to Neptune at higher velocity and therefore a shorter travel time. The potential impact on science for such a mission is high. The mission would explore both an ice giant planet and an almost-planet-sized, active satellite that is believed to be a captured Kuiper Belt object. Knowledge of Neptune would add significantly to the scientific understanding of the formation of the solar system. The science goals for a Neptune-Triton mission are those of the second stage of planetary exploration: thorough examination of a planetary system employing a broad suite of instruments over an extended interval. These objectives are similar to those that the Galileo mission sought at Jupiter and the Cassini-Huygens mission is pursuing at Saturn. The technical maturity of this mission is high. The instrument package, which is similar to that developed for Galileo and Cassini, has a high TRL rating and requires no new technology. A Triton lander/rover would borrow technology developed for Mars, although it would require adaptation to operate in the new environment. How- ever, the committee believes that aerocapture still offers a potentially significant enhancement even to an Ares V launched mission, although aerocapture at Neptune has a relatively low TRL. In summary, its combination of clearly defined, high-priority science goals and mature instrument technology places this mission in the “more deserving of further study” category. Further study should be primarily focused on evaluating the ability to use the Ares V to obtain the desired orbit and additional capabilities. Titan Explorer42 Scientific Objectives of the Mission Concept Knowledge and understanding of Titan, Saturn’s largest moon, have increased significantly as a result of remote sensing measurements obtained from the Cassini spacecraft following its orbital insertion around Saturn on June 30, 2004, and more recently with measurements from the descent of the Huygens probe through the atmo- sphere and onto the surface of Titan on January 14, 2005. More than 3 years of analysis of the Huygens data and more than 4 years of analysis of the Cassini data have dramatically improved the understanding of Saturn and its moons, particularly Titan and Enceladus. In the search for life in the solar system, Titan is in a unique position. Its density suggests that it is composed of a mixture of rock and ice in almost equal amounts, and Titan’s atmosphere 42The first two subsections are based on material in J.S. Levine, NASA Langley Research Center, “Titan Explorer: The Next Step in the Exploration of a Mysterious World,” Final Report for NASA Vision Mission Study per NRA-03-OSS-01, June 2005; and J.S. Levine, NASA Langley Research Center, “Titan Explorer: The Next Step in the Exploration of a Mysterious World,” presentation to the Committee on Science Opportunities Enabled by NASA’s Constellation System, February 21, 2008. See footnote 1 above. Note: This study should not be confused with the Titan Explorer study conducted for NASA during 2007. However, the two studies had similar science goals. The latter study can be found at http://www.lpi.usra.edu/opag/Titan_Explorer_Public_Report.pdf.

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 LAUNCHING SCIENCE 3.75 m diameter rigid biconic aeroshell TUFROC Forebody TPS – Peak heat rate = 308 W/cm2 (56 convective and 252 radiative) – 6.3 cm thick 5 m dia. Conical Ribbon Parachute FIGURE 2.13 Illustration of the deployment sequence for the Titan Explorer airship. NOTE: TUFROC, Toughened Uni-piece Fibrous Reinforced Oxidation-resistant Composite; TPS, Thermal Protection System; El, elevation; T, time; h, height; M, mass. SOURCE: Courtesy of Joel S. Levine, NASA. Figure 2.13.eps Includes low resolution bitmap image may reveal answers to questions about the chemical evolution on early Earth. The Titan Explorer mission focuses on nearly a dozen scientific questions, including the following: • What is the chemical composition of the atmosphere, including the trace gases? • What is the isotopic ratio of the gases in the atmosphere? • What prebiological chemistry is occurring in the atmosphere/surface of Titan today and what is its relevance to the origin of life on Earth? • What is the nature, origin, and composition of the clouds and haze layers? However, the mission’s objectives were developed before Cassini reached Titan, and they have not been altered significantly to take into account the results of Cassini and Huygens. Characteristics of the Mission Concept as Developed to Date The Titan Explorer mission (Figure 2.13), with a launch mass of 5,961 kg, would launch on a Delta IV Heavy rocket. The spacecraft consists of a Titan orbiter and a Titan airship that will traverse the atmosphere of Titan and can land on its surface. The airship is designed to have an operational lifetime of 4 months after entry, and the orbiter is designed to have a lifetime of 40 months after arriving in orbit around Titan. To answer the questions posed above, instruments on the Titan orbiter include a solar occultation spectrometer to measure atmospheric composition and isotopic ratios, a radar mapper to measure the nature of Titan’s surface, a magnetometer to search both for a planetary dipole field and surface magnetism, an ultraviolet spectrometer to

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 ANALYSIS OF SPACE SCIENCE MISSION STUDIES measure the escape of gases from Titan’s upper atmosphere, and a visual and infrared mapping spectrometer to measure cloud and haze layers and the nature of the satellite’s surface. Instruments on the Titan Explorer airship include an imager, mass spectrometer, surface composition spec- trometer, haze and cloud particle detector, Sun-seeking spectrometer, and a surface science package similar to that flown on the Huygens mission. Titan’s surface would be explored from an airship. Whereas other Titan exploration proposals would utilize a balloon, an airship offers the possibility of navigation to specific sites of interest. Success with this technique requires the development of cryogenically capable envelope materials for the airship. Relative Technical Feasibility of the Mission Concept The airship instruments (the imager, the mass spectrometer, and the surface composition spectrometer) and the orbiter instruments (the magnetometer and the radar mapper) all need additional development to result in improved resolution. The orbiter would require a second-generation multimission RTG, and for the airship a second-generation Sterling radioisotope generator. The spacecraft will be captured into orbit around Titan by means of aerocapture, which is one of the key enabling technologies for the mission. Owing to the lower entry velocity, aerocapture at Titan is not as challenging as at other planetary destinations. The mission also uses solar-electric propulsion, although mission requirements involve greater capabilities in this area than have previously been flown. General Cost Category in Which This Mission Concept Is Likely to Fall The project, as described in the final Vision Mission report, would have a total cost in FY 2008 dollars in the range of $3 billion to $4 billion. The mission was planned around the use of an existing EELV. Given recent esti- mates of outer-planet flagship missions significantly in excess of $2 billion, and considering that this mission would be significantly larger and more complex than those missions, the mission cost is likely to exceed $5 billion. Benefits of Using the Constellation System’s Unique Capabilities Relative to Alternative Implementation Approaches The Ares V with a dual-engine Centaur has significant excess capability to enable various mission design options. It has more than enough capability to deliver the Titan Explorer, without the use of Earth gravitational assist or the solar-electric propulsion module, within the proposed 6-year mission time line outlined in the baseline mission architecture. The excess capacity (nearly 10 metric tons) could be used to carry a conventional propulsion system to reduce or eliminate reliance on aerocapture. Further, the elimination of the solar-electric propulsion module is projected to save ∼$500 million, offsetting much of the cost differential between the Delta IV Heavy and Ares V launch systems. Alternatively, the basic Ares V (without the Centaur) could potentially eliminate the need for Earth gravity assist. It may also be possible to eliminate or dramatically reduce the reliance on the solar-electric propulsion module. Further study of the various mission design options is required to answer these questions. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Titan Explorer mission concept is worthy of further study as a Constellation-enhanced science mission. The use of an Ares V launch vehicle could eliminate the requirement for aerocapture. The Titan Explorer would provide a significant improvement in the knowledge of the evolution of prebiological chemistry. The committee notes that NASA is already conducting ongoing studies of possible Titan missions, although the most recent one studied uses a balloon instead of an airship to study Titan’s atmosphere and surface.

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 LAUNCHING SCIENCE Assessment of the Mission Concept for Further Study This mission is enhanced by the Constellation System. The Ares V with a dual-engine Centaur upper stage would have significant excess capability to allow various Titan exploration options, including reduced travel times and/or elimination of Earth gravity assist. Excess mass capability provided by the Ares V could be used to simplify the spacecraft design and therefore lower cost. The potential impact on science for the proposed mission is mixed. Although a Titan science mission could have significant impact on science, the science goals of the Titan Explorer as presented to the committee are obsolete, having become outdated following a new understanding of Titan with the results of the Cassini mis- sion and the Cassini Equinox Mission (Cassini’s extended mission). As currently conceived, this mission will require aerocapture, a technology that has not been used before. Also as currently conceived, the mission will require solar-electric propulsion and next-generation ion engines. Furthermore, the proposal did not address the planetary protection issues surrounding taking Advanced Sterling Radioisotopic Generators to Titan’s surface on the airship.43 By studying Cassini’s experience, it will be possible to improve on the science definition, justify the instrument selection, and make a compelling case for advanced technology such as a blimp(s), balloon(s), or other method for moving through Titan’s atmosphere. NASA is currently undertaking studies for a Titan mission that would use an EELV based on science-definition teams established in the wake of the Cassini mission. These efforts would naturally support any potential Ares V-based Titan exploration effort. 44 The technical maturity of the Titan Explorer mission concept is relatively high. The instrument package, although scientifically outdated, requires no new technology and has a high TRL rating. This mission would require aerocapture, a technology that has not been used before and is at TRL 3 to 6. The mission would require solar-electric propulsion and next-generation ion engines. In summary, although the potential scientific impact of a Titan Explorer mission using the Constellation System is potentially high, this particular mission concept was weakened because it did not incorporate recent Cas- sini scientific results, placing the mission in the “deserving of further study” category. The committee concluded that with an appropriate science discussion the concept would be more highly ranked, and the basic concept of a Titan mission should not be penalized for a poor presentation. NASA can revisit the potential for Ares V Titan missions after the agency has made a decision concerning the next outer-planets flagship mission. 43 For example, the warm radioisotope generator could melt part of the surface and create its own mini-environment. It is possible, albeit remotely so, that microorganisms brought from Earth on the spacecraft could then flourish in this environment, contaminating it. 44 J. Leary and the Titan Explorer Team, “Titan Explorer NASA Flagship Mission Study,” Johns Hopkins University Applied Physics Laboratory, Laurel, Md., August 2007; R.D. Lorenz, “A Review of Balloon Concepts for Titan,” Journal of the British Interplanetary Soci- ety 61:2-13, 2008; R.D. Lorenz, “Titan Bumblebee: A 1-kg Lander-Launched UAV Concept,” Journal of the British Interplanetary Society 61:118-124, 2008.