Passage to a Ringed World - NASA's History Office

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The satellites of Saturn comprise a diverse set of objects. This composite image shows, from top, Mimas, Rhea, Iapetus, Tethys, Enceladus and Dione. CHAPTER 5 The moons, or satellites, of Saturn represent a diverse set of objects. They range from the planet-like Titan, which has a dense atmosphere, to tiny, irregular objects only tens of kilometers in diameter. These bodies are all believed to have as major components some type of frozen volatile, prima- Overall Characteristics In 1655, Christiaan Huygens discovered Titan, the giant satellite of Saturn. Later in the 17th century, Jean- Dominique Cassini discovered the four next largest satellites of Saturn. It was not until more than 100 years later that two smaller moons of Saturn were discovered. As telescopes acquired more resolving power in the 19th century, the family of Saturn’s satellites grew. The Icy Satellites Most of the smallest satellites were discovered during the Voyager spacecraft flybys. The 18th satellite, Pan, was found nearly 10 years after the flybys during close analysis of Voyager images; it is embedded in the Aring of Saturn. Saturn’s ring plane crossings — when the obscuring light from Saturn’s bright rings dims, as the rings move to an edge-on orientation — represent the ideal configuration for discovering new satellites. Images rily water ice, but also other components such as methane, ammonia and carbon dioxide existing either alone or in combination with other volatiles. Saturn has at least 18 satellites; there may exist other small, undiscovered satellites in the planet’s system. obtained by the Hubble Space Telescope during the ring plane crossings in 1995 did not reveal any unambiguous discoveries of new satellites. Physical and Dynamic Properties Most planetary satellites present the same hemisphere toward their primaries, a configuration that is the result of tidal evolution. When two celestial bodies orbit each other, the gravitational force exerted on the near side is greater than that exerted on the far side. The result is an elongation of each body that forms tidal bulges, which can consist of either solid, liquid or gaseous (atmospheric) material. The primary tugs on the satellite’s tidal bulge to lock its longest axis on to the primary satellite line. The satellite, which is said to be in a state of synchronous rotation, keeps the same face toward the primary. Since this despun state occurs rapidly (usually within a few million years), most natural satellites are in synchronous rotation. Of Saturn’s icy satellites, two, Hyperion and Phoebe, are known to exhibit asynchronous rotation. THE ICY SATELLITES 53
The Hubble Space Telescope obtained these images of Saturn and its rings and satellites. The top image, obtained in August 1995, shows Titan (far left, casting a shadow) and (far right, from the left), Mimas, Tethys, Janus and Enceladus. In the middle image, obtained in November 1995, Tethys (upper left) and Dione appear. In the bottom image, obtained in December 1994, a storm is raging in the center of the planet. [Images courtesy of: top and middle — E. Karkoschka, University of Arizona Lunar and Planetary Laboratory; bottom — R. Beebe, New Mexico State University] 54 PASSAGE TO A RINGED WORLD
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Passage to a Ringed World The Cassi
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On the Cover: A collage of images s
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FOREWORD This book is for you. You
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ACKNOWLEDGMENTS This book would not
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Three separate images, taken by Voy
Page 12 and 13: descent to the surface. The Huygens
Page 14 and 15: address the challenges of the missi
Page 16 and 17: Participant Nations* * First flag i
Page 18 and 19: PROBING A MOON OF MYSTERY Huygens
Page 20 and 21: TOUR T18-3 TARGETED SATELLITE FLYBY
Page 22 and 23: SATURN CASSINI-HUYGENS MISSION SCIE
Page 24 and 25: Cassini-Huygens designed to determi
Page 26 and 27: In this diagram from his book, Syst
Page 28 and 29: This Voyager 1 image shows Saturn
Page 30 and 31: An Extended Atmosphere From Liquid
Page 32 and 33: Haze layers are also observed above
Page 34 and 35: IN THE EYE OF THE BEHOLDER Taken fr
Page 36 and 37: This spectacular view of the edge o
Page 38 and 39: the interaction was described as a
Page 40 and 41: have a signature of a tone decreasi
Page 42 and 43: CONSTITUENTS OF THE TITAN ATMOSPHER
Page 44 and 45: The processes of plate tectonics an
Page 46 and 47: tan was accreting, it is possible t
Page 48 and 49: Surface Science. The highest resolu
Page 50 and 51: This enhanced Voyager 2 image shows
Page 52 and 53: Voyager 1 captured this striking im
Page 54 and 55: Prometheus and Pandora, two tiny sa
Page 56 and 57: In the stellar occultation experime
Page 58 and 59: scattering than are larger particle
Page 60 and 61: In fact, Saturn’s rings — as we
Page 64 and 65: Before the advent of spacecraft exp
Page 66 and 67: The bright surface of Saturn’s mo
Page 68 and 69: Galilean satellites. Most of what i
Page 70 and 71: duced that one hemisphere is compos
Page 72 and 73: patches on the surface. Although it
Page 74 and 75: Artist’s conception of Ithaca Cha
Page 76 and 77: An artist’s rendition of Saturn
Page 78 and 79: The MAPS Instruments Coordinated ob
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Page 82 and 83: SATURN’S MAGNETIC FIELD Saturn’
Page 84 and 85: Solar Wind Magnetosheath Titan neut
Page 86 and 87: per atmosphere. Energetic particles
Page 88 and 89: and plasma wave emissions from narr
Page 90 and 91: CHAPTER 7 Space mission design aims
Page 92 and 93: (SOI) propulsive maneuver to initia
Page 94 and 95: the tour cannot continue. Of course
Page 96 and 97: crease inclination to this value. T
Page 98 and 99: This illustration shows the main de
Page 100 and 101: discussed earlier) and data handlin
Page 102 and 103: of the spacecraft! Below the oxidiz
Page 104 and 105: • Opening or shutting thermal lou
Page 106 and 107: management subsystem and the Probe
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Page 110 and 111: The Cassini-Huygens spacecraft stan
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The Imaging Science Subsystem (ISS)
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• Map the composition and thermal
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cise distance between the surface f
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• Study the interaction of the ma
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• Study the erosional and electro
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midway out on the magnetometer boom
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The RPWS instrument will be used to
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altitudes down to 40 kilometers abo
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dex. A group of platinum resistance
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CHAPTER 10 Mission operations are t
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PLANETARY MISSION OPERATIONS Functi
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two low-gain antennas 1 and will pr
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During much of Cassini’s orbital
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articulation control subsystem’s
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AFTERWORD It has been said that in
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Developments tied to the Cassini-Hu
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APPENDIX A Command. A data transact
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APPENDIX A 138 PASSAGE TO A RINGED
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APPENDIX A Plasma wave. A wave char
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APPENDIX A 142 PASSAGE TO A RINGED
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APPENDIX B H O 2 + Water molecule w
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APPENDIX C 146 PASSAGE TO A RINGED
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APPENDIX D 148 PASSAGE TO A RINGED
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APPENDIX D 150 PASSAGE TO A RINGED
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APPENDIX F 152 PASSAGE TO A RINGED
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APPENDIX F 154 PASSAGE TO A RINGED
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APPENDIX F Peralta, F. and S. Flana
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National Aeronautics and Space Admi
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