The International X-ray Observatory (IXO) is a collaborative venture between NASA, ESA and JAXA, which is under study for launch in 2020. Initial design studies have been performed in ESA's Concurrent Design Facility (CDF) at ESTEC and NASA's Mission Design Laboratory (MDL) at the GSFC. This article mainly describes the concept that has been derived by the CDL studies, but also presents the major differences between the two concepts.

IXO will have a launch mass of around 6600 kg and will be about 10 metres long and 4 metres in diameter in its launch configuration. The spacecraft will be about 23 metres long in its flight configuration. The nominal mission lifetime is five years, with consumables sized for 10 years of operations. A major science driver for the design of IXO is a long focal length, since this allows a greater photon-collecting capability at higher photon energies. A focal length of 20 metres has been selected for IXO as a balance between science requirements and engineering constraints. As no current launch vehicle is capable of accommodating a payload that is nearly 24 metres long, IXO will have a deployable structure to position the instrument module at the mirror focus after launch.

Illustrations of the IXO spacecraft. Credit: ESA

The IXO study includes the accommodation of six instruments:

• Wide-Field Imager – WFI
• Hard X-ray Imager – HXI
• X-ray Microcalorimeter Spectrometer – XMS
• High Time-Resolution Spectrometer – HTRS
• X-ray Polarimeter – XPOL
• X-ray Grating Spectrometer – XGS

From the output of the CDF study, the IXO spacecraft will be made up of five major assemblies:

• optics module – mirror assembly with straylight baffle assembly and deployable sunshield
• fixed telescope structure – a 6.7-metre long, 3.4-metre diameter connection between the optics module and the service module
• service module – contains the satellite systems: power distribution, attitude control, propulsion, telecommand and telemetry
• deployable telescope structure – moves the instrument module from the stowed position to the mirror focal point
• instrument module – carries the six instruments, their associated electronics and thermal control systems, and a mechanism for positioning one of WFI+HXI, XMS, HTRS, or XPOL at the focal point (the XGS CCD camera is positioned to the side of the focal plane and operates continuously)

Mirror assembly