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2 Discussion The discussion in the sections below is derived from text in the 2010 Astronomy and Astrophysics Decadal Survey (Astro2010) report New Worlds, New Horizons in Astronomy and Astrophysics (NWNH), 1 information conveyed to the panel at its November 7, 2010, meeting (see Appendixes A and B), and the panel’s own deliberations. THE BALANCED PROGRAM RECOMMENDED IN ASTRO2010 New Worlds, New Horizons in Astronomy and Astrophysics prioritized a set of missions and activities to advance the set of science priorities identified by the Astro2010 Science Frontier Panels. It is important to note that while the program was organized according to three science objectives—cosmic dawn, new worlds, and the physics of the universe—these science objectives themselves were not ranked. Rather, a program was constructed to optimize science return and to ensure progress on a much broader front as well (see Table 2.1), while also fostering unanticipated discovery. Importantly, NWNH does not recommend any specific science goal as its top priority. Moreover, NWNH prioritized missions and activities only within size categories and not across them. For example, in space, the recommended priority order for the large-scale activities were (1) the Wide-Field Infrared Survey Telescope (WFIRST); (2) the Explorer program augmentation; (3) the Laser Interferometer Space Antenna (LISA); and (4) the International X-ray Observatory (IXO). At the medium scale, the priorities were (1) a New Worlds Technology Development program and (2) an Inflation Probe Technology Development program. NWNH does not state that large-scale activities should be done ahead of medium-scale activities, or that medium-scale activities should be done ahead of small-scale ones, nor that the top large-scale priority is the top overall priority of the program. NWNH also does not state that the list of priorities within a category necessarily means that the highest priority must be completed before the next priority begins. A principle central to NWNH is the need for a balanced program, and thus small programs and core augmentations were also enumerated in NWNH.2 Some examples of such programs are “support of individual investigators, instrumentation, laboratory astrophysics, public access to privately operated telescopes, suborbital space missions, technology development, theoretical investigations, and collaboration on international projects.”3 These programs were not prioritized against medium or large activities. 1 National Research Council (NRC), New Worlds, New Horizons in Astronomy and Astrophysics [prepublication version], The National Academies Press, Washington, D.C., 2010. 2 The need for a balanced program is also given in National Research Council, A Performance Assessment of NASA’s Astrophysics Program, The National Academies Press, Washington, D.C., 2007. 3 NRC, New Worlds, New Horizons in Astronomy and Astrophysics [prepublication version], 2010, p. ES-2. 4

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TABLE 2.1 Summary of 2010 Astronomy and Astrophysics Decadal Survey Science Frontiers Panels’ Findings Area(s) of Unusual Panel Science Questions Discovery Potential Cosmology and CFP 1 How did the universe begin? Gravitational wave Fundamental Physics astronomy CFP 2 Why is the universe accelerating? (CFP) CFP 3 What is dark matter? CFP 4 What are the properties of neutrinos? Galactic GAN 1 What are the flows of matter and energy in the Time-domain Neighborhood circumgalactic medium? astronomy astrometry (GAN) GAN 2 What controls the mass-energy-chemical cycles within galaxies? GAN 3 What is the fossil record of galaxy assembly from the first stars to the present? GAN 4 What are the connections between dark and luminous matter? Galaxies Across GCT 1 How do cosmic structures form and evolve? The epoch of Cosmic Time (GCT) reionization GCT 2 How do baryons cycle in and out of galaxies, and what do they do while they are there? GCT 3 How do black holes grow, radiate, and influence their surroundings? GCT 4 What were the first objects to light up the universe, and when did they do it? Planetary Systems PSF 1 How do stars form? Identification and and Star Formation (PSF) characterization of PSF 2 How do circumstellar disks evolve and form nearby habitable planetary systems? exoplanets PSF 3 How diverse are planetary systems? PSF 4 Do habitable worlds exist around other stars, and can we identify the telltale signs of life on an exoplanet? Stars and Stellar SSE 1 How do rotation and magnetic fields affect stars? Time-domain surveys Evolution (SSE) SSE 2 What are the progenitors of Type Ia supernovas and how do they explode? SSE 3 How do the lives of massive stars end? SSE 4 What controls the mass, radius, and spin of compact stellar remnants? SOURCE: National Research Council, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., 2010, p. 247. Comparison of WFIRST and Euclid Capabilities and Science Goals WFIRST is a 1.5-meter space telescope with a near-infrared (NIR) imager and a near-infrared spectrometer-camera. Its NIR detectors have some sensitivity in the visible red, but WFIRST is primarily an infrared platform. In conception, it combines three overarching goals: (1) to use the three primary 5

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methods that have emerged to investigate dark energy and the validity of general relativity in describing cosmic acceleration (Baryon Acoustic Oscillations [BAO], weak lensing, and Type Ia supernovae);4 (2) to conduct a microlensing survey of exoplanets to derive the statistics of exoplanet system architectures; and (3) to provide a guest investigator mode to perform a deep-infrared survey of galaxies, quasars, and large- scale structure and a complete survey of the galactic plane. The panel notes that a guest investigator mode is a substantial addition to these IR surveys and one that makes WFIRST a community facility with a large potential “discovery space.”5 By comparison, Euclid is a 1.2-meter space telescope with an emphasis on dark energy. It uses the BAO and weak lensing approaches and foregoes the use of Type Ia supernovae. The Euclid Assessment Study Report (Yellow Book) characterizes Euclid as a precision cosmology mission with goals of dark matter, dark energy, initial conditions, tests of gravity and sharpening cosmological parameters.6 Although Euclid has a NIR imaging capability and NIR spectroscopic capability for BAO studies, it is primarily an optical instrument with charge coupled devices with a pixel scale of 0.1 arcseconds. However, a byproduct of the Euclid mission will also be a NIR imaging and spectroscopic survey, and Euclid has a modest planet microlensing capability. The multiple objectives of WFIRST—dark energy, microlensing planet search, infrared surveys, and the support of pointed observations proposed by guest investigators—are made possible by the large number of IR detectors and the fine resolution with which they sample the excellent images afforded from space. Euclid as currently envisioned has a smaller primary mirror, fewer IR arrays, and much coarser imaging scale in the infrared, so it cannot meet the NWNH science goals. The WFIRST design has a great advantage over the Euclid design in the microlensing search for planets because its high-resolution pixels are in the infrared, where the galactic bulge stars that will be monitored are intrinsically brighter and less affected by dust. As explained in NWNH, of the missions considered, only WFIRST can build up a deep statistical sample that will adequately complement the Kepler mission’s probe of Earth-sized planets close to their parent stars. For the program of dark energy research, another significant advantage of WFIRST is its capability for simultaneous spectroscopy and imaging. The present panel notes that a keystone of the integrated plan advocated in NWNH is exploration of dark energy over the full redshift range with multiple techniques and facilities. 7 Hence, it was concluded in NWNH that WFIRST and LSST would complement one another to improve overall accuracy and control systematics. The WFIRST and Euclid missions each have strengths and weaknesses with respect to the goals of a dark energy study, but NWNH concluded that the substantial advantages of the WFIRST configuration and its ability to address multiple priority science questions identified by the survey were decisive. Accordingly, NWNH ranked WFIRST highest in the large class of space-based activities. SPECIFIC STATEMENTS IN NWNH CONCERNING EUCLID The Astro2010 decadal survey committee was aware of the preliminary discussions on possible U.S. participation in Euclid that were inaugurated between NASA and ESA before NWNH was released, and it considered options for collaborations in its overall deliberations, albeit late in the process.8 There 9 are three references in NWNH to possible U.S. collaboration on Euclid: 4 NRC, New Worlds, New Horizons in Astronomy and Astrophysics [prepublication], 2010, pp. 1-5 and 7-17. 5 As noted by Roger Blandford at the November 7, 2010, meeting. 6 European Space Agency, Euclid: Mapping the Geometry of the Dark Universe—Assessment Study Report, ESA/SRE(2009)2, Paris, France, December 2009. 7 As noted by Roger Blandford at the November 7, 2010, meeting. 8 It is important to note that the Euclid mission was not proposed to the Program Prioritization Panels nor did it go through the cost, risk, and technical evaluation process. 9 NRC, New Worlds, New Horizons in Astronomy and Astrophysics [prepublication], 2010. 6

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• The European Space Agency (ESA) is considering an M-class proposal, called Euclid, with related goals. Collaboration on a combined mission with the United States playing a leading role should be considered so long as the committee’s recommended science program is preserved and overall cost savings result. (p. 1-6) • Euclid is a European mission concept aimed at cosmology and dark energy, which is competing for one of two M(edium)-class launch slots, with a decision expected in late 2011 and launches scheduled for 2018 and 2019.10 The overlap in goals and scope between the proposed U.S. and European missions is significant, and there is potentially a grand partnering arrangement involving NASA, DOE, and ESA if the expanded scientific priorities set by Astro2010 for such a mission can be aligned among the partners, and assuming that the arrangement is consistent with the United States playing a clear leadership role. (p. 3-14) • There have been discussions between the U.S. agencies and ESA about mounting a joint mission, which could be a positive development if it leads to timely execution of a program that fully supports all of the key science goals of WFIRST (planet microlensing, dark energy science, general investigations) and leads to savings overall. It is expected that the United States will play a leading role in this top-priority mission. (p. 7-18) As is clear from the quotes above, NWNH looked favorably on international participation with the Europeans on WFIRST/Euclid science, but only in the context of (1) “a clear leadership role” for the United States and (2) “a [timely] program that fully supports all of the key science goals of WFIRST (planet microlensing, dark energy science, general investigations) and leads to savings overall,” as articulated above. SPECIFIC STATEMENTS IN NWNH ON LARGE MISSION OVERRUNS In the most restricted budget scenario considered by the Astro2010 decadal survey committee, NWNH stated as follows:11 In the event that insufficient funds are available to carry out the recommended program, the first priority is to develop, launch, and operate WFIRST, and to implement the Explorer program and core research program recommended augmentations. . . . (p. 7-40) Even under such constrained circumstances, NWNH insisted on maintaining balance in the program and did not support sacrificing or significantly delaying any one element of the remaining components of its integrated plan for one large mission. In addition, consistent with this theme and related to the current situation with the James Webb Space Telescope, NWNH clearly objected to the use of funds to address overruns in large and medium missions at the expense of core activities. One relevant paragraph in NWNH contains the following 12 words: NASA’s core research programs, from theoretical studies to innovative technology development, are fundamental to mission development and essential for scientific progress. . . Maintaining these core activities has a high priority for the survey committee, and the budget allocations should not be allowed to decrease to address overruns in the costs of large and medium missions.” (p. 1-9) 10 As heard from the European Space Agency at the November 7, 2010, meeting, there are two M-class mission launch opportunities within 2017-2018, and the current budget would allow an M-class mission launch in 2022, assuming the first large-class mission will be launched in 2020. 11 NRC, New Worlds, New Horizons in Astronomy and Astrophysics [prepublication], 2010. 12 NRC, New Worlds, New Horizons in Astronomy and Astrophysics [prepublication], 2010. 7