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Nasa ’s Beyond Einstein Program: An Architecture for Implementation 5 Findings and Recommendations ASSESSING THE BEYOND EINSTEIN MISSIONS NASA and the Department of Energy (DOE) have requested that the National Research Council (NRC) assess the Beyond Einstein missions, with the following charge: Assess the five proposed Beyond Einstein missions (Constellation-X, Laser Interferometer Space Antenna, Joint Dark Energy Mission, Inflation Probe, and Black Hole Finder Probe) and recommend which of these five should be developed and launched first, using a funding wedge that is expected to begin in FY 2009. The criteria for these assessments include: Potential scientific impact within the context of other existing and planned space-based and ground-based missions; and Realism of preliminary technology and management plans, and cost estimates. Assess the Beyond Einstein missions sufficiently so that they can act as input for any future decisions by NASA or the next Astronomy and Astrophysics Decadal Survey on the ordering of the remaining missions. This second task element will assist NASA in its investment strategy for future technology development within the Beyond Einstein Program prior to the results of the Decadal Survey. Many NRC panels are tasked to judge scientific excellence within a single scientific discipline. NASA’s Beyond Einstein Program is designed to be at the intersection of physics and astronomy and is a subset of each discipline. Therefore, the Committee on NASA’s Beyond Einstein Program: An Architecture for Implementation had to take into account the goals of two scientific disciplines and their methods of working. Responding to the charge, the committee also based its conclusions on a second dimension: the technical and scientific readiness of the proposed missions. To deal with its complex charge, the committee included not only members who are experts in both physics and astronomy, but also individuals with great experience in spacecraft development and program implementation. The blend between scientists and engineers has led to an extraordinarily vigorous and productive assessment effort. A necessary tension between scientific attraction and timely implementation has been at the center of all the committee’s discussions.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation The five mission areas in NASA’s Beyond Einstein Program plan are in very different stages of technical development. Some of the mission candidates have been under study for more than 10 years, while others are at an early phase of conceptual design. Each mission candidate has its own balance of interest to the astronomy and physics research communities. The committee considered them all in as objective and transparent a way as possible, even if there is no perfectly commensurate basis for comparison. The committee recommends one mission area to be implemented first, but as noted in Chapter 2, each area makes an important contribution to Beyond Einstein research goals. Each mission area needs to receive appropriate support in order to prepare for consideration by NASA and the next astronomy and astrophysics decadal survey of the National Research Council. Some specific suggestions for providing such support are contained in the subsection below entitled “Beyond Einstein Cost Assessment Summary.” The committee considered many ways to approach the intertwined scientific, engineering, and programmatic issues implied by its charge, and it has endeavored to respond to its entire charge as faithfully as possible. The committee firmly believes that, while the statement of task required it to recommend one mission area for a fiscal year (FY) 2009 new start, all of the Beyond Einstein mission areas address key scientific questions that take physics and astronomy beyond where the century of Einstein left them. Furthermore, the scientific issues are so compelling that Beyond Einstein research will be pursued for many years to come. Therefore, the committee responds to the task in the conviction that it is recommending the first element of an enduring program, and not the only and last mission in Beyond Einstein science. How the Recommendations Evolved The committee started with systematic consideration of each of the 11 mission candidates identified thus far in the five mission areas in the Beyond Einstein Program. Since the task of the committee was to select one of the five mission areas, rather than one of the 11 potential mission candidates, the mission candidates were considered only as representatives of the capabilities that could be provided by a mission area. The committee was aware that NASA typically makes a broad request for proposals in order to encourage the most up-to-date scientific strategies and technological approaches. The committee heard at least two presentations from each mission candidate team, in addition to presentations from individual scientific leaders, and had conversations with the broader scientific community in town hall meetings across the United States (see Appendixes C and D). Subsequently, the committee asked clarifying questions of each team and included the team’s written responses in the assessment process. Agency leaders in NASA, DOE, and the European Space Agency (ESA) provided additional presentations. Using these inputs, the committee assessed each mission candidate for its scientific excellence, its response to Beyond Einstein goals, its competition from other space- and ground-based projects in the United States and abroad, its scientific and engineering complexity, its cost and related programmatic implications, its stage of development and overall readiness, and pertinent individual factors. In making its recommendations, the committee considered the potential contribution of ground-based capabilities to address the scientific questions posed to the Beyond Einstein Program. The committee assumed that existing and proposed ground-based capabilities such as the Large Synoptic Survey Telescope and the Laser Interferometer Gravitational Wave Observatory (both supported by the National Science Foundation [NSF]) will be funded and operated as planned. While it is impossible to predict what discoveries will be made by ground-based systems, the projected performance of ground-based systems was compared with the expected performance of Beyond Einstein missions. This assessment culminated in draft individual write-ups for each mission candidate. The committee carried out these steps before any formal discussion of the first part of its charge. The committee gave each mission candidate its full attention and developed a balanced view of the entire Beyond Einstein Program before addressing its main charge. Only after the drafting of the broad assessment of each mission as required in the second half of the charge did the committee start a comparative discussion to identify the main competitors for the FY 2009 start.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation The Beyond Einstein Program The committee found that all five Beyond Einstein mission areas contain scientifically pioneering, publicly appealing, and technically challenging mission candidates. As discussed in detail in Chapter 2, the committee assessed the 11 mission candidates according to their contributions to Beyond Einstein science and their broader scientific impact. For both Beyond Einstein science and the broader scientific impact, the committee assessed three factors: the potential for revolutionary science, science readiness and risk, and mission uniqueness. Chapter 3 contains the committee’s assessment of technical readiness for each of the 11 mission candidates. After the scientific and technical assessments of all five mission areas were completed, two stood out for the directness with which they address Beyond Einstein goals and their potential for broader scientific impact: the Laser Interferometer Space Antenna (LISA) and the Joint Dark Energy Mission (JDEM). The committee was unanimous that in fulfilling its charge, it should choose between these two. To put this result and the findings in the next subsection in context, the committee’s assessments of each of the Beyond Einstein areas are briefly recapitulated here. The proposed Black Hole Finder Probe (BHFP) mission candidates seek to detect thousands of hard x-ray sources and to determine the population distribution of massive black holes in external galaxies and of the more luminous x-ray binaries in our own Galaxy. The Inflation Probe (IP) mission candidates seek to study for the first time the conditions in the early universe when it suddenly expanded by 30 orders of magnitude, creating the particle populations that led to the particles and radiation observed today in the present universe. These two mission areas address important Beyond Einstein questions. However, because of scope and technical readiness issues, they fell behind the two leaders in the discussion. The Constellation-X (Con-X) mission candidate has been designed to be a general-purpose astrophysical observatory. It will unquestionably enable important progress in many fields of astrophysical research. Its broad significance to astronomy is highlighted by the fact that Con-X was second in priority to the James Webb Space Telescope (JWST), now under construction, in the 2001 astronomy and astrophysics decadal survey.1 The contributions of Con-X to Beyond Einstein science, though not the principal drivers of the mission design, will be significant, but not as decisive as the contributions of the two leaders. Con-X is a very well developed mission, and at present there exists a large pool of x-ray astronomers and technical expertise for building and using Con-X. One concern is that this workforce may dissipate if the construction of Con-X is delayed indefinitely; also, the very broad scientific contributions of Con-X, both within and beyond the Beyond Einstein areas, would be postponed. Similar concerns apply to potential delays in most mission projects. LISA is an extraordinarily original and technically bold mission concept. The first direct detection of low-frequency gravitational waves will be a momentous discovery, of the kind that wins Nobel Prizes. The mission will open up an entirely new way of observing the universe, with immense potential to enlarge understanding of both physics and astronomy in unforeseen ways. LISA could be the first to detect gravitational waves from the merger of massive black holes in the centers of galaxies or stellar clusters at cosmological distances, as well as waves generated by stellar mass compact objects as they orbit and fall into massive black holes. An optical identification of such sources would provide an absolute measurement of dark energy. If the committee’s charge had been to design a complete multiyear, multimission program addressing comprehensive Beyond Einstein goals, LISA would have been its flagship mission. Any leadership program addressing Beyond Einstein goals must have a state-of-the-art investigation of dark energy. With any mission clarifying previously unknown properties of 70 percent of the mass-energy in the universe, the potential for fundamental advancement of both astronomy and physics is quite high. For the United States, that mission will be the winner of the JDEM competition. Based on the mission candidates reviewed thus far, JDEM will set the standard in the precision and technical reliability of its determination of the distribution of dark energy in the distant universe. The key dark energy parameter will be measured with an improvement of at least a factor of 10 over today’s precision and is likely to exceed the precision attainable by the projects that will be completed in the next decade. Space observations have the potential to collect more data with fewer instrument 1 National Research Council, 2001, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation uncertainties than currently foreseen ground observations, so that a JDEM mission should be a technically secure platform for whatever comes after it in dark energy science. A JDEM mission would bring substantial benefits to general astronomy. All three JDEM mission candidates propose very large surveys by meter-class infrared (IR) space telescopes. Each proposes to collect an unprecedented volume of data, which would enrich the understanding of many topics in extragalactic astronomy, and especially galaxy formation and evolution. After the Hubble Space Telescope (HST) retires, there will be no diffraction-limited optical or near-IR telescope in space. The low backgrounds and large fields of view (FOVs) offered by two of the JDEM candidates would provide the largest quantity thus far of highly detailed information for understanding how galaxies form and acquire mass. The goal of determining the distribution of dark energy with unprecedented precision would drive astronomers’ understanding of supernovas and weak-lensing systematics to new levels of precision. There has never been a full-sky spectroscopic survey from space, so the broad discovery potential enabled by this third candidate approach to dark energy determination would be very large. The committee notes that the report Connecting Quarks with the Cosmos2 strongly supported both JDEM and Con-X. The report Astronomy and Astrophysics in the New Millennium3 ranked Con-X as the second-highest-priority new major space initiative (after JWST). However, no prioritization of all the Beyond Einstein missions against one another has ever been done. The committee compared the JDEM mission concepts with the future ground-based Beyond Einstein-type initiatives known to it (see the subsections “Scientific Context” and “Science Readiness and Risk” in Chapter 2). While some duplication in measurements was identified for the relevant time frame and while ground results are expected to improve over time, ground-based measurements will find difficulty competing with the sensitivity and volume of space measurements. Ground- and space-based measurements in combination, however, were found to be complementary. The success of LISA depends on the reliable operation of several critical technologies. One relates to the LISA proof masses that respond to gravitational waves and must be protected from nongravitational disturbances. Electrostatic sensors have to locate the proof mass and signal low-force micronewton thrusters to nudge the spacecraft and keep the proof mass at the center of its chamber in a purely gravitational orbit. The ESA, in collaboration with NASA, will launch a one-spacecraft LISA Pathfinder in late 2009 to evaluate in space the precision and reliability of the disturbance-reduction system. Assessing this technical risk is a precursor to ESA’s decision to proceed with the three-spacecraft LISA mission jointly with NASA. The LISA Pathfinder results will only be available after 2009, and a decision to propose a 2009 new start in the U.S. budgetary process would have to be made in the absence of Pathfinder results. The committee believes that it is more responsible technically and financially, and therefore more credible, to delay a decision on a LISA new start until after the results of the Pathfinder are taken into account. As discussed in the next major section, “Moving Forward with the Beyond Einstein Program,” it would be prudent for NASA to invest now in further LISA risk reduction and technology development, to help ensure that NASA is in a position to proceed with ESA to a formal LISA new start at the earliest opportunity after the Pathfinder flight. The JDEM mission candidates proposed thus far, while by no means routine, are based on instrument and spacecraft technologies that either have been flown in space or have been developed in other programs. In some ways, they have had their “pathfinders” already. These precursors give the committee confidence that a JDEM mission selected in 2009 could proceed smoothly to a timely and successful launch. Nonetheless, because the field of dark energy is developing rapidly, a request for proposals that is open to a broad range of mission concepts is advisable. The committee, mindful of its responsibility to the entire Beyond Einstein Program, is satisfied that a JDEM mission, given its fundamental significance and broad astronomical applicability, would be an excellent way to launch a new program of research that can produce important results for decades to come. 2 National Research Council, 2003, Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century, The National Academies Press, Washington, D.C. 3 National Research Council, 2001, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation Major Findings In light of the considerations summarized above and described in considerably more detail in the preceding chapters, the committee offers the following major findings and recommendations. The findings are not listed in order of priority, but rather in a sequence that conveys the committee’s reasoning. Finding 1. The Beyond Einstein scientific issues are so compelling that research in this area will be pursued for many years to come. All five mission areas in NASA’s Beyond Einstein Program address key questions that take physics and astronomy beyond where the century of Einstein left them. Finding 2. The Constellation-X mission will make the broadest and most diverse contributions to astronomy of any of the candidate Beyond Einstein (BE) missions. While Con-X can make strong contributions to Beyond Einstein science, other BE missions address the measurement of dark energy parameters and tests of strong-field general relativity in a more focused and definitive manner. Finding 3. Two mission areas stand out for the directness with which they address Beyond Einstein goals and their potential for broader scientific impact: LISA and JDEM. Finding 4. LISA is an extraordinarily original and technically bold mission concept that will open up an entirely new way of observing the universe, with immense potential to enlarge the understanding of physics and astronomy in unforeseen ways. LISA, in the committee’s view, should be the flagship mission of a long-term program addressing Beyond Einstein goals. Finding 5. The ESA-NASA LISA Pathfinder mission that is scheduled for launch in late 2009 will assess the operation of several critical LISA technologies in space. The committee believes that it is more responsible technically and financially to propose a LISA new start after the Pathfinder results are taken into account. In addition, Pathfinder will not test all technologies critical to LISA. Thus, it would be prudent for NASA to invest further in LISA technology development and risk reduction, to help ensure that NASA is in a position to proceed with ESA to a formal new start as soon as possible after the LISA Pathfinder results are understood. Finding 6. A JDEM mission will set the standard in the precision of its determination of the distribution of dark energy in the distant universe. By clarifying the properties of 70 percent of the mass-energy in the universe, JDEM’s potential for the fundamental advancement of both astronomy and physics is substantial. A JDEM mission will also bring important benefits to general astronomy. In particular, JDEM will provide highly detailed information for understanding how galaxies form and acquire their mass. Finding 7. The JDEM candidates identified thus far are based on instrument and spacecraft technologies that either have been flown in space or have been extensively developed in other programs. A JDEM mission selected in 2009 could proceed smoothly to a timely and successful launch. Finding 8. The present NASA Beyond Einstein funding wedge alone is inadequate to develop any candidate Beyond Einstein mission on its nominal schedule. However, both JDEM and LISA could be carried out with the currently forecasted NASA contribution if DOE’s contribution that benefits JDEM is taken into account and if LISA’s development schedule is extended and funding from ESA is assumed. Principal Recommendations Recommendation 1. NASA and DOE should proceed immediately with a competition to select a Joint Dark Energy Mission for a 2009 new start. The broad mission goals in the request for proposals should be (1) to determine the properties of dark energy with high precision and (2) to enable a broad range of astronomi-
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation cal investigations. The committee encourages the agencies to seek as wide a variety of mission concepts and partnerships as possible. Recommendation 2. NASA should invest additional Beyond Einstein funds in LISA technology development and risk reduction to help ensure that the agency is in a position to proceed in partnership with ESA to a new start after the LISA Pathfinder results are understood. Recommendation 3. NASA should move forward with appropriate measures to increase the readiness of the three remaining mission areas—Black Hole Finder Probe, Constellation-X, and Inflation Probe—for consideration by NASA and the next NRC decadal survey of astronomy and astrophysics. MOVING FORWARD WITH THE BEYOND EINSTEIN PROGRAM Summary Assessment of the Beyond Einstein Mission Set As indicated at the beginning of this chapter, the second task element of the committee’s charge was to “[a]ssess the Beyond Einstein missions sufficiently so that they can act as input for any future decisions by NASA or the next Astronomy and Astrophysics Decadal Survey on the ordering of the remaining missions.” This task element was intended to “assist NASA in its investment strategy for future technology development within the Beyond Einstein Program prior to the results of the [astronomy and astrophysics] Decadal Survey.” The committee’s assessment of the 11 mission candidates considered scientific importance, technical readiness, and probable cost. The candidates for JDEM, the committee’s first-priority mission area, need continued funding until NASA and DOE conduct a competition and selection for a JDEM. Furthermore, the committee believes that the competition to select a JDEM should be open to other mission concepts, launch opportunities, measurement techniques, and international partnerships. Additionally, LISA needs continued support until NASA initiates a post Pathfinder mission start for LISA. The scientific importance of the remaining three mission areas—Black Hole Finder Probe, Constellation-X, and Inflation Probe—was also all assessed by the committee as making an important contribution toward answering the Beyond Einstein questions as well as to other important issues in physics and astronomy. These mission areas warrant funding for technology development between now and the next astronomy and astrophysics decadal survey, although this funding may not fit into the Beyond Einstein funding wedge used in this assessment. Con-X has the potential to make enormously broad contributions to many areas of astronomy and physics. However, Beyond Einstein research is not its sole justification or its primary benefit to the science community. Although the funding would not fit within the current Beyond Einstein budget profile, an aggressive program of technology development should be continued for Con-X to prepare for a new start in the next decade if Con-X is ranked highly by the next astronomy and astrophysics decadal survey (as it was by the previous decadal survey4). The remaining BHFP and IP mission areas are most appropriately funded through other sources, such as the Astrophysics Research Grants Program, at least at the level needed to enable the mission teams to be competitive in the upcoming astronomy and astrophysics decadal survey. Beyond Einstein Cost Assessment Summary In order to evaluate the realism of the mission teams’ current cost estimates for their respective Beyond Einstein candidates, the committee developed an independent estimate and assessed the probable cost range for each mission. The committee assessment of the probable cost range for each candidate mission was also compared with previous missions of similar scope and complexity. The mission team’s estimate and the committee’s assessment of the probable cost range for each candidate Beyond Einstein mission are provided in Chapter 3. While not 4 National Research Council, 2001, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation exacting, the committee’s assessment indicates higher costs and longer schedules than currently estimated by the mission teams. As presented in the preceding section, the committee recommends that JDEM start development with the Beyond Einstein funding wedge that starts in FY 2009 and that NASA continue critical technology development for LISA to be ready for the results of the LISA Pathfinder mission. In addition to the probable cost range, the committee assessed the most probable development funding profile required for each of the candidate missions against the NASA Beyond Einstein funding wedge and used these data to assess how the JDEM and LISA profiles fit within this wedge. DOE expects to cofund JDEM up to approximately $400 million, and ESA is planning $500 million for LISA.5,6 The committee’s assessment showed that JDEM is the only mission that could be developed on its nominal schedule within the NASA Beyond Einstein funding wedge, based on the assumed DOE contribution. With a compatible fiscal year funding profile from DOE and ESA, or by adjusting the JDEM and LISA development schedules to better fit the NASA funding wedge, these missions could be carried out within the currently forecasted NASA contribution. The committee assessed two scenarios that could enable the recommended JDEM and LISA developments. The only quantitative funding profile data provided to the committee were those of the NASA Beyond Einstein funding wedge for FY 2009-FY 2012. Therefore, in order to analyze the scenarios, the committee extrapolated this profile through the development and launch of JDEM and LISA. For Scenario A, Figure 5.1 clearly shows that, based on the committee’s assessment, starting JDEM development in FY 2009 and launching in FY 2015 will not fit within the current NASA Beyond Einstein funding wedge, nor will it support concurrent NASA funding for critical technology development for LISA. Further, given the large mismatch between the probable JDEM budget fiscal year requirements and the available funding wedge, this will be the case even with no investment in LISA during the FY 2009-FY 2011 time frame. For Scenario B, Figure 5.2 shows that by delaying the full start of JDEM by 2 years and LISA until FY 2014, JDEM and LISA could fit within the committee’s forecast of the Beyond Einstein funding wedge. The committee does not recommend that this profile necessarily be followed, and leaves the program implementation to the agencies involved. This scenario is provided as evidence that there is at least one reasonable scenario for implementing the committee’s recommendations within the NASA Beyond Einstein funding wedge. According to information provided to the committee by DOE, DOE funds are expected to cover 7 years and to support a 2-3 year JDEM research and development phase and a 4-5 year construction phase.7 Depending on the fiscal year funding profile, the DOE contribution could enable a JDEM start date closer to and possibly in FY 2009. ESA told the committee that its funding was expected to be able to support a 2018 launch and therefore could be expected to be able to support a more aggressive LISA development schedule than the NASA budget alone would, possibly as early as FY 2014. Summaries of Mission Readiness This subsection summarizes the committee’s assessment of the scientific and technical readiness to begin development in FY 2009 toward the launch of the missions for each of the Beyond Einstein candidate mission areas: Black Hole Finder Probe, Constellation-X, Inflation Probe, JDEM, and LISA. As discussed below, the committee strongly believes that the future technology investment is required and warranted in all of the Beyond Einstein mission areas. The current Beyond Einstein budget profile will not support technology development beyond JDEM and LISA. In particular, the committee believes that after funding to start JDEM, the next-highest priority for funding from the funding wedge is for acceleration of the maturation of mission-critical LISA technologies that 5 Kathy Turner, Program Manager, Office of High Energy Physics, Department of Energy, “Note to BEPAC Regarding DOE’s JDEM Plans,” e-mail communication, March 30, 2007. 6 European Space Agency LISA budget data provided to the committee by David Southwood, ESA Director of Science, in discussions on ESA’s Astrophysics and Fundamental Physics Program, April 5, 2007. 7 Kathy Turner, Program Manager, Office of High Energy Physics, Department of Energy, “Note to BEPAC Regarding DOE’s JDEM Plans,” e-mail communication, March 30, 2007.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation FIGURE 5.1 Scenario A: Cost to NASA of the committee-recommended program versus the projected NASA Beyond Einstein budget. NOTE: JDEM, Joint Dark Energy Mission; LISA, Laser Interferometer Space Antenna. are currently at low Technology Readiness Levels (TRLs). Technology development for the other mission areas should continue to be supported in the broader astrophysics program. Black Hole Finder Probe Science Readiness Assessment Summary The Black Hole Finder Probe (BHFP) is one of the three Einstein Probes discussed in the original Beyond Einstein roadmap published in 2003. BHFP is designed to find black holes on all scales, from one to billions of solar masses. BHFP will observe high-energy x-ray emission from accreting black holes and explosive transients and address a key Beyond Einstein question, How did black holes form and grow? As described in Chapter 2, BHFP will be unique among current or planned missions in high-energy x-ray sensitivity combined with a large FOV and frequent coverage of the sky. The resulting hard x-ray sky maps, temporal variability data, and the large number of short-lived transient detections will directly impact a number of important astrophysical questions. BHFP will provide a unique window into the properties and evolution of astronomical objects whose physics is dominated by strong gravity. The committee was presented with two proposed missions, EXIST (Energetic X-ray Imaging Survey Telescope) and CASTER (Coded Aperture Survey Telescope for Energetic Radiation). These two missions are both wide-field coded-aperture hard x-ray survey telescopes, differing primarily in their selection of detector material.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation FIGURE 5.2 Scenario B: Cost to NASA of the committee-recommended program phased to fit within the projected NASA Beyond Einstein budget. NOTE: JDEM, Joint Dark Energy Mission; LISA, Laser Interferometer Space Antenna. The committee notes that the BHFP, as embodied in EXIST, is the only Einstein Probe that was specifically recommended in the 2001 decadal survey report, Astronomy and Astrophysics in the New Millennium.8 The science risk for BHFP is rather high (see the section “Black Hole Finder Probe” in Chapter 2). Although a census of massive black holes in galaxies can be achieved, only very-high-luminosity and high-mass black holes will be seen at high redshifts. In addition, the very uncertain conversion from x-ray luminosity to black hole growth rate implies that BHFP will not provide a unique value (to better than a factor of 10) of the black hole growth rate (e.g., in solar masses per year) in any individual galaxy or even in the entire universe. Finally, the difficulty in identifying host galaxies also yields significant risk in the interpretation of BHFP results. Both multiwavelength observational data and theoretical advances (e.g., in black hole accretion modeling) will be necessary for BHFP to realize its full scientific potential. Technical Readiness Assessment Summary CASTER and EXIST have both obtained program management and institutional support. CASTER has more technology maturity challenges, as the detector technology in general is at lower TRLs than that for EXIST, as discussed in Chapter 3. The large area of solid-state detectors and the enormous number of electronic readout channels will be a major implementation challenge for EXIST. Both programs have experienced instrument development teams and good risk-mitigation plans; however, more detailed design studies are needed to enable quantitative studies of how to reduce cost by reducing scope. The committee concludes that continued funding from the NASA Astrophysics Research Grants Program for detector development is consistent with the time scale for this mission and that the technology is sufficiently mature to allow an early selection of a single technology for a hard x-ray survey telescope. 8 National Research Council, 2001, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation The overall mission costs for both of the BHFP mission candidates are higher than originally envisioned. BHFP was originally proposed as one of the three Einstein Probes in the Beyond Einstein roadmap. These were envisioned as medium-scale missions that could be executed much more rapidly, and for considerably less money, than the flagship LISA and Con-X missions. However, the BHFP probe concepts now have costs that the mission teams estimate are in the vicinity of a billion dollars, and a much higher independent assessment of their probable cost range is described in Chapter 3; they are quite massive spacecraft that require expensive launch vehicles in the Atlas V class. The trade-off of sensitivity, detector area, and observing time should be carefully considered, and a smaller telescope should be studied to find less expensive ways to carry out the most important BHFP science within a smaller cost envelope. Constellation-X Science Readiness Assessment Summary As described in the section “Constellation-X” in Chapter 2, the committee’s assessment is that Con-X’s primary strength is in very high-spectral-resolution, high-throughput x-ray spectroscopy, representing an increase in these capabilities of roughly two orders of magnitude over missions currently flying. Although the capabilities of Con-X represent an evolution of x-ray satellite technology, Con-X’s very large collecting area and high-resolution spectrometry capability could lead to fundamental discoveries. In addition to the chances for serendipitous discoveries, the Con-X general observer program will harness the ingenuity of the entire astronomical community. The committee believes that, because of its heritage, Con-X does not involve a significant risk to being able to accomplish the planned key science project goals or to providing the x-ray community with a highly productive next-generation general-observer x-ray facility capable of both fundamental and serendipitous discoveries. Technical Readiness Assessment Summary Con-X is one of the best studied and tested of the missions presented to the committee. Much of this can be attributed to the heritage of the program management, flight technology, strong community support, and, finally, significant resources for technology and mission development. Aside from the well-known risks of satellite implementation, a number of technical risks have been called out by the Con-X candidate mission team and also discussed in Chapter 3. Chief among these is the achievement of the needed mirror angular resolution and the development of the position-sensitive microcalorimeters. The Con-X project has reasonable plans to mature both of these technologies, and, given adequate resources and time, there is little reason to expect that these technologies will limit the main science goals of the observatory. The committee notes that the technological requirements to achieve the mission goal appear to have been purposely kept conservative. The positive side is that the path to achieving the requirements (such as an angular resolution of ~15 arcsec) is well defined. The significant progress achieved both at the laboratories and by university-based groups indicates that a more aggressive influx of resources in key areas such as the mirror development, staged cooler system, and large microcalorimeter arrays will be of significant benefit to developments in these areas. Con-X development activities need to continue aggressively in areas such as achieving the mirror angular resolution, cooling technology, and x-ray microcalorimeter arrays to improve the Con-X mission’s readiness for consideration in the next astronomy and astrophysics decadal survey. The committee, however, does not believe that the current Beyond Einstein NASA funding wedge should fund these activities. Beyond Einstein is not the sole justification for Con-X, as its primary science capabilities support a much broader research program. Inflation Probe Science Readiness Assessment Summary “Inflation,” the term for an exponential expansion that, according to the Inflationary Big Bang Model, took place in an early era of the history of the universe, was proposed in order to solve several fundamental problems in cosmology. During the inflationary era, matter and radiation were created in the universe. The accelerating expansion that occurred during the era of inflation may have similarities with the accelerating expansion that is occurring today and that is attributed to the presence of dark energy throughout the
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation universe. A deeper understanding of the cause of inflation and dark energy is needed in order to allow an exploration of that similarity. Studying inflation may lead to an understanding of the source of the largest structures in the universe, which appear to be linked to quantum fluctuations and phenomena at the smallest scales. An understanding of the inflationary period would give profound insights into both physics and astronomy. Understanding this era is central to the Beyond Einstein goals. The Inflation Probe directly addresses the specific Beyond Einstein question, What powered the big bang? The theoretical framework for understanding the results of both the cosmic microwave background (CMB) and high-redshift galaxy observations is already in place. The observations made by the Inflation Probe will fit readily into models of the universe and provide useful constraints on cosmological parameters. The committee assessed four candidate IP missions for the Beyond Einstein Program. The science and measurement techniques for these probes are discussed in Chapter 2. Three of these are aimed at learning about the inflationary period using the signal imparted on the polarization of the CMB radiation by gravity waves induced during the inflationary period; the fourth candidate mission uses the effect that the inflation potential has on the primordial density-fluctuation power spectrum that describes the amount of structure in the universe at various length scales. The specific IP missions assessed by the committee are these: CMB experiments: Experimental Probe of Inflationary Cosmology (EPIC-F), which employs six 30-cm telescopes, each at a different frequency band, with a total of 830 bolometer detectors; Einstein Polarization Interferometer for Cosmology (EPIC-I), which is a Fizeau interferometric instrument with a synthesized beam resolution of 1° and 1,024 detectors; and Cosmic Microwave Background Polarimeter (CMBPol), which uses about 1,000 bolometers and has a spatial resolution of about 1°. Cosmic Inflation Probe (CIP), which consists of a 1.8-meter cooled telescope with a slitless grating spectrometer with a spectral resolution of 600 operating at wavelengths from 2.5 to 5 micrometers. The key measurement for the three CMB IP candidates is to determine the (B-Mode) CMB polarization due to gravity waves from the inflationary era. As discussed in Chapter 2, one concern about the B-mode polarization is that the B-mode power varies as the fourth power of the energy scale during inflation, so there is only a 3× range in energy scale between the current limits on the B-mode power and the likely detection limits of the Inflation Probe. Mitigating this concern is the fact that at the current best estimates for the spectral index of the primordial power spectrum, the energy scale for inflation might be in this range for typical inflation models, and the CIP mission team proposes to measure this spectral index to much greater precision. Technical Readiness Assessment Summary The CIP concept and mission design is a modification of existing missions. The detectors are very similar to the JWST Near-Infrared Camera (NIRCAM) long-wavelength detectors, but CIP requires eight times more detectors than NIRCAM does. The CMB polarization Inflation Probes collectively are in an earlier stage of development than CIP is. The three proposals outline detector and instrument concepts that are extrapolations from existing experiments. As discussed in Chapter 3, CIP and EPIC-F provided the committee with more mature program plans, management approaches, and technology risk-mitigation plans. Based on the information provided to the committee, EPIC-I and CMBPol are not as far along in their technology and programmatic developments; thus, the committee was not able to adequately assess these areas. The CMB polarization experiments EPIC-F, EPIC-I, and CMBPol all require extremely sensitive millimeter-wave continuum detectors and extremely effective rejection of the common-mode noise from the anisotropy signal. All three of these missions have proposed to use state-of-the-art detectors to reach the required high sensitivity. The polarization, stability, and characterization of the instrument needed to achieve a successful B-mode spectrum measurement is at levels far beyond what has been reached with currently existing instruments. A successful Planck mission will go a long way but not all the way toward proving the readiness of the detector technology. Significant continued support of detector and ultracool cryocoolers (sub-100 mK) is needed to push these missions along.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation The three CMB missions have proposed three different approaches for modulating the polarization signal to separate the desired polarized signal from the much larger temperature anisotropy. Given the state of development of the IP missions, it is not necessary to provide direct technology development to each of the mission teams. Investigations of different approaches for modulating the polarization signal may best be done with ground-based and balloonborne demonstrations. Although the state of CIP technology is more advanced than that of the polarization missions, it would benefit from advances in grating technologies. NASA’s Astrophysics Research and Analysis Program is already in place to fund these types of investigations. However, it should be noted that the scope of the Astrophysics Research Grants Program may need to be changed to accommodate aggressive IP development. JDEM Science Mission Readiness Assessment Over the past decade, conclusive evidence has been assembled indicating that the expansion of the universe is accelerating. Within the standard cosmological model, this implies that some 70 percent of the energy density of the universe is in the form of a mysterious “dark energy,” which counters the attractive gravitational force of matter and radiation. Little is known so far about this dark energy. Whether it is due to a cosmological constant, a dynamical evolving field, a modification of general relativity, or some other new physics cannot be determined from the data currently available. One of the goals of the Beyond Einstein Program is to provide answers to these compelling questions. Three missions to pursue these questions are being studied: the Supernova Acceleration Probe (SNAP), the Dark Energy Space Telescope (DESTINY), and the Advanced Dark Energy Physics Telescope (ADEPT). Each of the three candidate JDEM missions, described in more detail in Chapter 2, should be able to measure the time variance of dark energy at a level of precision that could have a profound impact on current understanding and would shape future research in this area. Such a result would be a major advance in basic astrophysics and cosmology and would have a broad impact across all of fundamental physics. The goal of the JDEM missions, as presented by the report of the Dark Energy Task Force,9 is to provide a factor-of-10 increase over the current accuracy of the dark energy ratio w(a). Given that the present accuracy is around 10 percent, the JDEM missions should provide percent-level measurements of w(a). Thus, the main science risk is being able to control the systematic errors to sub-percent levels. All techniques for measuring effects of dark energy will benefit greatly from both observational and theoretical studies to better understand systematic errors. If systematic errors cannot be controlled down to the sub-percent levels, the impact of JDEM could be compromised with only modest gains over ground-based studies. However, the committee believes that with substantial investment, theoretical and observational studies designed to calibrate the different distance estimators should lead to substantial progress within a few years. Although the ultimate sensitivity of JDEM is somewhat uncertain at present, factors that will limit its sensitivity will be addressed by intermediate-term projects and by control data collected by the mission itself and by other projects. Whereas the Dark Energy Task Force projects that a JDEM mission combining at least two techniques will produce at least a factor-of-10 improvement in sensitivity over present projects, it also projects an improvement of at least a factor of 8 under worst-case assumptions regarding the ability of JDEM to control systematic errors. Even such a worst-case improvement factor would represent a critical improvement in the understanding of the nature of dark energy. Technical Readiness Assessment Summary As described in Chapter 3, two of the three candidate missions for JDEM—DESTINY and SNAP—are relatively mature, and most of the critical technology is at TRL 5-6 or higher. The SNAP charge-coupled devices (CCDs), which are the exception, are at TRL 4-5, but there is a good plan to bring them to flight readiness. The ADEPT mission team did not provide the committee with adequate data for evaluating readiness, but in general ADEPT’s critical technology has flight heritage and no major challenges. It was stated by the ADEPT team that the mission would be based on technologies developed for missions such as Swift and GeoEye. “While there are differences, ADEPT has many similarities to the GeoEye-1 mission, 9 A. Albrecht et al., 2006, Report of the Dark Energy Task Force, Astro-ph/0609591, Batavia, Ill.: Fermi National Accelerator Laboratory.
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation which provides extensive heritage for ADEPT.”10 The mission team currently plans to use a Hawaii HgCdTe 2k × 2k infrared detector sensor. The cutoff frequency will be modified for ADEPT to 2 µm. There is some challenge to this modification, but there are ongoing programs that should demonstrate even lower cutoff frequencies. The information provided is not sufficient for the committee to perform realistic assessments of readiness, and there were insufficient data provided on the spacecraft for assessing the overall technical readiness. From the general statements made, ADEPT appears similar in complexity to the other JDEM mission candidates, with no obvious major instrument or spacecraft technical readiness challenges. The only identified challenges in the DESTINY technologies are in the precision pointing and stabilization, which are both recognized and being addressed by the mission team. The optics required for DESTINY are within the state of the art and can be built without any special challenge. The proposed detectors are 2k × 2k Hawaii-2RG devices. Although very similar to devices on JWST, there are differences—most notably, the cutoff wavelength. The new cutoff material has been demonstrated for the HST program, and this development will be leveraged in the DESTINY program. The information provided indicates that the DESTINY team is looking at investments required at Teledyne-Brown (the detector manufacturer) beyond those being made by JWST. The only challenge for the DESTINY spacecraft is a straightforward engineering one in the area of pointing and stabilization. Specifically, there are concerns with jitter from propellant slosh and other systematics that could present a problem for pointing repeatability. To prove out the proposed pointing and control concept, additional analysis will need to be completed to understand these issues more thoroughly. The proposed DESTINY mission concept has adequate technical margins for size, weight, power, and other non-pointing-system-related performance parameters that should provide flexibility to accommodate solutions to resolve any issues identified from the pointing performance analysis. The committee saw no major challenges to technical readiness for DESTINY. SNAP key technologies are either mature (TRL 6 or greater) or progressing toward TRL 6 in well-planned steps. Some changes to the HgCdTe detectors’ cutoff range have to be made, and these could present challenges. SNAP uses a 1.8 m composite telescope. The SNAP telescope development and primary mirror are seen as a straightforward engineering effort with no obvious challenges. SNAP uses two types of detectors: a Lawrence Berkeley National Laboratory-supplied, radiation-hardened CCD and a Rockwell- or Raytheon-supplied mercury cadmium telluride (MCT) IR detector. The CCDs appear to be a straightforward development effort with no major challenges or problems anticipated to achieving flight readiness. MCT detectors with the required cutoff and quantum efficiency have been demonstrated under DOE funding, and the required application-specific integrated circuit has been developed for JWST. Assuming that funding can be provided in the needed time frame, these devices should not be a challenge. The readout electronics for both detectors are claimed to be radiation-hard and to have adequate performance to meet mission objectives. The focal plane plate is about twice the size of existing devices and the material selected has extensive heritage, and no major development issues are envisioned. All components in the spectrograph are standard and should pose no development risk, with the exception of the Image Slicer. There is heritage from JWST (NIRCAM); however, if the prototype is a very close match and the testing was high fidelity with respect to SNAP requirements, there should be no major challenge to technical readiness. Finally, while most of the spacecraft bus technologies are proven and are well above TRL 6, the Ka-band transmitter is judged to be at TRL 5. With appropriate funding, this item can be brought to flight readiness in a timely manner. The SNAP mission team provided significant detail on the mission concept, showed adequate technical margins in all areas, and overall, SNAP was assessed by the committee to have no major challenges to achieving technical readiness. LISA Science Readiness Assessment Summary The science underlying LISA’s quest to detect and use gravitational waves is at a high level of readiness, as discussed in Chapter 2. Techniques for solving Einstein’s equations are sufficiently advanced to confidently predict the gravitational waves from the sources of interest and to interpret the data taken. A combination of analytical and numerical work has provided machinery to yield robust predictions 10 JDEM/ADEPT team response to the committee’s Request for Information (see Appendix E in this report).
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Nasa ’s Beyond Einstein Program: An Architecture for Implementation from general relativity for the gravitational-wave signal from massive black hole coalescences, and these methods are now being applied to the more complex and interesting case of mergers of rapidly spinning black holes. Substantial progress is likely during the next few years, well in advance of LISA. The signals from the galactic binary sources are assumed to exist based entirely on textbook general relativity. Event rates for massive black hole inspirals are uncertain by a factor of 10, while for inspirals of small objects into massive black holes, the rates are even more uncertain. These uncertainties result in a science risk factor should the mission fail to achieve its 5-year lifetime. Technical Readiness Assessment Summary LISA has had considerable technology development since entering Phase A development in 2004 and has had a baseline mission architecture in place for some time. Nevertheless, a number of critical technologies and performance requirements must be developed and verified before LISA has the technical readiness to move into the implementation phase; these techniques are discussed in Chapter 3. Some of these will be tested on the ESA LISA Pathfinder scheduled for launch in October 2009. Success of the Pathfinder is a prerequisite for LISA to proceed with implementation. Not all of the critical LISA technologies and performance will be tested on the Pathfinder. Therefore, given the scientific importance of LISA, the committee strongly believes that the next highest priority for allocation of the current NASA Beyond Einstein funding wedge after the JDEM start is funding to accelerate the maturation of the technical readiness of these remaining LISA technologies. Areas that are candidates for this funding and shown at TRL levels of 4 or less and discussed in Chapter 3 include micronewton thruster technology development and lifetime tests, the point-ahead actuator, the phase measurement system, and laser frequency noise suppression.
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