Passage to a Ringed World - NASA's History Office

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TOUR T18-3 TARGETED SATELLITE FLYBYS* Cassini Flybys, Saturn Voyager Closest Planned Satellite Approach, kilometers 1 Iapetus 909,000 4 Enceladus 87,000 1 Dione 162,000 1 Rhea 74,000 * Actual distances will be based on the type of observations planned. Titan is the only Saturn satellite that is large enough to enable significant gravity-assisted orbit changes. The smaller icy satellites can help sometimes with their small perturbations, which can be useful in trimming a trajectory. Designing the Cassini orbital tour is a complicated and challenging task that will not be completed for at least several years. One of the tours under consideration (called T18-3) can be used to illustrate the complexity involved in this type of navigational planning. T18-3 was the eighteenth tour designed for Cassini: This example is the third iteration of that tour. Tour T18-3 comprises 43 Titan flybys and seven TOUR T18-3 SERENDIPITOUS SATELLITE FLYBYS* Saturn 5000–25,000 25,000–50,000 50,000–100,000 Voyager Flyby Satellite kilometers kilometers kilometers Range, kilometers Mimas – 1 4 88,000 Enceladus 3 – 3 87,000 Tethys 1 3 6 93,000 Dione – 2 2 161,00 Rhea – 1 1 74,000 Phoebe – 1 – 2,076,000 * For flybys with closest approach on the sunlit side. “targeted” flybys of the icy satellites Iapetus, Enceladus, Dione and Rhea. “Targeted” means that the flyby distance can be chosen to best accommodate the planned observations — generally in the 1000-kilometer range. The closest approaches to the satellites made by the Voyager spacecraft in the early 1980s are used for reference. In addition to the targeted flybys, there are other, unplanned — serendipitous — flybys that occur as a result of the tour path’s geometry. If these flybys are close enough to the satellites, they will provide valuable opportunities for scientific observations. In our sample tour, T18-3, there are 28 of these flybys, with distances of less than 100,000 kilometers. At a range of 100,000 kilometers, the pixel resolution of Cassini’s narrowangle camera is about 0.6 kilometer for several of the icy satellites, providing better resolution than that achieved by Voyager. Science Objectives All the activities on Cassini–Huygens are pointed toward the achievement of the mission’s science objectives. The origin of the objectives can be traced all the way back to the meetings of the Joint Working Group in 1982. They were further developed during the Joint NASA–ESA assessment study in 1984-85. As a result of this study, the science objectives A MISSION OF DISCOVERY 11
12 PASSAGE TO A RINGED WORLD appeared for the first time in their present form in the group’s final report, issued by ESA in 1985. The objectives then became formally established with their inclusion in the NASA and ESA announcements of opportunity (ESA, 1989; NASA, 1989, 1991). The science objectives for Cassini–Huygens are organized by mission phase and target. Scientific Objectives During Cruise. Given the trajectory for the long voyage to Saturn, Cassini–Huygens has the opportunity to carry out a number of experiments during the cruise phase. After launch, there will be instrument checkouts and maintenance activities. Searches for gravity waves will be carried out during the three successive oppositions of the spacecraft, beginning in December 2001. These searches are radio experiments that involve using the DSN for two-way, K –band tracking of the a spacecraft. During two solar conjunctions of the spacecraft, a series of radio-propagation measurements obtained from two-way X-band and K -band DSN tracking will provide a a test of general relativity, as well as data on the solar corona. In early 1992, the Cassini project team at the Jet Propulsion Laboratory redesigned the mission and the Orbiter to meet NASA budgetary con- ENCOUNTERS WITH A CELESTIAL GIANT The actual makeup of Cassini’s fouryear “tour” of the Saturn system is yet to be finalized, but many possible scenarios are already under examination. This sample tour, named T18-3, contains over 70 orbits of Saturn, over 40 flybys of Titan and a number of close flybys of several other satellites. Titan Orbit Iapetus Orbit straints expected for the following years. As a result, most of the science objectives for targets of opportunity — asteroid flyby, Jupiter flyby and cruise — were deleted. This was part of the effort to control development costs and the cost of operation during the first few years in flight. In the current baseline plan, the scientific data acquisition will start two years before arrival at Saturn; that is, well after the Jupiter flyby. Scientific Objectives at Saturn. The list of scientific objectives for Cassini– Huygens is extensive. There are specific objectives for each of the types of bodies in the system — the planet itself, the rings, Titan, icy satellites and the magnetosphere. Not only is Initial Orbit
Page 1: Passage to a Ringed World The Cassi
Page 4 and 5: 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 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 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
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patches on the surface. Although it
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Artist’s conception of Ithaca Cha
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An artist’s rendition of Saturn
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The MAPS Instruments Coordinated ob
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field reverses direction across the
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SATURN’S MAGNETIC FIELD Saturn’
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Solar Wind Magnetosheath Titan neut
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per atmosphere. Energetic particles
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and plasma wave emissions from narr
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CHAPTER 7 Space mission design aims
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(SOI) propulsive maneuver to initia
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the tour cannot continue. Of course
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crease inclination to this value. T
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This illustration shows the main de
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discussed earlier) and data handlin
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of the spacecraft! Below the oxidiz
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• 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
Page 168:
National Aeronautics and Space Admi
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