The atmospheres of planets orbiting other stars may be studied by the technique of transit spectroscopy. This technique needs a very high SNR, so large space telescopes are needed, and bright target stars must be found. This paper uses planetary atmosphere models to discuss the SNR performance required to achieve specific science goals for both giant Jupiter-like planets and for small Earth-like planets. It discusses the space telescopes to survey bright stars to find suitable target stars, and the designs of large space telescopes to perform the transit spectroscopy.
Darwin was proposed in 1993 to the European Space Agency as a mid-IR (5-30 micron) interferometry observatory with baselines greater than 50 meters. It would be a long-duration general purpose radiatively cooled observatory, to be launched in the 2009-2018 timeframe. Since then ESA has started a study of such a mission, called the Infrared Space Interferometer (IRSI), as one of its candidate Cornerstone missions in its Horizons 2000 plan. This paper describes some of the aspects of the Darwin concept as presently conceived by the members of the Darwin Informal Team. This team is comprised of the original proposal authors and a number of additional persons.
Since 1990 the Edison program has studied designs for large, long-lived IR space telescopes incorporating intensive use of radiative cooling supplemented by mechanical refrigeration. This approach, which is now generally accepted as the most likely route to achieving large aperture and long lifetimes, led to proposals to ESA in 1993 and 1994 for a 1.7m observatory telescope operating at < 20 K as a Medium-sized mission and a Cornerstone, respectively. Extension of these ideas and the application of newer technology now indicate that a Cornerstone budget and an Ariane 5 launcher could accommodate mid- to far-IR telescopes of up to perhaps 3m aperture and/or achieve telescope temperatures of a few K--thereby attaining the full long-wavelength performance of cryogenic missions--in robust designs able to maintain their performance levels (i.e. low optics temperatures) for many years. These designs, too, have potential applications as the individual elements of spatial interferometers, for example, for searching for extrasolar terrestrial planets.
Previous designs for infrared space observatories assumed that the best method of cooling these systems was a single large tank of liquid helium. Although effective, as demonstrated by the IRAS and, soon, the ISO missions, this technique produces limited lifetime, small telescope aperture, and inflexibility in optical design. More modern spacecraft designs, including the Japanese IRIS and European Edison concepts, instead adopt multiple cooling strategies: radiative, cryogenic, and/or mechanical. This alternative philosophy permits each technique to be applied to the most appropriate task, taking advantage of strengths of each technology and minimizing weaknesses. In this paper we give a brief history of the development of some cooling technologies, emphasizing the advantages of the designs adopted for a pair of possible future infrared space missions, IRIS and Edison. We also briefly discuss negative aspects of the IRAS legacy in infrared spacecraft design and we speculate on what types of missions might follow IRIS and Edison.
We describe the current design for Edison, the first large radiatively-cooled infrared space observatory, now under consideration by the European Space Agency. Without the large cryogen tanks, more of the spacecraft can be filled with light-collecting optics and, of course, the observatory has no built-in lifetime. Our proposal is for a telescope with a 1.7 m primary to be launched by an Atlas, Ariane 5, or Proton. The baseline orbit for the observatory is a 'halo' around L2, a location which allows additional radiating area to be placed anti-sunward. Models of the temperature behavior of the observatory indicate an equilibrium temperature via radiation alone of about 20 K. Use of near-future cryo-coolers may allow optical system temperatures as low as approximately 15 K. Consequently, Edison will be limited in sensitivity by the celestial thermal background at wavelengths shortward of about 60 micrometers and by celestial source confusion at longer wavelengths.
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