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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. 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. 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 timeâthe 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: â¢ Advanced Compton Telescope (ACT), â¢ The Big Bang Observer, â¢ Generation-X (Gen-X), â 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. â 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. 20
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 21 â¢ Innovative Interstellar Explorer, â¢ Interstellar Probe, â¢ Kilometer-Baseline Far-Infrared/Submillimeter Interferometer, â¢ Modern Universe Space Telescope (MUST), â¢ Neptune Orbiter with Probes (2 studies), â¢ Palmer Quest, â¢ Single Aperture Far Infrared (SAFIR) Telescope, â¢ Solar Polar Imager, â¢ Stellar Imager, â¢ Titan Explorer, and â¢ 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 Councilâs (NRCâs) Committee on Science Opportunities Enabled by NASAâs 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 capabilitiesâprimarily 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 committeeâs 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., 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: â¢ Advanced Technology Large-Aperture Space Telescope (ATLAST), â¢ Dark Ages Lunar Interferometer (DALI), â¢ 8-Meter Monolithic Space Telescope, â¢ Exploration of Near Earth Objects via the Crew Exploration Vehicle, â¢ Solar Probe 2, and â¢ 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. â National Research Council, Science Opportunities Enabled by NASAâs Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. â 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.
22 LAUNCHING SCIENCE EVALUATION CRITERIA AND RESULTS The committee analyzed each of the 17 missionsâthe 11 Vision Mission concepts in hand from NASAâs 2004 solicitation and the 6 additional concepts received from the scientific and technical community in response to its request for informationâaccording to a standard format based on the committeeâs 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 Systemâs 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? âSignificantâ 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 capabilitiesâfor example: âDoes use of the Constellation Systemâs elements make a previously impossible mission technically feasible? âDoes use of the Constellation Systemâs elements reduce mission risk or enhance mission success for a previously complicated mission? âDoes use of the Constellation Systemâs elements enhance performance? âDoes use of the Constellation Systemâs 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. âRelative technical feasibilityâ 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 committeeâs 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
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 23 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 committeeâs 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 âmost deservingâ of further study, it divided the list of 12 mission concepts into âmore deservingâ and âdeserv- ingâ categories based on two considerations: â¢ Mission Impact on Science in the Field of StudyâThe 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 âcompellingâ 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. â¢ Technical MaturityâThe 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 âsuf- ficiently matureâ as indicating a technology readiness level (TRL) of 5 or higher (see Appendix D) for the major components and a form of âintegrated TRLâ 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 âmore deservingâ of further study. In addition, the committee evaluated whether a mission was âenabledâ or âenhancedâ by Constel- lation. If a mission was âenabled,â it could not be accomplished without the Constellation Systemâs capabilities. If a mission was âenhanced,â it could be accomplished without Constellation (for instance, by employing a smaller launch vehicle), but it might better accomplish its goals using Constellationâs capabilities: for instance, the mission could return much greater amounts of data or could return useful data faster if âenhancedâ by Constellation. Table 2.1 presents a summary of the committeeâs evaluations of the 12 individual mission concepts that it identified as being worthyâthat is, either âdeservingâ or âmore deservingââof further study as Constellation missions. As indicated in the table, the committee identified 5 missions that it determined are âmore deservingâ of further study. Recommendation:â 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 âdeservingâ 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.
24 LAUNCHING SCIENCE TABLE 2.1â 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 Sunâs 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.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 25 (a) (b) FIGURE 2.1â (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; ÎV, delta-v. SOURCE: Courtesy of NASA.
26 LAUNCHING SCIENCE Recommendation:â 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. 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:â 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. â The 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. â The first two subsections are based on material in M. Postman, T. Brown, A. Koekemoer, and the ATLAST Team, âAn Advanced Technol- ogy Large Aperture UV/Optical Space Telescope: A NASA Astrophysics Strategic Mission Concept and Technology Development Study,â in response to a request for information by the Committee on Science Opportunities Enabled by NASAâs Constellation System, May 2008. See footnote 1 above.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 27 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 â¼1 arc minute (arcmin) or at wavelengths shorter than â¼1 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 2.2â Illustration of the ATLAST 16-meter telescope. SOURCE: Courtesy of Northrop Grumman Space Technology. Figure 2.2.eps Includes low resolution bitmap images
28 LAUNCHING SCIENCE high stability of all of these performance parameters over tens to hundreds of hours of exposure time. 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 â8-Meter Monolithic Space Tele- scopeâ) 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 â Space is an optimal environment for optical observations for several reasons, including the following: (1) weatherâclouds 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) cleanlinessâdust and mechanical particulates affect ground-based telescopes.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 29 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 missionâs impact on science in the field and for its technical maturity, and therefore it is placed in the âdeserving of further studyâ category. This program could build on JWSTâs 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 Telescope 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 todayâs 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 NASAâs Marshall Space Flight Center (MSFC) considered two different telescope optical systems. An F/15 Ritchey-ChrÃ©tien 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 â The first two subsections are based on material in H. Philip Stahl, âDesign Study of 8 Meter Monolithic Mirror UV/Optical Space Tele- scope,â in response to request for information by the Committee on Science Opportunities Enabled by NASAâs 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.
30 LAUNCHING SCIENCE FIGURE 2.3â 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 âspace light-weighted.â 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
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 31 pounds per square inch (psi). A key design element of the structural concept is that all the mass of the observa- tory is carried through the main support structure to an interface ring, which attaches by means of another support system to the Ares V launch vehicle. This allows the use of a completely conventional spacecraft. Although the basic overall design and technology for this observatory are straightforward, for the most excit- ing science that can be performed by this missionâthe detection of exoplanetsâthe technical readiness is low and the coronagraph technology needs to be further developed. General Cost Category in Which This Mission Concept Is Likely to Fall This preliminary study claims that by eliminating complexity, it should be possible to design and build an 8-Meter Monolithic Space Telescope with seven times the collecting area of HST for less cost than JWST. This mission concept proposes to use existing ground-based mirror technology rather than ultralightweight mirror technology required for a large telescope to be launched on an EELV. While this adds 20,000 kg to the mass, this approach is estimated to save $0.7 billion to $2 billion in total program cost. The total cost for a 6- to 8-m observatory (excluding science instruments and operations) is estimated by its proposers to be $1 billion to $1.5 billion. The committee judges that the cost of this concept is likely to lie in the $1 billion to $5 billion range. Benefits of Using the Constellation Systemâs Unique Capabilities Relative to Alternative Implementation Approaches The unprecedented mass and volume capabilities of NASAâs planned Ares V cargo launch vehicle enable entire new mission concepts. First, the baseline 10-m fairing has an 8.8-m internal dynamic envelope diameter. This diameter is sufficient to accommodate a 6- to 8-meter-class monolithic circular primary mirror without the need for segmentationâwhich is compelling for specific science cases such as occultation studies of nearby stars performed to directly image host planets. A single primary mirror also provides a more uniform, symmetric, and stable point spread function. The payload mass of 55,800 kg to an L2 transfer orbit enables design simplicity, allowing the use of current-day technology for mirror and telescope design. The Ares V capacities allow the use of mass to reduce performance, cost, and schedule risk. This mission could also potentially benefit from either human or robotic servicing. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The 8-Meter Monolithic Space Telescope offers the possibility of a relatively easier and faster scientific use of the Ares V launch vehicle compared with other, more complex telescope designs. The reason for this fast-track ability is the use of present-day, proven, low-cost (compared with lightweight space design) technologies. The mission concept should therefore be studied further as a Constellation-enabled science mission. The various telescope designs, the initial and future instruments, the servicing concept, and schedule are examples of the details that are required before any final space telescope design and mission concept can be developed. Assessment of the Mission Concept for Further Study The 8-Meter Monolithic Space Telescope mission is enabled by the Constellation System. No existing launch vehicle has sufficient fairing volume or lift capacity to launch an 8-m monolithic space telescope into orbit. The scientific impact of this mission is similar to that for the ATLAST proposal in many respects. However, compared with ATLAST, an 8-m monolithic telescope potentially enables the direct observation of Earth-like planets around other stars in order to search for signs of life. However, to achieve this capability, the telescope must â To study extrasolar planetary systems directly, coronagraphs will have to achieve contrast ratios of nearly 10 billion to 1. These ratios are several orders of magnitude beyond the current state of the art and may not be possible with some kinds of telescope designs, for example those using segmented mirrors.
32 LAUNCHING SCIENCE be used as a coronagraph with a high rejection factor. Although the proposed mission concept mentioned but did not stress observations of Earth-like planets, the successful characterization of even one extra-solar-system planet would revolutionize astronomy and is seen by the committee as a compelling goal for science and a discriminat- ing factor in comparing the 8-Meter Monolithic Space Telescope with the two similar telescopesâATLAST and MUSTâboth of which have more difficult technical challenges to achieving this science goal. Furthermore, an 8-m space telescope is a larger version of HST, one of NASAâs most successful missions, and it would bring some of the same benefits to science as those from Hubble. Even if technical or operational problems on orbit prevented the characterization of Earth-like planets, this mission would still be an enormous scientific success for its impact on astronomy. The construction of large monolithic telescopes is a well-proven technology for ground-based observatories. It is likely that only modest improvements are required to produce space-qualified telescopes, allowing the mirror and structure to be rated as technically mature. The technology needed to create a coronagraph with a rejection factor on the order of 1010 is still immature and rated low. This technology is required for the most compelling science. Nevertheless, a monolithic mirror is at present foreseen as the best platform to enable a high-performance coronagraph, which makes the 8-Meter Monolithic Space Telescope a good candidate for further study. In summary, the 8-Meter Monolithic Space Telescope is currently seen as the best platform from which to observe Earth-like planets directly to search for signs of life, and the inherent capabilities of a large optical space telescope guarantee an enormous science return even if planet characterization falls short of its ambitious goals. For these reasons, the committee believes this project is âmore deserving of further study.â Dark Ages Lunar Interferometer (DALI)10 Scientific Objectives of the Mission Concept The Dark Ages Lunar Interferometer is a mission concept for a long-wavelength radio telescope on the lunar farside (see Figure 2.4). It is designed to probe the Dark Agesâthe cosmic epoch between post-big bang recom- bination and the formation of the first luminous objects. It was during this interval that baryonsâneutral hydrogen atomsâfirst began collapsing into matter-dominated, dense regions, eventually leading to the formation of stars and galaxies. DALI will detect, using interferometric techniques, neutral hydrogen signals (H I) in absorption against the cosmic microwave background through the highly redshifted 21-centimeter (cm) line. The primary science goal is to see into the Dark Agesâacquiring a three-dimensional view of the evolution of a large fraction of the universe. It should be possible to see the cosmic evolution of the H I line excitation temperature in absorption against the cosmic microwave background, thereby producing a precision probe of cosmology and of the first large-scale structures of the universe. DALI would allow for imaging of the H I line at different redshifts, producing a history of the growth of structure formation. The most powerful application of H I line observations is tomography, in which imaging spectral-line observations are acquired, allowing structures to be distinguished as a function of the redshift, z. Additionally, detection of radio emissions from extrasolar planetary magnetospheres may be possible. Some solar system planets such as Jupiter are significant radio emitters as a result of interactions between the solar winds and the planetâs magnetosphere. With sufficient sensitivity, extrasolar planetary radio emissions should be detectable over interstellar distances. Characteristics of the Mission Concept as Developed to Date DALI will be a radio interferometer on the lunar farside to be shielded at night from both human-generated and solar radio emissions. Its design is hierarchical, with individual antennas grouped into âstations,â and the signals from stations brought to a central processing facility. 10â The first two subsections are based on material in J. Lazio, for the DALI team, âThe Dark Ages Lunar Interferometer (DALI),â in response to request for information by the Committee on Science Opportunities Enabled by NASAâs Constellation System, May 2008. See footnote 1 above.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 33 FIGURE 2.4â Successively closer views of a Dark Ages Lunar Interferometer (DALI) concept instrument on the surface of the Moon. The image at lower right depicts a single instrument. Many instruments would be deployed in a crater on the lunar surface. SOURCE: Courtesy of NASA Goddard Space Flight Center. In simple interferometry, signals from two receivers are brought to a common location and combined. The resulting correlation coefficient is a sample of the visibility function (the Fourier transform of the sky brightness) seen by that pair of receivers. The sky brightness distribution can be determined by Fourier inversion of multiple samples, obtained by combing the signals from many receivers. Processing these observations to obtain numerous frequency channels across a large wavelength range is standard practice for Earth-based radio interferometers; it provides spectral analysis of the incident signal. Table 2.2 summarizes key technical requirements for the DALI mission concept. A key requirement is the collecting area, with preliminary estimates for imaging tomography on the order of 10 km 2, although the concept may be sufficiently flexible to allow for the gradual buildup to reach the full aperture. A significant challenge for DALI will be the removal of foreground signals. The required dynamic range is at least 105 (10 millikelvin [mK] versus 1,000 K galactic emission). The dominant foreground will be the nonthermal (synchrotron) galactic emission.
34 LAUNCHING SCIENCE TABLE 2.2â Dark Ages Lunar Interferometer (DALI): Scientific and Technical Requirements Parameter Requirement Technical Capability Comments Redshift zâ¼ 15-150a 10 < v < 100 MHz H I spin temperature decouples from cosmic microwave background at z â¼ 200a H I begins to be destroyed by first luminous objects at z ~ 20a Sensitivity â¼10 mK Collecting area â¼ 10 km2 Expected strength of H I absorption Mission lifetime â¼ 5 yr Imaging tomography is the most demanding collecting area No RFIâterrestrial or locally generated requirement Observations only during lunar night Angular â¼3 arcmin Baselines 0.5-50 km Expected size of H I structures resolution az = (square root (1 + v/c))/(square root (1 â v/c)) â 1. NOTE: H I, neutral hydrogen; RFI, radio frequency interference. SOURCE: Joseph Lazio, Naval Research Laboratory, âDark Ages Lunar Interferometer,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, June 10, 2008. The fundamental receiving element for celestial radio signals will be stations, each composed of 100 individual dipole antennas. The baseline mission concept has 1,000 antenna stations, to provide the imaging quality required. The equipment would be landed on a cargo variant of the planned Altair lunar lander and would be remotely deployed by one or more robotic rovers. The stations are distributed on the Moon over a region approximately 50 km in diameter. Each antenna station is conceived to be deployed by a mini-rover, which moves from a lunar lander to the desired location; the mini-rover remains as a station controller, or hub, after antenna station deployment. Signals from the individual dipoles imprinted on polyimide film are sent through imprinted strip lines to a hub to be summed and stored through the lunar night. The stored data are then relayed to a central location for cross- correlation with signals from all other stations. The correlated data are relayed to Earth, where imaging processing and analysis will proceed using methods already in use (or being developed) for ground-based arrays. Relative Technical Feasibility of the Mission Concept Most of the technology challenges for the DALI mission involve the engineering challenge of scaling up cur- rent components to large numbers and high complexity. In addition, the DALI components will need to survive very harsh lunar environments (temperature variations from approximately 100 to 400 K), and will need to be suitable for deployment by remote means on the lunar farside. Several key technologies have high TRLs because of substantial space experience or the Earth-based experience (50 years) with radio interferometers, although in smaller scale in numbers of individual antennas and receivers. Sustained investments over the coming decade will likely mature a number of major technology areas. Ongoing work under NASAâs Astrophysics Strategic Mission Concept Study program could be expected to produce a technology roadmap for DALI in the near future. General Cost Category in Which This Mission Concept Is Likely to Fall The DALI mission concept departs significantly from mission concepts whose costs can be estimated using standard methodologies. Because this mission is at least 15 years in the future, there are major uncertainties in the costs. The current mission concept envisions an imaging tomography baseline mission, with 1,000 stations and a dedicated data-relay satellite. Two Ares V launches would be required to deliver the complete payload to the surface of the lunar farside, including the rovers that would deploy the arrays. Although each element of this concept is, in itself, not particularly complex, the deployment of a large number of receivers on the lunar farside remotely makes this overall concept very complex, challenging, and costly. DALI will be a mission whose cost is likely to be above $5 billion.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 35 Benefits of Using the Constellation Systemâs Unique Capabilities Relative to Alternative Implementation Approaches The DALI observatory needs to be located in a radio-quiet region. The best candidate for such a radio-quiet environment is the lunar farside, out of view of terrestrial radio emissions. This makes Constellation eminently suitable to execute such a mission. The Constellation System, and specifically the Ares V and the Altair (cargo) lander, is an additional key enabling capability for the DALI concept. The estimated mass for the approximately 1,000 stations needed to receive the faint H I signal produces a large mass budget of around 35,000 kg, and an Ares V is needed to get such a mass to the Moon. Development of a cargo lander with the capability to land approximately 15,000 kg on the Moon would be required as a means for delivering DALI components to the lunar surface. Three DALI subsystems dominate the baseline design mass budget: the mini-rover/station/antenna subsystem, the correlator/central processor, and the power-storage subsystem. The estimated mass for a mini-rover, including antennas embedded in the polyimide film, is 25 kg. Each mini-rover has a battery mass of 10 kg, giving a total mass (for 1,000 stations) of 35 metric tons. While the exact number of antenna stations in the array may be modified as studies and analysis of relevant data progress, the number of antennas is likely to be large. The number depends on the redshift for which DALI is optimized, which will be based on results obtained from ground-based radio telescopes under construction, operating at higher frequencies albeit in a much noisier background environment, and on studies that may lead to much smaller arrays deployed during early expeditions to the Moon. At the present time, there is no information indicating that it is possible to significantly reduce the mass required to be delivered to the lunar surface while still meeting the scientific goals. Assessment of the Mission Concept for Further Study The DALI mission is enabled by the Constellation System, requiring several Constellation capabilities, includ- ing the Ares V and lunar lander technology. Only the Constellation System is projected to have the capability of placing interferometric arrays on the lunar farside, which is currently considered to be the quietest (from a radio- frequency point of view) accessible location on or near Earth. The scientific impact for this mission is high, providing the unique capability for viewing the universe at a distinctive period between about 10 million and 300 million years after the big bang. This period, known as the Dark Ages, represents the last frontier in cosmology, the era between the genesis of the cosmic microwave back- ground and the formation of the first stars and galaxies. The technical maturity for the DALI mission is rated as medium for the rovers and interferometrics and low for the need to reduce array mass and for deploying and operating in a remote location. Although the rover and interferometric technologies exist and are advancing, integrating them so that the rovers could emplace large arrays on the lunar farside would be extremely challenging. Adapting existing technology to operate in the harsh lunar thermal environment would also be difficult. A mission of this complexity to the lunar farside is likely to occur later rather than earlier in the queue of lunar missions employing Ares V, as NASA focuses on lunar near-side operations. In summary, although the implementation of the DALI mission has many daunting elements, the science is unique, and it cannot be addressed other than by placing radio arrays on the lunar farside. Therefore it is placed in the âdeserving of further studyâ category. The primary near-term challenge will be reducing the mass of the antenna arrays in order to enable significant numbers to be landed on the Moon.
36 LAUNCHING SCIENCE FIGURE 2.5â Artistâs conception of one possible version of the Generation-X x-ray telescope. SOURCE: Courtesy of Roger Brissenden, Smithsonian Astrophysical Observatory. Figure 2.5.eps Bitmap image - Low resolution Generation-X11 Scientific Objectives of the Mission Concept The proposed Generation-X space-based x-ray telescope (Figure 2.5) complements all the other wavelength future-generation telescopes, such as the Atacama Large Millimeter Array (ALMA), JWST, the Square Kilometre Array (SKA), and Thirty Meter Telescope (TMT). The Gen-X telescope is designed to study the new frontier of astrophysics: the birth and evolution of the first stars, galaxies, and black holes in the early universe. X-ray astronomy offers an opportunity to detect these by means of the activity of the black holes and the supernova explosions and gamma-ray burst afterglows of massive stars. The Generation-X Vision Mission is based on an x-ray observatory with a 50 m2 collecting area at 1 kilo- electronvolt (keV) (1,000 times larger than the Chandra X-ray Observatory) and 0.1-arc second (arcsec) angular resolution (several times better than Chandra and 50 times better than the Constellation-X 12 resolution goal). Such a high-energy observatory will be capable of detecting the earliest black holes and galaxies in the universe and will study the chemical evolution of the universe and extremes of density, gravity, magnetic fields, and kinetic energy, which cannot be created in laboratories. The goal of Gen-X is to observe the first black holes and stars at redshift z â¼ 10-20. The idea is to search for the effects of the early black holes on the formation of galaxies and to trace through time the evolution of galaxies, black holes, and the chemical elements. Characteristics of the Mission Concept as Developed to Date Various configurations have been studied for Gen-X: a single 20-m mirror, six 8-m mirrors, and a single 16-m mirror. The committee considers the 20-m concept and the six 8-m mirror concepts to be exceedingly complex, and it decided to evaluate only the 16-m mirror configuration employing the Ares V launch vehicle. 13 The Gen-X mission operates at Sun-Earth L2. A 16-m telescope will require either robotic or human-assisted 11â The first two subsections are based on material in R. Brissenden, Smithsonian Astrophysical Observatory, and the other members of the Gen-X Study Team, âGeneration-X Vision Mission Study Report,â March 2006; and R. Brissenden, Smithsonian Astrophysical Observatory, âThe Generation-X Vision Mission,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, February 21, 2008. See footnote 1 above. 12â Constellation-X is a proposed x-ray telescope. 13â There are numerous problems for the other configurations. For instance, the six 8-m mirror concept would require six launches of Delta IV Heavy rockets. The Delta IV Heavy requires 3 months of ground preparation prior to launch; therefore the launches alone would require 18 months of preparation, during which no other Delta IV rocketsânecessary for national security payloadsâcould be launched.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 37 in-flight assembly (see Chapter 4 for a discussion of these technologies). The required effective area, 50 m 2, implies that extremely lightweight grazing incidence x-ray optics must be developed. To achieve the required areal density, at least 100 times lower than in Chandra, technology development is needed to produce 0.2-mm- thick mirrors (approximately the width of two human hairs) that have active on-orbit figure control. The suite of detectors includes a large-field-of-view high-angular-resolution imager (wide-field imager), a cryogenic imaging spectrometer (microcalorimeter), and a reflection grating spectrometer. The technology relating to the mirrors would clearly require early development and investment. The mission concept proposes the delivery of a detailed technology development plan for the next decade that will get key optics and detector technologies evolved to TRL 6. The proposal suggests the need for a decade of development effort so that the mission could be realized in the following decade. The use of very thin mirrors with active control is a technological challenge. The success of the mission depends on this development effort. Initial adjustment and alignment, and then the maintaining of the alignment, will be challenging. The space assembly of the 16-m mirror has not been defined in detail. The proposers have suggested using robotic assembly, but human assembly remains another possibility. Relative Technical Feasibility of the Mission Concept The Chandra X-ray Observatory has a collecting area of 0.08 m2 and uses glass shell technology. Gen-X has a 50 m2 collecting area and uses thin foils with on-orbit figure adjustment of shape to reduce the mass. The Ares V launch vehicle would simplify the mission concept because of the large payload fairing and mass to L2 capability. It has the potential of further simplifying the mirror development by allowing thicker mirror shells than were studied for the earlier configurations. The technical challenges are the mirror development and in-space assembly. General Cost Category in Which This Mission Concept Is Likely to Fall A November 2007 Gen-X proposal that would employ the Ares V estimated a $4 billion cost (excluding launch costs).14 The committee believes that this is a low estimate. The technology development of the mirrors is the large unknown in the cost estimates. The committee believes that the cost of this mission will exceed $5 billion. Benefits of Using the Constellation Systemâs Unique Capabilities Relative to Alternative Implementation Approaches The Ares V capability results in a simplified and more cost-effective baseline mission concept for Gen-X. It would be able to deliver Gen-X directly to the Sun-Earth L2 point. A single launch would allow a 16-meter- diameter mirror to be placed into position, would permit piezoelectric control of the mirrorâs optical surface for final figure on-orbit to achieve â¼0.1-arcsec angular resolution, and would enable placing into position a 60-m extendable optical bench between the optics and the science instrumentation package. The estimated mass for the Gen-X Ares V configuration is 22,000 kg. The Ares V is capable of delivering 55,900 kg to translunar injection trajectory (essentially equivalent to that required to reach the Sun-Earth L2 point). This mass margin possibly could be used to reduce the risk in the mirror development by allowing the use of thicker mirrors. 14â R. Brissenden, Smithsonian Astrophysical Observatory, âA Concept Study of the Technology Required for Generation-X: A Large Area and High Angular Resolution X-ray Observatory to Study the Early Universe,â submitted in response to NNH07ZDA001N-ASMCS, No- vember 20, 2007.
38 LAUNCHING SCIENCE Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Gen-X mission concept is worthy of further study as a Constellation-enabled science mission. The major issue is the mirror development. The extra mass capability of the Ares V may enable the use of thicker mirrors and therefore significantly reduce risk. Assessment of the Mission Concept for Further Study In its interim report,15 the committee determined that the Gen-X mission would be enhanced by the Constella- tion System, but it has reevaluated that decision and determined that it is enabled by Constellation. The proposal, as defined several years ago, required a future version of the Atlas V that does not currently exist. The mission can only be accomplished with a significantly larger launch vehicleâthat is, an Ares Vâthan exists in the current EELV family. In addition, in order to fit on a smaller launch vehicle, the mission concept would require small and fragile mirrors that may be impossible to develop. The Ares V launch vehicle would simplify the mission concept because of its large payload fairing and mass capability to the L2 point. The scientific impact for this mission is high. The missionâs science goals are clearly defined. Such a high- energy observatory would be capable of detecting the earliest black holes and galaxies in the universe and could study the chemical evolution of the universe and extremes of density, gravity, magnetic fields, and kinetic energy, which cannot be created in laboratories. The technical maturity is rated as low for development of the mirror seg- ment. One potential option is in-space assembly, which also has a low level of technical maturity. In summary, although the potential scientific impact of Gen-X is high, very challenging technical maturity issues place the mission in the âdeserving of further studyâ category. Modern Universe Space Telescope16 Scientific Objectives of the Mission Concept The Modern Universe Space Telescope is a 10-m, diffraction-limited optical-ultraviolet telescope (Figure 2.6) that establishes a new threshold in terms of sensitivity, imaging resolution, and scientific return. 17 MUST would consist of a single large (4- to 6-m) central optical mirror surrounded by a number of â¼2-m petal-like segments. The segments would be robotically assembled around the primary mirror at the Sun-Earth L2 point, enabling a telescope architecture that would be scalable to larger mirror concepts in the future. The observatory would consist of four elements: the 10-m telescope, four or more science instruments, a spacecraft bus, and a sunshield/baffle. The proposed telescope would cover the wavelengths from 1,150 angstrom (Ã ) in the ultraviolet to approximately 2.5 Î¼m in the near-infrared. Capabilities investigated for the canceled robotic Hubble Space Telescope servicing mission and developed for the International Space Station (ISS) would be used to assemble the observatory in space at the Sun-Earth L2 point. MUSTâs resolution in the optical and ultraviolet wavelengths would isolate individual stars in other galaxies and extend the evaluative techniques that are used with objects in our own galaxy to distant ones. The telescope will focus on the following key scientific questions: 15â National Research Council, Science Opportunities Enabled by NASAâs Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. 16â The first two subsections are based on material in J. Green, J. Bally, M. Beasley, R. Brown, D. Ebbets, W. Freedman, J. Grunsfeld, J. Huchra, S. Kilston, R. Kimble, J. Morse, R. OâConnell, K. Sembach, M. Shull, O. Siegmund, and E. Wilkinson, âThe Modern Universe Space Telescope: A Vision Mission Concept Study for a Large UV-Optical Space Telescope,â Center for Astrophysics and Space Astronomy, Uni- versity of Colorado, Boulder, Colo.; J. Green, University of Colorado, Boulder, âThe Next Large UV-Optical Space Telescope: The Modern Universe Space Telescope,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, February 21, 2008. See footnote 1 above. 17â This mission concept has been referred to as the Large Ultraviolet/Optical Modern Universe Space Telescope, or LUVO-MUST. LUVO is considered to be a class of telescopes, and MUST is a specific proposal within that class.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 39 ATLAS X 6.5 m DIA BY 20.4 m LONG COMPOSITE PAYLOAD FAIRING A A USABLE PAYLOAD STATIC ENVELOPE 6500 mm 255.9 in MUST SECONDARY MIRROR STOWED MUST MIRROR HUB MUST PRIMARY VIEW A 14561.8 mm MIRROR 573.3 in SEGMENTS STOWED AVAILABLE VOLUME FOR PROPULSION MODULE LVA (SIP) B B VIEW B EELV 4394 - 5 PAF BOLTED I/F FIGURE 2.6â Artistâs illustration of the Modern Universe Space Telescope (MUST) inside its launch vehicle prior to assembly in space. SOURCE: Courtesy of NASA. Figure 2.6.eps Includes low resolution bitmap images Text is too small â¢ How are metals created and distributed through the modern universe? â¢ How are galaxies assembled, and how do they evolve? â¢ How do stars and planetary systems form, and how does this affect their likelihood of supporting life? â¢ Where are the baryons in the modern universe, and how are they distributed? The committee believes that this mission offers a significant advance in its scientific field. However, to receive broad community support to enable such a multibillion-dollar mission, any such telescope should address one or more of the emerging science areas that will have an impact that is qualitatively different from the impact of other contemporaneously proposed missions. Characteristics of the Mission Concept as Developed to Date MUST would be a ânext-generation Hubble Space Telescopeâ in many ways. Some of the technologies follow on from JWST in terms of large-mirror technology and construction and placement at L2. Coronagraphic perfor- mance at contrast levels needed for exoplanet science objectives (10â9 to 10â10) are likely to be limited owing to scattered light from the segmented mirrors. The currently planned detectors and optics requirements require minor advances from those available today. The mission is proposed to have a 10-year start-to-flight timescale. The only
40 LAUNCHING SCIENCE available mass estimate for this mission concept refers to the proposed Atlas V (with a 7.2-m fairing) variant, not an Ares V variant, and totals 16,500 kg (spacecraft bus [wet], 12,000 kg; dispenser for robotic assembly, 4,500 kg). MUST concentrates on science themes that are addressed largely by other observatories, such as JWST and ALMA, and that were the main themes of a similar mission, Space Telescope 2010, proposed during the 2001 astronomy and astrophysics decadal survey.18 The Space Telescope 2010 project was not highly ranked, owing to the limited advances of UV/optical performance when compared with less mature fields such as far-infrared astronomy, and also owing to the relative maturity of UV-unique observations when compared with emerging fields. The segmented-mirror design for MUST may preclude direct observation of extrasolar planets (one of its stated goals)âa scientific area that is one of the most exciting new possibilities in optical/infrared astronomy. This issue will merit further study for segmented-mirror optical telescopes in general. Relative Technical Feasibility of the Mission Concept The key technologies identified for MUST are large-format, buttable high-quantum-efficiency charge-coupled devices (CCDs) and high-resolution, large-format anodes for improved microchannel plate detectors. None of these is likely to be challenging. The current baseline design of a segmented primary mirror might have low-enough scattered light to allow progress for observations of Earth-like planets in habitable zones. On-orbit assembly and verification for the large optics may benefit from astronaut involvement (presumably at a location closer to Earth than the operating location of the Sun-Earth L2 point; see Chapter 4). Optic coat- ings and tunable filters need to be developed as well. However, these do not represent substantial technological improvements from current technology. General Cost Category in Which This Mission Concept Is Likely to Fall No cost estimate was provided for the MUST mission concept. However, extrapolating from the costs of HST and JWST, the committee estimates that MUST will cost more than $5 billion. Benefits of Using the Constellation Systemâs Unique Capabilities Relative to Alternative Implementation Approaches As indicated above, MUST was proposed for launch on an Atlas V Phase II launch vehicle with a 7.2-m fairing. Although this vehicle configuration has been identified as a potential growth option, it does not currently exist, and there are no plans at the moment to develop this upgraded capability. Current mission plans would use robotic assembly at the L2 point. Robotic assembly tools would be part of the launched telescope assembly payload. The 10-m primary mirror (4- to 6-m segment and surrounding 2-m petals) would be deployed using space shuttle and ISS robot arm technology. Reduction of risk and cost may be likely if astronaut participation in assembly at the Sun-Earth L2 point is used. Because the baselined launch vehicle does not exist, executing MUST would require that it be scaled back to fit within the existing capabilities of the Delta IV Heavy, or reconfigured to use the Ares V launch vehicle. The benefits of using the Constellation suite of launch vehicles would be the following: (1) an Ares V would allow a large (single) primary mirror (or a fully deployable larger single mirror) to be placed at the L2 point and (2) the Orion spacecraft, launched atop an Ares I rocket and then boosted out of low Earth orbit with the assistance of a departure stage (see Chapter 4), might provide the ability to assemble and/or service and upgrade the telescope at the Earth-Moon L1 or Sun-Earth L2 point. Although this would enhance the capabilities of the telescope, it would also increase costs. The long history of human servicing of HST offers lessons in the costs and advantages of this approach. 18â National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 41 Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The MUST mission concept is worthy of further study as a Constellation-enabled science mission. The robotic assembly technique proposed in this mission concept could be simplified, enhanced, or eliminated with the capa- bilities of the Constellation System, particularly a launch vehicle with a larger shroud diameter. The remarkable success of HST and its continued importance to many fields of astronomy indicate that there should be further study of a next-generation large UV/optical space-based telescope. Assessment of the Mission Concept for Further Study In its interim report,19 the committee determined that the MUST mission would be enhanced by the Constel- lation System, but the committee has reevaluated that decision and determined that it is enabled by Constellation. The proposal, as defined several years ago, required a future version of the Atlas V that does not currently exist. The mission can only be accomplished with a significantly larger launch vehicleâthat is, Ares Vâthan exists in the current EELV family. It is possible that a smaller version of this telescope could be flown on an EELV, but that is not what was proposed. As is the case with the ATLAST proposal, the scientific impact of this mission is unclear. The mission proposal listed several broad goals cutting across the ultraviolet and optical spectral regions. A detailed science case for this mission should be developed, with trade-off studies of its various options examined. For example, the goals of imaging and spectroscopy of exoplanets present challenges in terms of scattered light for a segmented-mirror telescope such as MUST. The proposal suggests that several mirror aperture sizes are possible, but it does not clearly explain the science advantages and disadvantages of each. Several key technologies for MUST are immature at this time. The production, deployment, mass, and stability of a large segmented mirror that would be assembled in space presents significant challenges. As with ATLAST, other items appear to be at reasonable TRLs to allow anticipation of development in time for such a mission. In summary, because the MUST mission has medium/low ratings for the missionâs impact on science in the field and for its technical maturity, it is placed in the âdeserving of further studyâ category. This program could significantly advance UV/optical astronomy. However, in order for this mission to move forward, the science goals for MUST have to be better defined. ASTRONOMY AND ASTROPHYSICS/HELIOPHYSICS MISSION Stellar Imager20 Scientific Objectives of the Mission Concept The complex dynamics of the interaction of solar magnetic fields and plasma can result in the release of coronal mass ejections (CMEs), large clouds containing up to a few billion tons of plasma with the embedded solar magnetic field and moving away from the Sun at speeds up to a few thousand kilometers per second. These eruptions are most common during the maximum of the solar activity cycle. The associated accelerated relativistic particles can damage satellites and be harmful to astronauts and airline passengers. When the CME-associated shock waves impact Earthâs magnetic field, the effects of the resulting magnetic storms affect satellites and Earth- based technology such as navigation systems, communications systems, and power grids. The goals of the Stel- lar Imager are to build on current solar physics and heliospheric physics research in order to understand stellar 19â National Research Council, Science Opportunities Enabled by NASAâs Constellation System: Interim Report, The National Academies Press, Washington, D.C., 2008. 20â The first two subsections are based on material in K.G. Carpenter and the Stellar Imager Vision Mission Team, âSIâThe Stellar Imager: A UV/Optical Deep-Space Telescope to Image Stars and Observe the Universe with 0.1 Milli-arcsec Angular Resolution,â NASA Goddard Space Flight Center, Greenbelt, Md., September 2005; K.G. Carpenter, NASA Goddard Space Flight Center, âStellar Imager (SI): Reveal- ing Our Universe at High Resolution,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, February 20, 2008.
42 LAUNCHING SCIENCE FIGURE 2.7â The Stellar Imager, which uses a large array of formation-flying mirrors (left) to focus light on a common beam- combining hub (center) to produce ultrahigh-resolution images of stars and other celestial targets (right). SOURCE: Courtesy of K. Carpenter, NASA Goddard Space Flight Center. magnetic activity and magnetic processes and their roles in the origin and evolution of structure and the transport of matter throughout the universe. The proposed Stellar Imager (Figure 2.7) will enable general astrophysics research. Its long-baseline interfero- metric capabilities would benefit astrophysics in many waysâallowing unprecedented images of active galactic nuclei, quasi-stellar objects, supernovae, interacting binary stars, supergiant stars, hot main-sequence stars, and protoplanetary disks. The Stellar Imager would provide information on the interiors of stars using astroseismology to contribute to the understanding ot internal structure, differential rotation, and large-scale circulations. Plans are for the Stellar Imager to use formation flying 1-m mirrors to form a Fizeau interferometer providing 0.1-milliarcsec resolution or, in âlight bucket modeâ (reflecting all of the photons from all of the elements into one focus), providing an equivalent 11-meter-mirror UV/optical telescope. The Stellar Imager would cover the ultraviolet and optical wavelengths (1,200-6,600 Ã ). It would provide images of dozens of stars over a period of up to a decade each. Stellar Imager would be composed of 20 1-m primary mirrors on a virtual surface (formation flying) that can have their baseline varied from 100 m up to 1,000 m depending on the target requirements. Stellar Imager would orbit at the Sun-Earth L2 point and would provide significant advances in the observation of stellar interiors and magnetic variability. Characteristics of the Mission Concept as Developed to Date The general idea of interferometry from many ground-based instruments is quite mature. Space-based interferometric platforms are still immature, and Stellar Imager would benefit greatly from early-demonstration
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 43 interferometric missions such as the Fourier-Kelvin Stellar Interferometer (FKSI) or Pegase (a proposed space mission to build a double-aperture interferometer composed of three free-flying satellites). The major technology requirements related to formation flying for Stellar Imager (see the subsection below) are all immature at the present time. Some small efforts in development have been undertaken through NASA and Air Force funding, but these efforts would need to be further developed in a timely manner to realize Stellar Imager operation in the 2020 time frame. Depending on the choice of the number of mirror satellites and other options, the payload mass ranges from 4,000 to 9,000 kg. Relative Technical Feasibility of the Mission Concept The Stellar Imager is a technically challenging mission, but it would be a natural follow-on to ground-based interferometers (e.g., Center for High Angular Resolution Astronomy [CHARA], European Southern Observatory Very Large Telescope Interferometer [VLTI]), and space-based interferometers (e.g., FKSI, Pegase). The Fizeau interferometer mode will reposition the same mirrors to form the interferometer. Baselines from the central hub (containing the secondary mirror and focal-plane instrument) would range from 100 to 1,000 m. Because each Stellar Imager operating mode requires all-formation flying of the many mirrors, there is a significant technology challenge for all the options. The Stellar Imager is a somewhat easier concept than the Space Interferometer Mission or the Terrestrial Planet Finder Interferometer (TPF-I) (these two being astrometric or nulling missions) because of its less stringent tolerances for the interferometry. Both the SIM and TPF-I missions may offer technology development useful for the Stellar Imager. The main technology challenge and/or development areas for the Stellar Imager are as follows: 1. Formation flying of 20 spacecraft â¢ Launch to and deployment at the Sun-Earth L2 point â¢ Positioning and control of formation elements to 3 nm â¢ Aspect control to tens of microseconds of arc â¢ Need for variable, noncondensing, continuous-use micro-Newton thrusters â¢ Large dynamic range of motion control required â¢ Autonomous real-time analysis and/or real-time wavefront sensing control needed 2. Onboard and ground-based methods for dynamic control testing and validation. There are additional, although less severe, challenges in technologies for mirror fabrication, spacecraft con- struction, and detector development. Key specific issues such as uncertainties about propellant (what propellant, how much propellant) and contamination issues involved with many free-flying mirrors (constantly firing thrusters) would need to be addressed as well. The Stellar Imager proposal team has a well-thought-out technology plan, at this early stage, for approaches to the needed technology development. However, this plan requires technology development by other programs that have not yet been flown. General Cost Category in Which This Mission Concept Is Likely to Fall The Stellar Imager is a large, flagship-class mission estimated by the proposers to cost approximately $3 billion if flown with approximately 20 1-m mirrors and a single hub. Larger formation-flying mirrors and an additional hub (to increase scientific and operational efficiency as well as provide redundancy for risk mitigation) would be possible with a single Ares V launch vehicle, but it is currently not possible to estimate cost. As discussed above, Stellar Imager is technically challenging, and as such, the initial cost estimate is likely to be too low. The mission depends critically on earlier testbed missions for technology development in certain areas. Thus, it is likely that the mission falls in the higher-than-$5 billion category. It is hard to determine any cost savings that may occur if a single Ares V launch is accomplished compared
44 LAUNCHING SCIENCE with the use of a pair of Delta IV Heavy launches. The trade-offs between larger fairing and weight limits versus the complexity of initiating formation flying once the missions reach the Sun-Earth L2 point are difficult to assess at present. However, it seems highly likely, based on past experience, that mission complexity and risk (and thus cost) are far reduced by the use of a single launch because it would allow the integration of as much of the hard- ware as possible prior to launch. Benefits of Using the Constellation Systemâs Unique Capabilities Relative to Alternative Implementation Approaches The scientific benefit and reliability of the Stellar Imager mission would be enhanced if an Ares V were avail- able as the launch vehicle. Larger formation-flying mirrors could be easily accommodated in the fairing at only a modest cost increment for the mission. Larger mirrors would increase the sensitivity and science productivity of the observatory by increasing the amount of light gathered and enabling much faster seismic measurements of the observed star (and therefore more stars to be studied in less time). Flying two central hubs would also be possible, thereby providing risk mitigation and easing the complexity of deployment. Cost savings may occur as well using an Ares V because the Stellar Imager could be flown in a single launch, placing the entire mission payload in orbit and at the Sun-Earth L2 point. The Stellar Imager could be launched using the current fleet of EELVs. However, two launches using the Delta IV launch system would be required for the full mission concept. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Stellar Imager mission concept is worthy of further study as a Constellation-enhanced science mission. A large launch vehicle such as the Ares V could allow for the full concept to be placed in orbit with a single launch, thereby reducing launch risks. This mission would allow a dramatic improvement in the scientific understanding of our Sun and similar stars. Assessment of the Mission Concept for Further Study This mission is enhanced by the Constellation System. Although a spacecraft could be launched on multiple smaller launch vehicles, this would increase risk and complexity. The Ares V launch vehicle would allow the spacecraft to be placed in orbit in a single launch. The scientific impact of this mission could be high, providing unprecedented study of stellar magnetism and activity cycles, potentially leading to a better understanding of the Sun and space weather. Additional science goals would study extreme physics environments unable to be re-created in laboratories. However, the developed science plan, as described, is fairly limited in scope and would likely only appeal to a small science community. An increase in scientific programs and goals to be investigated would greatly benefit the Stellar Imager mission concept. Additionally, it is unclear how a mission of limited durationâthat is, less than typical solar-like stellar cycles (the Sunâs solar cycle is 11 years, and the Stellar Imagerâs lifetime is 10 years)âwould accomplish the primary mission goals. The technical maturity of this mission is low for the free-flying mirrors required to accomplish the scientific goals. Although interferometry as practiced on the ground is a mature technology, controlling many mirrors in a constellation in space has not been achieved yet. The development of onboard and ground-based methods for the dynamic control, testing, and validation of the mirrors involves technical challenges also at low maturity. Stellar Imager will benefit from early demonstration of interferometric missions if they are developed and flown. Technology investment in the formation flying of multiple spacecraft and their metrology and control is needed. Additional challenges include mass production of the free-flying mirror spacecraft, development of larger-format detectors, and methods to ensure a long mission lifetime. In summary, although the potential scientific impact of Stellar Imager could be higher, it is limited by the developed science plan and the limited lifetime of the mission, and the technical maturity of controlling a large number of free-flying mirrors is low, placing the mission in the âdeserving of further studyâ category.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 45 FIGURE 2.8â Illustration of the Solar Probe 2 mission concept. SOURCE: NASA Goddard Space Flight Center. HELIOPHYSICS MISSIONS Solar Probe 221 Scientific Objectives of the Mission Concept The proposed Solar Probe 2 would be the first spacecraft to directly explore with high time resolution in situ measurements of the solar corona at altitudes above the photosphere as low as 4 solar radii (perihelia of 5 solar radii), the source region of the solar wind. This mission would provide the necessary observations to solve the long-standing question of solar wind acceleration and coronal heating. Frequent and close encounters with the Sun would provide sufficient data to address the equally puzzling phenomena of energetic particle acceleration that result in an adverse near-Earth space environment for astronauts and space systems (see Figure 2.8). A solar probe mission to explore the source regions of the solar wind has been studied for many decades; however, it has been limited by technology requirements. The most recent decadal survey by the NRC for solar 21â The first two subsections are based on material in A. Szabo, D.C. Folta, J.P. Downing, and C. Roberts, NASA Goddard Space Flight Center, âSolar Probe 2,â in response to request for information by the Committee on Science Opportunities Enabled by NASAâs Constellation System, May 2008. See footnote 1 above.
46 LAUNCHING SCIENCE TABLE 2.3â Orbit Characteristics of Ballistic Solar Probe Solutions Mission Parameter Ares V Direct Ares I Multiple Venus Gravity Assists Ares V Multiple Venus Gravity Assists Perihelion (Rs) 18.5 7.7 5.0 Aphelion (AU) 0.91 0.73 0.74 Final orbit period (days) 129 87 86 Launch C3 (km2/s2) 275 87 200 Payload mass (kg) 500 2,500 5,800 Cruise phase (years) 0.16 15.0 7.3 Number of gravity assists 1 (Venus) 11 (3, Earth; 8, Venus) 8 (all Venus) SOURCE: Information about the vehicleâs capabilities with a Centaur V2 upper (third) stage was obtained from Marc Timm, Ares Program Executive, Exploration Systems Mission Directorate, NASA, âConstellation Overview,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, February 29, 2008. and space physics, The Sun to the Earthâand Beyond, gave this mission high priority for the 2003-2013 decade.22 In its fiscal year (FY) 2009 budget, NASA initiated the start of the Solar Probe Plus mission, which would accom- plish some of the goals of the decadal survey. The Solar Probe 2 mission would employ a spacecraft similar to that proposed for Solar Probe Plus, but could send it closer to the Sun by taking advantage of the propulsive capabilities of an Ares I rocket with an upper stage (and nuclear electric propulsion), or, more likely, by using the Ares V rocket. The proposed European Space Agency (ESA) Solar Orbiter/NASA Sentinels (Heliophysical Explorers [HELEX]) mission would explore the inner heliosphere, providing solar photospheric and coronal remote sensing observations along with multipoint in situ measurements of the heliosphere. 23 Solar Probe 2 would provide short- duration but frequent dips into the solar corona, testing and validating the understanding and models developed on the basis of the long-duration systematic studies of HELEX. Thus, Solar Probe 2 would be part of the spacecraft fleet charged to develop the critical forecasting capability of the space radiation environment in support of human and robotic exploration. The 2005 Solar Probe Science and Technology Definition Team established the following detailed Solar Probe science objectives: determine the structure and dynamics of the magnetic fields at the sources of the solar wind, trace the flow of the energy that heats the solar corona and accelerates the solar wind, determine what mechanisms accelerate and transport energetic particles, and explore dusty plasma phenomena and their influence on the solar wind and energetic particle formation. Characteristics of the Mission Concept as Developed to Date The Solar Probe 2 mission concept retains the technological advancements developed for the Solar Probe and Solar Probe Plus missions. The primary innovation of the present concept is the exploitation of Ares I and Ares V launch capabilities, which might provide both a low perihelion distance (as low as 5 solar radii) and frequent revisit times without the use of radioisotope thermoelectric generators. The Constellation System offers several options for Solar Probe 2 to reach a low perihelion orbit. These are summarized in Table 2.3. In summary, several options exist using the Ares vehicles to obtain low perihelion orbit with short orbital periods, thus meeting the requirements to penetrate deeply into the solar corona and shorten the revisit times of this region. The Solar Probe 2 spacecraft bus design as proposed will follow very closely the Solar Probe Science and 22â National Research Council, The Sun to the Earthâand Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003. 23â See http://hubble.esa.int/science-e/www/object/index.cfm?fobjectid=41396.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 47 Technology Definition Team Report concept. It is a three-axis-stabilized spacecraft designed to operate in the extreme thermal environment of the solar corona. The spacecraft bus will be protected by a 2.7-meter-diameter carbon-carbon conical primary heat shield with a secondary shield attached to its base. The bus consists of a hex- agonal equipment module and a cylindrical adapter. The equipment module will house the spacecraft subsystems and all of the instruments. The Solar Probe 2 instrumentation is identical to the 2005 concept, and it consists of both in situ and remote sensing instruments serviced by a common data processing unit and low-voltage power supply. The in situ instrumentation includes the following: fast ion analyzer, two fast electron analyzers, ion composition analyzer, energetic particle instrument, magnetometer, plasma wave instrument neutron/gamma-ray spectrometer, and coronal dust detector. The remote sensing package consists of a polar source region imager for extreme ultraviolet and magneto- graphic imaging of the solar wind source regions and a hemispheric imager for the white-light imaging of coronal structures. To facilitate data telemetry and commanding, the spacecraft will be equipped with one high-gain antenna, one medium-gain antenna, and two low-gain antennas for emergencies. X band will be used for both data downlink and commanding. Optionally, the higher-telemetry-rate Ka band can be employed for larger-data-volume downlink. The estimated total power requirement of the spacecraft is â¼230 watts (W). Solar Probe 2 would be powered by retractable solar panels. During solar close encounters, the primary solar panels will be retracted and the spacecraft will rely on the combination of smaller high-temperature secondary solar panels and batteries. The total spacecraft mass is estimated to be â¼900 kg, well inside the lift capacities of the Ares I and V launch vehicles. Relative Technical Feasibility of the Mission Concept The mission concept and instruments required for the Solar Probe 2 mission do not present any technical challenges. The thermal control technology is apparently well developed. The ability of Ares V in particular to achieve the mission goals still requires further review but appears feasible. General Cost Category in Which This Mission Concept Is Likely to Fall Precise cost estimation of this mission without a detailed study is difficult. Previous Solar Probe studies estimated the total cost of the mission at between $700 million and $1.4 billion, including the launch vehicle. The present estimate does not include the cost of the launch vehicle: based on earlier studies of Solar Probe missions, the total mission cost would be expected to be more than $1 billion. Benefits of Using the Constellation Systemâs Unique Capabilities Relative to Alternative Implementation Approaches The use of Constellation System vehicles as described in this mission concept proposal would enable a Solar Probe 2 mission. Ares I or Ares V launch vehicles, together with gravity assists by Venus, would get the probe into the 4 to 7 solar radii position. This would result in an orbital period around the Sun of under 100 days, thus allowing multiple passes through the lower solar corona. This can be accomplished without the need for radioiso- tope thermoelectric generators or electric propulsion, although adding these would provide enhancement of the mission. The currently developed Constellation System uniquely enables a Solar Probe 2 mission with an orbital configuration that combines the advantages of the 2005 and 2008 Solar Probe studies, significantly enhancing the returned science data compared with the data from any of the previous mission designs. Assessment of the Mission Concept for Further Study The Solar Probe 2 mission is enabled by the Constellation System, which allows insertion of the satellite into a short-period orbit as close as 5 solar radii to the surface of the Sun. The mission would be enhanced by the use of additional propulsion, which would be accommodated with the large Ares V lift capabilities.
48 LAUNCHING SCIENCE The potential impact of the Solar Probe 2 on scientific knowledge of the Sun and solar wind is very high. The science goals of Solar Probe 2 have been a high priority in the heliophysics community for many decades, and the mission has continually been endorsed by decadal surveys and NASA roadmaps. For budgetary and programmatic reasons, a version of the mission known as Solar Probe Plus, which only reaches 10 solar radii but has a shorter orbital period than that of the original design, is currently being planned by NASA. Solar Probe 2 would provide the opportunity to recover the fundamental science discoveries enabled by reaching the much closer approach to the Sun (4 to 5 solar radii, depending on the specific implementation) with the short (less than â¼100-day) orbital period. In addition, some implementations of Solar Probe 2 could reach approximately 50 degrees latitude, thus recovering the higher-latitude observations. The Constellation System would provide the propulsion needed to enable the mission without the need for new technologies. The technical maturity for this mission is rated as high. The instrument payload is based on existing instru- ments. Technology development, including heat shields, has already been done. A number of different mission concepts were examined. The mission concept employing only conventional propulsion and power is well developed and at high TRL. There are also implementations requiring nuclear electric propulsion, which provide significant improvements over the conventional implementations. These have low technology readiness levels. Because the mission payload is based on one developed many years ago, it would be useful to revisit the optimal payload in light of recent instrument development. It seems likely an enhanced payload could be accommodated with the Constellation capability. In summary, Solar Probe 2âs combination of clearly defined, high-priority science goals of major interest to the community and mature instrument and spacecraft technology places this mission in the âmore deserving of further studyâ category. Further study should be narrowly focused on evaluating the ability to use the Ares V to obtain the desired orbit. Interstellar Probe24 Scientific Objectives of the Mission Concept The heliosphere, the bullet-shaped bubble created by the interaction of the solar wind and solar magnetic field with the interstellar medium, shields our solar system from most interstellar plasma, cosmic rays, and dust. The Interstellar Probe (Figure 2.9) will travel beyond the heliopause (the boundary of the heliosphere) to study cosmic rays, plasma, neutral particles, and dust that constitute the galactic environment close by our solar system. It is designed to answer the following fundamental questions: 1. What is the nature of the nearby interstellar medium? 2. How do the Sun and galaxy affect the dynamics of the heliosphere? 3. What is the structure of the heliosphere? 4. How did matter in the solar system and interstellar medium originate and evolve? The heliosphere moves through the interstellar medium at supersonic velocity relative to the medium creat- ing a shock bounding the heliosphere at about 180 to 200 astronomical units (AU). The Interstellar Probe would explore interstellar space located beyond this boundary. To reach interstellar space in a reasonable amount of time will require a spacecraft with unprecedented pro- pulsion capability. The initial NASA-funded Vision Mission concept studies included two implementations of 24â The first two subsections are based on material in T.H. Zurbuchen, University of Michigan, and the ISP [Interstellar Probe] Vision Mis- sion Team, âNuclear-Powered Interstellar Probe, Final Report 2006,â 2006; and R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, âEnabling a Faster, Better Innovative Interstellar Explorer with NASAâs Constellation System,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, February 21, 2008. See footnote 1 above. Note that the presentation to the committee was not made by a member of the team that delivered the final report. However, the presentation was made at the recom- mendation of the principal investigator for âNuclear-Powered Interstellar Probe, Final Report 2006,â and both studies had nearly identical scientific recommendations.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 49 FIGURE 2.9â One possible Interstellar Probe configuration during its flyby of Jupiter. SOURCE: Reprinted with permission of the Johns Hopkins University Applied Physics Laboratory. the Interstellar Probe, one employing nuclear reactor technology25 and one using solar sails.26 Although a final report was written only for the nuclear option, the committee received a presentation from R.L. McNutt on three others: a Delta IV launch vehicle with a spacecraft using a solar sail (the original Vision Mission proposal prepared by McNutt), a Delta IV launch vehicle with a spacecraft using radioisotope electric propulsion, and an Ares V launch vehicle with a spacecraft also using radioisotope electric propulsion. 27 These three versions (collectively referred to as Innovative Interstellar Explorer) differ primarily only in propulsion options. The spacecraft would weigh approximately 1,230 kg. All four approaches have almost identical science goals and very similar instru- ment payloads. The main difference is that, because of its large size, the nuclear reactor implementation included 25â T.H. Zurbuchen, University of Michigan, and the ISP [Interstellar Probe]Vision Mission Team, âNuclear-Powered Interstellar Probe, Final Report 2006,â 2006. 26â R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, âEnabling a Faster, Better Innovative Interstellar Explorer with NASAâs Constellation System,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, Febru- ary 21, 2008. 27â R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, âEnabling a Faster, Better Innovative Interstellar Explorer with NASAâs Constellation System,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, Febru- ary 21, 2008.
50 LAUNCHING SCIENCE remote sensing instruments in the visible, ultraviolet, and infrared wavelengths, allowing it to observe the universe from a unique vantage point beyond the zodiacal light and far from Earth. Because of the similarity of the primary instruments and science goals, the concept maturity, technology readiness, and advantages gained through use of the Constellation System, the four approaches are collectively discussed together below. The Interstellar Probe (previously called Interstellar Explorer) was endorsed by the 2003 NRC decadal survey The Sun to the Earthâand Beyond.28 The mission concept is included in the 2005 Heliophysics Roadmap as a Vision Mission. It has also been discussed in earlier roadmaps and decadal surveys. In addition, an ESA Technol- ogy Reference Studies report, funded through the Science Payload and Advanced Concepts Office 29 and published in April 2007, describes a solar sail mission with two close flybys of the Sun. This mission may therefore provide an opportunity for international cooperation. The Interstellar Probe mission logically follows on the limited in situ observations by the Voyager spacecraft and the upcoming remote sensing observations to be obtained by the Interstellar Boundary Explorer (IBEX), which will image the large-scale shape of the boundaries separating the heliosphere from the interstellar medium. Because of its complete instrument payload and data rate, the Interstellar Probe would provide a very significant improvement over existing missions, potentially enabling revolutionary discoveries about our local galaxy. Characteristics of the Mission Concept as Developed to Date The scientific concept, instrument package, and mission plan of the Interstellar Probe are quite mature. The goal is to leave the solar system within â¼20Â° of the incoming interstellar wind and to reach 200 AU in as short a time as possible. Most of the scientific instruments on the Interstellar Probe have substantial heritage. These include a fast plasma instrument to measure the solar wind, pickup ions, and interstellar plasma; a magnetometer to measure the quasi-static magnetic field; and a plasma/radio-wave receiver. There are also instruments to mea- sure energetic particles, an instrument to measure the composition of the dust in the interstellar medium, and a neutral imager. The Interstellar Probe requires either a nuclear reactor or radioisotope thermoelectric generator/radioisotope power system (RTG/RPS). The mission also requires high-capability propulsion. Four options were considered: conventional, nuclear reactor electric, radioisotope electric, and solar sail. Given the current lack of technology development programs for nuclear propulsion and for solar sails, the conventional propulsion or the radioisotope electric propulsion (REP) provides the most mature mission concept. The major disadvantages are longer flight time to 200 AU, â¼30 years with Jupiter gravity assist versus 20 years for nuclear, or 15 years for solar sail. There are also trade-offs in the use of a Jupiter assist, which requires the use of radiation-hardened parts and shielding. Relative Technical Feasibility of the Mission Concept The scientific concept, instrument package, and mission plan for the Interstellar Probe are quite mature. For all four implementations, the primary technology issues are (1) the need for radioactive power sources, (2) qualifying parts and ensuring instrument operations for a 30- to 50-year lifetime, and (3) a Deep Space Network upgrade to a phased array of many antennas. All four implementations have propulsion technology issues. The nuclear reactor option described in the Vision Mission report by Zurbuchen and the ISP Vision Mission Team is the more complex of the missions because of the low technology readiness level of the nuclear reactor and the relatively low technology readiness level of the nuclear electric propulsion options. The mission plan also incorporated a mother spacecraft with the nuclear reactor and remote sensing instruments and two identically instrumented daughter spacecraft with in situ particles and fields instruments that would be released after most of the reactor burnout. This approach provides additional science capability, but at a higher cost. Because there is no nuclear reactor development program at this time, this option is not considered in the next subsections. 28â National Research Council, The Sun to the Earthâand Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003. 29â See http://sci.esa.int/sciencee/www/object/index.cfm?fobjectid=36022.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 51 General Cost Category in Which This Mission Concept Is Likely to Fall Conventional propulsion (using the Ares V with an additional Centaur upper stage) and the REP place this mission in the $1 billion to $5 billion category.30 Benefits of Using the Constellation Systemâs Unique Capabilities Relative to Alternative Implementation Approaches Although three different options were considered, the solar sail option was not presented to the committee in great detail because it requires the most new technology development. The other two options were described in greater detail (see Table 2.4). One was a Delta IV Heavy with two solid rocket motors as upper stages; the other was an Ares V with Centaur upper stage (a capability that NASA has no plans for at the moment). Both options require an RTG power source and electric propulsion. For the same spacecraft dry weight, the Ares option reached 200 AU in â¼23 years, compared to 30 years for the Delta IV option, and was traveling 1.9 AU/year faster at that time. The benefits of using Constellation are avoiding nonconventional propulsion (solar sail or a nuclear reactor with electric propulsion). The Delta IV option is a unique configuration that has never been flown. It would require further study to determine if it is feasible and, if so, the cost relative to the Ares V option. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Interstellar Probe mission concept is worthy of further study as a Constellation-enabled science mission. Using an Ares V, the mission may provide a very significant improvement over earlier mission concepts. The use of the Ares V may enable the mission to be flown using only conventional propulsion, thus removing the need for solar sail technology or REP, and it could shorten the time to reach 200 AU. Shortening the mission lifetime would reduce costs and risk. Additional capability may also enable instrument package redundancy, further reduc- ing risk. The science motivation for the mission has continued to receive the endorsement of decadal surveys and other studies for two decades. Assessment of the Mission Concept for Further Study In its interim report, the committee determined that the Interstellar Probe would be enhanced by the Constel- lation System, but it has reevaluated that decision and determined that it is enabled by Constellation. Reaching interstellar space in a reasonable time requires a spacecraft with extremely high propulsion capabilities. One of the two original Vision Mission studies used a nuclear reactor, and the other used solar sails. Preliminary studies indicate that using the Constellation System will enable the mission to reach the desired distance of 200 AU in a reasonable time (approximately 25 years) using only conventional propulsion. Without the propulsion capability of the Ares V launch vehicle, such a mission would take from 30 to 50 years, a time period that the committee determined to be unrealistic for effective science return. The potential impact of the Interstellar Probe mission on science in the field is very high. The Interstellar Probe would travel outside the heliopause (the boundary of our solar system) to study cosmic rays, plasma, neutral particles, and dust that constitute the galactic environment close by our solar system. It is designed to determine the nature of the nearby interstellar medium, the structure of the heliosphere, how the galaxy affects the dynamics of the heliosphere, and how matter in the solar system and interstellar medium originated and evolve. The Inter- stellar Probe has been endorsed for more than 20 years by both NRC and NASA panels, and its science goalsâto provide fundamental new observations of the interstellar mediumâare highly rated. The technical maturity for this mission is relatively high. The planned instrument payload has substantial heritage, and similar instruments have flown on numerous heliophysics missions. The primary technology issues 30â This estimate is based on the presentation to the committee by R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, âEnabling a Faster, Better Innovative Interstellar Explorer with NASAâs Constellation System,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, February 21, 2008.
52 LAUNCHING SCIENCE TABLE 2.4â Comparison of the Two Innovative Interstellar Explorer Options Using the Delta IV Heavy and Ares V Delta IV Heavy + 2 Star 48AsâOption 1 Ares V + CentaurâOption 1 Launch date October 22, 2014 December 3, 2015 Gravity assist body Jupiter Jupiter Gravity assist date February 5, 2016 October 10, 2016 Gravity assist altitude 75,150 km 128,855 km Gravity assist radius 2.05 Rj 2.80 Rj Gravity assist 2â 23.8 km/s 25.1 km/s Burnout date October 13, 2032 July 3, 2030 Burnout distance 104 AU 116 AU Burnout speed 7.9 AU/year 9.8 AU/year Date 200 AU reached December 31, 2044 February 14, 2039 Minimum trip time to 200 AU 30.2 years 23.2 years Speed at 200 AU 7.8 AU/year 9.7 AU/year Right ascension at 200 AU 263.8Â° 248.7Â° Declination at 200 AU 0.0Â° 0.58Â° Launch mass 1,230 kg 1,230 kg Propellant mass 440 kg 440 kg Final mass 790 kg 790 kg Power 1.0 kW 1.0 kW Isp (specific impulse) 3,800 s 3,410 s EP system efficiency 53.8% 53.5% Total stack C3 123.3 km2/s2 270.0 km2/s2 EP 2â 16.5 km/s 14.8 km/s Thrust time 18.0 years 14.6 years NOTE: Both spacecraft would also require radioisotope electric propulsion. SOURCE: Adapted from R.L. McNutt, Jr., Johns Hopkins University Applied Physics Laboratory, âEnabling a Faster, Better Innovative Interstellar Explorer with NASAâs Constellation System,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, February 21, 2008. are (1) the need to have radioactive power sources, (2) the qualifying of parts and ensuring of instrument operations for a longer-than-20-year lifetime, and (3) a Deep Space Network upgrade to a phased array of many antennas. In summary, the Interstellar Probeâs combination of clearly defined, high-priority science goals and mature instrument technology places this mission in the âmore deserving of further studyâ category. Further study should be primarily narrowly focused on evaluating the ability to use the Ares V to obtain the desired orbit. Solar Polar Imager31 Scientific Objectives of the Mission Concept The current understanding of the Sun and its atmosphere is severely limited by the lack of good observations of the polar regions. The Solar Polar Imager is a spacecraft (Figure 2.10) in a 0.48-AU circular orbit around the Sun with an inclination of 75Â° to the ecliptic plane, intended to provide remote sensing observations of the Sun and in situ observations (measurements of the local properties of the plasma and electromagnetic fields) of this critical region. Observations of the Sunâs poles, not possible from the usual ecliptic viewpoint, may revolutionize 31â The first two subsections are based on material in P.C. Liewer, D. Alexander, J. Ayon, L. Floyd, G. Garbe, B. Goldstein, D. Hassler, A. Kosovichev, R. Mewaldt, N. Murphy, M. Neugebauer, A. Sandman, D. Socker, S. Suess, R. Ulrich, M. Velli, A. Vourlidas, and T.H. Zurbu- chen, âSolar Polar Imager: Observing Solar Activity from a New Perspective, Vision Mission Study Final Report,â Jet Propulsion Laboratory, Pasadena, Calif., December 2005; and D. Socker, U.S. Naval Research Laboratory, for the Solar Polar Imager Vision Mission Study Team, âSolar Polar Imager: Observing Solar Activity from a New Perspective,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, February 21, 2008. See footnote 1 above.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 53 FIGURE 2.10â Illustration of the Solar Polar Imager spacecraft. SOURCE: Courtesy of NASA, the Jet Propulsion Laboratory, and California Institute of Technology. scientific understanding of the internal structure and dynamics of the Sun and its atmosphere. Through the use of remote sensing and in situ instrumentation, the Solar Polar Imager would investigate the physical connection between the Sun, the solar wind, and solar energetic particles. The primary science questions motivating the Solar Polar Imager mission are the following: 1. What is the relationship between the magnetism and dynamics of the Sunâs polar regions and the solar cycle? 2. What is the three-dimensional global structure of the solar corona, and how is this influenced by solar activity and coronal mass ejections? 3. How are variations in the solar wind linked to the Sun at all solar latitudes? 4. How are solar energetic particles accelerated and transported in radius and latitude? 5. How does the total solar irradiance vary with solar latitude? 6. What advantages does the polar perspective provide for space weather prediction? Because the Solar Polar Imager will make the first close-up remote sensing measurements of the polar regions of the Sun, the possibility exists for surprising discoveries and greatly improved understanding. The Solar Polar Imager would serve as a pathfinder for permanent high-ecliptic-inclination solar sentinels, if it is deemed that the polar perspective is important for space weather prediction. The Solar Polar Imager will have a Doppler-magnetograph imager, a white light coronagraph, an extreme ultraviolet imager, a total solar irradiance monitor, an ultraviolet spectrograph, a magnetometer, a solar wind ion composition and electron spectrometer, and an energetic-particle instrument. In the original Vision Mission study, the spacecraft used a Delta IV launch vehicle and required 6.7 years of flight time to reach its science orbit. Once it reached its intended orbital distance from the Sun, it would use a solar sail to achieve the highly inclined orbit of 75Â°. The use of the solar sail determines the mission duration, the choice of launch vehicle, the spacecraft con- figuration and control subsystems, and the operations cost during the long cruise. It also complicates the thermal, power, and telecommunications designs. Use of the Ares V might permit use of solar-electric propulsion or per- haps direct injection with consequent simplification and shortening of the time to arrive at the science orbit. The science goals for the mission require simultaneous observations in the ecliptic plane for the helioseismology and magnetographic studies, and the cost for these is not included in the Solar Polar Imager mission description.
54 LAUNCHING SCIENCE The only measurements obtained to date of the Sunâs polar regions are the observations of the solar wind made by the Ulysses spacecraft (in a polar orbit around the Sun) at 2 to 3 AU. The structure of the solar wind at the poles was very different at solar maximum compared with solar minimum, but Ulyssesâs long (8 year) orbital period permitted only snapshots of the solar wind speed and magnetic field at the poles. Ulysses did not have any remote sensing instruments to probe solar structure and magnetic fields. In comparison, the Solar Polar Imager orbit would provide measurements of the poles every 4 months, closer to the Sun, and with more complete diagnostics. Although Solar Polar Imager is not mentioned by name in the NRCâs 2003 solar and space physics decadal survey, a multi-spacecraft mission to provide imaging of the poles of the Sun is described as an important next step in providing understanding of the solar structure, magnetic field, and dynamics, and a multi-spacecraft mission is a possible enhanced version of the Solar Polar Imager mission.32 The committee believes that this mission does offer a significant advance in its scientific field, but it will have to be further reviewed in a future decadal survey. The ESA Solar Orbiter mission is planned to reach latitudes of approximately 32Â°. ESA has performed a preliminary study of the Solar Polar Orbiter, making this mission a candidate for international cooperation. 33 Characteristics of the Mission Concept as Developed to Date The scientific concept, instrument package, and mission plan of the Solar Polar Imager are mature. The pri- mary technology issues are (1) the need to use solar sails to reach the desired polar orbit, (2) the weight and power requirements for a total solar irradiance monitor, and (3) the flight test needed for the Doppler magnetograph and the UV spectrograph. The mission concept as described depends on the use of solar sails, which are an untested technology that currently has no development path. If use of the Ares V would allow the high-inclination orbit to be attained with solar-electric propulsion or conventional propulsion, it would greatly simplify the mission. Relative Technical Feasibility of the Mission Concept The instrument payload of the Solar Polar Imager is relatively mature, with essentially all instruments, except the Doppler magnetograph imager, having heritage from many missions, including Solar and Heliospheric Observa- tory, Solar Terrestrial Relations Observatory, Ulysses, Cassini, Fast Auroral Snapshot/Time History of Events and Macroscale Interactions during Substorms (FAST/THEMIS), Mars Reconnaissance Orbiter, and Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER). The Doppler magnetograph imager design is based on a ground instrument that has received study as a flight instrument. There is no existing total irradiance monitor that can meet the low weight and power requirements, but there is an ongoing instrument development program. The telemetry assumes the use of X band and a new Deep Space Network using 180 12-meter-diameter anten- nas, although the Solar Polar Imager could use Ka band and 30-m antennas with an increase in power and mass. The mission concept for the Solar Polar Imager is mature. Propulsion to enable the spacecraft to reach the desired high-inclination orbit is the only element that is not mature. General Cost Category in Which This Mission Concept Is Likely to Fall The cost estimated for the initial Delta IV version of the Solar Polar Imager was less than $1 billion. The cost of this mission is likely to be approximately $1 billion. Although there are cost benefits in building multiple spacecraft, adding two extra spacecraft would increase the overall mission cost. 32â National Research Council, The Sun to the Earthâand Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003. 33â See http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36025.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 55 Benefits of Using the Constellation Systemâs Unique Capabilities Relative to Alternative Implementation Approaches The Solar Polar Imager would require the development of solar sail technology to reach the desired orbit using a Delta IV launch vehicle. However, with a launch on Ares V, the mission may be achievable without solar sails, but using an upper stage and solar-electric propulsion with advanced ion thrusters. The result would be a shorter time to mission orbit (5 years compared with 8 years). Eliminating the solar sails would also simplify the mission. Because power and mass were drivers in the initial design of instruments, there may also be some simplification in instruments if the mission switches to the Ares V approach. However, the benefits of using Ares V are still not fully studied. Another interesting possibility would be the launch of multiple satellites to monitor solar activity simultane- ously. The baseline Solar Polar Imager concept observes a particular point on the Sun every 4 months; however, activity on the Sun occurs on much shorter timescales. The optimum would be to insert three identical satellites into equally spaced polar orbits, thereby ensuring continuous coverage of the Sun. Given the projected launch capability of the Ares V with a dual-engine Centaur, it appears that two identical spacecraft could be inserted into complementary orbital inclinations, providing enhanced coverage of the Sun. (Note that NASA has no current plans to adapt the Centaur to the Ares V.) Additional study is required to determine if a third spacecraft could be launched on a single mission in order to provide more continuous coverage of the polar region. The cost of the project would increase primarily by the cost of the additional spacecraft. Some of the Solar Polar Imager sci- ence goals require simultaneous observations from the ecliptic plane, which could possibly be provided by these additional spacecraft. The Vision Mission report suggests that astronauts could assemble the satellite for the solar sail implementa- tion, an activity that would not be necessary if this mission is launched on an Ares V. Should This Concept Be Studied Further as a Constellation-Enabled Science Mission? The Solar Polar Imager mission concept is worthy of further study as a Constellation-enhanced science mis- sion. The Ares V launch vehicle could allow the elimination of the unproven solar sail technology. The Solar Polar Imager would make unique observations of the Sunâs polar regions, dramatically improving and possibly changing current understanding of the behavior of the star closest to Earth. This better understanding could be applied to the study and understanding of stars elsewhere in the universe. The enhanced understanding may enable predictive capabilities about the behavior of the Sun, needed for space weather and climate prediction. Assessment of the Mission Concept for Further Study The Solar Polar Imager mission is enhanced by the Constellation System. Without Ares V, the desired orbit could be achieved through the use of solar sails, an unproven technology (discussed further in Chapter 4). Ares V offers the potential to place the spacecraft (one or more) into the proper orbit without solar sails. The potential impact of this mission on science in the field is large. The Solar Polar Imager would provide the first imaging of the Sunâs poles with repeated observations at short intervals due to the close-in orbit, yielding comprehensive diagnostics that may dramatically improve understanding of the structure and dynamics of the Sun and solar magnetic field. Although Solar Polar Imager has not specifically been endorsed by the decadal surveys or roadmaps, the need for imaging of the polar regions of the Sun was clearly elucidated by the 2003 solar and space physics decadal survey. The science goals are important and compelling. The Solar Polar Imager provides a significant improvement over currently available observations of the Sun. The technical maturity for this mission is relatively high overall. The instrument payload has strong heritage; most instruments have flown on previous heliophysics missions. Reaching the desired polar orbit over the Sun with a short orbital period requires large propulsion capabilities. Two possibilities for accomplishing this involve using solar-electric propulsion or undeveloped solar sail technology. Using the Constellation propulsion capabilities and therefore eliminating the solar sails would simplify the mission. However, the required solar-electric propul-
56 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 Imagerâs combination of clearly defined, high-priority science goals and mature instrument technology places this mission in the âmore deserving of further studyâ 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 Earthâs 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 asteroidâs weak gravitational field. In fact, landing equipment on and retrieving samples from an asteroidâs surface has already been accomplished robotically. A study sponsored by the Advanced Projects Office within NASAâs 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 NEOâs 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 NEOâs 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 34â The 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, âExploration of Near-Earth Objects via the Crew Exploration Vehicle,â NASA Johnson Space Center, Houston, Tex., in response to request for information by the Committee on Science Opportunities Enabled by NASAâs Constellation System, May 2008. See footnote 1 above.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 57 FIGURE 2.11â 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.
58 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 âlower-bookendâ option, (2) the midvolume Ares IV single-launch option,35 (3) the midvolume Ares V single-launch option, and (4) the âupper-bookendâ 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 asteroidâs 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 targetâs 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. 35â The 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.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 59 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 proposerâs cost estimate is that one should expect to spend funds similar to those spent for an extended lunar sortieâthat 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 Systemâs 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 objectâs trajectory. The emplacement of an array of suitable instruments would provide an understanding of the objectâs 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. 36â The Ares V and Orion spacecraft launch option remains a potential future development for NASA. See Steve Cook, âLunar Program Industry Briefing: Ares V Overview,â September 25, 2008, available at http://www.nasa.gov/pdf/278840main_7603_Cook-AresV_Lunar_ Ind_Day_Charts_9-25%20Final%20rev2.pdf.
60 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 NASAâs 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 âdeserving of further studyâ category. However, the most compelling rationale for a human NEO mission is for its value as an intermediate step between lunar exploration and interplanetary travelâto 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 âmore deserving of studyâ 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 NASAâs 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, whichâowing to its retrograde orbitâis 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 âother important objects,â after the top four choices, in the NRCâs 1994 report An Integrated Strategy for the Planetary Sciences 1995-2010, and was the focus for two of the nine âdeferred high-priority flight missionsâ 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 NASAâs 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). 37â The first two subsections are based on material in B. Bienstock, The Boeing Company, and the Neptune Orbiter with Probes Team, âNASA Vision Mission Neptune Orbiter with Probes,â 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), âStudy of a Neptune Orbiter with Probes Mission, Final Report,â May 2006; T.R. Spilker, JPL, and A.P. Ingersoll, Caltech, âAerocapture Implementation of NASAâs âNeptune Orbiter with Probesâ Vision Mission,â presentation to the Committee on Science Opportunities Enabled by NASAâs Constellation System, February 20, 2008; and B. Bienstock, The Boeing Company, and D. Atkinson, University of Idaho, âNeptune Orbiter with Probes,â 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 NASAâs 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. 38â National Research Council (NRC), An Integrated Strategy for the Planetary Sciences 1995-2010, 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. 39â National Research Council, Grading NASAâs Solar System Exploration Program: A Midterm Review, The National Academies Press, Washington, D.C., 2007.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 61 FIGURE 2.12â 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 Neptuneâs 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 missionâs 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 40â Delta IV Payload Planners Guide, 06H0233, United Launch Alliance, Littleton, Colo., September 2007, available at http://www.ulalaunch. com/docs/product_sheet/DeltaIVPayloadPlannersGuide2007.pdf. 41â Delta IV Payload Planners Guide, The Boeing Company, Huntington Beach, Calif., 2002.
62 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 Neptuneâs stratosphere down to pressure levels of â¼100 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 Tritonâs atmosphere and surface globally in order to establish the composition and age of the surface, the inventory of volatiles, the satelliteâs current geologic activ- ity, and the atmosphereâs 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 Tritonâs 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 Titanâs 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 Systemâs Unique Capabilities Relative to Alternative Implementation Approaches The nuclear-electric propulsion mission concept significantly exceeds the capabilities of the largest rocket currently in any nationâs 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.
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 63 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 Neptuneâs 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 missionâs 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 âmore deserving of further studyâ 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, Saturnâs 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 Titanâs atmosphere 42â The first two subsections are based on material in J.S. Levine, NASA Langley Research Center, âTitan Explorer: The Next Step in the Exploration of a Mysterious World,â Final Report for NASA Vision Mission Study per NRA-03-OSS-01, June 2005; and J.S. Levine, NASA Langley Research Center, âTitan Explorer: The Next Step in the Exploration of a Mysterious World,â presentation to the Committee on Science Opportunities Enabled by NASAâs 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.
64 LAUNCHING SCIENCE 3.75 m diameter rigid biconic aeroshell TUFROC Forebody TPS â Peak heat rate = 308 W/cm2 (56 convective and 252 radiative) â 6.3 cm thick 5 m dia. Conical Ribbon Parachute FIGURE 2.13â 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: â¢ What is the chemical composition of the atmosphere, including the trace gases? â¢ What is the isotopic ratio of the gases in the atmosphere? â¢ What prebiological chemistry is occurring in the atmosphere/surface of Titan today and what is its relevance to the origin of life on Earth? â¢ What is the nature, origin, and composition of the clouds and haze layers? However, the missionâs 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 Titanâs surface, a magnetometer to search both for a planetary dipole field and surface magnetism, an ultraviolet spectrometer to
ANALYSIS OF SPACE SCIENCE MISSION STUDIES 65 measure the escape of gases from Titanâs upper atmosphere, and a visual and infrared mapping spectrometer to measure cloud and haze layers and the nature of the satelliteâs 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. Titanâs 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 Systemâs 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 â¼$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 Titanâs atmosphere and surface.
66 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 (Cassiniâs 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 Titanâs surface on the airship.43 By studying Cassiniâs 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 Titanâs 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 âdeserving of further studyâ 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. 43â 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. 44â Leary and the Titan Explorer Team, âTitan Explorer NASA Flagship Mission Study,â Johns Hopkins University Applied Physics J. Laboratory, Laurel, Md., August 2007; R.D. Lorenz, âA Review of Balloon Concepts for Titan,â Journal of the British Interplanetary Soci- ety 61:2-13, 2008; R.D. Lorenz, âTitan Bumblebee: A 1-kg Lander-Launched UAV Concept,â Journal of the British Interplanetary Society 61:118-124, 2008.