X -ray Evolving Universe Spectroscopy ( XEUS ) was a project of the European Space Agency for a new X-ray observatory in space.

With an effective collecting area of ​​5 m² at a photon energy of 1 keV and developed instruments XEUS should be about a hundred times more sensitive than the current X-ray Observatory, ESA XMM- Newton. With a diameter of the main mirror assembly of 4.2 m and a focal length of 35 m XEUS was too large for a single spacecraft. Instead, a precisely tuned formation flight of a main mirror probe was provided with a detector probe. Both should be sent in a halo orbit around the Lagrangian point L2 of the Sun-Earth system, which is about 1.5 million kilometers from the sun is seen behind the earth.

XEUS was one of the nine missions of ESA proposals that were selected in the fall of 2007, a total of 50 proposals under the Cosmic Vision process for detailed studies. If successful in the selection process XEUS could start earlier than 2018. In July 2008, however, the XEUS project was abandoned in favor of the International X -ray Observatory ( IXO ), the ESA wanted to develop together with NASA and JAXA, as successor to XMM -Newton and Chandra. Since an early realization of IXO on NASA's side was not to be expected, the ESA went in 2011 to study a smaller observatory under its own leadership under the name ATHENA. However ATHENA could not prevail in the selection of the next big ESA mission in 2012. Instead, the probe JUICE has been selected; which is to explore the Jupiter system and the special moon Ganymede.