XEUS

XEUS (X-ray Evolving Universe Spectroscopy) was a space observatory plan developed by the European Space Agency as a successor to the successful XMM-Newton X-ray satellite telescope. It was merged to the International X-ray Observatory around 2008, but as that project ran into issues in 2010, the ESA component was forked off into Advanced Telescope for High Energy Astrophysics (ATHENA).