THE TECHNOLOGY OF DARWIN

摘要

DARWIN is an optical space interferometer, and a cornerstone mission in the Scientific Satellites Programme of ESA, with a foreseen launch in 2015. It has the ambitious mission objectives of: a) detection and analysis of planets orbiting nearby stars, searching for earth like conditions and life and b) high resolution imaging by aperture synthesis. The mission is implemented on several free-flying telescope spacecraft and one beam-combining spacecraft, possibility augmented by a spacecraft dedicated to ground communication and metrology. The beam combiner and the telescope spacecraft fly in one plane with the telescope spacecraft at equal distance from the beam combiner. The resolution of the interferometer is adjusted by changing the inter-satellite distance. Analysis of the planetary light requires that the stellar light be suppressed to a high degree. This is done by a technique called nulling interferometry, in essence this means that achromatic phase shifts are applied to the beams before recombination such that the on-axis light, i.e. stellar light, is cancelled by destructive interference, while the much weaker planetary light interferes constructively. The required stellar light rejection ratio (10~5 - 10~6), imposes severe optical requirements: a) the optical path differences between arms of the interferometer must not deviate by more than a few 10 nanometers, b) the telescopes should have an attitude control better than 8.5 mas and c) the interfering beams should have the amplitudes matched to better than 1%. The spectral range between 6 and 18 μm has been selected for DARWIN's extra-terrestrial planet search due to the favourable ratio between planetary and stellar light, which is improved by roughly 3 orders of magnitude compared to the visible spectrum. Furthermore, absorption bands of important tracers of life (H_2O, O_3 and CO_2) are present in this range. Operating in the infrared band necessitates a temperature of below 40K for all optical components, which can be achieved by passive cooling. challenges of DARWIN. A summary is provided of the main areas of technology development, with emphasis on formation flying and nulling interferometry.