TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 7
the instrumentation requirements and search strategies for TPF-I, and they will ultimately provide a
theoretical framework for analysis of the mission data.
7.2 Phase A
In Phase A the emphasis will be on specifying the detailed design of the optics of TPF-I, leading to the
Preliminary Mission and System Review. The mission and system definition studies will refine the
architecture so that it is ready for preliminary designs in Phase B. The overriding question to be resolved
by the end of Phase A will be to define the capability of the mission and determine to what scale the
observatory must be built. What volume of space should be searched for planets How large of a
collecting area will be needed What angular resolution will be required Scaling the architecture and
defining the scope of the mission will be necessary before TPF-I continues to Phase B, C, and D.
7.2.1 Priority 1: Frequency of Terrestrial Planets
The scale of the TPF-I—the distance between the furthest collecting apertures of the interferometer—will
determine the number of stars that are attainable. Defining the capability of a mission will involve a trade
between the desire to explore a larger number of extrasolar planetary systems and the technological
difficulty of building a larger observatory.
To better understand this trade, preparatory science activities prior to Phase A will include detections of
planets by a variety of techniques. Coupled with advances in astrophysical theory, these data (including
transit detections of several-Earth mass planets from CoRoT for nearby stars) will provide improved
estimates of the frequency of Earth-like planets. The programmatic implications will be wide-ranging.
More certain knowledge of the frequency of Earths will not only help set the scale of the observatories but
will later assist in setting priorities in the initial phases of the missions.
7.2.2 Priority 2: Target Stars
The refined target list, to be developed for the mission during its Phase A, will assist in optimizing the
trades (taking into account the expected performance limits) that will ultimately define the capability of
TPF-I. The number of attainable target stars of different spectral types, their distances, and their detailed
characteristics will largely determine the scientific return of each mission. This target list is more than
just an output from a catalog search. There are many characteristics of stars that must be measured, and
then carefully weighed in importance, before such a refined list can be constructed.
All nearby stars should be well characterized before the target lists for TPF-I can be chosen. A refined
target list, with the farthest target identified, will be needed in Phase A to set the scale of the observatory.
The distance to the farthest planet in the survey will be determined by the size of the observatory (its
angular resolution) and the area of its collecting apertures (its sensitivity).
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