Cosmic Background Explorer (COBE) discovered that the cosmic microwave background (CMB) has measurable fluctuations, demonstrated that these tiny variations imprint precise information about the early universe. WMAP is credited with obtaining the best measurements of the age, geometry, and content of the universe. The Swift mission has transformed understanding of explosive gamma-ray burst events, and it holds the record for detecting the most distant object in the universe. The WISE mid-infrared survey, extending over the entire sky, is studying the coolest stars, the universe’s most luminous galaxies, and some of the dimmest near-Earth asteroids and comets. Small Explorer (SMEX) missions, as well as Mission of Opportunity contributions to non-NASA missions, have made essential advances in understanding of phenomena ranging from the explosive release of energy in flares on the Sun (with the Reuven Ramaty High Energy Solar Spectroscopic Imager) to the assembly of galaxies (with the Galaxy Evolution Explorer). The promise of future Explorer missions is as great as ever, and this program will be essential to enabling new opportunities, and to maintaining breadth and vibrancy in NASA’s astrophysics portfolio in a time of budgetary stress. This survey recommends that the annual budget of the astrophysics component of the Explorer program be increased from $40 million to $100 million by 2015.

The categorization of the recommended Explorer program augmentation as a large-category activity reflects the total cost of the augmentation for the decade 2012-2021, and its high ranking is motivated by the committee’s view that expanding the Explorer program is a very effective way to maximize scientific progress for a given outlay.

Laser Interferometer Space Antenna (LISA)

LISA employs three separated spacecraft to detect long-wavelength ripples in the fabric of space-time, thereby opening a new window on the universe. LISA will detect the mergers of black holes with masses ranging from 10,000 to 10 million solar masses at cosmological distances, and will make a census of compact binary systems throughout the Milky Way. LISA promises new discoveries as well as progress on central questions such as understanding the growth of galaxies and black holes. LISA will also test general relativity with exquisite precision in regimes inaccessible on Earth. LISA complements the search for gravitational radiation being made at shorter wavelengths by the ground-based Advanced LIGO. LISA is a partnership with ESA, and so its schedule is dependent on ESA’s selection of the next L-class mission opportunity—LISA is one of three contenders for this opportunity. LISA’s key technologies will be demonstrated on the ESA-led LISA Pathfinder mission, due for launch in 2012. With the success of Pathfinder and a decision by ESA to move forward, LISA could launch by 2025. Independent review found LISA’s technical risk, assuming Pathfinder success, to be medium, and

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