Passage to a Ringed World - NASA's History Office

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scattering than are larger particles, so this tells us that these areas of the rings have an abundance of small, micrometer-size particles. The D, E, F and G rings all show such forwardscattering properties as do the spokes of the B-ring. These data, combined with ring occultation data from the photopolarimeter, ultraviolet spectrometer and radio-science experiments, give us some information about the variation of particle-size distribution across Saturn’s ring system. Cassini will have the opportunity to conduct extensive studies of these diffuse rings and their particle distributions. Ring Composition Ground-based infrared spectral studies of the A and B rings show that they are composed largely of very nearly pure water ice. The spectral characteristics of the rings are also very similar to those of several of Saturn’s inner satellites. The extended wavelength range over which Cassini will be able to make spectral measurements will yield information on both the nature of the chemical contaminants in the rings’ water ice as well as on the relationship between ring and satellite composition. Studies of the color distribution (as a sign of compositional variation) in the main rings show that the ring system is not completely uniform in its makeup and that some sorting of materials within A and B rings exists. Why such a nonuniform composition exists is unknown; Cassini data will provide some clues toward solving this problem. The other, diffuse rings of Saturn are much more difficult to study in this manner, but we do know that the E-ring is somewhat bluish in color — and thus different in makeup from the main rings. Speculation exists that the moon Enceladus is the source of E-ring material, but this is unconfirmed. Since ring particles larger than about one millimeter represent a considerable hazard to the Cassini spacecraft, the mission plan will include efforts to avoid dense particle areas of Saturn’s ring plane. The spacecraft will be oriented so as to provide maximum protection for itself and its sensitive instrumentation packages. Even with such protective steps, passage through the ring plane will allow the Cassini particle measurement experiments (the Cosmic Dust Analyzer, the Ion and Neutral Mass Spectrometer and the Cassini Plasma Spectrometer) to perform important studies of the particles making up the less dense regions of the Cassini ring plane. Such measurements could provide insight into the composition and environment of the ring system and Saturn’s icy satellites. This false-color image of Saturn’s B and C rings reveals fine details and subtle color differences. THOSE MAGNIFICENT RINGS 49
Voyager 1 looked back on November 16, 1980, four days after flying by the planet, to capture Saturn and its spectacular rings from this perspective. Formation and Evolution 50 PASSAGE TO A RINGED WORLD A century after their discovery, the origin of Saturn’s rings was a point of speculation. Then, for many years, the matter was actually thought to be well understood. Today’s view is that the origin of those magnificent rings involves a complex set of issues. The size, mass and composition of the rings make their formation and evolution rather difficult to explain. Among the early notions about the rings’ origin was a theory by Edouard Roche. He suggested that the rings were fragments left over from a moon that had at one time orbited Saturn. This moon had broken up, Roche explained, because of the tremendous stresses placed upon it by Saturn’s huge gravitational field. The Roche theory does not hold up well, however, under scrutiny. For instance, did the rings form out of the initial solar system nebula, or after one or more satellites were torn apart by Saturn’s gravity? Moreover, if the rings were the result of the numerous comets captured and destroyed by Saturn’s gravity, why are Saturn’s rings so different in nature from the rings of the other giant planets? Over their lifetime, the rings must have been bombarded continually by comets and meteors — they should have accumulated a great amount of carbonaceous and silicate debris — yet their composition seems almost entirely to be water ice. Another issue concerns the stability of the ring system. The effects of torque and gravitational drag, along with the loss of momentum via collisional processes should have produced a system only one-tenth to one-hundredth the age of the solar system itself. If this hypothesis is correct, then we cannot now be observing a ring system around Saturn that formed when the solar system coalesced.
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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
Page 8 and 9: ACKNOWLEDGMENTS This book would not
Page 10 and 11: 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 60 and 61: In fact, Saturn’s rings — as we
Page 62 and 63: The satellites of Saturn comprise a
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
Page 80 and 81: field reverses direction across the
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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falls below its storage temperature
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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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