Wide Field Infrared Survey Telescope (WFIRST) Starts Mission Formulation Phase

Roeland P. van der Marel, marel@stsci.edu

The Wide Field Infrared Survey Telescope (WFIRST) got its formal start in February 2016, when NASA advanced it into the mission Formulation Phase, with launch aimed for the mid 2020s. This marked the completion of several years of pre-formulation work, capped by a successful Mission Concept Review in December 2015.

Figure 1: WFIRST will be the next mission after Webb in a long line of successful large NASA space missions recommended by the Decadal Surveys in Astronomy and Astrophysics.

WFIRST was the highest ranked large space mission in the 2010 Astronomy & Astrophysics Decadal Review. It will provide fundamental new constraints on dark energy, a repulsive force that is pushing the universe apart at an ever-faster rate; on the large-scale distribution of dark matter, which is most of the matter in the universe; and on the demographics and properties of exoplanets, which are planets around other stars. It will also build on Hubble’s legacy by providing major advances in all areas of astrophysics through competed Guest Observer and funded archival Guest Investigator programs.

While originally envisioned to be a 1.5-meter telescope, the WFIRST concept now makes use of an existing optical telescope assembly with a 2.4-meter-diameter primary mirror. This “AFTA” (Astrophysics-Focused Telescope Assets) telescope was donated to NASA in 2012 by the National Reconnaissance Office. The observatory design, instrument capabilities, and observing programs may all continue to evolve as the mission matures. However, the present mission concept clearly shows WFIRST’s potential for revolutionizing many areas of science.

Figure 2: The Hubble-sized WFIRST is now in the NASA mission Formulation Phase, with launch aimed for the mid 2020s. The Institute will be a partner in the mission science operations, and will lead the systems for the Wide Field Instrument, dark energy surveys, mission scheduling, and data archive.

The observatory will have two instruments to execute its science program, a near-infrared imaging camera and a visible-light coronagraph. The prime mission will be six years, but mission consumables will be sized to enable a potential extended mission phase.

The Wide Field Instrument (WFI) will operate in the near-infrared 0.7–2.0 micron range. Eighteen 4k × 4k detectors will yield an unprecedented field of view, 100 times that of Hubble. This will allow WFIRST to produce large-scale maps of the night sky at Hubble resolution. The present WFI concept has 6 imaging filters, a grism, and an Integral Field Channel (IFC).

Figure 3: Overlay of the WFIRST field of view on top of a mosaic of the Andromeda galaxy. The mosaic was created from over 400 individual pointings obtained by the Panchromatic Hubble Andromeda Treasury (PHAT) program. A single ACS/WFC field of view is shown for comparison. By contrast, the WFIRST Wide Field Instrument, with its 18 4k × 4k near-IR detectors, will be able to image most of this field in just one pointing. This potential for wide field surveys at space-based resolution will allow WFIRST to revolutionize many areas of astrophysics.

Three large surveys will be executed with the WFI: a High-Latitude Survey (2 years) and a Supernova Survey (0.6 years) for studies of dark energy and the large-scale structure of the Universe; and a Bulge Microlensing Survey (1 year) to complete the census of exoplanets in a mass-radius regime that is complimentary to that surveyed by the Kepler mission.

The High-Latitude Survey will cover over 2,200 square degrees with imaging and low-resolution (grism) spectroscopy. The imaging, in four NIR bands (Y, J, H, and F184), will reach J = 26.7 AB for point sources. The spectroscopy will measure redshifts for over 15 million sources at redshift 1.1 to 2.8.

The Supernova Survey will have both imaging and IFC spectroscopy. The imaging survey is designed in three tiers, shallow, medium, and deep, to find supernovae at redshifts below 0.4, 0.8, and 1.7, respectively. The three tiers will cover approximately 27, 9, and 5 square degrees, respectively, with observations repeated with a cadence of 5 days, in filters Y and J for the shallow tier, and J and H for the medium and deep tier. IFC spectrophotometric observations will be used to fully characterize the type and light curve of a subset of 2700 supernovae, chosen to sample the full redshift range.

These surveys will measure the equation of state of dark energy and its time evolution, helping determine whether it is a cosmological constant, through all of the major methods suggested thus far. The HLS will enable weak lensing shape and photometric redshift measurements of hundreds of millions of galaxies, which will in turn yield precise measurements of distances and matter clustering through measurements of cosmic shear, galaxy-galaxy lensing, and the abundance and mass profiles of galaxy clusters. The wide-area HLS grism survey will determine million of redshifts for galaxies between z = 1 and 3, thus measuring the evolution of the size of the Universe and constraining the scale of Baryon Acoustic Oscillations to 0.3%, as well as measuring the growth of structure via redshift-space distortions. The SNS will constrain dark energy by discovering and measuring precise distances to thousands of Type Ia supernovae up to redshift z = 2.

In the Bulge Microlensing Survey, ten fields (an area of over 2 square degrees) will be imaged every 15 minutes over contiguous 72-day periods, to create highly-sampled light curves of 56 million stars brighter than H = 21.6 (AB). Six such campaigns will be executed over the mission lifetime, resulting in the expected discovery, through their microlensing signature, of over 2000 bound planets in the range 0.1–1000 Earth masses, including about 400 of Earth mass and below. These planets will sample orbital major axes from 0.03 to 30 AU, including the habitable zone, the outer regions of planetary systems, and free-floating planets. Another 20,000 giant planets in short-period orbits will be detected from their transit signatures.

All of these surveys will yield a treasure trove of data for Archival Guest Investigator studies in other areas of astrophysics. Examples of science projects enabled by the data in the High-Latitude Survey include: mapping the formation of cosmic structure in the first billion years after the Big Bang via the detection and characterization of over 10,000 galaxies at z > 8; finding over 2,000 QSOs at z > 7; quantifying the distribution of dark matter on intermediate and large scales through lensing in clusters and in the field; identifying the most extreme star-forming galaxies and shock-dominated systems at 1 < z < 2; carrying out a complete census of star-forming galaxies and the faint end of the QSO luminosity function at z ∼ 2, including their contribution to the ionizing radiation; and determining the kinematics of stellar streams in the Local Group through proper motions.

Moreover, 25% of the primary mission (1.5 years) is set aside for Guest Observer studies. Observations will be competitively selected through peer review, in the same spirit as for other NASA Great Observatories. These programs can use any of the available instruments and modes to study topics in any area of astrophysics. Examples include: studying young clusters and embedded star-forming regions within the Galaxy; reaching the very faint end of the stellar luminosity function via very deep observations of Local-Group galaxies; or mapping the core of the Virgo cluster.

The second WFIRST instrument is the Coronagraph Instrument (CGI), which is specially designed for studying planets orbiting other stars. It will use deformable mirrors to reach groundbreaking new contrast levels of around 1 in a billion, several orders of magnitude better than the current state of the art with ground- and space-based observatories. It will operate in the 0.4–1.0 micron range, and will have both an imaging detector for exoplanet detection, and an IFC for exoplanet spectroscopy. The associated technology development may also pave the way for future missions aimed at detecting signs of life in the atmospheres of Earth-like exoplanets.

The CGI can measure planets similar to those in our Solar System, and also measure for the first time the photometric properties of the ‘mini-Neptune’ or ‘super-Earth’ planets—objects that Kepler has shown to be the most common planets in our galaxy, but with no analog in our own Solar System. With a 1-year program, dozens of exoplanets can be targeted with the CGI. Initial observations will focus on discovery and characterization of planets around pre-selected target stars. When a previously known or unknown planet is detected, additional observations will be made for longer time periods, with full spectral resolution for planet characterization.

Figure 4: WFIRST has an ambitious science program. This includes a multi-pronged approach to measuring dark energy, the discovery and census of planets down to sub-Earth masses via their microlensing signatures, and the imaging and spectroscopy of planets via coronagraphy. Moreover, the WFIRST instruments will enable many exciting studies in general astrophysics, through the combination of Guest Observer programs and archival Guest Investigator studies of the wide-area and time-resolved WFIRST surveys.

NASA competitively selected eleven Science Investigations Teams and two Adjutant Scientists for the WFIRST mission. The leading members of these teams, together with NASA and Science Center representatives, make up the Formulation Science Working Group. This group will advise the Project on mission, observation, and analysis concepts, systems and designs that will optimally enable WFIRST to meet its science goals.

The WFIRST mission will provide tremendous synergy with Hubble and Webb. It will extend the legacy of Hubble-quality imaging to much wider fields, and is expected to find many unique objects suitable for detailed follow-up study by Webb (which will have enough propellant to remain active throughout the 2020s). For example, WFIRST survey discoveries might include rare early galaxies and bright supernovae explosions from early generations of stars. Webb observations using multi-band, high-resolution imaging and sensitive infrared spectroscopy can reveal the detailed nature of such sources.

Work on the WFIRST mission is a collaboration among several different partners. The WFIRST Project Office is at Goddard Space Flight Center (GSFC), which also oversees the work on the WFI instrument, the Spacecraft Bus, and System Integration, and which will host the Mission Operations Center. The Jet Propulsion Laboratory (JPL) is developing the CGI instrument. WFIRST Science Operations will be a shared responsibility between GSFC, the Institute, and the Infrared Processing and Analysis Center (IPAC).

The Institute science operations responsibilities will include the mission’s observation scheduling system, data archive, dark energy survey products, and WFI data processing system. The Barbara A. Mikulski Archive for Space Telescopes (MAST) at the Institute already holds the astronomical data from some 20 astronomy missions, and the addition of the WFIRST data will add considerably to its scientific discovery potential. The IPAC science operations responsibilities will include the proposal peer-review process, microlensing survey products, and CGI data processing system. GSFC will provide overall science operations management and strategic guidance.

Institute work on the WFIRST science operations planning will build on expertise with similar instruments and modes on Hubble and Webb. Work in prior years has focused on a range of activities. We have released software simulation tools, building on synergy with ongoing Webb development, to help astronomers assess what the Universe looks like through the eyes of WFIRST. For example, WebbPSF calculates the field-dependent point spread function, while Pandeia simulates small galactic or extragalactic scenes. We also published technical reports on a range of topics, including mission scheduling and guiding, and the operations and data analysis for the coronagraph and wide-field grism modes.

Institute tools and results are being distributed through our WFIRST web site (http://www.stsci.edu/wfirst), which is part of our broader efforts to engage the astronomical community in this exciting new mission. Other WFIRST information is available on the mission websites of our partners at GSFC (http://wfirst.gsfc.nasa.gov/) and IPAC (https://wfirst.ipac.caltech.edu/).