IRSI/Darwin: Peering through the interplanetary dust cloud

ESA has identified interferometry as one of the major goals of the Horizon 2000+ Programme. Infrared interferometers are highly sensitive astronomical instruments that enable us to observe terrestrial planets around nearby st ars. It is in this context that the infrared space-interferometry mission I RSI/Darwin is being studied, The current design calls for a constellation o f six free-flying telescopes using 1.5 metre mirrors, plus one hub and one master spacecraft. As the baseline trajectory, an orbit about the second co linear libration point of the Earth-Sun system has been selected. The thermal radiation from the interplanetary dust cloud that surrounds the Sun, the so-called 'zodiacal infrared foreground', is a major concern for any high-sensitivity infrared mission, The most reliable information about this radiation comes from the measurements made by the Cosmic Background Ex plorer (COBE) mission. There are various ways to detect faint terrestrial p lanets despite the bright foreground. We find that, using integration times in the order of 30 h, the baseline mission scenario is capable of detectin g Earth-sized exo-planets out to 11 pc, Increasing the heliocentric distanc e of the instrument would make the observing conditions even better. A dust model that was fitted to the COBE measurements shows that an observing loc ation for Darwin in the outer Solar System would potentially reduce the zod iacal foreground by a factor of 100, effectively increasing the number of p otential target stars by almost a factor of 30,