and flight of the first-priority large mission WFIRST, and a vigorous ground-based research program. The second need is to characterize the level of zodiacal light present so as to determine, in a statistical sense if not for individual prime targets, at what level starlight scattered from dust will hamper planet detection. Nulling interferometers on NASA-supported ground-based telescopes (for example, Keck, and the Large Binocular Telescope) and/or on suborbital, SMEX, or MIDEX platforms could be used to constrain zodiacal light levels. A range of measurement techniques must be strongly supported to ensure that the detections extend to the relevant Earth-Sun distance range17 for a sufficient sample of systems. After these essential measurements are made, the need for a dedicated target finder can be determined and the approach for a space-imaging mission will be clear. The programs above will enable the optimal technologies to be selected and developed.
For the direct-detection mission itself, candidate starlight suppression techniques (for example, interferometry, coronagraphy, or star shades) should be developed to a level such that mission definition for a space-based planet imaging and spectroscopy mission could start late in the decade in preparation for a mission start early in the 2020 decade. The committee envisions that this program can be implemented at moderate funding levels early in this decade, but that it will require augmentation over current support levels for all of these activities. From the above considerations, a budget of $4 million per year is recommended in the first several years of the decade, in addition to the generally available technology development funds. If the scientific groundwork has been laid and the design requirements for an imaging mission have become clear by the second half of this decade, a technology down-select should be made. Furthermore, mission development should be supported at an appropriate level for the mission design and scope to be well understood. Initiating this activity will require significantly greater resource levels than the early-decade mission-enabling activities described above. It is currently difficult to anticipate the developments that could justify initiating this mission-specific development program, and the committee therefore recommends that a decadal survey implementation advisory committee be convened mid-decade to review progress both scientifically and technically to determine the way forward, and in particular whether an increased level of support associated with mission-specific technology development should commence. In this case a notional decadal budget of $100 million is proposed. However, the level of late-decade investment required is uncertain, and the appropriate level must be determined by a decadal survey implementation advisory