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Review of Science Requirements for the Terrestrial Planet Finder: Letter Report
in the discussion were Marc Kuchner (Princeton University), Alan Boss (Carnegie Institution of Washington), Dan Coulter (NASA), and Garth Illingworth (University of California, Santa Cruz).
The 2000 decadal survey report ranked the Terrestrial Planet Finder third in its list of major NASA missions behind the James Webb Space Telescope (then called the Next Generation Space Telescope) and the Constellation-X Observatory and sixth overall:
The main goal of TPF is nothing less than to search for evidence of life on terrestrial planets around nearby stars. The present concept calls for a space-based infrared interferometer of enormous sensitivity, capable of nulling out the light from the host star. TPF’s angular resolution will also enable it to peer into the innermost regions of protoplanetary disks, galactic nuclei, starburst galaxies, and galaxies at high redshift. By a large margin, TPF is the most costly and the most technically challenging mission discussed in this report. Both SIM and NGST involve key technologies that must be demonstrated if TPF as currently envisioned is to go forward. The committee’s recommendation of this mission is predicated on the assumptions that TPF will revolutionize major areas of both planetary and nonplanetary science, and that, prior to the start of TPF, ground- and space-based searches will confirm the expectation that terrestrial planets are common around solar-type stars. NASA should pursue a vigorous program of technology development to enable the construction of TPF to begin in this decade. (p. 39)
The original mission that was considered by the 2000 decadal survey ranked highly based on its potential science impact on terrestrial planet finding2 and on the astrophysics reach afforded by the high angular resolution at infrared wavelengths. However, the widely recognized technical challenges of the interferometer prohibited the decadal survey committee from prioritizing it as a flight mission. Rather, that committee gave TPF its high ranking as a technology development activity with the aim of pushing the technology forward in this decade, and enabling the mission to be flown in the following decade. Specifically, “The committee attributes $200 million [in FY2000 dollars] of the $1,700 million total estimated cost of TPF to the current decade ….” (p. 37).
At the time of NASA’s initial request in January 2004 for the current vision, the TPF project was considering both a free-flying infrared interferometer and an optical coronagraph, with the goal of downselecting to a single architecture in the near future. The course of the TPF project has since changed in order to take advantage of the new opportunities presented by NASA’s new space exploration goals3 and to maximize the scientific potential for terrestrial planet finding. Specifically, the TPF project team is now proposing to fly TPF-C (an optical telescope with a coronagraph) followed by TPF-I (a free-flying infrared interferometer) within its planet-finding portfolio. The level-1 requirement for TPF-C’s wavelength coverage is proposed to be 0.5 to 0.8 µm, with “stretch” goals of 0.5 to 1.05 µm. The level-1 requirement for TPF-I’s wavelength coverage is proposed to be 6.5 to 13 µm, with “stretch” goals of 6.5 to 17 µm.
The primary scientific goal of the TPF mission (direct detection and spectroscopic analysis of Earth-like planets in orbit about some of the nearest main-sequence stars) arguably requires both TPF-C and TPF-I. This requirement was not well understood at the time the TPF mission was presented to the decadal survey committee, because understanding was imperfect then concerning the spectrum that our own Earth would present to a nearby solar system. Furthermore, the identification of biomarkers (i.e., spectroscopic features indicative of chemical balances attributable to biogenic activity) requires observations in spectra that span not only the optical but also the mid-infrared (IR) bands. Assuming there are planets to be
Terrestrial planets are planets similar in size and composition to Earth. In our solar system Mercury, Venus, and Mars (as well as Earth) are considered to be terrestrial planets.
National Aeronautics and Space Administration, The Vision for Space Exploration, NP-2004-01-334-HQ, NASA, Washington, D.C., 2004.