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independent advice committee review the IXO case and examine progress in the mission design and
readiness. If the review is favorable, NASA should be prepared to invest immediately in technology
development at a high level, and work with the project to define the partnership agreements.
On the basis of the above considerations, a budget of $4 million per year is recommended in the
first several years of the decade to allow for risk reduction and mission definition, with an increase in the
last half of the decade to a level of $20 million to $30 million per year, the minimum the committee
estimates is necessary to develop critical technologies and prepare IXO to be mature and ready for
consideration by the next decadal survey for a start soon thereafter. Descopes should be considered to
ensure that the cost to NASA remains below $2 billion but reviewed to ensure that the baseline science
requirements are still achieved. Investing 10 percent of NASA’s eventual cost is consistent with the
committee’s other recommendations regarding mission-specific technology development. Prior to a start,
NASA, in coordination with ESA and JAXA, should ensure that IXO’s principal risks are retired,
including a down-select of the critical mirror technology, with sufficient maturation to demonstrate the
performance, mass, and cost.
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. However, many high-priority
science questions require an X-ray observatory on this scale, continuing the great advances made by
Chandra and XMM-Newton. Furthermore, the science of IXO is quite complementary to that of LISA.
The committee therefore recommends that NASA begin by mid-decade an aggressive program to mature
the mission and develop the technology so that this high-priority science mission can be realized.
Recommendations for New Space Activities—Medium Projects
Priority 1 (Medium, Space). New Worlds Technology Development Program for a 2020 Decade
Mission to Image Habitable Rocky Planets
One of the fastest growing and most exciting fields in astrophysics is the study of planets beyond
our solar system. The ultimate goal is to image rocky planets that lie in the habitable zone of nearby
stars—at a distance from their star where water can exist in liquid form—and to characterize their
atmospheres. Detecting signatures of biotic activity is within reach in the next 20 years if we lay the
foundations this decade for a dedicated space mission in the next.
Achieving this ultimate goal requires two main necessary precursor activities. The first is to
understand the demographics of other planetary systems, in particular to determine over a wide range of
orbital distances what fraction of systems contain Earth-like planets. To this end, the committee
recommends, as discussed earlier in this chapter, combined exploitation of the current Kepler mission,
development and flight of the first-priority large mission WFIRST, and a vigorous ground-based research
program. The second need is to characterize the level of zodiacal light present so as to determine, in a
statistical sense if not for individual prime targets, at what level starlight scattered from dust will hamper
planet detection. Nulling interferometers on NASA-supported ground-based telescopes (for example,
Keck, and the Large Binocular Telescope) and/or on suborbital, SMEX, or MIDEX platforms could be
used to constrain zodiacal light levels. A range of measurement techniques must be strongly supported to
ensure that the detections extend to the relevant Earth-Sun distance range 16 for a sufficient sample of
systems. After these essential measurements are made, the need for a dedicated target finder can be
determined and the approach for a space-imaging mission will be clear. The programs above will enable
the optimal technologies to be selected and developed.
For the direct detection mission itself, candidate starlight suppression techniques (for example,
interferometry, coronagraphy, or star shades) should be developed to a level such that mission definition
16 The Spitzer Space Telescope was sensitive to dust located at wide separations from stars, analogous to the
solar system’s outer Kuiper belt, but not to analogs of the inner asteroid belt or the zodiacal dust close to Earth.
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