Passage to a Ringed World - NASA's History Office

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In the stellar occultation experiment, the Voyager 2 spacecraft had a single opportunity to observe as light from a star was occulted by the ring system. As the light passed through the rings, it was attenuated and extinguished by the particles comprising the system. The velocity of the spacecraft, the brightness of the star, the sensitivity of the instrumentation and the sampling rate of the instruments provide the limitations for such stellar occultation experiments. The primary instruments used to conduct the stellar occultation study were Voyager’s photopolarimeter and ultraviolet spectrometer, with the photopolarimeter providing the higher resolution data set. Resolving features as small as about 100 meters, the stellar occultation experiments detected many small-scale ring structures and found that the F-ring was far more complex than images had suggested. The data set, furthermore, showed that the B-ring was quite opaque in many regions and confirmed that the Cassini “division” was not at all empty. It also provided a direct measurement of the maximum thickness of the ring system in several locations, finding it to be much less than 100 meters thick. In the radio occultation experiment, Voyager 1 had a single opportunity to watch as the spacecraft’s radio beacon was occulted by the ring system. As this radio signal passed through the ring material, its signal was also attenuated. The antennas of the Deep Space Network received the attenuated signal, allowing scien- tists to study the complex data being returned. While the radio-science data set was complicated to process and interpret, it yielded information on particle size distribution in the ring system not available from stellar occultation studies. From the radioscience data set, we know something of the size distribution of the larger particles in the ring system. We know that much of the ring mass is in particles from a few centimeters to a few meters in diameter; boulders more than 10 meters in diameter do not comprise much of the mass of Saturn’s ring system. Both occultation experiments described here detected features in Saturn’s rings caused by the gravitational effects of the planet’s satellites. Locations where gravitational resonance effects had partially cleared material were identified by these experiments and are also visible at lower resolution in imaging data. While the existence of resonances had been recognized since 1866 — when it was pointed out that the Cassini division lay near a gravitational resonance point — the extent to which the structures occur was largely unexpected. At these resonances, gravitationalwave effects were detected in the photopolarimeter, ultraviolet spectrometer and radio-science occultation data sets — known as density and bending waves — that are similar in behavior to the arms of spiral galaxies. While some of these structures were later detected in the imaging data, the high-resolution data sets of the photopolarimeter, ultraviolet spectrometer and radio-science experiments provided the details that allowed Voyager science teams to derive information about the mass, density and viscosity4 of the ring material in their vicinity. Much of the fine structure of the A-ring is wellexplained by such gravitational resonances with satellite orbits. However, the resonance theory does not provide a universal solution and much of the structure of the B-ring is left unexplained. The occultation results, along with imaging studies, showed that a great many of the narrow ringlet features at Saturn are slightly eccentric — not circular in shape — and that these eccentric ringlets lie embedded in the mass of nearly circular rings that constitute the majority of Saturn’s ring system. These same studies also showed that many small ringlets are asymmetrical in structure and that there are very few truly empty “gaps” in the ring system. Results from the Voyager experiments provided a wealth of information to explain some physical properties of Saturn’s rings, but at the same time, they also created many mysteries that were unexpected and not understandable through Voyager data. For instance, Voyager data did not detect the number of moonlets we expected. Are these moonlets nonexistent or just not yet discovered? Cassini will have far more time to THOSE MAGNIFICENT RINGS 47
The numerous “spoke” features in the B-ring are evident in this image obtained by Voyager 2. 48 PASSAGE TO A RINGED WORLD perform ring searches and will provide higher resolution data sets, both required to answer this question. Ring Spokes As Voyager approached closer to Saturn, images began to show dark, radial structures on the rings. While these features had been sighted by Earth-based observers over the decades, the Voyager images showed the “spokes” in great detail, allowing them to be studied as they formed and rotated about the planet. Spokes seemed to appear rapidly — as a section of ring rotated out of the darkness near the dawn terminator — and then dissipate gradually, rotating around toward the dusk ring terminator. A spoke’s formation time seemed to be very short; in some imaging studies one was seen to grow over 6000 kilometers in distance in just five minutes. As it passed from the day side to the night side of the planet, Voyager provided unique views of Saturn’s rings and was able to observe the rings and spokes in forward-scattered light where they appeared as far brighter features than in backscattered light. This light-scattering property showed that the spokes were largely composed of very small, micrometer-sized particles. The spokes in Saturn’s rings form at the distance from Saturn where the rotational speed of the ring particles matches that of the planet’s magnetic field lines. Further studies of this possible interaction revealed a link between the spoke formation and a region on the planet associated with intense long-wavelength radio emissions (now referred to as the Saturn kilometric radiation, or SKR, zone). All these studies pointed to a sizable interaction between the ring system and Saturn’s electromagnetic fields. But what was it? Once again, Voyager had provided a tantalizing tidbit of information, but had left a riddle. Ring Particle Properties The Voyager trajectories allowed the rings to be viewed from a much wider range of angles than could ever be achieved by Earth-based observers. This allowed for extensive observations of the light scattering, or photometric, properties of the rings. The proximity of the spacecraft to Saturn also provided for higher resolution photometric and spectral studies than could ever have been achieved by Earth-based observers. These studies yielded a great deal of information about the nature of the ring particles and the composition of the rings. Observations in forward-scattered light made while the spacecraft was on the dark side of the planet showed that some areas of the rings were much brighter in forward-scattered light than others. Small particles in the micrometer-size range are much more efficient in producing forward
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Passage to a Ringed World The Cassi
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On the Cover: A collage of images s
Page 6 and 7: 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 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 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
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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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