ar X iv : a st ro - p h / 01 03 28 8 v 1 1 9 M ar 2 00 1 IRSI - DARWIN : How to see through the interplanetary dust cloud

ESA has identified interferometry as one of the major goals of the Horizon 2000+ programme. Infrared interferometers are a highly sensitive astronomical instruments that enable us to observe terrestrial planets around nearby stars. In this context the Infrared Space Interferometry Mission (IRSI)/ DARWIN is studied. The current design calls for a constellation of 6 free flying telescopes using 1.5 m mirrors, plus one hub and one master spacecraft. As the baseline trajectory an orbit about the second collinear 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 by NASA's Cosmic Background Explorer (COBE) mission. There are various ways to detect faint terrestrial planets despite the bright foreground. We find that, using integration times in the order of 30 h, the baseline mission scenario is capable of detecting earth-sized exo-planets out to 14 pc. We seize the suggestion that increasing the heliocentric distance of the instrument would make the observing conditions even better. A dust model that was fitted to the COBE measurements shows that an observing location of DAR-WIN in the outer solar system would potentially reduce the zodiacal foreground by a factor of 100, effectively increasing the number of potential target stars by almost a factor of 30.