dent cost appraisal and technical evaluation process, the committee developed the ranked program described below for ground-based and spaced-based astronomy in the United States. In each category, the discussion proceeds with ranked large and ranked medium priorities followed by unranked smaller priorities. A large space activity is one with total cost estimated to exceed $1 billion; a medium space activity is one with total cost estimated to range from $0.3 billion to $1 billion. A large ground-based activity is one with total cost of construction and acquisition of capital assets estimated to exceed the threshold for the NSF’s MREFC program (currently $135 million in FY2010 for projects from the Directorate for Mathematical and Physical Sciences); a medium ground-based activity is an initiative for which the total cost would fit into the Mid-Scale Innovations Program range, $4 million to $135 million as defined by this committee. The committee has not ranked the core-sustaining activities described in Chapter 5 except in the sense that it has recommended funding augmentations to some relative to the current levels of support. The committee’s priorities have varying degrees of relevance to DOE, NASA, and NSF, given that some projects are envisioned as being supported by more than one agency.

WFIRST¹⁴ is a wide-field-of-view near-infrared imaging and low-resolution spectroscopy observatory that will tackle two of the most fundamental questions in astrophysics: Why is the expansion rate of the universe accelerating? And are there other solar systems like ours, with worlds like Earth? In addition, WFIRST’s surveys will address issues central to understanding how galaxies, stars, and black holes evolve. WFIRST will carry out a powerful extrasolar planet search by monitoring a large sample of stars in the central bulge of the Milky Way for small deviations in brightness due to microlensing by intervening solar systems. This census, combined with that made by the Kepler mission, will determine how common Earth-like planets are over a wide range of orbital parameters. To measure the properties of dark energy, WFIRST will employ three different techniques: it will image about 2 billion galaxies and carry out a detailed study of weak lensing that will provide distance and rate-of-growth information; it will measure spectra of about 200 million galaxies in order to monitor distances and expansion rate using