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P. C. Fisher (Ruffner Associates, Inc.), T. W. Stuhlinger (Ruda and Associates, Inc.), J. B. Houston, Jr. (Houston Technology International, Inc.)
Before launching NASA's Terrestrial Planet Finder (TPF) mission, see Angel (1996), prudence would suggest making a survey for earth-like planets associated with accessible F, G, and K stars within 13 pc of the sun. Although exozodiacal emission associated with a star may obscure a planet's thermal emission, the survey should still be done at a sensitivity level that would permit detecting earth-like planets.
A study like this can be conducted with a ground-based system constructed from a linear array of coronagraphs, see Fisher, Houston, Jr., and Stuhlinger (1997). As compared to large dual telescope systems, like the Keck system, the change to multiple small-aperture coronagraphs permits: 1) angular resolution in one direction that can be comparable or better; 2) telescope emissivity that can be significantly lower; 3) planet signal to parent star noise that can be significantly better. Some characteristics of one particular instrument will be described. For an earth-like planet at 13 pc, a (chopped) planet signal three standard deviations above background can be secured in a two-week observation, provided an evacuated pipe at cryogenic temperature facilitates adequate dilution of atmospheric and telescope backgrounds. Presuming calculation and verification of adequate dilution and atmospheric turbulence effects are encouraging, then at least one type of ground-based instrument of "reasonable" cost and adequate sensitivity appears feasible. Some efforts to develop a consortium that could govern, build, and use such a facility will be noted.
This work was supported by Ruffner Associates, Inc.
Angel, R. 1996, ExNPS Report - Exploration of Neighboring Planetary Systems (Jet Propulsion Laboratory: Pasadena) D9. Fisher, P. C., Houston, Jr., J. B. and Stuhlinger, T. W., 1997, IAU Colloquium No. 161, eds.: C. B. Cosmovici, S. Bowyer, and D. Wertheimer (Editrice Compositori: Bologna), 367.