PROJECT ORI'on a Design Study of a System for Detecting Extrasolar Planets

PROJECT ORI'ON A Design Study of a System for Detecting Extrasolar Planets . NASA NASA SP-436 PROJECT ORION A Design Stud? of a System for Detecting Extrasolar Planets David C.Black, Editor Arnes Research Center Nat~cnalAeroriautics and Space Adminislrat~on Scientific and Technical Information Branch 1986) Cover: Orion Nebula XI1 Photograph courtesy of Lick Observatory. Ode to Apodization Twinkle, twinkle, little star Thirty parsecs from where we are. Does your wobble through the sky Mean a planet is nearby? Or has the result come into being Because of one arcsecond seeing? -Raymond P. Vito Library of Congress Cataloging in Publication Data Main entry under title: Project Orion. (NASA SP ; 436) Bibliography: p. 1. Project Orion. I. Black, David C. 11. Series: United States. National Aeronautics and Space Administration. NASA SP ;436. QB602.9.P76 523.1'13 80-11728 For sale by the Sul~erintenclentof Documents. U.S. Gorevnmc~ntPrinting Olficc \X7a\hinrrton n C 70402 TABLE OF CONTENTS Page PREFACE ......................................... ix 1 . INTRODUCTION ................................. 1 Discovery of Our Planetary System .................. 1 Efforts .to Detect Other Planetary Systems ............. 4 Project Orion ................................... 6 2 . TOWARD DESIGN CONCEPTS ..................... 11 Remarks Concerning the Term "Planet" .............. II Detection Problem .Astrophysical Aspects ........... 12 Detectioil Problem .Terrestrial Aspects .............. 24 Detection Problem .Hardware Aspects ............... 30 Summary ...................................... 43 3 . DBRECT DETECTION SYSTEMS .................... 49 Direct Detection at Visual Wavelengths ............... 49 Direct Detection at Infrared Wavelengths .............. 71 Summary ...................................... 85 4 . ORIlON IMAGING STELLAR INTERFEROMETER: A DESIGN CONCEPT ............................. 89 Light-Collectil~gSystem: Alignment and Guidance ...... 96 Signal Detection. Processing. and Tracking ............. 116 IS1 Building .................................... 138 Site Selection and Development ..................... 140 Options and Additional Considerations ............... 145 Summary ...................................... 148 5 . TOWARD A NEW FRONTIER ..................... 155 Conclusio~~sof the Orion Study ..................... 155 Recornmendations for Future Activity ................ 157 Appendix A .Variability of Optical Paths Through the Atmosphere Due to Thermal Turbulence ......... 158 Appendix B .Analysis of Imaging Stellar Interferometer Data to Determine Relative Star Positions ........... 174 Appendix C .Sonine Apodization Using Masks ........ 186 Appendix D .Direct Imaging of Extrasolar Planets With a Space Telescope ......................... 188 REFERENCES ...................................... 194 LIST OF PARTICIPANTS ............................ 202 EDITOR'S NOTE ................................... 203 xii Stonchenge, Sal~aburyPlain, England Photograph courtesy of Beulah P. Gossett PREFACE The motivation behind Project Orion can perhaps be best under- stood by answering the question "Why is a search for other planetary systems important?" In large measure, the answer to that question is contained in two topics: the origin of the solar system and the existence of extraterrestrial intelligence - topics that seem at first glance unrelated. Speculation concerning the origin of the solar system is as old as man himself. The "modern" era of such speculation is generally con- sidered to have begun with Copernicus, who showed that Earth is not the center of the universe. The major cosmogonic hypotheses of this early period were those of Laplace and Kant. Although differing from each other in detail, their hypotheses were similar in that both men envisioned the planets as having formed from material that was spun off the Sun. These hypotheses were discarded a little over a century after they were advanced when it was realized that the Sun, which has about 99 percent of the mass in the solar system, has only about 2 percent of the angular momentum of the solar system. The principal reason these hypotheses fell into disfavor was that, at the time of this obsel-vational discovery, there was no known mechanism that would remove angular momentum from the Sun, thereby making it hard to understand why the planets have so much angular nlo~nentumrelative to the Sun if they were originally part of the Sun. The next generation of cosmogonic hypotheses was directed specifically at the problem of the angular momentum distribution in the solar system. Again, there were variations among the hypotheses, but they all tended to rely on singular or catastrophic events, such as a supernova or a stellar collision, to account for the solar system. Most of these "catastrophic" hypotheses, which were popular in the first few decades of this century, have subsequently been shown to be physically untenable or, at the very least, highly unlikely. Most current hypotheses concerning the origin of the solar system are in many ways similar to the early hypotheses of Laplace and Kant. They envision the planets forming from a nebula of gas and dust which is thought to have surrounded the Sun early in its history. This return to a "nebular" hypothesis has come about pn- marily for two reasons. First, unlike our counterparts of thc previous century, we now know of several reasonable physical mecha~~isms that could have removed angular momentum from the Suil. Combin- ing observational studies of the rotational velocities of stars on the main sequence (stars of various masses which, like the Sun, are deriv- ing their luminous energy mainly from conversion of I~ydrogeninto helium) with theoretical models of stellar structurs, a co~ssistei~tppic- ture emerges which indicates that the Sun has very little angular momentum because most of it was removed by thc solar wind, a plasma that continuously flows off the Sun's surface. T11e seco~ad reason for a return to "nebular" hypotheses is that the advent of high-speed electronic computers permits us to perform numerical experiments or modeling of the collapse of an interstellar cloud under the influence of its own gravity. It is generally felt that stars form by virtue of such a collapse process. Although these numerical experiments are not definitive, they strongly suggest that when a star is born it is likely to have a circumstellar nebula, and that condi- tions in such a nebula would be highly conducive to the foi-matio~~of planetary bodies. An equivalent way of expressing present cosmogonic hypotheses is to say that planetary system formation seems lo be a natural, if not inevitable, aspect of the star l'omation process. The important distinction between the catastrophic and llebular cosmogonic hypotheses is that, if the former is correct, planetary systems are the exception rather than the rule, whei-eas if the latter is correct, planetary systems are the rule. A systematic study of the frequency of occurrence of planetary systems would thus provide a val- uable observational check on present theories of star formatioi~. The possible existence of extraterrestrial intelligence (ETI) 11as long piqued man's curiosity. This curiosity has been the basis for a number of science fiction efforts. A graphic demonstration of this fascination with ETI is Orson Welles' radio dramatization in 1939 of the work by H. G. Wells entitled, "War of the Worlds." Until rela- tively recently, ETI had been the plaything of science fiction, largely ignored as a subject of scientific inquiry. However, (Cocconi and Morrison in 1959 took the first major step in cha.nging the attitude of the scientific community toward the question of ETT. They pointed out that there is a natural signpost in the electromagnetic spectrum that would be known to any advanced civilization, and that such civilizations might send radio signals at or near the frcqucncy of this natural marker. This signpost is the 2 1-cm-wavelerigth radiat~on arising from a hyperfine transition in atomic hyclrogen, the most abundant element in the universe. Shortly after the paper by Cocconi and Morrison, Frank Drake conducted a search, known as Project Ozma, for such signals. Drake's search was ui~s~accessful,brrt its importance cannot be overlooked as it was the first serious attempt at detecting ETI signals. The relevance of a SET1 (search for kTT) effort to a search for other planetary systems lies in the fact that the only known intelligent life form, namely ourselves, developed and was nurtured on a planet. If planets are required for the cxistencc of ETI, knowledge of the frequency of occurrence of planetary sysleii~s is clearly highly desirable. A systematic search for other planetary systeins would thus reveal whether there is justification in arguing that a nal~rral,perl~aps even causal, relation exists between the phenomenon of star fonna- tion, which has occurred some 10' times in the galaxy, anci the existence of ETI. The detection of other planetary syst~riisis cllffi- cult; present observational techniques and instrumen tdtion arc at best marginal in terms of their ability to carry out such a search. T11c purpose of this design study was to apply inoderil tech~~ologyto tllc problem in the form of specific design concepts for syrtenls that could successfully mount a search for other planetary systeriis. Tkc Earth is an object of exquisite beauty, and to tlie extent that tlic material on the ensuing pages is instrumental in the discovery of another such object, it

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