4
Universe Exploration and the Search for Earth-like Planets

The Universe Exploration roadmap and The Search for Earth-like Planets roadmap represent the two major research components of the Universe Division at NASA headquarters.1,2 These research components are supported by the most recent NRC astronomy decadal survey in this area and related reports.3-5

These two roadmaps make a strong case for exploring the fundamental physics associated with the beginning of the universe and the nature of space-time and for searching for Earth-like planets. They do not, however, present the most robust case possible for the suite of missions that address the important broad range of astrophysical questions that are at the forefront of astrophysical research but do not fall conveniently in the scope of the Beyond Einstein and the Search for Earth-like Planets programs as presented in these roadmaps (see the next two sections). It is imperative that a proper balance be struck along the complete spectrum of astrophysical research. The division of topics between these two roadmaps also tends to deemphasize the capability that some of the proposed missions critical to the search for Earth-like planets have to do broader astrophysical research. Finally, the partitioning into two roadmaps has deemphasized the value of shared technology, facilities, and infrastructure.

UNIVERSE EXPLORATION ROADMAP: FROM THE BIG BANG TO LIFE

The Beyond Einstein mission suite addresses exciting and fundamental physics questions designed to test whether or not there are observable limits to Einstein’s theory of gravity. A strong case is made for exploring the beginnings and evolution of the universeone of the 5 fundamental questions identified in the NRC’s Astronomy and Astrophysics in the New Millennium6 (the AAp decadal survey) and one of the 11 identified in Connecting Quarks with the Cosmos.7 A strong emphasis on studying the details of black hole and space-time structure as well as dark energy reflects the endorsement of these activities in the NRC’s AAp decadal survey, Connecting Quarks, and Physics of the Universe8 reports. The roadmap’s emphasis on the Laser Interferometer Space Antenna (LISA) and Con-X missions is in line with priorities of the AAp survey as well as a recent NRC letter report.9

The roadmap discusses science-driven (not budget-driven) branch points and decision rules. The science criteria are designed to provide flexibility in defining, prioritizing, and designing future missions. There is clearly a need to maintain options to be addressed in future budget and scientific contexts.

In the Beyond Einstein program, there is provision for only one Beyond Einstein Probe mission before the middle of the 2020 decade. At present, missions of the scale of the Dark Energy Probe and the Inflation Probe cannot be funded. Such missions are needed to accomplish some of the specific high-priority science objectives (such as the Joint Dark Energy Mission in the Universe Exploration roadmap) and to enable a flexible response to new scientific opportunities.

The elements of NASA’s former Structure and Evolution of the Universe and Origins programs that have not been put into the Search for Earth-like Planets roadmap have been awkwardly incorporated into the Pathways to Life (PTL) theme in the Universe Exploration roadmap, so that they are represented by only one of the four primary roadmap objectives. The majority of forefront astronomy and astrophysics research endorsed by the AAp decadal survey is meant to be represented under this umbrella



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Review of Goals and Plans for NASA’s Space and Earth Sciences 4 Universe Exploration and the Search for Earth-like Planets The Universe Exploration roadmap and The Search for Earth-like Planets roadmap represent the two major research components of the Universe Division at NASA headquarters.1,2 These research components are supported by the most recent NRC astronomy decadal survey in this area and related reports.3-5 These two roadmaps make a strong case for exploring the fundamental physics associated with the beginning of the universe and the nature of space-time and for searching for Earth-like planets. They do not, however, present the most robust case possible for the suite of missions that address the important broad range of astrophysical questions that are at the forefront of astrophysical research but do not fall conveniently in the scope of the Beyond Einstein and the Search for Earth-like Planets programs as presented in these roadmaps (see the next two sections). It is imperative that a proper balance be struck along the complete spectrum of astrophysical research. The division of topics between these two roadmaps also tends to deemphasize the capability that some of the proposed missions critical to the search for Earth-like planets have to do broader astrophysical research. Finally, the partitioning into two roadmaps has deemphasized the value of shared technology, facilities, and infrastructure. UNIVERSE EXPLORATION ROADMAP: FROM THE BIG BANG TO LIFE The Beyond Einstein mission suite addresses exciting and fundamental physics questions designed to test whether or not there are observable limits to Einstein’s theory of gravity. A strong case is made for exploring the beginnings and evolution of the universeone of the 5 fundamental questions identified in the NRC’s Astronomy and Astrophysics in the New Millennium6 (the AAp decadal survey) and one of the 11 identified in Connecting Quarks with the Cosmos.7 A strong emphasis on studying the details of black hole and space-time structure as well as dark energy reflects the endorsement of these activities in the NRC’s AAp decadal survey, Connecting Quarks, and Physics of the Universe8 reports. The roadmap’s emphasis on the Laser Interferometer Space Antenna (LISA) and Con-X missions is in line with priorities of the AAp survey as well as a recent NRC letter report.9 The roadmap discusses science-driven (not budget-driven) branch points and decision rules. The science criteria are designed to provide flexibility in defining, prioritizing, and designing future missions. There is clearly a need to maintain options to be addressed in future budget and scientific contexts. In the Beyond Einstein program, there is provision for only one Beyond Einstein Probe mission before the middle of the 2020 decade. At present, missions of the scale of the Dark Energy Probe and the Inflation Probe cannot be funded. Such missions are needed to accomplish some of the specific high-priority science objectives (such as the Joint Dark Energy Mission in the Universe Exploration roadmap) and to enable a flexible response to new scientific opportunities. The elements of NASA’s former Structure and Evolution of the Universe and Origins programs that have not been put into the Search for Earth-like Planets roadmap have been awkwardly incorporated into the Pathways to Life (PTL) theme in the Universe Exploration roadmap, so that they are represented by only one of the four primary roadmap objectives. The majority of forefront astronomy and astrophysics research endorsed by the AAp decadal survey is meant to be represented under this umbrella

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Review of Goals and Plans for NASA’s Space and Earth Sciences theme of PTL, a move apparently motivated by an unnecessarily narrow interpretation of the vision for space exploration. A much broader vision of NASA’s science mission in the exploration vision is described by the president’s commission’s report A Journey to Inspire, Innovate, and Discover10 and the NRC report Science in NASA’s Vision for Space Exploration.11 A February 2005 NRC letter report explicitly echoes the president’s commission that “maintaining the breadth of the astronomy and astrophysics enterprise at NASA is consistent with the new exploration vision.”12 The designation “Pathways to Life” has no previous heritage in NASA or NRC documents. The panel recommends that the nomenclature “Pathways to Life” be abandoned and that NASA develop coherent new themes in the Universe Division that will do justice to the Beyond Einstein program, the Search for Earth-like Planets program, and all the other science highlighted in the AAp decadal survey that the roadmap has agglomerated into Pathways to Life, but that represents vibrant science deserving of separate emphasis. The current roadmap does not enunciate well all the transformative science objectives within the PTL theme and hence may not adequately follow NRC recommendations. For example, among the five fundamental questions outlined by the AAp decadal survey are, How do galaxies first arise and mature?, How are stars born and how do they live and die?, and How do planets form and change as they age? The Connecting Quarks report also lists understanding how the heavy elements were made as one of the key science questions for the new century. Dark matter is mentioned only briefly in the roadmap and gamma-ray bursts not at all, whereas these topics are significant elements of the AAp decadal survey and Connecting Quarks reports. Whether and how these areas of astrophysics are to be addressed in the NASA mission plan outlined in this roadmap are not made clear. This roadmap does not give a comprehensive PTL strategy with a well-sequenced PTL mission progression. Some areas of astrophysics (e.g., those with more tenuous connections to PTL) may thus be vulnerable to exclusion from planning in the Universe Division. The panel recommends that while maintaining appropriate objectives such as Beyond Einstein, the Search for Earth-like Planets, and other topics as distinct entities, NASA should always consider them in combination for purposes of strategic planning, technology development, and budgeting. This approach would allow a more effective statement of the science justification for the broadly capable missions and a more accurate assessment of technology needs and the costs of achieving Universe Division science objectives. The Universe Exploration roadmap was clearly constructed with a rigorously self-imposed, narrowly focused approach, with the intention of protecting a core suite of missions within a presumed flat budget. This approach led to omission of important science objectives as outlined above and restricted discussion of missions. The panel is concerned that the circumstances under which this roadmap and the Search for Earth-like Planets roadmap were produced (pressure to align the roadmaps to a narrow view of the objectives of the vision for space exploration, changes in direction to the roadmap teams during the task, time pressure, and absence of peer review) led to some emphasis on missions that were known to NASA roadmap committee participants but that did not necessarily reflect the consensus of the AAp decadal survey. This relatively narrow focus also led to some pressure to name and describe some notional missions (e.g., the Inflation Probes) but not others in an attempt to get “in the queue.” Planning on single-decade timescales has been productive scientifically, but on longer timescales it is quite difficult to plan without knowing what will have been learned and what technologies will have become available. Therefore suggestions by the roadmap committee for missions in the far term cannot be interpreted as current community endorsement of particular missions. The laudable strategy of preserving and supporting the Beyond Einstein mission line nevertheless marginalizes the PTL theme. All missions discussed in the roadmap in the PTL timeline are already in progress, apart from a notional “Pathways to Life Observatories” that appears two decades in the future. Beyond missions in the next decade, the roadmap balance between these two types of missions is not consistent with previous NRC recommendations. The scientific cases for the James Webb Space Telescope (JWST) and the Stratospheric Observatory for Infrared Astronomy (SOFIA) are only weakly connected to the PTL theme in the roadmap and are much more strongly articulated in the AAp survey.13

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Review of Goals and Plans for NASA’s Space and Earth Sciences That the roadmap committee was “circumspect about mentioning missions whose primary residence is in another roadmap”a is a direct result of the desire to support the Beyond Einstein program, but it threatens support for NASA missions (e.g., Space Interferometry Mission (SIM), Terrestrial Planet Finder-Coronagraph (TPF-C), Terrestrial Planet Finder-Interferometer (TPF-I)) that share technology challenges with this roadmap (see “Crosscutting Capability Issues,” below) and potentially support PTL themes through their broader astrophysical components beyond exo-planets.b The strong linkage of this roadmap to the Search for Earth-like Planets roadmap is mentioned, but it is not implemented in any practical way in the mission timeline. As stressed in an NRC letter report,14 general astrophysics applications of exo-planet missions are critical to enabling transformational science in a range of disciplines. The NRC has previously noted that the new focus on an accelerated and an exclusively exo-planet-driven design for TPF-C also has adverse effects on other Universe Division goals, including Beyond Einstein.15,16 These various problems result from “stove-piping” as reflected in both the Universe Exploration and the Search for Earth-like Planets roadmaps and are at least partly attributable to the current partitioning of the Universe Division program. The panel recommends that NASA support the exciting fundamental research outlined in this roadmap for the Beyond Einstein program, but also give due consideration to the transformative science objectives outlined by the AAp decadal survey and encompassed in the broad origins, evolution, and fate themes articulated by the President’s Commission on Implementation of United States Space Exploration Policy, which are not well enunciated in the Pathways to Life theme. A significant issue that this roadmap does not address is the future of the Hubble Space Telescope. In a recent report the NRC laid out a continuing science role for Hubble that includes both Beyond Einstein and PTL questions.17 The fate of Hubble is intimately connected to the fate of other NASA missions. The panel recommends that NASA’s planning take into account the high priority accorded to the Hubble Space Telescope in past NRC decadal survey reports and the recent NRC report Assessment of Options for Extending the Life of the Hubble Space Telescope.18 The AAp decadal survey report encouraged cooperation among agencies. Opportunities for relationships with non-NASA participants are already in place for some missions in the timeline (e.g., with the Department of Energy for the Gamma-ray Large Area Space Telescope (GLAST) and the Joint Dark Energy Mission (JDEM)), and others not mentioned in the roadmap are also possible.19 The AAp survey report also encourages international collaboration, and it cites a joint NRC-European Science Foundation report, U.S.-European Collaboration in Space Science, which addresses many practical aspects of collaboration.20 GLAST, LISA, and JWST are explicitly mentioned in this context, as well as the TPF connection to the Darwin mission in Europe. SEARCH FOR EARTH-LIKE PLANETS ROADMAP The Search for Earth-like Planets roadmap presents two science objectives of strong merit: (1) the search for and direct detection and characterization of Earth-like extrasolar planets and (2) the study of the formation and evolution of extrasolar planetary systems from stellar disks. The discovery of truly Earth-like planets around nearby stars would be transformational; the discovery of spectroscopic signatures of possible Earth-like life on planets around other stars would be revolutionary. Objective 2 is required to give the deepest understanding of the context of objective 1. The rationale for the mission suite of Kepler, SIM, TPF-C, and TPF-I and the notional Life Finder mission is well laid out and corresponds to the exo-planet investigation priorities defined in the AAp survey report. However, the important broad astrophysical applications of these missions, as outlined in that report, are not addressed in either this roadmap or the Universe Exploration roadmap. The a   Kathy Flanagan, reply to “Questions for Roadmap Briefers: Universe Division,” June 14, 2005. b   Kathy Flanagan, “Universe Exploration: From the Big Bang to Life,” presentation at Review of NASA Strategic Roadmaps: Science Panel, Washington, D.C., June 13, 2005.

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Review of Goals and Plans for NASA’s Space and Earth Sciences roadmap’s discussion and justification of missions related to the overall problem of star and planet formation, such as the current Spitzer Telescope and Hubble Space Telescope and the future JWST, SOFIA, and the Single Aperture Far-Infrared (SAFIR) mission, are much sparser and less well connected to community priorities than were the discussion and justification presented in the AAp decadal survey report. The panel notes that JWST was the top priority in the AAp decadal survey report for all ground-and space-based astronomy.21 The proposed mission line consists of a Discovery-class mission (Kepler) followed by a series of large, complicated, and expensive missions (SIM, TPF-C, TPF-I). The roadmap attempts to discuss the interdependencies of these missions and to introduce important decision points, but the only real options presented are (1) to cancel TPF-C and proceed directly with SIM and accelerate TPF-I if Kepler shows that (the frequency of habitable Earth-like planets around solar-type stars) is 0.01 or less and (2) “to solicit proposals for rapid development low-cost missions” if a very nearby terrestrial planet should be discovered. With a Kepler launch in 2008 and a determination of coming 4 years later in 2012, the first decision point will be moot, given the current schedule of the start of TPF-C phase A in 2006 and the goal of a launch in 2014. Similarly, the first scientific results from SIM will be too late to influence the design of TPF-C. The need for science precursor missions to drive the design of both TPF-C and TPF-I and the importance of the astrophysics goals of TPF as stated in the AAp decadal survey report were discussed in a recent NRC letter report.22 The roadmap shows little, if any, acknowledgment of these concerns. For example, the descoping of SIM that is currently under consideration by NASA would have strong negative effects on the value of TPF for areas of astrophysics beyond the mission’s goals for planet searching. The panel urges NASA to ensure that appropriate TPF precursor science is done and that astrophysics is well integrated into these planet-detection missions. The roadmap outlines an ambitious plan of large, expensive, and technologically challenging missions. However, the roadmap contains very little discussion of the current mission costs and technological challenges and milestones that must be met for each mission to be successful. The realism of the proposed mission timeline and the ability of the proposed missions to fit into the budget line are serious concerns. The panel recommends that broad-based community input be sought to guide decisions if any significant revision to the Search for Earth-like Planets roadmap mission sequence becomes necessary. A stronger case could have been made for research and missions that were not a part of the central mission line aimed at the detection of habitable terrestrial planets. The discussion of desired theoretical work is limited, as is the discussion of astrobiology and of the problem of integrated research on planet formation, star formation, and planetary system evolution. As an example, recent research has shown that some extrasolar planetary systems contain gas giants very close to the central star, a configuration very unlike our solar system. An important theme of extrasolar planetary system detection and characterization could be the determination of whether or not our solar system really is different from other demographically common solar systems. Appropriate research should focus on gaseous, icy, and water worlds in addition to terrestrial planets. The roadmap has little discussion of the important research on planetary system evolution with the current Hubble and Spitzer missions or that promised by future missions such as SOFIA, the Wide-field Infrared Survey Explorer (WISE), JWST, Herschel, and SAFIR. Similarly, the discussion of ground-based planet detection and characterization activities is limited primarily to the successes of radial velocity surveys, and it fails to include techniques that will be very important during the next decade such as Keck interferometric astrometry (using the Keck outrigger telescopes), other ground-based astrometry, planetary transit photometry, and microlensing observations, as well as detailed study and characterization of planet-host stars and potential TPF target stars. The panel recommends that NASA construct a balanced program that both seeks Earth-like planets and fosters research on broader issues of solar system formation and evolution in the context of realistic budget constraints and the full suite of facilities and techniques that are appropriate to this line of research. The roadmap acknowledges that TPF-I will probably be combined with the European Space Agency development of its Darwin mission, but there is no obvious plan to do this at an early stage. In

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Review of Goals and Plans for NASA’s Space and Earth Sciences fact, the roadmap lauds international competition rather than cooperation. Truly productive international cooperation must be very carefully fostered and nurtured from the earliest opportunity. The report U.S.-European Collaboration in Space Science advocates such cooperation.23 The panel recommends that international cooperation be part of the baseline planning for this research. Furthermore, the panel concluded that astrobiology was included in both the Universe Exploration and Search for Earth-like Planets roadmaps as an afterthought and should have been a more integral part of the roadmaps. CROSSCUTTING SCIENTIFIC ISSUES There are numerous crosscutting scientific opportunities with other roadmap areas. Taking advantage of these may result in more fruitful science and higher efficiency in the scientific programs. The Sun-Solar System Connection roadmap makes the strong case that the Sun and solar system are prime laboratories for fundamental plasma physics, especially magnetic field reconnection.24 The Stellar Imager has clear implications for stellar astrophysics. The solar heliosphere modulates cosmic rays that affect life on host planets and possibly affect climate through atmospheric ionization and influence on cloud cover. Thus the Interstellar Probe discussed in the Sun-Solar System Connection roadmap has obvious relevance to topics of the Universe Division and perhaps Earth science. The solar-system exploration and Mars programs are rife with crosscutting issues in astrobiology, comparative planetology, and the origin and evolution of star and planetary systems.25,26 The notional Biosignature mission mentioned in the Earth Science roadmap is of clear potential astrobiological significance, but was not considered at all in the Universe Exploration and Search for Earth-like Planets roadmaps, and was not given strong scientific motivation in the Earth Science roadmap.27 CROSSCUTTING CAPABILITY ISSUES Most of the capabilities needed to enable the missions presented in these two roadmaps are properly addressed even though these roadmaps did not benefit from the planned integration with NASA’s capability roadmaps. The Universe Exploration roadmap provides a sound outline of how to organize technology support so that adequate technology exploration and development of enabling technology can proceed to the point at which desired missions can continue on to flight development. This requires investment in research and analysis programs to explore new concepts, including opportunities for proof-of-concept observations from suborbital platforms, and funding for continued engineering development of concepts aligned with mission objectives to bring the technology to spaceflight readiness. Obtaining support for the second step has historically been difficult in the NASA program. The panel recommends that NASA invest in research and analysis programs to explore new concepts, including opportunities for proof-of-concept observations from suborbital platforms, with particular attention to funding for continued engineering development to bring the technology to spaceflight readiness. The Universe Exploration roadmap committee explicitly attempted to address “critical” technology needs and “avoided listing ‘ desirements’” that could replace requirements if they should appear in many roadmaps. Bearing that in mind, the following issues arise: Two roadmaps with overlapping capability needs may leave the impression that the development costs are more expensive than is actually the case. The use of different document structures and presentation styles in these two roadmaps makes interpreting the common needs more difficult. Little attention is given to the potential use of infrastructure and other capabilities now being planned by the Exploration Systems Mission Directorate. These resources may include in-space

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Review of Goals and Plans for NASA’s Space and Earth Sciences construction by humans or robots and in-space depot and servicing facilities, the availability of which could substantially change the engineering and operational approaches chosen for the missions outlined in the two roadmaps. These capabilities could also enable servicing to extend the useful life of future missions and to permit periodic upgrades of instrumentation. Future in-space capability could also eventually provide in-space integration and testing, thereby avoiding the problems associated with Earth’s gravity and atmosphere. There is no attention to the timing of the capability development so that there can be high confidence that technical capabilities will be available sufficiently before mission design to allow their full integration. The panel recommends an assessment of the trade space between launch vehicle lift mass, fairing size, length of mission, aperture size and weight, detector quantum efficiency, and other factors to ensure that the mission options for these two roadmaps are fully understood. The panel recommends that NASA develop plans to use the long-range technology developed in the Exploration Systems Mission Directorate in service of the goals of the Science Mission Directorate. REFERENCES 1. National Aeronautics and Space Administration (NASA), Advanced Planning and Integration Office. 2005. Universe Exploration: From the Big Bang to Life. NASA, Washington, D.C. Available at <www.hq.nasa.gov/office/apio/pdf/universe/universe_roadmap.pdf>. 2. NASA, Advanced Planning and Integration Office. 2005. The Search for Earth-like Planets. NASA, Washington, D.C. Available at <www.hq.nasa.gov/office/apio/pdf/earthlike/earthlike_roadmap.pdf>. 3. National Research Council (NRC). 2001. Astronomy and Astrophysics in the New Millennium. National Academy Press, Washington, D.C. 4. NRC. 2003. Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century. The National Academies Press, Washington, D.C. 5. National Science and Technology Council. 2004. Physics of the Universe. Office of Science and Technology Policy. Available at <www.ostp.gov/html/physicsoftheuniverse2.pdf>. 6. NRC. 2001. Astronomy and Astrophysics in the New Millennium. 7. NRC. 2003. Connecting Quarks with the Cosmos. 8. National Science and Technology Council. 2004. Physics of the Universe. 9. NRC. 2005. “Review of Progress in Astronomy and Astrophysics Toward the Decadal Vision: Letter Report,” letter from C. Megan Urry, chair of the Committee to Review Progress in Astronomy and Astrophysics Toward the Decadal Vision to Mr. Alphonso V. Diaz, NASA Associate Administrator for Science, and Dr. Michael S. Turner, National Science Foundation Assistant Director. February 11. The National Academies Press, Washington, D.C. 10. President’s Commission on Implementation of United States Space Exploration Policy. 2004. A Journey to Inspire, Innovate, and Discover. Available at <govinfo.library.unt.edu/moontomars/>. 11. NRC. 2005. Science in NASA’s Vision for Space Exploration. The National Academies Press, Washington, D.C. 12. NRC. 2005. “Review of Progress in Astronomy and Astrophysics Toward the Decadal Vision: Letter Report,” p. 11. 13. NRC. 2001. Astronomy and Astrophysics in the New Millennium. 14. NRC. 2004. “Review of Science Requirements for the Terrestrial Planet Finder: Letter Report,” letter from Wendy L. Freedman, chair of the SSB/BPA Panel to Review the Science Requirements for the Terrestrial Planet Finder, to Anne L. Kinney, director of the Universe Division,

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Review of Goals and Plans for NASA’s Space and Earth Sciences Science Mission Directorate, NASA Headquarters, September 23. Available at <www.nap.edu/html/terrestrial/NI000566.pdf>. 15. NRC. 2005. “Review of Progress in Astronomy and Astrophysics Toward the Decadal Vision: Letter Report.” 16. NRC. 2004. “Review of Science Requirements for the Terrestrial Planet Finder: Letter Report.” 17. NRC. 2005. Assessment of Options for Extending the Life of the Hubble Space Telescope. The National Academies Press, Washington, D.C. 18. NRC. 2005. Assessment of Options for Extending the Life of the Hubble Space Telescope, p. 7. 19. NRC. 2001. Astronomy and Astrophysics in the New Millennium. 20. NRC. 1998. U.S.-European Collaboration in Space Science. National Academy Press, Washington, D.C. 21. NRC. 2001. Astronomy and Astrophysics in the New Millennium. 22. NRC. 2004. “Review of Science Requirements for the Terrestrial Planet Finder: Letter Report.” 23. NRC. 1998. U.S.-European Collaboration in Space Science. 24. NASA, Advanced Planning and Integration Office. 2005. Sun-Solar System Connection. NASA, Washington, D.C. Available at <www.hq.nasa.gov/office/apio/pdf/sun/sun_roadmap.pdf>. 25. NASA, Advanced Planning and Integration Office. 2005. SRM 3—The Solar System Exploration Strategic Roadmap. NASA, Washington, D.C. Available at <www.hq.nasa.gov/office/apio/pdf/solar/solar_roadmap.pdf>. 26. NASA, Advanced Planning and Integration Office. 2005. Robotic and Human Exploration of Mars. NASA, Washington, D.C. Available at <www.hq.nasa.gov/office/apio/pdf/mars/mars_roadmap.pdf>. 27. NASA, Advanced Planning and Integration Office. 2005. Exploring Our Planet for the Benefit of Society: NASA Earth Science and Applications from Space Strategic Roadmap. NASA, Washington, D.C. Available at <www.hq.nasa.gov/office/apio/pdf/earth/earth_roadmap.pdf>.