The public announcements have begun. NASA's Constellation-X mission (*) and ESA's XEUS X-ray mission are merging, along with Japan's JAXA, to consider a joint mission imaginatively named the International X-ray Observatory.
In May 2008 ESA and NASA established a coordination group involving ESA, NASA and JAXA, with the intent of exploring a joint mission merging the ongoing XEUS and Constellation-X efforts. The coordination group met twice, first in May 2008 at ESTEC, then in June 2008 at the Center for Astrophysics. As a result of these meetings a joint understanding was reached by the coordination group on a proposal to proceed towards the goal of developing an International X-ray Observatory (IXO).Presumably the hope is that a joint multinational mission would be more likely to gain funding and be built than either of the separate US and European/Japanese missions, perhaps because the joint mission would have a broader range of capabilities than the single missions and perhaps because missions based on multinational agreements are harder to cancel politically. I say presumably because I personally am somewhat skeptical about this course of action, although I'm prepared to change my mind is someone can provide a convincing argument.
The coordination group proposed the start of a joint study of IXO. A single merged set of top level science goals and derived key science measurement requirements were established. The starting configuration for the IXO study will be a mission featuring a single large X-ray mirror and an extensible optical bench with a 20-25m focal length, with an interchangeable focal plane. The instruments to be studied for the IXO concept will include an X-ray wide field imaging spectrometer, a high spectral resolution non-dispersive X-ray spectrometer, an X-ray grating spectrometer, plus allocation for further payload elements with modest resource demands. The study will explore how to enhance the response to high-energy X-rays. This plan establishes an IXO study, which will be the input to the US decadal process and to the ESA selection for the Cosmic Vision Plan. The IXO study supersedes the ongoing XEUS and Constellation-X activities.
At a bilateral ESA-NASA meeting 2008, July 15 and 16 in Annapolis this plan was endorsed by David Southwood the ESA Director for the Science and Robotic Exploration Program and Ed Weiler the NASA Associate Adminstrator of the Science Mission Directorate. A letter signed by Jon Morse (NASA HQ Astrophysics Division Director) and Fabio Favata (ESA Coordinator for Astronomy and Fundamental Physics Missions) records the details of the agreed plan.
As part of this plan the Agencies will establish an IXO coordination group (IXO-CG) charged with the definition of the science requirements for the IXO study, scientific supervision on the IXO study activities and providing inputs to the agencies. Further details will be presented and discussed at planned upcoming meetings including this NASA IXO (previously Con-X) FST meeting and the ESA IXO (previously XEUS) workshop to be held at MPE in Garching, Germany on Sept 17-19. These are open meetings and scientists from Europe, Japan and the US are encouraged to attend both meetings.
What this does for the potential launch date of the observatory is currently unclear (it probably pushes it further into the future), but I'd guess it would be unlikely to be earlier than 2020.
In the mean time X-ray astronomy will have to make do with the aging existing major X-ray observatories Chandra and XMM-Newton (both launched in 1999, and still functioning well beyond their original nominal mission lifetimes) and smaller niche missions such as NeXT and NuSTAR.
(*) Full disclosure: I am a member of one of the volunteer Constellation-X Facility Science Team Science Panels.