The high throughput X-ray astrophysics mission is the second cornerstone in ESA's long term space science programme. The long duration X-ray observatory consists of three heavily nested X-ray imaging telescopes coupled to X-ray CCD cameras and reflection gratings which provide a high throughput facility for cosmic X-ray spectroscopy. Details of the spacecraft, mirror modules and possible instrument designs are described. The mission is due for launch in 1998 with an anticipated lifetime of over ten years. This observatory will make possible major advances in X-ray astrophysics at the turn of the century.