  XEUS, the X-ray Evolving Universe Spectroscopy mission, is at present an
ESA-ISAS initiative for the study of the evolution of the hot Universe in the
post-Chandra/XMM-Newton era. The key science objectives of XEUS are: 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 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^2 below 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-m focal length, comprising two
laser-locked (separate) mirror and detector spacecraft's. The telescope is
injected in a low earth orbit with an inclination commensurate with the ISS. At
present an on-orbit growth of the mirror spacecraft is foreseen with the aid of
the ISS, raising the mirror diameter from 4.5 to 10 m. The detector spacecraft
will be replaced at 5 year intervals after run-out of consumables with an
associated upgrade of the focal plane package.
