Passage to a Ringed World The Cassini–Huygens Mission to Saturn and Titan Passage to a Ringed World The Cassini–Huygens Mission to Saturn and Titan On the Cover: A col- lage of images shows the destination of the Cassini–Huygens mis- sion — the Saturn sys- tem. The insets, from left, are Enceladus, Saturn and Titan. The collage represents the mission’s five chief areas of scientific investigation: icy sat- ellites, Saturn, Titan, rings and the mag- netosphere. (Mag- netospheric and plasma processes produce the “spokes” that run out across the rings as diffuse, dark markings.) Passage to a Ringed World The Cassini–Huygens Mission to Saturn and Titan Linda J. Spilker, Editor Jet Propulsion Laboratory California Institute of Technology NASA SP-533 National Aeronautics and Space Administration Washington, D.C. October 1997 FOREWORD This book is for you. You will find that it lives up to the promise of its cover and title. The text is authoritative, but at the same time easy to understand. It is written for any layperson who is interested in space exploration. In this book, we will give you a good look at what is involved in sending a large spacecraft to the outer solar system. Join us and share in the excitement of this extended voyage of discovery! One of the first things you will learn is that Cassini–Huygens is an inter- national mission. Seventeen countries are involved. You will also learn that Huygens is an atmospheric probe that the Cassini spacecraft will deliver to Titan, the largest moon of Saturn. Titan has a gaseous atmosphere that is thicker and more obscuring than the atmosphere here on Earth. We will look through Titan’s atmosphere with Cassini’s radar and discover surface details for the first time. The mission will find out if there are really liquid hydrocar- bons on Titan’s surface in the form of lakes or seas. The Cassini spacecraft itself will spend four years in orbit about Saturn. We will examine the rings and visit many of the satellites. We will sample spe- cial locations in the magnetosphere that are believed to harbor interesting — some might say strange — plasma processes (that is, electromagnetic interac- tions involving electrons, protons and ions). You can see in the table of contents the “menu” we have prepared for you. One chapter explains the mission, another the spacecraft. Other chapters tell you about Saturn, Titan, the rings and the various other parts of the Saturn system. These chapters reflect the facts and theories as we know them today. The chapters were prepared by expert researchers in each of the areas cov- ered. Professional writers then edited the text for clarity and to make sure that you would not be overwhelmed with jargon and technospeak. Working hand- in-hand with them were graphic designers and illustrators who created the page layouts and the many informative diagrams. Finally, to assure that the fi- nal product was accurate, the book was reviewed by scientists and engineers who work on the Cassini–Huygens mission. Come with us now as we set out on this voyage of exploration. We have over three billion kilometers to go! It will take more than six years to reach Sat- urn. We will get there on July 1, 2004. By the end of the Cassini–Huygens mis- sion, four years later, we will have completed the most complicated scientific experiment ever performed. We wish you good reading! Richard J. Spehalski Dennis L. Matson Cassini Program Manager Cassini Project Scientist ACKNOWLEDGMENTS This book would not have been possible without the outstanding efforts of many talented people, from the authors who wrote the chapters, to the editors and reviewers, to the graphic designers, artists and finally the printer. I would like to thank all of them for their excellent work. Many long hours went into develop- ing the exciting Cassini–Huygens book you hold in your hand. Cassini–Huygens plans to reap a bountiful harvest of new information about the Saturn system, so stay tuned! Many individuals worked diligently on writing the chapters in this book. These dedicated authors include Vince Anicich, Scott Bolton, Jim Bradley, Bonnie Buratti, Marcia Burton, Roger Diehl, Stephen Edberg, Candy Hansen, Charley Kohlhase, Paulett Liewer, Dennis Matson, Ellis Miner, Nicole Rappaport, Laci Roth, Linda Spilker and Brad Wallis. I also thank Stephen Edberg, Charley Kohlhase, Ellis Miner and Randii Wessen for their support in creating the Appendices. The creative design of this book is the ingenious work of the Jet Propulsion Laboratory’s Design Services team. The team worked tirelessly to complete this work before the Cassini–Huygens launch. My thanks to Audrey Steffan-Riethle for design and art direction. Special thanks go to Sanjoy Moorthy who spent many long hours writing and editing and organizing the material. My thanks also to David Hinkle for illustration and production, David Seal for computer artwork, Adriane Jach and Elsa King-Ko for design and production, Marilyn Morgan for editing and Robert Chandler for printing coordination. Many people provided reviews of the chapters. Special thanks for detailed reviews and editorial support go to Stephen Edberg, Charley Kohlhase, Jean-Pierre Lebreton, Ellis Miner, Nicole Rappaport, Laci Roth and Randii Wessen. My thanks to Marjorie Anicich, Michel Blanc, Jeff Cuzzi, Luke Dones, Michele Dougherty, Charles Elachi, Larry Esposito, Bill Fawcett, Ray Goldstein, Don Gurnett, Len Jaffe, Bill Kurth, Janet Luhman, Jonathan Lunine, Carl Murray, Andy Nagy, Toby Owen, Carolyn Porco, Dave Young and Philippe Zarka for serving as reviewers. Linda J. Spilker Cassini Deputy Project Scientist Contents Chapter 1 A Mission of Discovery 1 Chapter 2 The Ringed World 17 Chapter 3 The Mysterious Titan 27 Chapter 4 Those Magnificent Rings 41 Chapter 5 The Icy Satellites 53 Chapter 6 A Sphere of Influence 67 Chapter 7 Planning a Celestial Tour 81 Chapter 8 Vehicle of Discovery 89 Chapter 9 Tools of Discovery 101 Chapter 10 An Ensemble Effort 121 Afterword 131 Appendices A Glossary of Terms 135 B Acronyms & Abbreviations 143 C General Index 146 D Solar System Characteristics 148 E Program & Science Management 151 F Bibliography 152 CHAPTER 1 A Mission of Discovery Saturn, the second most massive planet in the solar sys- sphere. Some 17 additional icy satellites are known tem, offers us a treasure of opportunities for exploration to exist — each a separate world to explore in itself. and discovery. Its remarkable system of rings is a sub- Saturn’s magnetosphere is extensive and maintains ject intensively studied, described and cataloged. Its dynamic interfaces with both the solar wind and planet-sized satellite, Titan, has a dense, veiled atmo- with Titan’s atmosphere. From Observation to Exploration use of instruments to accurately serving the heavens by Galileo Ga- Saturn was known even to the an- measure celestial positions, enabling lilei, the pace of progress quickened. cients. Noting the apparitions of the astronomers to catalog the planets’ planets and recording other celestial positions. Today, the Cassini–Huygens mission events was a custom of virtually every to Saturn and Titan is designed to early civilization. These activities Developments in mathematics and the carry out in-depth exploration of the knew no oceanic bounds, suggesting discovery of the theory of gravitation- Saturn system. A payload of science a curiosity universal to humankind. al attraction were also necessary instruments will make in situ measure- Real progress in understanding the steps. With the invention of the tele- ments or observe their targets under planets came with the invention and scope and its first application in ob- favorable geometric and temporal circumstance. For example, an instru- ment might make observations over a range of various angles of illumina- tion and emission, or study events such as occultations or eclipses. Three separate Cassini–Huygens’ interplanetary jour- images, taken by ney starts in October 1997, with the Voyager 2 through launch from Cape Canaveral in Flo- ultraviolet, violet rida. Upon arrival at Saturn, Cassini– and green filters, Huygens will go into orbit about the respectively, were combined to cre- planet. The spacecraft consists of two ate this false-color parts — the Cassini Orbiter and a image of Saturn. smaller spacecraft, the Huygens Probe, which is targeted for Titan, Saturn’s largest moon. Huygens will arrive at Titan in No- vember 2004. After using its heat shield for deceleration in Titan’s up- per atmosphere, Huygens will deploy a parachute system. Six instruments will make scientific measurements and observations during the long A MISSION OF DISCOVERY 1 GETTING TO KNOW SATURN A TIMELINE OF DISCOVERY ~ 800 BC Assyrian and Babylonian observations ~ 300 AD Mythological view of Saturn the god 1610 Galileo notes the “triple planet” Saturn with his telescope 1655–59 Huygens discovers Saturn’s largest satellite, Titan 1671–84 Cassini discovers a division in the ring; he also discovers the satellites Iapetus, Rhea, Dione and Tethys 1789 Herschel discovers satellites Mimas and Enceladus — and notes thinness of rings 1848 Bond and Lassel discover the satellite Hyperion 1850 Bond, Bond and Daws discover inner ring 1857 Maxwell proves that rings are not solid 1895 Keeler measures ring velocities 1898 Pickering discovers satellite Phoebe 1932 Wildt discovers methane and ammonia on Saturn 1943–44 Kuiper discovers methane and ammonia on Titan 1979 Pioneer 11 flies past Saturn 1980 Voyager 1 encounters Saturn 1981 Voyager 2 encounters Saturn 1989 Hubble Space Telescope’s Wide Field and Planetary Camera images Saturn 1995 Wide Field and Planetary Camera 2 images ring plane crossing 1997 Cassini–Huygens launches 2004 Cassini–Huygens enters Saturn orbit; Huygens explores Titan 2 PASSAGE TO A RINGED WORLD descent to the surface. The Huygens space agency (Agenzia Spaziale sion, will span nearly 30 years! Probe data will be transmitted to the Italiana, or ASI), through a bilateral The formal beginning was in 1982, Orbiter and then to Earth.