WFIRST EXECUTIVE SUMMARY design elements (e.g., propellant supply) that intrinsical- The Wide Field Infrared Survey Telescope ly limit the lifetime to less than 10 years. (WFIRST) is the highest ranked recommendation for a DRM2 was requested by NASA in response to the large space mission in the New Worlds, New Horizons European selection of the Euclid dark energy mission, (NWNH) in Astronomy and Astrophysics 2010 Decadal which is scheduled to launch in 2020. DRM2 is de- Survey. The most pressing scientific questions in astro- signed to be non-duplicative with the capabilities of the physics today require a very wide field survey in order Euclid mission and/or planned ground based facilities to be answered, and existing telescopes such as the such as LSST, and also to represent a lower cost alter- Hubble Space Telescope (HST) or the Keck Telescope native to DRM1, though possibly with a reduced sci- cannot make these kinds of observations due to their ence return compared to DRM1. A recent committee of optical designs and narrow fields of view. The first gen- the National Research Council examined the potential eration of wide field surveys from the ground (e.g., overlap in the WFIRST and Euclid missions and found SDSS) has resulted in significant advances in astrono- that there was no duplication in their science capabili- my and astrophysics, and the value of such surveys is ties (NRC 2012). However, to reduce mission costs, recognized by the parallel decadal recommendation of DRM2 employs a 1.1 m telescope (still a three mirror LSST, a ground-based telescope designed for survey anastigmat), utilizes a smaller number of higher perfor- work. Translating wide field design principles into space mance infrared sensors operating from 0.7 – 2.4 m will revolutionize astronomical surveys in much the (H4RG-10’s, which require further development before same way that the HST revolutionized the imaging of being ready for launch, but have four times as many individual astronomical objects and galaxies. The ab- pixels in a similar sized package), uses an observatory sence of atmospheric distortion and absorption, the design with only selective redundancy, and fits within a darkness of space, and the opportunity for continuous lower cost, lower throw weight launch vehicle. DRM2 observations enable space-based surveys to go deep- assumes that WFIRST will be deployed in an Earth-Sun er, with more precision and accuracy, than can ever be L2 libration point orbit, and have a minimum operational possible from the ground. WFIRST will be the most lifetime of 3 years. With the improved sensors, DRM2 sensitive infrared space telescope designed for preci- has similar sensitivity to DRM1 for a fixed observation sion wide-field survey work, and will produce the most time, and can achieve any, but not all, of the NWNH powerful and informative astronomical data set for prob- science objectives within its 3 year design lifetime. It is ing the nature of dark energy, cataloguing the variety of assumed that the actual observing plan for a WFIRST exoplanet systems, and mapping the distribution of utilizing the DRM2 mission concept will be optimized matter across cosmic time. based on the data available at that time. The WFIRST Science Definition Team (SDT) was Both DRM1 and DRM2 represent exciting and formed to refine the science case for the mission, opti- viable mission concepts. DRM1 would accomplish mize the design and implementation scheme, and de- all of the goals of the decadal survey, at a slightly velop two Design Reference Missions (DRMs) to serve lower cost than originally anticipated. DRM2 would as the basis for further programmatic and technical re- accomplish a substantial fraction of these, and view. The first DRM (DRM1) represents the completion would be considerably less expensive. The WFIRST of the Interim Design Reference Mission (IDRM), and SDT recommends expedited development of both was scoped to meet all of the scientific and observa- concepts, with a decision to go ahead with one or tional requirements outlined in the decadal survey. It the other prior to the NRC’s mid-decadal review. utilizes existing, proven technology, and could move The SDT’s immediate short term recommendation into design and development immediately if a new start is to further improve the IR detector capability as were to be approved. the most cost effective way to improve the science DRM1 incorporates a 205K, 1.3m, unobstructed, return of either design, and to continue this effort three mirror anastigmat telescope. It utilizes current as the top priority for WFIRST in the years 2013- technology infrared sensors operating from 0.7 – 2.4 2015. m, and fits within a medium lift class launch vehicle The SDT has refined the scientific objectives of fairing. DRM1 assumes that WFIRST will be deployed WFIRST to four cornerstone goals, of equal im- in an Earth-Sun L2 libration point orbit, and have a min- portance: imum operational lifetime of 5 years, but will have no 1) Complete a statistical census of planetary systems in the Galaxy, from the outer habitable zone to free Executive Summary 1 WFIRST floating planets, including analogs to all of the Box 1 planets in our Solar System with the mass of Mars or greater. 2) Determine the expansion history of the Universe “WFIRST is a wide-field-of-view near-infrared and the growth history of its largest structures in imaging and low-resolution spectroscopy obser- order to test possible explanations of its apparent vatory that will tackle two of the most fundamen- accelerating expansion including Dark Energy and tal questions in astrophysics: Why is the expan- modifications to Einstein's gravity. This objective sion rate of the universe accelerating? And are must be achieved through multiple techniques, and there other solar systems like ours, with worlds must include a galaxy redshift survey, a SN Ia sur- like Earth? In addition, WFIRST’s surveys will vey, and a gravitational weak lensing survey. address issues central to understanding how 3) Produce a deep map of the sky at near-infrared galaxies, stars, and black holes evolve. ….. wavelengths, enabling new and fundamental dis- WFIRST will settle fundamental questions about coveries ranging from mapping the Galactic plane the nature of dark energy, the discovery of to probing the reionization epoch by finding bright which was one of the greatest achievements of quasars at z>10. U.S. telescopes in recent years. It will employ 4) Provide a general observer program utilizing a min- three distinct techniques—measurements of imum of 10% of the mission minimum lifetime. weak gravitational lensing, supernova distances, and baryon acoustic oscillations—to determine The SDT re-affirms, even after the selection of the the effect of dark energy on the evolution of the Euclid mission, that WFIRST must be capable of exe- universe. An equally important outcome will be cuting any and all of its cornerstone science objectives, to open up a new frontier of exoplanet studies including all of the observational techniques listed by monitoring a large sample of stars in the cen- above, which are necessary to achieve a highly reliable tral bulge of the Milky Way for changes in result. However, the SDT recognizes that in the inter- brightness due to microlensing by intervening vening years between now and launch, the actual allo- solar systems. This census, combined with that cation of observing times to the various approaches will made by the Kepler mission, will determine how be determined in response to the work that will be car- common Earth-like planets are over a wide ried out in space and on the ground before WFIRST range of orbital parameters. It will also, in guest begins its science operations. investigator mode, survey our galaxy and other Many of the most pressing questions in astrophys- nearby galaxies to answer key questions about ics today, such as the nature of dark energy, the mech- their formation and structure, and the data it ob- anisms of galaxy evolution, and the demographics of tains will provide fundamental constraints on planetary systems, require large, uniform statistical how galaxies grow.” samples -- observations of thousands of square de- From the New Worlds, New Horizons Decadal grees of sky or rapid-cadence observations of a few Survey in Astronomy and Astrophysics square degrees. WFIRST is designed to provide these samples, taking advantage of the stability and dark in- frared sky available only in space. It is a very different mission from the James Webb Space Telescope (JWST), which is designed to provide much more de- The SDT carefully considered the relationship be- tailed information on small numbers of objects. No other tween WFIRST, the Euclid mission, and the ground- existing or planned instrumentation can produce a wide based LSST. Euclid has two instruments: a high- field infrared sky survey that approaches the depth and resolution visible imager, and a NIR imag- resolution that WFIRST will achieve. ing/spectroscopy instrument. Euclid's design was opti- mized to maximize survey area, and other objectives (such as reaching diffraction-limited resolution in the NIR) were traded away to make this possible. Although its survey footprint will be smaller, WFIRST will observe 2 magnitudes deeper, achieve diffraction-limited resolu- tion for a 1.1-1.3 m telescope, and include the K band. Executive Summary 2 WFIRST The K band is a key instrument capability with science implications ranging from charting the expansion history of the Universe at z>2 to mapping the highly reddened Galactic Plane. LSST is dedicated to imaging surveys in the optical bands (out to ~1 micron). WFIRST was specifically de- signed not to duplicate the optical survey capabilities of LSST. Instead, with the well-matched depths of the sur- veys (Figure 19), LSST optical imaging and WFIRST infrared imaging will provide an unprecedented pan- chromatic view of several thousands of square degrees of sky.