The Wide-Field Infrared Survey Telescope (WFIRST) mission was the highest-ranked large space mission in the New Worlds, New Horizons in Astronomy and Astrophysics1 (NWNH) decadal survey due to its combination of highly compelling science, moderate cost, and medium-low risk. The near-infrared imaging and low-resolution spectroscopy mission would address some of the most fundamental questions in astrophysics. Using three techniques—weak gravitational lensing, supernova distances, and baryon acoustic oscillations (BAOs)—WFIRST would probe the nature of dark energy. By monitoring a large sample of stars in the central bulge of the Milky Way, it would use microlensing to study the architecture of other solar systems. WFIRST is also expected to perform wide-field surveys to advance understanding of how galaxies, stars, and black holes evolve. After considering the WFIRST science program in the context of recent rapid scientific advances, and in light of the selection of Euclid and the confirmation of the Large Synoptic Survey Telescope, as well as the realization of other synergistic ground-based instruments, the committee offers the following finding:
Finding 3-1: The science case for the WFIRST wide-field infrared telescope concept generally specified by NWNH is as compelling today as when it was considered by NWNH.
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1 National Research Council, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., 2010.
Compared to the WFIRST/Interim Design Reference Mission (IDRM) implementation studied by NASA to respond to the WFIRST envisioned by NWNH, the larger aperture and better spatial resolution of WFIRST/Astrophysics Focused Telescope Assets (AFTA) enable it to reach to higher redshifts and achieve greater source densities. The WFIRST/AFTA supernova program, including spectroscopic follow-up with an Integral Field Unit added to the mission, will measure supernova redshifts over a range from z = 0.2 to z = 1.7. Compared to the IDRM, AFTA obtains more spectra (2,700 compared to 1,500) with better photometry out to higher redshift. For weak lensing, WFIRST/AFTA will cover a smaller area than the IDRM but will have a factor 1.5 more galaxies, allowing detailed dark matter maps. For the redshift survey to measure BAOs, WFIRST/AFTA will be somewhat better than the IDRM overall, but much better for the high-redshift range. WFIRST/AFTA will also carry out a microlensing survey to measure exoplanet system architectures of comparable power to WFIRST/IDRM, and AFTA drift scans would aid in removing systematic effects in interpretation of the microlensing data. With a 2.5-times larger aperture, double the number of pixels, and 60 percent better spatial resolution compared to the IDRM, WFIRST/AFTA will be a significantly more powerful telescope for the galaxy and black hole evolution surveys and for guest investigator observations.
Finding 3-2: The opportunity to increase the telescope aperture and resolution by employing the 2.4-m 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.
NWNH recommended an exoplanet technology development program as its top priority in the medium space category. This program has multiple components all aimed at preparing for a future Earth-like-planet imaging mission. Preparatory observations aimed at understanding the distribution of Earth-like planets, choosing targets, and measuring the levels of exozodiacal light that could hamper planet detection would retire key uncertainties. A multifaceted technology program to investigate methods of starlight suppression through coronagraphy and star shades would prepare to choose the eventual instrument implementation.
With the larger aperture of WFIRST/AFTA, a coronagraph could be added that would potentially address some of the recommendations in the NWNH exoplanet development program, as well as accomplish some of its planet-imaging scientific objectives. As a result, NASA is considering a coronagraph for WFIRST/AFTA. The design is currently immature, with two options for the optical configuration and three different mask designs being considered. All options being considered would demonstrate advanced techniques such as high-order wavefront control, low-order
wavefront sensing, and high-throughput spectroscopy that are very likely to be directly applicable to any future Earth-like-planet imaging mission that employs a coronagraph.
Finding 1-7: The WFIRST/AFTA coronagraph satisfies some aspects of the broader exoplanet technology program recommended by NWNH by developing and demonstrating advanced coronagraph starlight suppression techniques in space.
Because multiple coronagraph implementations are still being considered, the coronagraph’s performance is currently quite uncertain. For some implementations and technical assumptions, the addition of the coronagraph could make measurements of exozodiacal dust distributions that would fully inform target choice and design of a future Earth-like-planet imager. For less aggressive mask designs and technical assumptions, the advance over other ground-based techniques is modest.
Finding 1-8: Whether the WFIRST/AFTA coronagraph satisfies the NWNH goal to establish exozodiacal light levels at a precision required to plan an Earth-like-exoplanet imaging mission is uncertain due to the immaturity of the coronagraph design and uncertainty in the ultimate performance.
The decadal survey committee planned the NWNH program in a cost-constrained environment that has worsened since the report’s publication. The maturity of the WFIRST design and technologies and the relatively low technical, cost, and schedule risks associated with the WFIRST implementation considered by NWNH were important to the missions’ top ranking in the large category. NWNH placed strong emphasis on a balanced program of activities with significant increased investment in the Explorer and research and analysis (R&A) programs.
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.
Considering WFIRST/AFTA without the coronagraph, the assessment of the Aerospace Corporation cost and technical evaluation (CATE) is that WFIRST/AFTA is a $2.1 billion mission with a risk of cost growth significantly increased over WFIRST/IDRM (medium-low risk for the IDRM, compared to medium risk for AFTA). It is the opinion of the committee that the Aerospace CATE does not take into account the increased risk associated with the inherited hardware, which limits design and descope options at this early stage of mission planning.
Finding 2-2: The use of inherited hardware designed for another purpose results in design complexity, low thermal and mass margins, and limited
descope options that add to the mission risk. These factors will make managing cost growth challenging.
Following an evaluation informed by the Aerospace CATE, the committee offers the following finding:
Finding 2-4: The risk of cost growth is significantly higher for WFIRST/AFTA without the coronagraph than for WFIRST/IDRM.2
The WFIRST/AFTA coronagraph was presented to this committee as an instrument with significant technology development components. Advancing the technologies is, in fact, the principal rationale for the coronagraph’s inclusion on the mission. The coronagraph design itself is uncertain, with several different approaches being pursued. The very approximate cost of the instrument and the additional year of mission operations to accommodate the observing program provided by the project is $0.3 billion, making the total cost of WFIRST/AFTA with the coronagraph $2.4 billion. The cost impact to WFIRST/AFTA is, however, very uncertain because limited study has been completed to assess the cost associated with accommodating the coronagraph on the mission. No independent CATE of the mission with coronagraph has been performed.
Finding 2-5: The coronagraph design is immature, it involves immature technologies, and there has been limited study of accommodating the instrument on the mission. It is, therefore, not possible to quantitatively assess the cost and risk impact to the WFIRST/AFTA program.
In general, technology demonstration missions accept higher risk and have higher schedule uncertainty than science-driven missions. As such, the committee finds the following:
Finding 2-6: 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.
Finding 2-7: WFIRST’s moderate cost, low technical risk, and mature design were important to its ranking as the top priority for a large space mission in NWNH. The inclusion of the coronagraph compromises this rationale.
NWNH provided NASA with advice on the relative priority of programs in the large, medium, and small categories in the event of a budget that was reduced rela-
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2 See also Finding 2-2.
tive to the planning wedge provided by NASA. NWNH stated that the implementation of the WFIRST primary science, the Explorer augmentation, and the R&A program enhancements were all ranked above the exoplanet technology development program.3 Because neither the Explorer or R&A program recommendations have been fully implemented, the committee finds the following:
Finding 2-8: Without corresponding augmentation to other NASA programs accompanying funding to include the coronagraph on WFIRST, the inclusion of the coronagraph is not consistent with stated priorities in NWNH. In a time of reduced budgets, the first priority in NWNH is “to develop, launch, and operate WFIRST and to implement the Explorer program and core research program recommended augmentations.” Implementing the coronagraph would address some aspects of the exoplanet technology development; the exoplanet technology development program was considered a lower priority by NWNH.
The committee recognizes the high scientific importance of and public excitement surrounding a mission that could image Earth-like planets. Implementing a coronagraph on WFIRST/AFTA would advance this eventual goal and potentially provide scientific return responsive to NWNH exoplanet goals. However, because of the priority NWNH placed on the WFIRST primary science and the implementation of a balanced program, the committee recommends the following:
Recommendation 2-1: NASA should move aggressively to mature the coronagraph design and develop a credible cost, schedule, performance, and observing program so that its impact on the WFIRST mission can be determined. Upon completion of this activity, and a cost and technical evaluation of WFIRST/AFTA with the coronagraph, an independent review focused on the coronagraph should be convened to determine whether the impact on WFIRST and on the NASA astrophysics program is acceptable or if the coronagraph should be removed from the mission.
After the completion of the review focused on the coronagraph, an external evaluation of the cost and technical risks of the resulting implementation that is more detailed than a CATE would help NASA to ensure the mission is properly scoped and includes adequate contingency to match the available budget. The committee therefore recommends the following:
Recommendation 3-1: NASA should sponsor an external technical and cost review of the WFIRST/AFTA mission that NASA plans to propose as a new
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3 National Research Council, New Worlds, New Horizons in Astronomy and Astrophysics, 2010, p. 237.
start. This review should be independent of NASA’s internal process. The objective of the review should be to ensure that the proposed mission cost and technical risk are consistent with available resources and do not significantly compromise the astrophysics balance defined in the 2010 National Research Council report New Worlds, New Horizons in Astronomy and Astrophysics. This review should occur early enough to influence the exercising of a re-scoping of the mission if required.
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