The Wide-Field Infrared Survey Telescope (WFIRST) mission as envisioned by the 2010 National Research Council (NRC) decadal survey report New Worlds, New Horizons in Astronomy and Astrophysics1 (NWNH) is a near-infrared imaging and low-resolution spectroscopy mission designed to address some of the most fundamental questions in astrophysics. Using three techniques—weak gravitational lensing, supernova distances, and baryon acoustic oscillations—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 was also expected to perform wide-field surveys to advance understanding of how galaxies, stars, and black holes evolve. The hardware implementation assumed in NWNH was the same telescope and camera that had been studied for a mission called the Joint Dark Energy Mission-Omega (JDEM-Omega), which assumed the use of a 1.5-m telescope equipped with a large near-infrared focal plane. The primary change between JDEM-Omega and WFIRST (as recommended by NWNH) was the broadening of the observational program to include exoplanet system architecture studies using microlensing and wide-field surveys (which had both been separately proposed to the decadal survey as standalone missions requiring similar telescopes to JDEM), with little or no change to the hardware implementation. NWNH ranked WFIRST as the highest-priority recommendation for large space missions due to the compelling science, but also
1 National Research Council, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., 2010.
because of the moderate cost ($1.6 billion in fiscal year (FY) 2010 dollars for the JDEM-Omega implementation) and the medium-low technical risk. Due to these factors, NWNH envisioned a launch by the end of the decade.
After NASA received the NWNH recommendations, it chartered a science definition team to optimize the mission hardware to address all three science goals. This exercise resulted in a modified mission design that would employ a 1.3-m off-axis focal telescope, deployed at L2 and operated at 240 K, with a long wavelength cut-off of 2.0 microns. Because of the unobscured off-axis nature of the telescope, a slightly smaller aperture (1.3 m) could achieve the same science goals as JDEM. This version was named the WFIRST/Interim Design Reference Mission (WFIRST/IDRM). The IDRM was subjected to an independent cost and technical evaluation (CATE). The CATE was performed by Aerospace Corporation under contract with NASA. The IDRM is close enough in implementation to the WFIRST implementation recommended by NWNH that the performance, cost, and risk were evaluated to be comparable.
Funding limitations to date have prevented any version of WFIRST, as recommended by NWNH and studied as the WFIRST/IDRM, from undergoing a detailed design study, such that implementation could begin and launch occur close to the end of the decade, on a schedule originally envisioned by NWNH. In the summer of 2012, NASA announced that the National Reconnaissance Office had given NASA hardware for two 2.4-m telescopes to be used to support NASA science programs. The gifted hardware is not a complete fore-optics assembly: the electronics and actuators were not included, the primary and secondary mirror assemblies require disassembly and recoating due to their age, and other modifications are needed to tailor the design for a NASA science mission. The telescopes were also not intended for use much below room temperature. However, even with these limitations, the 2.4-m aperture was immediately recognized as being a possible attractive attribute for NASA missions such as WFIRST. NASA began a study of how to best employ this telescope hardware, eventually named the Astrophysics Focused Telescope Assets (AFTA).
A modified WFIRST mission employing the AFTA hardware was studied in late 2012 and 2013, resulting in the report Wide-Field InfraRed Survey Telescope Astrophysics Focused Telescope Assets: Final Report by the Science Definition Team (SDT) and WFIRST Project.2 This study recognized that the larger aperture was attractive for including a coronagraph on the baseline mission, which could potentially advance exoplanet objectives that were highly ranked in NWNH.
2 WFIRST-AFTA Science Definition Team and WFIRST Project, Wide-Field InfraRed Survey Telescope Astrophysics Focused Telescope Assets: Final Report by the Science Definition Team (SDT) and WFIRST Project, May 23, 2013, http://wfirst.gsfc.nasa.gov/science/sdt_public/WFIRST-AFTA_SDT_Final_Report_Rev1_130523.pdf.
With the James Webb Space Telescope launch approaching in 2018, a funding wedge may open up in the next few years that makes it timely to consider how to advance the priorities given in NWNH. However, the WFIRST/AFTA implementation, a larger telescope with the possible addition of a coronagraph, differs in science scope and also in technical complexity and cost from the WFIRST implementation considered by NWNH. As a result, NASA asked the NRC to convene a study to address the following:
- Compare the WFIRST mission described in New Worlds, New Horizons to the WFIRST/AFTA design reference mission, with and without the coronagraph, on the basis of their science objectives, technical complexity, and programmatic rationale, including projected cost.
- Based on the above comparison and taking into account any relevant scientific, technical, and programmatic changes that have occurred since the release of New Worlds, New Horizons:
- Assess the responsiveness of the WFIRST/AFTA mission, with and without the coronagraph, to the overall strategy to pursue the science objectives of NWNH—and in particular, WFIRST; and
- Assess the responsiveness of the WFIRST/AFTA mission with the coronagraph to the precursor science and technology objectives of the New Worlds technology development program described in NWNH.
See Appendix A for the committee’s full statement of task.
The committee was informed that NASA is no longer studying the implementation of WFIRST with a 1.3-m telescope (WFIRST/IDRM), and the committee was not asked to assess the scientific or programmatic rationale of the WFIRST/IDRM implementation. Therefore, the committee was not asked to recommend which version of WFIRST should be implemented. The committee was also not asked to recommend whether the coronagraph should be added to WFIRST/AFTA, but rather was asked to assess how the coronagraph might advance NWNH technology development and science goals, and to comment on whether its addition is consistent with the programmatic rationale that led to WFIRST’s top ranking in the large space mission category. It is with the understanding of this restricted charge that the committee wrote its report.
In addition to presentations to the committee (see Appendix B), the committee relied on the NWNH survey committee’s report, the WFIRST/AFTA Science Definition Team report, and responses to follow-up questions submitted to the project to complete its evaluation. Recommendations regarding the WFIRST implementation and program were made in the NWNH panel reports3 and, in particular, the Report of the Panel on Electromagnetic Observations from Space; however, not all of these recommendations were adopted in the main NWNH report. The current
3 National Research Council, Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., 2011.
committee, therefore, refers only to recommendations made in the main decadal survey report.
COMPARISON OF THE WFIRST/IDRM AND WFIRST/AFTA IMPLEMENTATIONS
Table I.1 provides a basic comparison of WFIRST/IDRM (the implementation studied by NASA to respond to the NWNH recommendation) to WFIRST/AFTA. In addition to the larger on-axis mirror, WFIRST/AFTA has added an Integral Field Unit (IFU) primarily for spectroscopic follow-up of supernovae, which on WFIRST/IDRM would have been performed with a slitless prism. For AFTA, slitless spectroscopy will be performed with a grism rather than a prism. The AFTA wide-field imager employs 18 larger-format near-infrared detectors as opposed to 28 smaller format devices. WFIRST/IDRM was to be deployed in an L2 orbit, while WFIRST/AFTA will be in geosynchronous orbit. A further requirement from NASA is that WFIRST/AFTA be designed so as to be robotically serviceable. The mission lifetime is 5 years for both missions, unless the coronagraph is added, in which case WFIRST/AFTA will be designed and costed for 6 years of operations to accommodate the time required for coronagraph-specific observing.
While the IFU is an additional instrument not envisioned for WFIRST by NWNH, its addition to AFTA enables the simplification of the instrument complement, with the removal of a guider and two instrument channels. The IFU additionally reduces roll angle constraints, allowing for reduced solar array size and slightly simpler overall thermal design. There are no new developments for this instrument, and as a result of the design simplifications and removal of a guider, the project does not consider this addition to add significant complication or cost to the mission.
The option of adding a coronagraph is being studied by NASA, but it is currently not part of the baseline mission. If added, 1 year of operations would have to be added for coronagraph observations. The plans for the 5-year mission lifetime, devoted to the core science, would be unaffected.
TABLE I.1 Basic Comparison of WFIRST/Interim Design Reference Mission (IDRM) to WFIRST/Astrophysics Focused Telescope Assets (AFTA)
|Mirror diameter||1.3 m, off-axis||2.4 m, on-axis|
|Image PSFa||Diffraction limited at 1 micron||Diffraction limited at 1 micron|
|Spectral PSF||Diffraction limited at 3 micron|
|Instrument list||Wide-field imager (includes prism) Two spectrographs (slitless prisms) Guider||Wide-field imager (includes grism) IFU No separate guider Possible coronagraph|
|Imager wavelength||0.6-2.0 micron||0.6-2.0 micron|
|Imager pixel scale||0.18′′/pixel||0.11′′/pixel|
|Imager detectors||28 H2RGs (2k × 2k)||18 H4RGs (4x × 4k)|
|Filters||5, including a “wide”||6, including a “wide”|
|Pixel size (physical)||18 micron||10 micron|
|Pixel number (imaging)||120 million||300 million|
|Imager FOV||0.291 sq deg||0.281 sq deg|
|Grism/prism (Imager filter wheel)||Prism, R = 75 0.6-2.0 microns||Grism, R = 550-800 1.35-1.95 microns|
|Spectrograph detectors||8 H2RGs (2k × 2k) (2 separate channels)||1 H2RG (2k × 2k) (IFU)|
|Spectrograph resolution||Slitless, R = 180-270b 1.1-2.0 microns||IFU R = 100 0.6-2.0 microns|
|Spectrograph FOV||0.26 sq deg × 2||3′′ × 3.15′′ (for supernova and its host galaxy)|
|Guider||Prime: 2 pair HgCdTe Auxiliary Fine Guidance Sensor (during spect)||No separate guider|
|Telescope temperature||240 K||TBD: 277 K|
|Electrical power capacity||2500 W solar arrays 80 A-hr battery||TBD: 2000 W solar array TBD: 160 A-hr battery|
|Mission Life||5 years||5 years (6 if coronagraph is added)|
|NOTE: FOV, field of view; IFU, Integral Field Unit; PSF, point spread function.|
a The instruments on IDRM were designed to meet these science requirements; the imaging channel and the spectral channels have different requirements.
b Using quantities from the IDRM science definition team report: 160-240′′ = R − Θ at 0.45′′/pixel where spectral resolution λ/Δλ = R = R − Θ/(2 pixel scale (Wide-Field Infrared Survey Telescope Science Definition Team, Wide-Field InfraRed Survey Telescope (WFIRST) Interim Report, July 11, 2011, http://wfirst.gsfc.nasa.gov/science/sdt_public/WFIRST_Interim_Report.pdf).