In this chapter, each of the “large,” “medium,” and “small” recommendations in New Worlds, New Horizons in Astronomy and Astrophysics1 (NWNH) are considered, in turn, for the space-based program. As in Chapter 3, the progress that has been made toward the NWNH goals is evaluated, including the programs adopted by the agencies and their plans for the remainder of the decade. Finally, an overview of the space-based program is provided and its balance is considered.
For reference, Table ES.5 from NWNH is reproduced below (Table 4.1), listing the priorities for large-scale space-based activities. These included, in rank order, the Wide-Field Infrared Survey Telescope (WFIRST), an augmentation to NASA’s Explorer program, the Laser Interferometer Space Antenna (LISA), and the International X-ray Observatory (IXO).
WFIRST was NWNH’s highest-ranked large space initiative, with a science program that incorporated precision measurements of cosmic acceleration from large imaging, spectroscopic, and supernova monitoring surveys, statistical char-
1 National Research Council (NRC), 2010, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C.
TABLE 4.1 Large-Scale, Spaced-Based Recommended Activities from the 2010 Astronomy and Astrophysics Decadal Survey
|Recommendation||Launch Dateb||Science||Technical Riskc||Appraisal of Costsa|
|Total (U.S. Share)||U.S. Share, 2012-2021||Cross-Reference in Chapter 7|
|1. WFIRST —NASA/DOE collaboration||2020||Dark energy, exoplanets, and infrared survey-science||Medium low||$1.6B||$1.6B||Page 205|
|2. Augmentation to Explorer Program||Ongoing||Enable rapid response to science opportunities; augments current plan by 2 Medium-scale Explorer (MIDEX) missions, 2 Small Explorer (SMEX) missions, and 4 Missions of Opportunity (MoOs)||Low||$463M||$463M||Page 208|
|3. LISA —Requires ESA partnershipd||2025||Open low-frequency gravitational-wave window for detection of black-hole mergers and compact binaries and precision tests of general relativity||Mediume||$2.4B ($1.5B)||$852M||Page 209|
|4. IXO —Partnership with ESA and JAXAd||2020s||Black-hole accretion and neutron-star physics, matter/ energy life cycles, and stellar astrophysics||Medium high||$5.0B ($3.1B)||$200M||Page 213|
a The survey’s cost appraisals for Wide-Field Infrared Survey Telescope (WFIRST), Laser Interferometer Space Antenna (LISA), and International X-ray Observatory (IXO) are based on the survey’s cost, risk, and technical readiness evaluation (i.e., the cost appraisal and technical evaluation, or CATE, analysis) and project input, in FY2010 dollars for phase A costs onward; cost appraisals for the Explorer augmentation and the medium elements of the space program are committee-generated, based on available community input. The share for the U.S. government is shown in parentheses when it is different from the total. The U.S. share is based on the United States assuming a 50 percent share of costs and includes an allowance for extra costs incurred as a result of partnering.
b The survey’s appraisal of the schedule to launch is the earliest possible based on CATE analysis and project input.
c The risk scale used was low, medium low, medium, medium high, and high.
d Note that the LISA and IXO recommendations are linked—both are dependent on mission decisions by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA).
e Technical risk assessment of “medium” is contingent on a successful LISA Pathfinder mission.
SOURCE: National Research Council, 2010, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., Table ES.5.
acterization of the demographics of exoplanet systems from a gravitational microlensing survey of the galactic bulge, a large area survey of the galactic plane, and a wide range of galactic and extragalactic investigations enabled by guest investigator studies of the survey data sets and by a Guest Observer (GO) program. The version of WFIRST that has now entered Phase A formulation is significantly different and, in most ways, more powerful than the facility described by NWNH, with a larger aperture telescope, a larger infrared focal plane, and an additional instrument for coronagraphic observations of exoplanets and protoplanetary and zodiacal disks. However, WFIRST as currently designed will neither cover as large an area of the sky as envisaged by NWNH, nor get as far into the infrared, due to the heated mirrors. In addition, the telescope will be time-shared between imaging and spectroscopy, which will reduce the amount of spectroscopic survey data obtained. The history of these changes is reviewed below, and the importance of cost control is emphasized, which will be essential to maintain balance in the NASA program as WFIRST proceeds toward launch in the mid-2020s. These remarks are made in the context of strong endorsement of the scientific promise of WFIRST.
FINDING 4-1: The 2.4-meter telescope, larger infrared detectors, and addition of a coronagraph make the 2016 design of WFIRST an ambitious and powerful facility that will significantly advance the scientific program envisioned by NWNH, from the atmospheres of planets around nearby stars to the physics of the accelerating universe.
Table 4.2 summarizes the changes in WFIRST design from the one described by NWNH to the one approved for Phase A study, including the projected cost of these designs scaled to fiscal year (FY) 2015 dollars. These changes and the cost estimates are discussed in more detail in the following subsections.
While not endorsing a specific hardware implementation, NWNH noted that the design of the proposed Joint Dark Energy Mission Omega (JDEM-Omega) satellite, a 1.5-meter telescope equipped with large focal plane arrays of H2RG detectors with a 2.0 μm long-wavelength cutoff, had capabilities “essentially identical to those envisaged for WFIRST.”2 Based on the cost and technical evaluation (CATE3) of this design, NWNH adopted a cost of $1.6 billion (FY2010 dollars) for WFIRST and emphasized the moderate cost and “medium-low” assessment of its technical risk as important factors in its top ranking.
2 NRC, 2010, New Worlds, New Horizons, p. 207.
3 The CATE, or cost and technical evaluation process, was created by the Astro2010 Survey Committee, and a detailed description of the CATE process can be found in the NWNH report.
TABLE 4.2 Changes to WFIRST Since New Worlds, New Horizons in Astronomy and Astrophysics
|Reference Mission||Projected Cost (FY2015 dollars)|
|JDEM-Omega Design—2010||$1.8 billion|
|1.5 m on-axis telescope|
|36 H2RG detectors: 1 imaging array + 2 spectroscopic arrays|
|2.0 μm long wavelength cutoff|
|L2 orbit, Atlas V launch vehicle|
|Interim Design Reference Mission—2011||$1.8 billion|
|1.5 m on-axis telescope → 1.3 m off-axis telescope|
|Design Reference Mission 1—2012||$1.8 billion|
|3 focal plane arrays → 1 larger array with grism in filter wheel|
|2.0 μm cutoff → 2.4 μm cutoff|
|WFIRST-AFTA 2013 Design Reference Mission—2013||$1.9 billion to $2.1 billion (includes GO)|
|1.3 m telescope → 2.4 m AFTA telescope (on-axis)|
|36 H2RG detectors → 18 H4RG detectors|
|2.4 μm cutoff → 2.0 μm cutoff|
|Supernova IFU option → baseline|
|GO program increased to 25-30% of observing time|
|On-axis coronagraph as an option (with 6-year prime mission)|
|L2 orbit → inclined GEO orbit|
|WFIRST-AFTA 2015 Design Reference Mission—2015a||$2.0 billion to $2.3 billion (includes coronagraph)|
|Coronagraph (with 6-year prime mission) option → baseline|
|Coronagraph development and technology downselect|
|Atlas V launch vehicle → Delta IV Heavy|
|WFIRST at KDP-A—2016b|
|2024 launch||$2.3 billion to $2.5 billion|
|2025 launch||$2.6 billion to $2.8 billion|
|Inclined GEO orbit → L2|
NOTE: Acronyms defines in Appendix C.
a Paul Hertz, NASA, “NASA Astrophysics: Progress toward New Worlds, New Horizons,” presentation to committee on October 8, 2015.
b Paul Hertz, NASA, “Astrophysics,” presentation to committee February 26, 2016.
Following NWNH, NASA established a WFIRST Project Study Office and a Science Definition Team (SDT). The Interim Design Reference Mission (IDRM) adopted a hardware implementation similar to that of JDEM-Omega, but with the substitution of an unobstructed, 1.3-meter, off-axis telescope for the 1.5-meter on-axis design, providing the same light-gathering power with a smaller primary mirror and cleaner optical path.4 The Design Reference Mission 1 (DRM1) presented in the SDT’s final report5 differed in two important respects—combining the IDRM’s separate imaging and spectroscopic arrays into a single focal plane array with a dispersive element in the filter wheel, and extension of the long-wavelength cutoff to 2.4 μm. By the time this report was completed, the schedule delays and increased cost of the James Webb Space Telescope (JWST) made it clear that a Phase A start for WFIRST would be delayed by several years, and might not happen at all.
Completion of the SDT report6 coincided with a dramatic new development, the announcement that two 2.4-meter optical telescope assemblies (colloquially known as “the NRO telescopes”) were being made available to NASA following the discontinuation of the program for which they were originally built. NASA commissioned a new SDT and Project Study Office to examine the feasibility of implementing WFIRST using one of these telescope assemblies (the “Astrophysics Focused Telescope Assets,” or AFTA) and to consider the optional addition of an on-axis coronagraphic instrument for direct imaging and spectroscopy of exoplanets. Advances in detector technology also enabled the WFIRST-AFTA SDT to baseline 18 H4RG detectors in place of 36 H2RG detectors, doubling the total pixel count of the wide-field instrument. However, the AFTA mirror assembly was designed for operation at ≈ 290 K, degrading the long-wavelength performance relative to earlier WFIRST designs with a colder telescope. The 2013 SDT report7 proposed an implementation with a 270 K telescope operating temperature and a 2.0 μm long wavelength cutoff, and several options for the coronagraphic instrument. An integral field unit (IFU) was also added. Recognizing the broad range of investigations enabled by the 2.4-meter aperture, this report also recommended an expanded GO program encompassing 25-30 percent of the prime mission, and it recommended that the addition of a coronagraph be accompanied by a 1-year extension of the prime mission (from 5 to 6 years) so that coronagraphic studies
4 NASA Goddard Space Flight Center, 2011, Wide-Field InfraRed Survey Telescope WFIRST Interim Report, Science Definition Team, http://wfirst.gsfc.nasa.gov/science/sdt_public/WFIRST_Interim_Report.pdf.
5 NASA, 2012, Wide-Field InfraRed Survey Telescope WFIRST Final Report, Science Definition Team, https://arxiv.org/ftp/arxiv/papers/1208/1208.4012.pdf.
6 NASA, 2012, WFIRST Final Report.
7 NASA, 2013, Wide-Field InfraRed Survey Telescope Astrophysics Focused Telescope Assets WFIRST-AFTA Final Report, Science Definition Team and WFIRST Project, https://arxiv.org/ftp/arxiv/papers/1305/1305.5422.pdf.
could be executed without reducing the length of the dark energy, microlensing, and GO programs.
At NASA’s direction, the 2015 SDT report8 adopted the coronagraph as a (descopable) part of the baseline WFIRST-AFTA mission, rather than an optional addition, and adopted a 6-year prime mission lifetime. The 2015 SDT report, which is the most up-to-date public description of WFIRST-AFTA, introduced a number of refinements based on integrated modeling and coronagraph technology evaluation, but no major changes to the hardware complement. However, partly in response to concerns raised in the 2014 NRC report Evaluation of the Implementation of WFIRST/AFTA in the Context of New Worlds, New Horizons in Astronomy and Astrophysics,9 the baseline telescope operating temperature was changed from 270 K to 282 K, and the baseline launch vehicle was changed from an Atlas V to a Delta IV Heavy.
The 2015 SDT report discussed (in its Appendix C) the option of changing from an inclined geosynchronous orbit to an L2 orbit.10 The principal technical advantage identified for the geosynchronous orbit was the availability of continuous data downlink and thus higher data transmission capability. The most important advantage identified for the L2 orbit was the greater efficiency and flexibility in scheduling microlensing and coronagraph observations—in geosynchronous orbit, microlensing observations of the galactic bulge must be interrupted 4-5 days per month because of Moon observing constraints, and coronagraph observations would typically suffer cutouts of a few hours per day because of Earth observing constraints, depending on target location. Other significant advantages identified for L2 were the substantially lower cosmic ray background (and consequently lower shielding requirement) outside of Earth’s trapped electron belt and the greater stability from avoiding variable thermal loading from Earth. This analysis suggested that the advantages of an L2 orbit outweighed the disadvantages, but it also emphasized that “the SDT deliberately avoided writing requirements that could not be met at either location.”11
Since the completion of the 2015 SDT report, WFIRST has been through one further design cycle, leading to the design presented at the Mission Concept Review in December 2015. The most significant change in this design cycle was to baseline an L2 orbit, with a number of consequent alterations such as addition of data recorders and propellant tanks and a larger antenna for data transmission.
8 NASA, 2015, Wide-Field InfraRed Survey Telescope-Astrophysics Focused Telescope Assets WFIRST-AFTA 2015 Report, Science Definition Team and WFIRST Study Office, https://arxiv.org/ftp/arxiv/papers/1503/1503.03757.pdf.
9 NRC, 2014, Evaluation of the Implementation of WFIRST/AFTA in the Context of New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C.
11 NASA, 2015, WFIRST-AFTA 2015 Report, p. C-1.
Thanks to (1) JWST remaining on schedule and budget since its 2011 reprogram, (2) the adoption of the 2.4-meter National Reconnaissance Office (NRO) telescope, and (3) the addition of the coronagraph, WFIRST-AFTA has enjoyed stronger support in Congress, within NASA, and in the astronomical community, compared to the previous implementations of WFIRST. All three of these developments have played a role in garnering this stronger support, and it is difficult to disentangle their individual contributions. In FY2014, FY2015, and FY2016, Congress allocated more funds to WFIRST than requested by the administration. Following the direction of the FY2016 Omnibus Spending Bill, NASA initiated the Phase A start of WFIRST in February 2016. According to presentations to the committee by Paul Hertz, Neil Gehrels, and Kevin Grady, the projected schedule and NASA’s Astrophysics Division (NASA-APD) budget enable a 2025 launch of WFIRST, paced by the availability of funding. A steeper funding profile would enable an earlier launch and would reduce the total mission cost, but it might also have a negative impact on program balance in the near term.
In summary, with the necessary funding wedge opened by the approaching ramp-down of JWST construction, NASA is now proceeding with NWNH’s top-ranked large space priority, although in a form that is different from, and in many ways more powerful, than the one considered by NWNH. The currently anticipated launch date is delayed by approximately 5 years relative to that envisioned by NWNH.
The 2014 National Research Council Report and the Current WFIRST Budget Estimate
The Committee on Assessment of the Astrophysics Focused Telescope Assets (AFTA) Mission Concepts was tasked with assessing the responsiveness of the WFIRST-AFTA DRM, with and without the coronagraph, to the science objectives and strategy of NWNH. This committee finds itself in general agreement with the 2014 report of that committee, Evaluation of the Implementation of WFIRST/ AFTA, regarding both the promise and the risks of the AFTA implementation of WFIRST and the addition of the coronagraph, although it was not able to review the WFIRST mission in nearly as great detail as the 2014 committee. The committee reiterates and emphasizes the following three findings in Evaluation of the Implementation of WFIRST/AFTA:12
Finding 3-2: The opportunity to increase the telescope aperture and resolution by employing the 2.4-meter AFTA mirror will significantly enhance the scientific power of the mission, primarily for cosmology and general survey science, and will also positively impact the exoplanet microlensing survey. WFIRST/AFTA’s planned observing program is responsive to all the scientific goals described in NWNH. (p. 37)
12 NRC, 2014, Evaluation of the Implementation of WFIRST/AFTA.
Finding 2-4: The risk of cost growth is significantly higher for WFIRST/AFTA without the coronagraph than for WFIRST/IDRM. (p. 39)
Finding 3-3: If implementing WFIRST/AFTA compromises the program balance, then it is inconsistent with the rationale that led to the high-priority ranking in NWNH. (p. 38)
On behalf of the WFIRST Project Office, Neil Gehrels presented to the committee a detailed response to findings in Evaluation of the Implementation of WFIRST/ AFTA, addressing cost and technical risks, progress in maturation of coronagraph technology, and the role of the coronagraph in the mission.
The projected imaging and spectroscopic depth of WFIRST-AFTA in the 2015 SDT report are lower than those in the 2013 DRM that was assessed in Evaluation of the Implementation of WFIRST/AFTA, in part because of an increase in the assumed telescope operating temperature from 270 K to 282 K.13 Nonetheless, compared to the WFIRST IDRM,14 the larger aperture and higher pixel count of WFIRST-AFTA make it a substantially more powerful facility for wide-field imaging, with larger étendue and higher angular resolution, improving the performance for weak lensing, microlensing, astrometry, and a wide range of potential GO programs. For long wavelength (>1.7 μm) observations and wide-field spectroscopy, the gains from aperture and pixel count have been at least partly offset by the increased telescope operating temperature and by the elimination of dedicated spectroscopic focal plane arrays in favor of a single larger array used for both imaging and spectroscopy. For the microlensing science, the yield of detected planets for WFIRST-AFTA is expected to be 2,600 (of which 420 would be three times the Earth’s mass or smaller), compared to 1,700 (with 230 small planets) for the IDRM mission, in a smaller allocation of observing time. Even more significantly, the higher spatial resolution and integral field spectrograph mode will significantly increase the number of systems in which the lens star (which hosts the planet) can be directly detected, allowing its spectral type, distance, and perhaps metallicity to be determined.15
Evaluation of the Implementation of WFIRST/AFTA recommended an external technical and cost review of WFIRST-AFTA. NASA commissioned Aerospace Corporation to carry out a CATE of the 2015 WFIRST-AFTA DRM, which was completed in February 2015. Informed by this evaluation, NASA projected the cost of this DRM, with the coronagraph, to be $2.0 billion to $2.3 billion in FY2015 dollars.16 Of this cost, $0.35 billion (in FY2015$) was ascribed to the coronagraph, including the extra year of mission operations, and $0.1 billion was ascribed to sup-
14 NASA Goddard Space Flight Center, 2011, WFIRST Interim Report.
15 NASA, 2015, WFIRST-AFTA 2015 Report.
16 Paul Hertz, NASA, “NASA Astrophysics: Progress Toward New Worlds, New Horizons,” presentation to committee on October 8, 2015.
port of the expanded GO program recommended by the WFIRST-AFTA SDT.17 The estimated cost of the “original” WFIRST scope envisioned by NWNH was $1.8 billion, very close to the NWNH value of $1.6 billion after adjustment from FY2010 to FY2015 dollars, and as stated by P. Hertz, “thereby validates NASA’s expectation that the cost of a larger telescope is offset by the savings of using an existing telescope.” In other words, the growth in estimated cost between 2010 and 2015 was fully attributable to the combination of the coronagraph, the GO funding, and inflation.
However, changes to the mission design between the 2015 DRM and the version of WFIRST presented at the Mission Concept Review and approved at Key Decision Point A led to an increase of the estimated cost by approximately 25 percent ($550 million).18 An unknown portion of this change is associated with the change from geosynchronous orbit to L2, and another portion is associated with an increased estimate for the cost of the Delta IV Heavy launch vehicle (the choice of launch vehicle did not change, just its estimated cost). Some of it may simply reflect more accurate assessment as the mission design matures. NASA’s current cost projection for WFIRST is $2.6 billion to $2.8 billion in FY2015 dollars for a 2025 launch. A key uncertainty is the launch vehicle cost, which is difficult to project 8-10 years into the future at this time. It is projected that an accelerated funding profile leading to a 2024 launch would save approximately $0.3 billion relative to the “in-guide” profile that leads to a 2025 launch.
Independent of details, and independent of the coronagraph, the risk of cost growth for WFIRST-AFTA remains higher than for the NWNH version of WFIRST. This increased risk arises from the use of inherited hardware, which increases the mass of the telescope system and removes some of the options that would otherwise be available to meet mass or technical margins or to implement cost-saving descopes.
FINDING 4-2: Because of the risk of cost growth, the concern raised in Evaluation of the Implementation of WFIRST/AFTA that WFIRST could distort the NASA program balance remains a concern. In addition, the delay in the implementation of WFIRST over the schedule anticipated in NWNH means that cost growth in WFIRST would limit options for the next decadal survey.
The WFIRST Coronagraph
The addition of a coronagraph is a major change to WFIRST not anticipated by NWNH.
17 NWNH explicitly noted that the cost of the guest investigator program was not included in its WFIRST cost estimate.
18 Paul Hertz, NASA, “Astrophysics,” presentation to committee February 26, 2016.
The WFIRST coronagraph does directly address the highest-priority medium-scale space activity identified by NWNH, a “New Worlds Technology Development Program for a 2020 Decade Mission to Image Habitable Rocky Planets.” While the program envisioned by NWNH was laboratory based, the shift to a 2.4-meter telescope allows a coronagraph flown on WFIRST to execute a strong science program—imaging and spectroscopy of gas giant and perhaps “super-Earth” planets around nearby stars—in addition to demonstrating starlight suppression technology in space. Simulations of the current coronagraph performance show that the instrument could photometrically or spectroscopically characterize ~16 known giant planets, while a search for new systems would detect and characterize ~12 planets of Neptune size or below, including (if the measured Kepler frequency of planets extends out to 1 AU scales) ~4 super-Earths.19 The WFIRST coronagraph can also characterize the level of zodiacal light around potential target stars, thus accomplishing one of the essential tasks identified by NWNH as a precursor to a future planet-imaging mission. While ground-based observations are also examining this question, the WFIRST coronagraph measurements will likely be more directly applicable.
FINDING 4-3: The WFIRST coronagraph responds to an opportunity that arose after NWNH, the availability of the 2.4-meter AFTA telescope. This development allows a space-borne coronagraph to carry out an exciting exoplanet science program, in addition to demonstrating technology that would be needed for a future mission capable of imaging Earth-like planets around nearby stars. The addition of the coronagraph, therefore, addresses NWNH’s highest medium-scale space-based priority of a New Worlds Technology Development program.
The total cost of exoplanet-related precursor science and technology development, including technology development for the WFIRST coronagraph, significantly exceeds the $100 million to $200 million envisioned by NWNH for the New Worlds Technology Development Program. Given the pressure of budget constraints on realizing the overall scientific program recommended by NWNH, funding any specific program well above the level recommended by NWNH is a concern. However, exoplanet research has progressed significantly since NWNH, and, as evident from the title that the decadal survey committee selected for its report, it is a central scientific focus of NWNH. It is also a science area that resonates strongly with the public and with NASA stakeholders in Congress and the administration. Furthermore, the survey stated that “[i]t is currently difficult to anticipate the developments that could justify initiating this mission-specific development program, and the committee therefore recommends that a decadal survey implementation advisory committee be convened mid-decade to review progress
19 NASA, 2015, WFIRST-AFTA 2015 Report.
both scientifically and technically to determine the way forward, and in particular whether an increased level of support associated with mission-specific technology development should commence. In this case a notional decadal budget of $100 million is proposed.”20 Taken together, these developments and the survey’s language on the matter justify the currently anticipated expenditure discussed above.
FINDING 4-4: At the currently estimated cost, NASA’s decision to add a coronagraph to the AFTA implementation of WFIRST is justifiable within the scientific goals of NWNH. The broader societal interest in the possibility of life beyond Earth is also compelling. However, an increase in cost much beyond the currently estimated $350 million would significantly distort the science priorities set forth by NWNH.
The 2014 NRC report committee found that “introducing a technology development program onto a flagship mission creates significant mission risks resulting from the schedule uncertainties inherent in advancing low technical readiness level (TRL) hardware to flight readiness.”21 It recommended that NASA move aggressively to mature the coronagraph design to a level that would allow credible assessments of its expected scientific performance and its cost and schedule impact on WFIRST. WFIRST support has led to rapid progress on coronagraph technology and performance forecasts, as documented in the 2015 SDT report22 and in presentations to the committee.23 In the current WFIRST schedule, the coronagraph is not on the critical path, and recent design modifications make the coronagraph performance relatively insensitive to stability of the telescope optics. The WFIRST Project Team reported that NASA Headquarters has directed that the coronagraph will not impose driving requirements onto the mission design.24 Nonetheless, as the newest and least technologically mature element of WFIRST, the coronagraph inevitably increases the risk of schedule delays and cost growth.
FINDING 4-5: Coronagraph technology has matured rapidly over the past 2 years, addressing one of the key recommendations of the 2014 report Evaluation of the Implementation of WFIRST/AFTA in the Context of New Worlds, New Horizons in Astronomy and Astrophysics. The coronagraph remains a schedule, cost, and technical risk for WFIRST.
20 NRC, 2010, New Worlds, New Horizons, p. 216.
21 NRC, 2014, Evaluation of the Implementation of WFIRST/AFTA, Finding 2-6.
22 NASA, 2015, WFIRST-AFTA 2015 Report.
23 Scott Gaudi, Ohio State University, “Planetary Systems and Stars,” presentation to committee on December 12, 2015; Jeremy Kasdin, Princeton University, “Exoplanet Imaging Technology,” presentation to committee on December 13, 2015.
24 Neil Gehrels, NASA GSFC, “WFIRST Response to Harrison Committee Findings,” presentation to the committee on October 14, 2015.
The Way Forward
With its 2016 Phase A start, WFIRST is entering a phase of critical design decisions that will affect its capabilities and its cost. The costs associated with funding guest investigators and guest observers are distinct in character from other WFIRST costs because they maintain support for individual investigators and for training of young researchers. The $0.1 billion increase in estimated mission cost associated with WFIRST’s expanded guest investigator and GO program does not, therefore, tilt the balance of the NASA-APD program in the direction of large projects, despite its association with a large mission. Costs associated with the coronagraph and the sixth year of prime mission operations are likewise in a distinct category, as the scientific rationale for the coronagraph program is distinct from the NWNH rationale for WFIRST. In assessing the alignment of WFIRST with NWNH priorities, it is valuable to distinguish these three cost categories as descopes are considered, because the first category is directly comparable to the NWNH cost estimate, the second was called out in NWNH as expected but not included in their cost estimate, and the third represents new scope for the WFIRST mission.
Many of the decisions during Phase A study will take the form of trade choices in which increased performance can be achieved with increased cost or risk. In addition, assessing the value of potential descopes is a substantial part of the project work in Phase A. To remain consistent with the rationale for WFIRST’s high ranking in NWNH, trade decisions and scope definitions during Phase A must be made with control of cost and risk as a central consideration. NASA Astrophysics Division Director Paul Hertz discussed with the committee the plans for management, reviews, and cost control during WFIRST formulation and development. In the committee’s assessment, these plans maintain a proper emphasis on the importance of cost control in all phases of the project.
A more accurate mission cost assessment will be possible at the end of Phase A, when the mission goals and requirements are fully defined and the mission architecture is more completely understood. After entering Phase B, cost reductions are difficult to achieve because they would typically require changes in scope or requirements. Therefore, Key Decision Point B (marking the entry into Phase B) is a critical time at which to evaluate the WFIRST mission cost estimate and, if necessary, consider changes in mission scope that would maintain the cost and risk at a level that is consistent with the overall program envisioned by NWNH. This action is intended to preserve the scientific priorities of NWNH by enabling Recommendation 4-3 (the Explorer program), Recommendation 4-4 (LISA), and Recommendation 4-5 (Athena), and would preserve a balanced astrophysics program by maintaining support for individual investigator programs (see discussion of a balanced program in Chapter 2).
RECOMMENDATION 4-1: Prior to Key Decision Point B, NASA should commission an independent technical, management, and cost assessment of the Wide-Field Infrared Survey Telescope, including a quantitative assessment of the incremental cost of the coronagraph. If the mission cost estimate exceeds the point at which executing the mission would compromise the scientific priorities and the balanced astrophysics program recommended by the 2010 report New Worlds, New Horizons in Astronomy and Astrophysics, then NASA should descope the mission to restore the scientific priorities and program balance by reducing the mission cost.
As previously noted, a faster development schedule would reduce the total mission cost for WFIRST, in addition to advancing its scientific impact. These considerations favor a steeper funding profile if budgets allow it, but it is important not to sacrifice program balance (e.g., Explorer Announcements of Opportunity [AOs] and core research grant programs) within the decade. One strong consideration favoring an accelerated schedule is the desirability of overlap between WFIRST and JWST. As discussed in the 2015 SDT report (Appendix B),25 the wide field of WFIRST and enormous sensitivity of JWST give the two missions a unique synergy. WFIRST can discover rare objects that are ideal targets for JWST spectroscopic follow-up, such as the brightest z = 12 galaxies, pair instability supernova candidates, and the most extreme gravitational lenses. The combination of large-scale context and detailed measurements over small fields is essential for other investigations, such as the study of stellar streams in nearby galaxies.
FINDING 4-6: The unique scientific opportunity afforded by combined WFIRST/JWST observing programs favors development and launch of WFIRST on the earliest schedule that is technically and financially feasible.
As of this writing, there appears to be significant Canadian and Japanese interest in WFIRST participation, with both countries having appointed members to the WFIRST-AFTA SDT. International partnerships could lower the U.S. cost for WFIRST.
U.S. Participation in Euclid
NWNH noted that the science program of the Euclid mission, then in definition phase and competing for a European Space Agency (ESA) M-Class launch slot, overlapped the dark energy program of WFIRST, using similar techniques but a substantially different hardware implementation and strategy. NWNH noted the possibility of a combined mission, provided that the United States would have a
“leading role” and that all of the WFIRST scientific objectives would be addressed. Soon after the completion of NWNH, it became clear that JWST schedule delays and cost increases would severely impact the development schedule of WFIRST as envisioned by NWNH. In this context, the Office of Science and Technology Policy requested an NRC Panel on Implementing Recommendations from the New Worlds, New Horizons Decadal Survey to assess several options for coordinating WFIRST and Euclid, including a plan being negotiated by NASA for a 20 percent U.S. share in Euclid. The panel concluded that a 20 percent U.S. Euclid share would be inconsistent with the recommendations of NWNH because it would not achieve the objectives of WFIRST and would have a significant negative impact on NASA’s ability to implement NWNH priorities.26
NASA subsequently requested an NRC Committee on the Assessment of a Plan for U.S. Participation in Euclid to evaluate a proposed U.S. hardware contribution (near-infrared detectors) to Euclid of approximately $20 million. That committee concluded that “the NASA proposal [U.S. participation in Euclid] would represent a valuable first step toward meeting one of the science goals (furthering dark energy research) of NWNH for WFIRST.”27 It recommended that NASA make a hardware contribution of approximately $20 million “in the context of a strong U.S. commitment to move forward with the full implementation of WFIRST,”28 and that NASA negotiate with ESA to secure a U.S. position on the Euclid Science Team and inclusion of a team of U.S. scientists in the Euclid Consortium. It also recommended that any hardware contribution exceeding $30 million be subject to an independent community review. The committee also recognized that “additional support for this science team will cost an additional ~$2 million per year for about 10 years, for a total similar to the hardware investment.”
At present, NASA has purchased $30 million worth of near-infrared detectors and is also investing approximately $30 million in packaging and testing of these detectors before transfer to ESA. NASA has selected one U.S. Euclid science team comprised of three groups of investigators totaling more than 50 scientists who are now members of the Euclid Consortium, and the principal investigator (PI) of the largest investigator group is a member of the 12-person Euclid Science Team and the Euclid Collaboration Board. NASA has budgeted $17 million of funding for support of these science teams through the 2020 Euclid launch and budgeted further funding for the science team after launch brings the total to $50 million. NASA also plans to fund a U.S. node of the science ground system for $50 million. U.S.
26 NRC, 2012, Report of the Panel on Implementing Recommendations from the New Worlds, New Horizons Decadal Survey, The National Academies Press, Washington, D.C.
27 NRC, 2014, Assessment of a Plan for U.S. Participation in Euclid, The National Academies Press, Washington, D.C., p. 1.
scientists making use of archived Euclid data would do so in guest investigator mode, but it has not yet been determined how support of those scientists will be structured. Thus, at present the total cost of U.S. participation in Euclid is likely to be in the range of $150 million to $200 million, with the majority of that cost going to support of U.S. science teams and archive activities.29 This level of participation in Euclid is significantly in excess of the $40 million to $50 million recommended by the NRC Committee on the Assessment of a Plan for U.S. Participation in Euclid.
FINDING 4-7: NASA’s investment in Euclid, expected to total between $150 million and $200 million by the end of the mission, is a significant augmentation of the dark energy science program budget beyond the level envisioned by NWNH and by the NRC Committee on the Assessment of a Plan for U.S. Participation in Euclid.
This augmentation has a number of salutary effects. Most importantly, it allows early participation by U.S. scientists in space-based dark energy investigations, and the presence of U.S. investigators on both Euclid and WFIRST science teams enables much better coordination of the two mission’s science programs. It will eventually allow joint analyses of data sets to test for systematics and improve statistical constraints. However, this investment necessarily reduces NASA’s ability to address other NWNH priorities.
RECOMMENDATION 4-2: In the remainder of the decade, NASA should treat support of Euclid participation beyond the existing commitments to the European Space Agency as lower priority than support of the Explorer program, gravity wave technology development, and X-ray technology development.
Table 4.2 provides a summary of changes to WFIRST, beginning with the JDEM-Omega design described as a template by NWNH. Only the changes that most impact mission architecture or scientific capabilities are listed. From DRM1 to WFIRST-AFTA, pixel count changes from 151 Mpix (0.18′′/pixel) to 302 Mpix (0.11′′/pixel). Projected costs from 2013 onward include $0.1 billion for a funded GO program. Projected costs from 2015 onward include coronagraph and additional year of prime mission operations. Cost projections are based on combinations of WFIRST Study Office analyses and CATEs performed by Aerospace Corporation.
Augmentation to the Explorer Program
The augmentation to the Explorer Program was the second priority among the space-based, large-class priorities in NWNH. The recommendation was to augment the “current plan by 2 Medium-scale Explorer (MIDEX) missions, 2 Small Explorer
29 Input from NASA, March 25, 2016.
(SMEX) missions, and 4 Missions of Opportunity (MoOs).”30 The committee finds the NWNH recommendation for the Explorer program to be ambiguous, with reasonable interpretations implying four or six missions (and equal number of MoOs) over the decade. The committee believes that the intent of NWNH was that this recommendation be for an augmentation of four missions during the time period 2012 to 2021, in addition to the two missions that would otherwise have been deployed, for a total of six.31 The Explorer program augmentation was one of the recommendations that NWNH prioritized even in the eventuality that the NASA budget available for new projects was not as high as NWNH assumed.
Historically, Explorer missions have been remarkably successful in delivering groundbreaking scientific results at moderate cost. Recently completed missions include the Wilkinson Microwave Anisotropy Probe (WMAP), the Galaxy Evolution Explorer, the Rossi X-ray Timing Explorer, and the Wide-Field Infrared Survey Explorer. The Nuclear Spectroscopic Telescope Array (NuSTAR) and Swift are the Explorer missions currently in operation, and they continue to make notable scientific contributions. ASTRO-H (a MoO in collaboration with the Japan Aerospace Exploration Agency [JAXA]) was launched on February 12, 2016. Explorer missions in development are the Neutron-star Interior Composition Explorer (NICER; another MoO to be launched February 2017 to the International Space Station) and the Transiting Exoplanet Survey Satellite (TESS; launch planned for December 2017).
Recognizing that the Explorer program’s small- and medium-sized missions “enable rapid response to new discoveries and provide platforms for targeted investigations essential to the breadth of NASA’s astrophysics program,”32 NWNH ranked an augmentation to the Explorer program as its second priority in the category of large space-based projects. In comparison to the Explorer program’s original intent of deploying a SMEX or MIDEX every other year, NWNH noted the rate of two per decade represented a significant lost scientific opportunity. Therefore, NWNH recommended that NASA support the selection of “two new astrophysics MIDEX missions, two new astrophysics SMEX missions, and at least four astrophysics MoOs over the coming decade.”33 The recommended augmentation was based on the “high level of scientific return on relatively moderate investment . . . that provides the capability to respond rapidly to new scientific and technical breakthroughs.”34
30 NRC, 2010, New Worlds, New Horizons, p. 8.
31 Based on input from Roger Blandford, Lynne Hillenbrand, and Marcia Rieke, February 4, 2016.
32 NRC, 2010, New Worlds, New Horizons, p. 208.
33 NRC, 2010, New Worlds, New Horizons, p. 209.
34 NRC, 2010, New Worlds, New Horizons, p. 3.
NASA’s Science Mission Directorate separated the budgeting and selection of the heliospheric and astrophysics Explorer programs to enable implementation of this NWNH recommendation. In September 2014, NASA-APD released the first AO for astrophysics Explorers in this decade (an earlier AO did not proceed to selection). The 2014 AO was for one SMEX and one MoO. The SMEX mission budget was capped at $175 million (FY2015 dollars) (including the cost of access to space, but not including any contributions; NASA-provided launch services may be proposed at a charge of $50 million in FY2015 dollars against the PI-managed mission cost; launch date of selected mission no later than end of 2020). The MoO call was capped at $65 million and suborbital-class missions were capped at $35 million (FY2015 dollars).
As a result of the 2014 AO, three SMEX projects are undergoing Phase A studies for a down-select to one mission expected for summer 2016: IXPE (Imaging X-ray Polarimeter Explorer), PRAXyS (Polarimeter for Relativistic Astrophysical X-ray Sources), and SPHEREx (an all-sky near-infrared spectral survey). In addition, two MoOs are undergoing Phase A studies in response to the same AO issued in 2014, also with a downselect to one MoO planned in summer 2016. These are U.S. participation in JAXA’s LiteBIRD cosmic microwave background polarization survey and GUSTO (Gal/Xray U/LDB Spectroscopic Stratospheric Terahertz Observatory).
For the rest of the decade, NASA plans to release a MIDEX AO in FY2016 (but no earlier than September), (including one MoO), another SMEX AO (with one MoO) in 2019, and a MIDEX (with one MoO) in 2021.35
Because of budget constraints, NASA’s implementation of the augmented Explorer program did not begin as early in the decade as originally planned. The current NASA-APD plan places a high priority on selecting four missions, despite the budget constraints. Even if fully executed, however, the plan does not result in the full augmentation recommended by NWNH.
The committee is concerned that growth in NASA-APD’s large programs may prevent even this reduced implementation of the NWNH Explorer program. Therefore, the committee emphasizes the high priority it places on the implementation of the plan with the following recommendation:
RECOMMENDATION 4-3: NASA’s Astrophysics Division should execute its current plan, as presented to the committee, of at least four Explorer Announcements of Opportunity during the 2012-2021 decade, each with a Mission of Opportunity call, and each followed by mission selection.
Regrettably, the full augmentation recommended by NWNH may not be executable in the current environment. However, if budgets increase, then restoring the full Explorer augmentation would be consistent with the priorities of NWNH.
35 Paul Hertz, NASA, “Astrophysics,” presentation to committee February 26, 2016.
LISA was ranked third among the space-based, large-class priorities in NWNH. The survey described LISA as employing three spacecraft to detect long-wavelength gravitational waves. At the time, LISA was a partnership with ESA, and key technologies remained to be demonstrated, and so the NWNH ranking of LISA was based on an equal partnership between NASA and ESA in the execution of the mission and on the success of the LISA Pathfinder (LPF) mission.
The first half of the decade has brought tremendous progress in gravitational wave astronomy. The first direct detection of gravitational waves by Advanced LIGO (Advanced Laser Interferometry Gravitational-wave Observatory) is a ground-breaking achievement and establishes gravitational wave astronomy at tens of hertz to kilohertz frequencies as a new probe of a wide range of astrophysical phenomena, including black hole growth and evolution, endpoints of stellar evolution, nucleosynthesis of the heaviest elements and nuclear equations of state, and precision tests of general relativity. In the low-frequency regime (10−7 to 10−9 Hz), the steady accumulation of pulsar timing data, the discovery of many new stable millisecond pulsars, and advancements in instrumentation have improved sensitivity to the point that new upper limits are challenging understanding of the populations of supermassive black hole binaries. The impressive early results from the LPF mission36 have demonstrated the key technologies needed for a future space mission to cover the source-rich millihertz portion of the gravitational wave spectrum.
FINDING 4-8: The first direct detection of gravitational waves by Advanced LIGO is a ground-breaking achievement that establishes gravitational wave astronomy as a revolutionary new probe of astrophysical phenomena.
Because of the JWST schedule delay and cost increase, and LISA’s ranking behind WFIRST in NWNH, it became clear early in the decade that NASA would not have the resources to begin a gravitational wave space mission in the 2010s. Therefore, in 2011 the ESA/NASA co-equal partnership was dissolved. Both NASA and ESA subsequently sponsored studies of possible missions that might meet the scientific goals of LISA as described in NWNH but at significantly less cost. Several concepts were costed by the United States, but none resulted in a mission cost of less than $1.2 billion. The European efforts resulted in the European-led Evolved Laser Interferometer Space Antenna (eLISA) concept with two arms, a reduced arm length and reduced mission time. In the ESA Cosmic Visions competition
36 M. Armano, H. Audley, G. Auger, J.T. Baird, M. Bassan, P. Binetruy, M. Born, et al., 2016, Subfemto-g free fall for space-based gravitational wave observatories: LISA Pathfinder results, Physical Review Letters 116:231101.
for large mission launch opportunities, a planetary mission was selected for L1 (the first opportunity). The L2 and L3 launch opportunities were competed as science themes, rather than specific missions, and supported by notional mission architectures. The “Hot and Energetic Universe” (notionally the Athena X-ray mission) theme was selected for the L2 opportunity with launch in the 2020s, and the “Gravitational Universe” (notionally the eLISA mission) theme was selected for the L3 opportunity with launch in the 2030s. The call for mission concepts to address the Gravitational Universe science theme is scheduled for late 2016, with selection in 2017. This call is expected to allow for international contributions of up to 20 percent of the mission cost, with the stipulation that the success of the mission not be compromised by a failure to deliver on the promises made by the international partners. The Chinese and Japanese agencies have expressed interest in participating, as has NASA.
The U.S. groups working on LISA at NASA Goddard Space Flight Center, the Jet Propulsion Laboratory, and in the university community were vastly reduced in size following the 2011 decision by NASA not to proceed with LISA in this decade. During FY2010 and FY2011, the LISA project was funded by NASA at a level of approximately $3 million per year. Since then, the groups have secured only competed funding at a level of about $300,000 per year, with the total for FY2012 to FY2015 (the NWNH timeframe) at $1.3 million.
FINDING 4-9: The dissolution of the U.S. LISA project, and the attendant loss of science and technology funding, has severely impacted preparations for a space gravitational wave mission. If this situation persists, the options for significant U.S. participation in this revolutionary discovery area will be limited.
NASA is currently authorized to plan for U.S. participation in an eLISA mission at the reduced cost of $150 million, which equates to a 10 percent stake in the mission. The current expectation is that some subsystems will be contributed by NASA, and NASA will continue to support a small group of scientists to plan and support the science of eLISA.
In November 2015, LPF was successfully launched, 3 years later than the launch date assumed by NWNH. LPF is now at the Sun-Earth L1 Lagrange point, its intended operating point where gravitational gradients are minimized. LPF tests microNewton thrusters, the inertial control reference system for the proof masses, and picometer interferometry. The mission entered primary science mode at the end of February 2016, and within hours demonstrated disturbance-free flight at residual noise levels that meet or exceed the level-1 science requirements.
The NWNH ranking of LISA was based on an equal partnership between NASA and ESA in the execution of the mission and on the success of LPF. Specifically, NWNH recommended that a decadal survey implementation advisory committee
(DSIAC)37 review the situation mid-decade if LPF “is not a success or if a roughly equal partnership is not possible.”38 While Europe has now taken the lead, the opportunity exists for the United States to play a significant role in a gravitational wave mission if the community acts quickly and decisively in the next few years.
FINDING 4-10: Results of the LPF mission have demonstrated the feasibility of many of the key technologies needed to carry out a space gravitational wave mission, and ESA has selected a gravitational wave theme for the L3 large mission opportunity. These developments address two of the main conditions identified in NWNH for U.S. participation in a gravitational wave mission.
The science of LISA is even more compelling than in 2010 with the success of Advanced LIGO in making a direct detection of gravitational waves. NWNH envisioned a major U.S. role in this exciting scientific opportunity. It seems appropriate for NASA and ESA to rethink their strategies—namely, to ensure that the new science enabled by a gravitational wave detector operating at low frequencies, at which some of the most interesting sources are found, is carried out successfully. Steps to reduce mission risk and to improve the scientific return to eLISA are as follows: (1) restore instrumentation for the third arm of the triangle, (2) use a larger rocket delivery system for the required increase in mass, (3) increase the lifetime of the mission, and (4) offer the services of U.S. spaceflight centers for the system engineering and integration of the mission. This would require a significantly larger U.S. contribution than the $150 million technology currently being considered. The newly formed NASA L3 study team would best serve its function by participating in the planning and organization with ESA scientists and by identifying a range of options for U.S. participation in the L3 mission.
RECOMMENDATION 4-4: NASA should restore support this decade for gravitational wave research that enables the U.S. community to be a strong technical and scientific partner in the European Space Agency (ESA)-led L3 mission, consistent with the Laser Interferometer Space Antenna’s high priority in the 2010 report New Worlds, New Horizons in Astronomy and Astrophysics (NWNH). One goal of U.S. participation should be the restoration of the full scientific capability of the mission as envisioned by NWNH.
37 The DSIAC was envisioned as a standing body to oversee the implementation of NWNH. The Academies’ Committee on Astronomy and Astrophysics is currently filling part of this role, while separate, independent study committees of the Academies fill in another part. In practice, the mid-decadal committee is carrying out the mid-decadal assessment that the DSIAC was envisioned to perform.
38 NRC, 2010, New Worlds, New Horizons, p. 19.
IXO was ranked fourth among the space-based, large-class priorities in NWNH. The mission was described as a versatile, large-area, high-spectral resolution X-ray telescope. The X-ray mission IXO addressed many of the high-priority science goals of NWNH, but because of “the technical cost, and programmatic uncertainties associated with the project at the current time,”39 the mission was ranked fourth by NWNH. Nonetheless, given the importance of the science addressed, NWNH recommended a technology development program with an estimated cost of about $200 million for the 2010 decade. Furthermore, NWNH stated that if IXO were selected as the first L-class mission by ESA, then “NASA should proceed immediately with a DSIAC review to determine an appropriate path forward to realize IXO as soon as possible with acceptable cost and schedule risk.”40
As described above, ESA selected the Hot and Energetic Universe science theme for an L2 mission with launch in 2028 and proceeded to develop an IXO-like mission as a European-led project with restrictions on international participation. The resulting mission, named Athena, is a down-scoped, reduced-cost version of IXO. Performance in some key areas has been maintained at near-IXO levels, and in addition, the mission has the capability to perform deep and wide X-ray surveys. Athena is currently in an early stage of formulation and details of the international partnership (which includes Japan) are being negotiated. An ESA decision to formally start Athena is expected in late 2018 or early 2019.
The Athena design features a large-area X-ray mirror with 2 square meters of collecting area and 5 arcsecond angular resolution. The focal plane has two instruments—a microcalorimeter and a wide-field imager. Compared to IXO, Athena lacks X-ray gratings, an X-ray polarimeter, and a hard X-ray instrument. However, the core capability recommended by NWNH for a next-generation X-ray mission is retained. A GO program open to the international community for Athena is also planned, as well as targeted key projects.
Soon after ESA selected the Hot and Energetic Universe science theme, NASA announced its intention to contribute to ESA’s mission and is currently planning a contribution of $150 million in hardware. A guiding principle is that a U.S. contribution must enhance the science value of the Athena mission. As of this writing, the exact U.S. contribution has not been finalized, but under discussion are contributions to the microcalorimeter and the wide-field imager, consistent with technology development goals for a future U.S.-led X-ray mission.
The Athena capabilities are a compelling subset of those of IXO, and Athena should execute much of the IXO science described by NWNH. Its core capability—-
high-throughput, high-resolution, spatially resolved X-ray spectroscopy—has been the defining feature of X-ray missions highly ranked in both the 2000 and 2010 U.S. decadal survey reports. NASA technology investments over the past 5 years, and the U.S. contributions to Athena currently under discussion, align well with the NWNH recommendation for the IXO technology investment.
RECOMMENDATION 4-5: NASA should proceed with its current plan to participate in Athena, with primary contributions directed toward enhancing the scientific capabilities of the mission.
The ongoing preliminary mission design for Athena indicates that a descope may be necessary if the total cost is to remain within the €1 billion ESA target. The most likely descope currently discussed involves reducing the mirror effective area, leading to about a 30 percent lower efficiency for most of the science observations envisioned for Athena. The committee believes that the currently planned NASA contribution to Athena is still merited with such a descope. Furthermore, it is possible that an increased level of NASA funding for the Athena mission can prevent a descope and thus significantly increase the amount of science that can be accomplished in the nominal life of Athena. The committee does not recommend a specific action because, as of this writing, the exact nature and the cost of such an increased contribution, which would prevent a descope, are unknown. A future decision on increased U.S. funding for Athena would be inconsistent with NWNH if it distorted the overall balance of the NASA-APD program.
For reference, Table ES.4 of NWNH is reproduced below (Table 4.3). The ranked recommendations in this category were a New Worlds Technology Development Program and an Inflation Probe Technology Development Program.
New Worlds Technology Development Program
NWNH identified exoplanetary science as one of the fastest-growing and exciting fields of astrophysics. An ultimate goal in this field is to image rocky planets in the “Habitable Zones” of nearby stars and to characterize their atmospheres with spectroscopy. Two key precursor activities were noted. First, the demographics of planetary systems should be measured accurately enough to predict in a statistical sense how common Earth-like planets are around nearby stars. This demographic survey is being realized by Kepler, WFIRST microlensing, and ground-based Doppler measurements. Second, observations are needed to characterize the level of zodiacal light around nearby stars to determine at what level starlight scattered from dust will hamper planet detection. This will be inves-
TABLE 4.3 Medium-Scale, Space-Based Recommended Activities from the 2010 Astronomy and Astrophysics Decadal Survey
|Recommendation||Science||Appraisal of Costsa||Cross-Reference in Chapter 7|
|1. New Worlds Technology Development Program||Preparation for a planet-imaging mission beyond 2020, including precursor science activities||$100M to $200M||Page 215|
|2. Inflation Probe Technology Development Program||Cosmic microwave background (CMB)/ inflation technology development and preparation for a possible mission beyond 2020||$60M to $200M||Page 217|
a The survey’s cost appraisals are in FY2010 dollars and are committee-generated and based on available community input.
SOURCE: National Research Council, 2010, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., Table ES.4.
tigated by NASA’s investment in the Long Baseline Telescope Interferometer and could be satisfied by WFIRST coronagraphy. Finally, NASA’s support of an Extreme Precision Doppler Spectrograph capability helps address a key need identified in NWNH for exoplanet science and precursor investigations in advance of a large exoplanet mission, although not at the full level of precision for velocity measurements recommended by NWNH.
To prepare for a direct-detection mission, NWNH recommended a New Worlds Technology Development Program to advance starlight suppression technology. Funding was envisioned to be at $4 million per year early in the decade, in addition to generally available technology development funds. In addition, mission development was recommended at an appropriate level for the mission design and scope to be well understood. NWNH envisioned a technology down-select around mid-decade when the design requirements of an imaging mission have become clear. The report recognized that significant funding at a currently uncertain level would be needed for this mission. NWNH recommended that a mid-decadal committee be convened to determine the way forward, including the level of support associated with mission-specific technology development (notionally in the $100 million to $200 million range, but at the appropriate level as determined at mid-decade). The NWNH-proposed program was intended to allow a habitable-exoplanet imaging mission to be well formulated in time for consideration by the 2020 decadal survey.41
41 NRC, 2010, New Worlds, New Horizons, pp. 195 and 216.
Events played out rather differently than envisioned by NWNH. WFIRST was delayed significantly by JWST, and the mission was redesigned to take advantage of the 2.4-meter AFTA mirror. NASA funded technology development in coronagraphy and starshades, as well as probe mission concept studies for both technologies, and a coronagraph augmentation is now included in the WFIRST-AFTA mission. Selection of an L2 orbit for WFIRST-AFTA would enable the possible addition of a starshade in the future. Following congressional direction, WFIRST began Phase A in February 2016.
The committee reiterates Finding 1-7 from Evaluation of the Implementation of WFIRST/AFTA:
Finding 1-7: The WFIRST/AFTA coronagraph satisfies some aspects of the broader exoplanet technology development program recommended by NWNH by developing and demonstrating advanced coronagraphic starlight suppression techniques in space. (p. 38)
The total investment in NWNH-recommended technology development and precursor science, including expenditures in the first half of the decade and those planned for the second half, includes programs in coronagraph and starshade technology development and in precursor science. In addition, the coronagraph addition on WFIRST-AFTA is a significant investment in exoplanet technology development and science.
FINDING 4-11: The current planned decadal investment in NWNH-recommended technology development and precursor science exceeds the level envisioned in NWNH.
The committee believes that NASA’s continued development of coronagraph and starshade technology at a modest level for mission design, scope, and capability is a positive step and that this activity would be profitably evaluated by the next decadal survey. However, given the substantial advances already enabled by WFIRST coronagraph development, the committee assigns higher priority to supporting adequate gravitational wave technology development than to further exoplanet technology development beyond WFIRST.
Inflation Probe Technology Development Program
NWNH’s second-ranked, space-based activity in the medium-scale category was a technology development effort (with a view to preparing a mature proposal for a dedicated space mission to study inflation through cosmic microwave background [CMB] observations for consideration by the 2020 decadal survey) contingent on having made a positive B-mode detection from the epoch of inflation. The report also recommended that the NASA Astronomy and Physics Research
and Analysis program should augment support for CMB technology development “at a modest level.”42
A major research goal from the time shortly before NWNH to present has been a search for B-mode polarization as evidence for primeval gravitational waves created during inflation. The strength of the B-mode waves indicates the energy associated with the field causing inflation, and through this connection, the measurement of B-modes has become a strong test of the inflation hypothesis and a possible way to distinguish between models. The measurement of B-mode polarization is much more difficult than measuring the temperature anisotropies. The temperature anisotropies have amplitudes of tens of microkelvin, while B-mode components in an inflation model with tensor-to-scalar ratio of r ~ 0.1 would be of order 10 nanokelvin, and the actual level of B-mode polarization might be much smaller. Further difficulties are strong foreground emission and the conversion of E-mode patterns into B-mode patterns by gravitational lensing.
At the time of NWNH, WMAP had made measurements of E-mode polarization patterns associated with the adiabatic density and temperature fluctuations as well as cross-correlation maps between the temperature anisotropy and the polarization. The polarization from both the epoch of recombination at a z = 1000 and reionization at z ~ 6 was identified, and electron optical depth at these epochs was determined. The lower limit for r < 0.22 came from the power exponent of the density fluctuations, ns, which was measured to be less than unity, allowing some contribution from gravitational waves. NWNH stated that
[t]he convincing detection of B-mode polarization in the CMB produced in the epoch of reionization would represent a watershed discovery. . . . If these fingerprints of inflation are detected, then a decadal survey implementation advisory committee (DSIAC) . . . could determine whether a technology development program should be initiated with a view to flying a space microwave background mission during the following decade that would be capable of improving the accuracy by a further factor of 10 and elucidating the physical conditions at the end of inflation.43
While this has not occurred, significant new results have come from the Planck mission; the WMAP and Planck CMB maps agree, and the cosmological parameters derived from them agree within the errors. The Planck polarization data is consistent with ground-based results from ACTpol, SPTpol, BICEP2/Keck, and POLARBEAR lensing B-modes.
The most recent measurements reflect enormous advances in detector systems. New detector systems consist of arrays of superconducting transition-edge bolometers with associated single-mode frequency filters and polarizers with large formats, including as many as 500 detectors connected by multiplexers to SQUID
42 NRC, 2010, New Worlds, New Horizons, p. 217.
43 NRC, 2010, New Worlds, New Horizons, p. 198.
amplifiers. The sensitivity of the individual detectors of 300 microK is close to the background limit on the ground.
Measurements from the South Pole as well as from the Atacama site in Chile are continuing. The emphasis is to increase observing sensitivity with even larger formats, to increase the wavelength coverage, and to cover more of the sky. The target is to achieve sensitivity to the gravitational waves that would be present at a tensor-to-scalar ratio of r ~ 0.01. Planck results for B-modes are anticipated soon and may also be able to measure or set limits at r ~ 0.01. On the ground, a new community-based effort has come together to define the science goals and instrument definition of CMB-S4: a stage IV program to deploy approximately 500,000 detectors spanning 30-300 GHz using multiple telescopes and sites to map ≳70 percent of the sky. CMB-S4 was highly ranked by the Department of Energy (DOE)- and National Science Foundation (NSF)-chartered Particle Physics Project Prioritization Panel (P5).
NASA is supporting a near-term plan for higher sensitivity with observations from long-duration balloon flights. To date, several groups have made long-duration balloon flights around the South Pole with polarization sensitive radiometers at multiple wavelengths. It is conceivable that balloon-borne payloads will reach r ~ 0.001 with large-format detectors developed for both ballooning observations and a possible space mission by NASA. The ballooning program can also test polarization modulators and space worthy cryogenic systems. In the Explorer program, the MoO (with JAXA) LiteBIRD has been selected for Phase A study, with a downselect expected in summer 2016.
FINDING 4-12: The Inflation Probe Technology Development program is well aligned with the recommendations of NWNH, with NASA, NSF, and DOE supporting technology development and precursor science. Third-generation ground-based efforts and a suborbital program are taking place, targeting CMB B-mode polarization. The proposed CMB-S4 program would push the limits of what can be achieved from the ground and advance understanding of the technology and science requirements for a possible future space mission.
The next decadal survey could consider a larger space mission, which may be the best way to achieve the most sensitive full-sky measurements with good control of systematic errors. In principle, it should be possible to measure the tensor-to-scalar ratio to the level limited only by the ability to remove the foreground contamination. As more information becomes available in the next 5 years, critical design decisions, such as the optimum frequency bands and the best angular scale for the beam, will become clearer.
NWNH identified several areas for augmented support in the small-scale space category, listed in alphabetical (not-ranked) order. The lower-than-expected available budget has meant that some of the recommended augmentations have not occurred. However, some programs have seen increases in excess of those recommended. Assessing which programs have seen increases or decreases is sometimes difficult, due to changes in accounting associated with folding elements of existing programs into new initiatives. In this section, each of the small-scale recommendations from NWNH is addressed in turn.
It was recommended that the Astrophysics Theory Program (ATP) be augmented by $35 million over the decade. This has not happened, and ATP funding has remained flat in real-year dollars since 2008. The continued eroding of support in this area threatens the science yield of the current and future mission because modest investments in theory often have very large impact.
An augmentation of $40 million over the decade was recommended for ultraviolet/optical (UV/O) technical development. The actual projected augmentation over the decade is $54 million. The funding increases in this area have been focused on exoplanet-related technologies such as coronagraphs and star shades. It is difficult to disentangle the increases in this area from those coming in response to the medium-scale recommendation for additional support for exoplanet imaging. The committee could not identify funding for non-exoplanet UV/O technical developments as recommended for a future ultraviolet space telescope.
NWNH identified a “mid-technology readiness level (TRL)” gap in support for technology development and recommended that spending on intermediate technology development be increased by $2 million per year early in the decade and $15 million per year by the end of the decade. NASA responded by establishing a new program, the Strategic Astrophysics Technology (SAT), which provided $17 million in FY2015; it is slated to grow to $30 million per year in FY2018. It is difficult to trace how much of this is new funding, because several existing programs were rolled into this one new program.
The $2 million per year augmentation of laboratory astrophysics augmentation has not occurred, and funding in this area is flat or slightly down. However, NASA reports that all proposals rated “excellent” or “excellent/very good” have been funded.
NWNH recommended a $150 million U.S. contribution to the JAXA-led Space Infrared Telescope for Cosmology and Astrophysics (SPICA) mission, but noted that a “reduced budget scenario would not permit participation in the JAXA-SPICA mission.”44 SPICA is now being proposed as a joint JAXA/ESA mission for ESA’s
44 NRC, 2010, New Worlds, New Horizons, p. 238.
2015 M5 AO. U.S. participation may be possible as a MoO in response to the 2016 MIDEX AO.
The Suborbital program was boosted by $7 million per year over FY2011-2012 to a total of $32 million per year in FY2013-2015, broadly in line with the $15 million per year augmentation recommended in NWNH. The augmentation has allowed for the development of ultralong-duration balloon flight capabilities and for an expanded program for mounting suborbital class payloads to the International Space Station. The development of orbital sounding rockets was investigated, but deemed cost prohibitive at this time.
NASA and NSF have established the Theoretical and Computational Astrophysics Networks (TCAN) program in response to the NWNH recommendation. The TCAN program started in FY2014, and was reviewed for continuation in 2015. Implementation has come late in the decade, and at a level significantly below the recommendation. The current NASA contribution is $1.5 million per year, while the recommended level was $5 million per year.
FINDING 4-13: NASA’s implementation of NWNH’s recommended small-scale activities has been mixed. Some recommended augmentations have not occurred and there have been cuts in some programs recommended for augmentation. Other programs, in particular the suborbital and exoplanet areas, have seen increases in excess of what was recommended by NWNH.
As noted earlier, NWNH put a premium on balance in both the ground-based and space-based programs. In the context of NASA, balance is achieved through a diversified portfolio that includes large flagship missions, medium-scale Explorer missions and technology development, and smaller suborbital, data analysis, theory, and laboratory astrophysics programs.
Since FY2011, NASA has reported increased funding in the core research and analysis programs by 22 percent. This category incorporates APRA, ATP, the Exoplanet Research Program (XRP; formerly Origins of Solar Systems), Astrophysics Data Analysis Program (ADAP)/Long Term Space Astrophysics (LTSA), and a modest new investment in the TCAN program at the level of $1.5 million per year. NASA investment in laboratory astrophysics program is included in the APRA component of the budget. Funding for the GO programs has remained relatively flat since FY2011. The launch of JWST is projected to lead to an overall increase in the GO budget toward the end of the decade.
It is instructive to consider, within the core research and analysis programs, those that directly support individual investigators. There are estimates that NASA participation in the U.S. astronomical enterprise provides about two-thirds of
the grant funding to individual investigators.45 Presentations to the committee by Paul Hertz and James Ulvestad reported $145 million in this area supplied by NASA and $75 million by the NSF in FY2015. Thus, any change in NASA’s funding profile has a major impact on the astronomical community. Public information provided to the committee shows that NASA’s funding for small and medium-sized programs aimed at data analysis and theory, defined here to be the sum of the GO programs (which include the programs for the Explorers, Hubble Space Telescope, and Chandra and international programs such as Herschel and the X-ray Multi-Mirror Mission, as well as the Astrophysics Data Analysis program and ATP) dropped from a high of $107 million in FY2006 to a low of $78 million in 2016. This drop of 26 percent in inflation-adjusted dollars has had a major impact on the support of the community and is likely a major contributor to a sharp drop in proposal success. Most of the drop is due to reductions in the budget for GO programs as existing missions exit prime operations. Budget constraints have implications for the scientific productivity of missions as expressed by the 2014 NASA Senior Review:
The operation of the nation’s space borne observatories is so severely impacted by the current funding climate in Washington that the SRP feels that American pre-eminence in the study of the Universe from space is threatened to the point of irreparable damage if additional funds cannot be found to fill the projected funding gaps.46
Likewise, a constant level of funding in the ADAP program has not kept pace with the growth in the volume of archival data available. Dr. Hertz reported to the committee that GO funding will increase later this decade as new missions go into operation.
One of the critical components of the balance in the NASA portfolio is the medium-scale Explorer missions and associated technology development. In response to NWNH, NASA has used the SAT program to support technology development directed at future strategic missions. Specific initiatives have focused on exoplanet, CMB, gravitational wave, and X-ray science, in addition to optics and detector development. Total funding over the first half of the decade has exceeded $64 million. Funding for coronagraph technology development has been included in the overall commitment to WFIRST. Funding for the Suborbital program has also been well supported. As discussed elsewhere in this report, however, support for Explorer missions in the first half of the decade has been minimal, although planned AOs between now and the end of the decade would remedy this shortfall.
45 NRC, 2000, Federal Funding of Astronomical Research, National Academy Press, Washington, D.C.
46 2014 NASA Astrophysics Senior Review, Rest of Missions Panel, Washington, D.C., p. 8.
FINDING 4-14: Despite a challenging budget environment, NASA-APD has maintained a balanced portfolio through the first half of the decade and, with the assumption of successful completion of an ambitious Explorer schedule, will do so during the second half of the decade as well. This stability, however, has been preceded by a decline in individual investigator funding during the last part of the previous decade.
The greatest challenge to maintaining a balanced portfolio is the cost of the JWST and WFIRST missions. The risk of cost growth remains higher for the current WFIRST design than for the NWNH version of WFIRST. For example, the FY2016 congressional appropriation, while providing full support for WFIRST, JWST, Hubble, and SOFIA (Stratospheric Observatory for Infrared Astronomy),47 has necessitated a $36 million reduction in the rest of the Astrophysics portfolio. As emphasized previously, growth in the cost of WFIRST beyond current estimates could seriously compromise NASA’s program balance going forward.
47 NWNH recommended that SOFIA participate in the senior review process to evaluate its role in NASA’s portfolio.