THE NEXT GENERATION SPACE TELESCOPE Visiting a Time When Galaxies Were Young The NGST Study Team Edited by H.S. STOCKMAN Space Telescope Science Institute June 1997 ©1997 The Association of Universities for Research in Astronomy, Inc. All rights Reserved. Contents Preface and Acknowledgments . .i Authorship . .iii Executive Summary . .v 1 Studying the Early Universe: The Dark Ages . .1 2 Seeing Beyond the Hubble Space Telescope . .17 3 The NGST Challenge: Establishing Feasibility . .34 4 Concepts for the Next Generation Space Telescope . .45 5 Launch and Orbit . .53 6 Large Space Telescopes: An Issue of Transportation . .62 7 The Science Instrument Module: Recording the Light from Stars and Galaxies . .82 8 The NGST Spacecraft Support Module . .97 9 Operating and Using NGST . .109 10 Technology for the Next Generation Space Telescope . .121 Appendices A Contributors to the NGST Study . .136 B References and Suggested Reading . .143 C The Design Reference Mission . .146 D The NGST-δ Concept . .154 Acronyms . .157 Preface and Acknowledgments n the spring and summer of 1996, three independent teams stud- ied the feasibility of a large aperture space telescope to follow Ithe Hubble Space Telescope. The scientific goals for the new telescope had been laid out in a report by the HST & Beyond Committee, a group appointed by the Association of Universities for Research in Astronomy to consider the needs of the astronomical com- munity after the nominal end of the HST mission in 2005. The techni- cal capabilities and constraints on the new observatory were daunting: the telescope optics should be at least 4 meters in diameter and pas- sively cooled to achieve optimum sensitivity in the near-infrared por- tion of the spectrum. Moreover, the costs should be kept within a frac- tion of those for the HST: approximately $500M for construction and $900M for lifetime costs, not including support for scientific data analy- sis. In their presentations to NASA on 19-21 August, 1996, the teams led by Lockheed Martin, TRW, and the Goddard Space Flight Center con- cluded that a Next Generation Space Telescope (NGST) was not only feasible and affordable, but that it could be made more powerful using recent breakthroughs in space technologies. Coming on the heels of breathtaking HST observations of distant galaxies in the process of for- mation, such an NGST could bridge the gap in our understanding of the earliest origins of stars, galaxies, and the elements that are the founda- tions of Life. This report presents the findings of the three teams and the technological roadmap which will guide us to the successful devel- opment of the NGST over the next decade. We have made liberal use of the written material, tables, and dia- grams prepared by the three study teams. We have also taken advan- tage of the knowledge and ideas of our colleagues in government, industry and academia. In Appendix A, we list the members of the three study groups, the NGST Science Working Group and the NGST Scientific Oversight Committee. We deeply appreciate their assistance, advice, and enthusiasm. The scientific and technological goals of NGST are part of the Origins initiative in the Office of Space Science, NASA Headquarters. We are ii NEXT GENERATION SPACE TELESCOPE pleased to acknowledge the support and leadership of Edward Weiler, a steadfast friend of HST, Harley Thronson, a proponent of all things infrared, and Mike Kaplan, a tireless advocate of new technology. John Campbell, Project Manager for HST, initiated the NGST study at GSFC and we deeply appreciate his formative efforts and continued support. We are also grateful for the foresight of Riccardo Giacconi, Director General of the European Southern Observatory (ESO), and the efforts of his staff. We note that European Space Agency (ESA) staff at the Space Telescope Science Institute and ESO played important roles in the NGST scientific and technical studies. We look forward to future col- laboration with ESA, ESO, and other international partners. iii Authorship HIS REPORT summarizes the contributions from the hundreds of Tindividuals and dozens of institutions listed in Appendix A. We are pleased to acknowledge the useful comments and suggestions received from colleagues and, in particular, the members of the Scientific Oversight Committee. Editor H. S. (Peter) Stockman, Jr. Space Telescope Science Institute Contributing Editors Pierre Bely Space Telescope Science Institute Dan Coulter Jet Propulsion Laboratory Keith Kalinowski Goddard Space Flight Center Mike Krim Hughes Danbury Optical Systems Charles Lillie TRW Corporation Michael Margulis Lockheed Martin Missiles and Space Company John Mather Goddard Space Flight Center Michael Mesarch Goddard Space Flight Center Lloyd Purves Goddard Space Flight Center Bernie Seery Goddard Space Flight Center Patricia Pengra, Managing Editor Space Telescope Science Institute Lori Keesey, Assistant Editor Keesey & Associates Executive Summary EARCHING FOR THE ORIGIN of the universe is very much like arche- ology. Astronomers, like archeologists, must peel away the stra- Sta of time to find clues. Over the past few decades, astronomers have made great progress doing just that — peering farther and farther back in space and time to study objects that existed when our universe was still very young. Astronomers have uncovered tantalizing clues in images taken by the Hubble Space Telescope (HST). In one image (Hubble Deep Field, below, and detail at top of next page), they found a myriad of galaxies that formed perhaps 5 billion years after the Big Bang. Surprisingly, the fledgling galaxies seem very well-developed and exhibit many of the features of current galaxies. From this, astronomers have deduced that the galaxies formed much earlier — perhaps only a few billion years after the cataclysmic explosion that gave birth to the universe. The Hubble Deep Field. Several hundred never- before-seen galaxies are visible in this “deepest- ever” view of the universe, made with NASA’s Hubble Space Telescope. Besides the classical spiral and elliptical shaped galaxies, there is a bewildering variety of other galaxy shapes and colors that are important clues to understanding the evolution of the universe. vi NEXT GENERATION SPACE TELESCOPE Hubble Deep Field (detail). This detail shows distant spirals and spherical galaxies as well as blue, disturbed galaxies that are presumably large star forming regions, perhaps within a larger undetected host galaxy. COBE reveals the Beginning of Structure. This Cosmic Background Explorer (COBE) false-color map of the sky shows tiny differences in the density of matter in the uni- verse soon after the Big Bang. High-density regions (blue) are believed to have evolved into the largest scale structure seen in the universe today. Astronomers also have uncovered clues in data gathered by the Cosmic Background Explorer (COBE) (above, right). The explorer- class observatory detected the seeds of galaxies and other large-scale structures that began to evolve just 300,000 years after the Big Bang. How did these seeds condense into the stars and galaxies observed by Hubble? This period of time might be called the “dark zone” — a gap in the history of our universe that holds the secrets of its evolution. EXECUTIVE SUMMARY vii Visiting a Time When Galaxies Were Young. NGST will observe the “dark zone,” a period when primordial seeds began to evolve into the galaxies and stars we see today. The Answer To see the first generations of stars, the science community believes it will need a successor to Hubble. Even with new instruments, Hubble’s observations are limited to “adolescent” objects. The younger objects, which are receding from us at an even faster rate, are redshift- ed into the near infrared (past 2 microns) where Hubble loses sensitiv- ity. Known as the Next Generation Space Telescope (NGST), the obser- vatory will be sensitive to infrared radiation and, with its large light- gathering mirror and superb resolution, capable of detecting faint sig- nals from the first billion years — the period when galaxies formed (above). For such observations, the new telescope will be chilled to the low temperatures of outer space and placed in an orbit beyond the viii THE NEXT GENERATION SPACE TELESCOPE L2 1X3 AU NGST Orbit. To enhance its performance, scientists hope to place the obser- vatory as far from the Earth-Moon system as possible to reduce stray light and to maintain the telescope’s relatively cool temperature. Two orbits being considered are the second Lagrange point (L2) and a 1 x 3 AU solar orbit. Moon (above). The location and low temperatures make the obser- vatory thousands of times more sensitive than Earth-bound tele- scopes, and enables astronomers to see how and when the first generations of stars appeared and how quickly those stars manu- factured the heavy elements that eventually became the material for worlds like Earth. With such a capability, astronomers will finally lift the veil that now obscures the dark zone of the universe’s first billion years. “HST and Beyond” Report Today, astronomers have at their disposal a variety of ground- and space-borne telescopes and instruments, operating at a wide range of wavelengths. Given the variety, and the intense competition for fund- ing, the science community is mindful that a solid scientific case is needed to support a follow-on mission to the enormously successful Hubble Space Telescope. In its report, “HST and Beyond,” the blue ribbon committee appoint- EXECUTIVE SUMMARY ix ed by the Association of Universities for Research in Astronomy (AURA) recommended such a follow-on mission. The report urged the devel- opment of a general-purpose, near-infrared observatory equipped with a primary mirror larger than 4 meters. Able to maintain a cool temper- ature of 70 K or lower, the observatory would be up to 1,000 times more sensitive than any existing or planned facility in the 1-5 micron region.