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The TPF-C mission (Terrestrial Planet Finder - Coronagraph) is a NASA project (Fig. 8.21). The aim is to build a space telescope operating in the visible and the near infrared that allows imaging of planetary systems consisting of terrestrial-type planets orbiting nearby stars (typically for some thirty targets), and that is also capable of carrying out spectral analysis of each of the components in a planetary system, over the range 0.6-2 |m, with a spectral resolution of at least 70. The idea is to study the composition of the atmospheres of terrestrial exoplanets to detect -as is the case with TPF-I/DARWIN - biological tracers. TPF-C fits into the same scheme as TPF-I/DARWIN and is complementary to it. The spectral information gained in the visible and near infrared on the one hand, and in the thermal infrared on the other, will allow us to set constraints on atmospheric models, and give an indication of the biological or abiotic origin of any atmospheric gasses that may be detected (cf. Chaps. 7 and 9).

To provide effective spectroscopy of terrestrial-type planets in the visible and near infrared, the instrument needs to have an extremely wide dynamical range (typically 1010, or 25 magnitudes!), which amounts to observing objects of magnitude 30-32 alongside objects of magnitude 5-7.

Such an instrumental concept cannot function without permanent control of the wavefronts and without a high-performance coronagraph. TPF-C, which is currently being defined, pushes space technology to its ultimate limits in terms of:

• control of the surface of the mirrors to better than a few tens of picometres.

To achieve this requires an adaptive system to correct for deformation of the wavefront,

• superfine polishing (to control scattering) - almost to the atomic level,

Fig. 8.21 Artist's impression of the TPF-C mission (image credit: courtesy NASA)

• control and correction of mechanical and thermal distortions,

• guiding accuracy.

Initially foreseen for 2014 in its coronagraphic version and 2019 in the interferometer version, TPF-C, like all ambitious projects of this sort, faces extremely high development costs, incompatible with the current state of finances of the various space agencies. This project, like its interferometer counterpart, will be forced to adapt.

Preliminary versions of TPF-C, with scientific objectives limited to spectroscopy of giant planets and Super-Earths (i.e., bodies of several Earth-radii) are also being drawn up. They are Ellipse (in the USA) and SEE-COAST (Super-Earth Explorer, Coronagraphic Off-Axis Space Telescope (in Europe).

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