constitutes the majority of ordinary matter in clusters of galaxies. Large-aperture, Large-aperture, time-resolved, high-resolution X-ray spectroscopy is required for future progress on all of these fronts, and this is what IXO can deliver.

The IXO mission, a collaboration among NASA, ESA, and JAXA, will revolutionize X-ray astronomy with its large-aperture, energy-resolving imager. IXO is a relatively young mission concept that resulted from the merger of two longstanding proposals, ESA’s XEUS mission and NASA’s Constellation-X mission (which was recommended by AANM). At the heart of IXO is a 3-square-meter-aperture, lightweight focusing X-ray mirror with 5-arcsecond angular resolution. The key component of the IXO focal plane is an X-ray microcalorimeter spectrometer—a 40 × 40 array of transition-edge sensors covering several arcminutes of sky that measure X-ray energy with an accuracy of roughly 1 part per 1,000 (depending on energy). It will be launched to Lagrange point L2.

The independent cost and readiness analysis indicates a total appraised project cost of $5.0 billion (at 70 percent confidence), and the estimated time to completion is about 9.5 years. The survey’s independent analysis concluded that the technical risk is medium high. Areas of particular concern include the challenge of successfully manufacturing the large-aperture mirror and achieving an angular resolution of 5 arcseconds. Uncertainties in total mass combined with a low-mass margin could require a larger, more expensive launch vehicle. In addition, several of the secondary instrument components are technologically immature (technology readiness level 3 or 4). Retiring this risk will require a substantial directed technology development program, estimated to cost about $200 million.

The path forward has two key decision points. The first relates to technical readiness. For IXO to be ready for a mission start, technology readiness must have progressed to the point that a down-select for the mirror technology can be made and cost uncertainties are reduced. The committee considers that in the current budget climate, allowing any major mission to exceed $2 billion in total cost to NASA would unacceptably imbalance NASA’s astrophysics program. If the technology development program has not been successful in bringing cost estimates below this level, the committee recommends that descope options be considered to ensure that NASA costs remain below $2 billion.

The second decision point relates to ESA’s choice for its next L-class mission slot. Since both IXO and LISA are close to 50-50 partnerships with ESA, the phasing of their development must be decided jointly. If LISA is selected for the first L-class launch slot, the investment in IXO this decade, although still substantial, can be limited to technology development sufficient to bring IXO to a technology readiness level of 5 or greater by 2020. This ordering would be consistent with the committee’s priorities. However, if IXO is selected for the first L-class launch, NASA should request that a decadal survey implementation advisory committee review the IXO case and examine progress in the mission design and readiness. If the

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