Passage to a Ringed World - NASA's History Office

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per atmosphere. Energetic particles reaching the atmosphere create the auroral emission by exciting gases in the upper atmosphere (molecular and atomic hydrogen lines in the case of Saturn; oxygen and nitrogen in Earth’s atmosphere). Saturn’s aurora was first detected by the Voyager ultraviolet spectrometer. While is it not clear which magnetospheric particles (electrons, protons or heavy ions) create the aurora, it is clear that planets with higher fluxes of energetic particles have stronger auroral emissions. Cassini’s MAPS instruments will make coordinated studies of Saturn’s aurora, with the Ultraviolet Imaging Spectrometer providing images. Saturn Kilometric Radiation. For about 20 years prior to the Voyager visits to Saturn, radio astronomers had been searching for Saturn’s radio emissions. We now know that Saturn is a much weaker radio source than Jupiter. Confirmation of radio emissions from Saturn came only when Voyager 1 approached within three astronomical units of the planet. Saturn emits most strongly at kilometric wavelengths. Like the radio emission of other planets, Saturn kilometric radiation (SKR) comes from the auroral regions of both hemispheres and the radio beams are fixed in Saturn’s local time. However, the emitting regions are on the night side for Earth and on the dayside for Saturn. The emission appears to come from localized sources near the poles — one in the north and one in the south — that “light up” only when they reach a certain range of local times near Saturn’s noon. The periodicity of these emissions is about 10 hours, 39 minutes, assumed to be the rotation rate of Saturn’s conducting core. This is somewhat longer than the atmospheric rotation rate of 10 hours, 10 minutes observed at the cloud tops near the equator. The periodicity in SKR emission is unexpected for a planet with such a symmetrical magnetic field. Possibly, a magnetic anomaly exists that allows energetic electrons to penetrate further down into the polar region (at some point) and here the SKR radiation is generated at the electron’s natural frequency of oscillation. Based on the local time of emission, the source energy for the SKR appears to be the supersonic solar wind and, in fact, changes in the solar wind strongly control the SKR power. For example, a solar wind pressure increase by a factor of about 100 results in an increase by a factor of Energetic neutral imaging. Simulation of an energetic neutral atom (ENA) image of the type that will be obtained by Cassini’s Magnetospheric Imaging Instrument. The Saturn magnetosphere appears close to the center of the image and Titan is on the left. A SPHERE OF INFLUENCE 77
THE FOURTH STATE OF MATTER Plasma is the fourth state of matter. A plasma is an ionized gas containing negatively charged electrons and positively charged ions of a single or many species; it may also contain neutral particles of various species. Examples are the Sun, the supersonic solar wind, Earth’s ionosphere and the interstellar material. Plasmas behave differently from neutral gases; the charged particles interact with each other electro- magnetically and with any electric and magnetic fields present. The charged particles also create and modify the electric and magnetic fields. In a highly conducting plasma, the magnetic field lines move with (are “frozen to”) the plasmas. The X-ray image here shows a million-degree plasma, the solar corona, which is the source of the supersonic solar wind plasma that pervades the solar system. [Image from the Yohkoh satellite] 78 PASSAGE TO A RINGED WORLD about 10 in the SKR power. For a period of about two to three days following the Voyager 2 encounter, no SKR emission was detected. It is thought that since Saturn was immersed in Jupiter’s long magnetotail at this time, the planet’s magnetosphere was shielded from the solar wind. One of the main objectives of the Radio and Plasma Wave Science instrument aboard Cassini is to make measurements of the SKR, study its variation with variations in the solar wind and map the source region. Other Magnetospheric Emissions We are all familiar with waves in a vacuum (electromagnetic waves) and waves in a gas and fluid (electromagnetic waves, sound waves, gravity waves). A magnetized plasma supports all these waves and more. Because of the electromagnetic interactions between charged plasma particles and the magnetic field, new types of waves can propagate that have no counterpart in a neutral gas or fluid. Waves in the magnetosphere can be produced via various processes, for example by ionization of atmospheric neutral atoms in the magnetospheric plasma or by currents flowing between different plasma populations. These waves, as well as other types of waves (Alfvén waves, magnetosonic waves and ion and electron cyclotron waves, to name a few) can propagate in a plasma and be detected by sensors such as the magnetic and electrical antennas of Cassini’s Radio and Plasma Wave Science instrument. These waves are trapped within the magnetosphere and thus can only be sampled inside it. Saturn produces a variety of radio
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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
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In this diagram from his book, Syst
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This Voyager 1 image shows Saturn
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An Extended Atmosphere From Liquid
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Haze layers are also observed above
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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 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 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
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Page 104 and 105: • Opening or shutting thermal lou
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Page 110 and 111: The Cassini-Huygens spacecraft stan
Page 112 and 113: The Imaging Science Subsystem (ISS)
Page 114 and 115: • Map the composition and thermal
Page 116 and 117: cise distance between the surface f
Page 118 and 119: • Study the interaction of the ma
Page 120 and 121: • Study the erosional and electro
Page 122 and 123: midway out on the magnetometer boom
Page 124 and 125: The RPWS instrument will be used to
Page 126 and 127: altitudes down to 40 kilometers abo
Page 128 and 129: dex. A group of platinum resistance
Page 130 and 131: CHAPTER 10 Mission operations are t
Page 132 and 133: PLANETARY MISSION OPERATIONS Functi
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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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