Passage to a Ringed World - NASA's History Office

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An artist’s rendition of Saturn’s immense magnetosphere. Ω is the S planet’s rotation axis, closely aligned with the magnetic axis. [Image courtesy of Los Alamos National Laboratory] CHAPTER 6 Saturn, its moons and its awesome rings sit inside an enormous cavity in the solar wind created by the planet’s strong magnetic field. This “sphere of influence” of Saturn’s magnetic field — called a magnetosphere — resembles a similar magnetic bubble surrounding Earth. Inside the Magnetosphere Inside Saturn’s vast magnetospheric bubble is a mixture of particles, including electrons, various species of ions and neutral atoms and molecules, several populations of very energetic charged particles (like those in Earth’s Van Allen Belts) and charged dust grains. The charged particles and dust grains all interact with both the steady and the fluctuating electric Solar Wind Solar Wind Fast Neutrals Pickup Ions A Sphere of Influence Bow Shock Aurora Ring Current H + , He ++ and magnetic fields present throughout the magnetosphere. These ionized gases contain charged particles (electrons and ions) such as occur in the solar wind and planetary magnetospheres and are called plasmas. The steady fields can cause organized motions of the charged particles, creating large currents in the plasma. Cusp Polar Wind SKR H + , He ++ Aurora Ω S Titan Tail Titan Neutral Torus Magnetosheath H + , He ++ H + + + , H , H3 2 Magnetopause The region is not at all spherical; rather, the supersonic solar wind, flowing at 300–1000 kilometers per second against Saturn’s magnetic field, compresses the magnetosphere on the side facing the Sun and draws it out into a long magnetotail in the direction away from the Sun. H + + + , H , H3 2 Planetary Ions Plasma Sheet Plasma behavior is more complex than that of neutral gases because, unlike neutral particles, the charged particles interact with each other electromagnetically as well as with any electric and magnetic fields present. The plasma’s fluctuating fields (including wave fields) can “scatter” the charged particles in a manner similar to collisions in a neutral gas and cause a mixing of all the magnetospheric components. A SPHERE OF INFLUENCE 67
Sources of particles in Saturn’s magnetosphere. 68 PASSAGE TO A RINGED WORLD Most of what we know about Saturn’s magnetosphere comes from the brief visits by Pioneer 11 and Voyagers 1 and 2, but remote observations by the Hubble Space Telescope and other spacecraft have also provided us with intriguing information. Magnetospheric Particle Sources Saturn has a variety of sources for the particles in its magnetosphere. Particles can escape from any moon, ring or dust particle surface, or they can be “sputtered” off by energetic particles or even micrometeoroid impacts. The primary particle sources are thought to be the moons Dione and Tethys. But, the solar wind, iono- Main Ring F-Ring Mimas Enceladus Tethys G-Ring E-Ring sphere, rings, Saturn’s atmosphere, Titan’s atmosphere and the other icy moons are sources as well. Recent Hubble Space Telescope results show large numbers of neutral hydrogen atoms (the neutral hydrogen cloud in the illustration above) throughout the magnetosphere that probably come from a number of these sources. It has even been proposed that water ions and molecules may form a dense “ionosphere” above Saturn’s rings. Recent Hubble Space Telescope results show large numbers of neutral hydrogen atoms throughout the magnetosphere that probably come from a number of the sources mentioned. Determining the relative importance of the varied sources in different parts of Saturn’s space environment is a prime objective for the Magnetospheric and Plasma Science (MAPS) instruments aboard the Cassini spacecraft. Neutral particles can escape from any moon, ring or dust particle sur- Dione H 2 O Rhea face — or they can be “sputtered” off by energetic particles or even micrometeoroid impacts. When these particles become ionized, they can excite electromagnetic waves with a frequency that can be used to determine their type. The icy rings absorb the energetic particles inward of the moon Mimas. Energetic particles can be created by processes within the magnetosphere or they can leak in from the solar wind. These and many other magnetospheric phenomena were seen by the three earlier spacecraft. The mysterious “spokes” in the rings of Saturn, clearly seen in Voyager images, are probably caused by electrodynamic interactions between the tiny charged dust particles in the rings and the magnetosphere. Auroras, which exist on Saturn as well as Earth, are produced when trapped charged particles precipitating from the magnetosphere collide with atmospheric gases. Neutral Hydrogen Cloud Titan Despite many exciting discoveries, many more questions about the physical processes in Saturn’s magnetosphere remain unanswered. This chapter examines the current state of knowledge about Saturn’s magnetosphere and discusses the observations we expect to make with Cassini’s instruments and the knowledge we expect to gain from forthcoming explorations.
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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
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descent to the surface. The Huygens
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address the challenges of the missi
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Participant Nations* * First flag i
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PROBING A MOON OF MYSTERY Huygens
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TOUR T18-3 TARGETED SATELLITE FLYBY
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SATURN CASSINI-HUYGENS MISSION SCIE
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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
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Page 34 and 35: IN THE EYE OF THE BEHOLDER Taken fr
Page 36 and 37: This spectacular view of the edge o
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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
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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
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Page 58 and 59: scattering than are larger particle
Page 60 and 61: In fact, Saturn’s rings — as we
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Page 74 and 75: Artist’s conception of Ithaca Cha
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Page 82 and 83: SATURN’S MAGNETIC FIELD Saturn’
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Page 90 and 91: CHAPTER 7 Space mission design aims
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Page 118 and 119: • Study the interaction of the ma
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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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