Darwin (ESA)
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Darwin is a proposed European Space Agency (ESA) mission designed to directly detect Earth-like planets orbiting nearby stars, and search for evidence of life on these planets. The launch date will be at or after 2020 according to space.com. The current design envisions three free-flying space telescopes, each at least 3 meters in diameter, flying in formation as an astronomical interferometer. These telescopes will redirect the light to the main spacecraft which will contain the beam combiner, spectrographs and cameras for the interferometer array, and which will also act as a communications hub.
The space telescope will observe in the infrared part of the electromagnetic spectrum. As well as studying extrasolar planets, the instrument will probably have a general purpose imaging mode which will produce very high resolution (i.e. milliarcsecond) infrared images, allowing detailed study of a variety of astrophysical processes. The infrared spectrum was chosen because an Earth-like planet is only outshone by its star by a factor of a million there; in the visible spectrum an Earth-like planet is outshined by its star by a factor of a billion.
The planet search would use a nulling interferometer configuration. In this system, phase shifts would be introduced into the three beams, so that light from the central star would suffer destructive interference and cancel itself out. However, light from any orbiting planets would not cancel out, as the planets are offset slightly from the position of the star. This would allow planets to be detected, despite the much brighter signal from the star.
For planet detection, the instrument would operate in an imaging mode. To detect an Earth-like planet would require about 10 hours of observation in total, spread out over several months. Once a planet is detected, a more detailed study of its atmosphere would be made by taking an infrared spectrum of the planet. By analysing this spectrum, the chemistry of the atmosphere could be determined, and this could provide evidence for life on the planet. The presence of oxygen along with water vapor in the atmosphere would be evidence for life, as water vapor is extremely effective in reducing oxygen back to other gases. If nevertheless large amounts of oxygen exist in the atmosphere, it must be continually reproduced "artificially", i.e. via biological photosynthesis. The presence of oxygen alone could not be seen as evidence. Numerical simulations showed that under proper conditions it is possible to build up an oxygen atmosphere via photolysis of carbon dioxide.
The interferometric version of NASA's Terrestrial Planet Finder mission is very similar in concept to Darwin, and also has very similar scientific aims. Given this, and the technical difficulties inherent in such a project, it seems unlikely that both instruments will be built; a collaborative project is more likely.
Antoine Labeyrie has proposed a much larger space-based astronomical interferometer similar to Darwin, but with the individual telescopes positioned in a spherical arrangement and with an emphasis on interferometric imaging. The spherical geometry reduces the amount of pathlength compensation required in re-pointing the interferometer array. This "Hypertelescope" project would be much more expensive and complex than the Darwin and TPF missions, involving many large free-flying spacecraft.
