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IXO mission graphic

International X-ray Observatory

Phase: Studied

Mission Project Home Page -

Program(s):Physics of the Cosmos

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The International X-ray Observatory (IXO) was an X-ray astronomy mission with joint participation from NASA, the European Space Agency (ESA), and the Japan Aerospace Exploration Agency (JAXA). IXO was a merger of NASA's Constellation-X and ESA's XEUS mission concepts.

In mid-2008, a officials from ESA, NASA, and JAXA headquarters agreed to conduct a joint study of IXO with a single merged set of top-level science goals. This agreement established key science measurement requirements. The spacecraft configuration for the IXO study was a mission featuring a single large X-ray mirror and an extendible optical bench, with a focal length of ~20 m, and a suite of six focal plane instruments.

The X-ray instruments under study for the IXO concept included: a wide field imaging detector, a high-spectral-resolution imaging spectrometer (calorimeter), a hard X-ray imaging detector, a grating spectrometer, a high timing resolution spectrometer, and a polarimeter. The IXO mission concept was submitted to the 2010 U.S. Decadal Survey committee and to ESA's Cosmic Vision process.

In the 2010 Astrophysics Decadal Report, IXO was ranked fourth among the large-scale missions. In 2011, ESA decided on a new way forward for their L-class mission candidates. During the ESA reformulation exercise, the most important elements of IXO were captured in the ATHENA mission.

Last Updated: April 28, 2015

Related Links
  • ESA IXO Website -
  • ESA ATHENA Website -