unresolved questions. How do planetary systems form and evolve? Are planetary systems like our solar system common in the universe? Do any extrasolar planetary systems harbor life? Even a familiar object like the Sun poses many mysteries. What causes the small variations in the Sun’s luminosity that can affect Earth’s climate? What is the origin of the eruptions on the solar surface that cause “space weather”?

To seek the answers to these questions and many others described in this report, the committee recommends a set of new initiatives for this decade that will substantially advance the frontiers of human knowledge. Table ES.1 presents these initiatives, combined for both ground- and space-based astronomy, in order of priority. The committee set the priorities primarily on the basis of scientific merit, but it also considered technical readiness, cost-effectiveness, impact on education and public outreach, and the relation to other projects. The initiatives were divided into three categories—major, moderate, and small—that were defined separately for ground- and space-based projects based on estimated cost (see Chapter 1). The estimated cost of the recommended program for the decade 2000 to 2010 is $4.7 billion in FY2000 dollars, about 20 percent greater than the $3.9 billion inflation-adjusted cost of the recommendations of the 1991 survey. Two of the recommended projects, the Terrestrial Planet Finder (TPF) and the Single Aperture Far Infrared (SAFIR) Observatory, could start near the end of this decade or at the beginning of the next. The committee has assumed that about 15 percent of the total estimated cost for these two projects will fall in this decade.

• The Next Generation Space Telescope (NGST), the committee’s top-priority recommendation, is designed to detect light from the first stars and to trace the evolution of galaxies from their formation to the present. It will revolutionize understanding of how stars and planets form in our galaxy today. NGST is an 8-mclass infrared space telescope with 100 times the sensitivity and 10 times the image sharpness of the Hubble Space Telescope in the infrared. Having NGST’s sensitivity extend to 27 µm would add significantly to its scientific return. Technology development for this program is well under way. The European Space Agency and the Canadian Space Agency plan to make substantial contributions to the instrumentation for NGST.