  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 1.5m 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 30h, the baseline mission
scenario is capable of detecting earth-sized exo-planets out to 14pc. 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 DARWIN 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.
