the NASA appraised cost, based on a 50 percent participation and including the costs of partnering at such a level, to be $1.4 billion. The cost and schedule risk classification is medium high. If Pathfinder is not a success or if a roughly equal partnership is not possible, the committee recommends that NASA request advice from a decadal survey implementation advisory committee (DSIAC) to review the situation mid-decade. LISA presents a compelling scientific opportunity, and there is readiness to address its remaining technical challenges.

Overall the recommendation and prioritization for LISA reflect its compelling science case and the relative level of technical readiness.

International X-ray Observatory (IXO)

IXO is a versatile, large-area, high-spectral-resolution X-ray telescope that will make great advances on broad fronts ranging from characterization of black holes to elucidation of cosmology and the life cycles of matter and energy in the cosmos. Central to many of the science questions identified by this survey, IXO will revolutionize high-energy astrophysics with more than an-order-of-magnitude improvement in capabilities. IXO is a partnership among NASA, ESA, and the Japan Aerospace Exploration Agency (JAXA), and, like LISA, it is a candidate for the next L-class ESA launch opportunity. On the basis of a 50 percent participation, it has an appraised cost to NASA, including the cost of partnering, of $3.1 billion, and the cost and schedule risk is medium high. The technical risk is also medium high. Cost threats and uncertainties due to the immaturity of some of the required technologies have added considerably to the cost appraisal. The budget profiles used by the committee to define an overall program are unlikely to permit a start before the end of the decade—allowing time for the necessary technology maturation and risk reduction. However, this situation does not diminish the committee’s assessment of the importance of the discoveries that IXO would make. Because of IXO’s high scientific importance, a technology development program is recommended for this decade with sufficient resources—estimated to be on the order of $200 million—to prepare IXO for favorable consideration in the next survey in 2020. The committee thinks that allowing IXO, or indeed any major mission, to exceed $2 billion in total cost to NASA would unacceptably imbalance NASA’s astrophysics program, given the present budgetary constraints. If the technology development program is not successful in bringing cost estimates below this level, descope options must be considered. Should ESA select IXO as the first L-class mission, NASA should proceed immediately with a DSIAC review to determine an appropriate path forward to realize IXO as soon as possible with acceptable cost and schedule risk.

The ranking of IXO as the fourth-priority large space mission reflects the technical, cost, and programmatic uncertainties associated with the project at the current time. Many high-priority science questions require an X-ray

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