XEUS: approaches to mission design

The x-ray Evolving Universe Spectroscopy mission (XEUS) is an ambitious project under study by the European Space Agency (ESA), which aims to probe the distant hot universe with comparable sensitivity to NGST and ALMA. The effective optical area and angular resolution required to perform this task is 30m2 and <5" respectively at 1 keV. The single Wolter-I x-ray telescope having these characteristics will be equipped with large area semiconductor detectors and high-resolution cryogenic imaging spectrometers with 2 eV resolution at 1 keV. A novel approach to mission design has been developed, placing the detector instruments on one dedicated spacecraft and the optics on another. The International Space Station (ISS) with the best ever available infrastructure in space will be used to expand the mirror diameter from 4.5 m to 10 m, using robotics and extravehicular activities. The detector spacecraft (DSC) uses solar-electric propulsion to maintain its position while flying in formation with the mirror spacecraft. The detector instruments are protected from straylight and contamination by sophisticated baffles and filters, and employ the earth as a sun shield to make the most sensitive low energy x-ray observations of the heavily red-shifted universe. Detailed approaches, including alternatives to the baseline mission design of XEUS, have been and continue to be addressed, ensuring an efficient concept to be available for the eventual mission implementation. Both the development of the XEUS baseline scenario and complementary work conducted on some alternative mission designs are discussed.