ar X iv : a st ro - p h / 02 07 28 3 v 1 1 2 Ju l 2 00 2 1 THE XEUS MISSION

XEUS, the X-ray Evolving Universe Spectroscopy mission, constitutes at present an ESA-ISAS initiative for the study of the evolution of the hot Universe in the postChandra/XMM-Newton era. The key science objectives of XEUS can be formulated as the: – Search for the origin, and subsequent study of growth, of the first massive black holes in the early Universe. – Assessment of the formation of the first gravitationally bound dark matter dominated systems, i.e. small groups of galaxies, and their evolution. – Study of the evolution of metal synthesis up till the present epoch. Characterization of the true intergalactic medium. To reach these ambitious science goals the two salient characteristics of the XEUS observatory entail: 1. Its effective spectroscopic grasp, combining a sensitive area > 20 m below a photon energy of 2 keV with a spectral resolution better than 2 eV. This allows significant detection of the most prominent X-ray emission lines (e.g. O-VII, Si-XIII and Fe-XXV) in cosmologically distant sources against the sky background. 2. Its angular resolving power, between 2 and 5 arc seconds, to minimize source confusion as well as noise due to the galactic X-ray foreground emission. To accommodate these instrument requirements a mission concept has been developed featuring an X-ray telescope of 50 meter focal length, comprising two laser-locked spacecraft, i.e. separate mirror and detector spacecraft’s. The telescope is injected in a low earth orbit with an inclination commensurate with the ISS, a so-called fellow traveler orbit. At present an on-orbit growth of the mirror spacecraft is foreseen through a robotic upgrade with the aid of the ISS, raising the mirror diameter from 4.5 to 10 meter. The detector spacecraft, formation flying in a non-Keplerian orbit in tandem with the mirror spacecraft will be replaced at 5 year intervals after run-out of consumables with an associated upgrade of the focal plane package.