In May 2008, ESA and NASA established a coordination group involving all three agencies, with the intent of exploring a joint mission merging the ongoing XEUS and Constellation-X Observatory (Con-X) projects.

X-ray images reveal hot spots in the Universe – regions where particles have been energized or raised to very high temperatures by strong magnetic fields, violent explosions, and intense gravitational forces.

Closer to home, IXO observations would have constrained the equation of state in neutron stars, black holes spin demographics, when and how elements were created and dispersed into the Outer space, and much more.

[12][13][14] To achieve these science goals, IXO requires extremely large collecting area combined with good angular resolution in order to offer unmatched sensitivities for the study of the high-z Universe and for high-precision spectroscopy of bright X-ray sources.

[7] The heart of IXO mission was a single large X-ray mirror with up to 3 square meters of collecting area and 5 arcsec angular resolution, which is achieved with an extendable optical bench with a 20 m focal length.

Therefore, IXO would have carried a range of detectors, which would have provided complementary spectroscopy, imaging, timing, and polarimetry data on cosmic X-ray sources to help disentangle the physical processes occurring in them.

[3] Two high-resolution spectrometers, a microcalorimeter (XMS or cryogenic imaging spectrograph (CIS) and a set of dispersive gratings (XGS) would have provided high-quality spectra over the 0.1–10 keV bandpass where most astrophysically abundant ions have X-ray lines.

[20] IXO's imaging X-ray polarimeter would have been a powerful tool to explore sources such as neutron stars and black holes, measuring their properties and how they impact their surroundings.

Therefore, the IXO Instrument Platform would have featured a large shield that blocks the light from the Sun, Earth, and Moon, which otherwise would heat up the telescope, and interfere with the observations.