Passage to a Ringed World - NASA's History Office

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have a signature of a tone decreasing with time (because the high frequencies arrive before the low frequencies). Lightning whistlers have been detected in both the Earth and Jupiter magnetospheres and, besides being detectable from large distances, they offer an opportunity to estimate the frequency of lightning flashes. Titan’s Magnetic Field. The question of whether or not Titan has an internal magnetic field remains open, although Voyager results did not suggest the presence of one. Recent Galileo results from Jupiter indicate the possibility of a magnetic field associated with the moon Ganymede. For Titan there are two possibilities — a magnetic field could be induced from the interaction of Titan’s substantial atmosphere with the flow of Saturn’s magnetosphere (such as at Venus, with the solar wind), or a magnetic field could be generated internally from dynamo action in a metallic molten core (such as at Earth). SIX GIANT SATELLITES Satellite Titan Moon Io Europa Ganymede Callisto (Planet) (Saturn) (Earth) (Jupiter) (Jupiter) (Jupiter) (Jupiter) Distance from 1,221,850 384,400 421,600 670,900 1,070,000 1,883,000 Parent, kilometers Rotation Period, 15.945 27.322 1.769 3.551 7.155 16.689 days Radius, 2575 1738 1815 1569 2631 2400 kilometers Average Density, 1.88 3.34 3.57 2.97 1.94 1.86 grams per cubic centimeter In addition to being important to understanding the Titan interaction with Saturn’s magnetosphere, a Titan magnetic field, if generated internally, would place strong constraints on its interior structure. Titan’s Atmosphere Background. The discovery of methane absorption bands in Titan’s spectrum in 1944 was the first confirmation that Titan has an atmosphere. Theoretical analysis followed — would Titan be enshrouded within a warm, atmospheric greenhouse or possess a thin, cold atmosphere with a warm layer at high altitudes? The reddish appearance of Titan’s atmosphere led scientists to suggest that atmospheric chemistry driven by ultraviolet sunlight and/or the interaction with Saturn’s magnetospheric plasma produced organic molecules. The term “organic” refers here to carbonbased compounds, not necessarily of biological origin. Later, laboratory experiments and theoretical research were aimed at reproducing the appearance of Titan’s spectrum and learning if Titan’s atmosphere could indeed be considered a prebiological analog to Earth’s atmosphere. Researchers filled laboratory flasks with various mixtures of gases, including methane, and exposed the flasks to ultraviolet radiation. The experiments produced dark, orange–brownish polymers dubbed “tholins,” from the Greek word meaning “muddy.” Voyager Results. In 1980 and 1981, the Voyager spacecraft flybys returned a wealth of data about Titan. Voyager 1 skimmed by at a distance of just 4000 kilometers. Voyager images revealed an opaque atmosphere with thin, high hazes. There was a significant difference in brightness between the northern and southern hemispheres and polar hoods, attributed to seasonal variations. THE MYSTERIOUS TITAN 31
The change in Titan’s temperature with altitude is like Earth’s: decreasing with increasing altitude up to 50 kilometers, then increasing again above that. This illustration shows atmospheric structure, the location of the surfaceobscuring ethane haze and the possible location of clouds. 32 PASSAGE TO A RINGED WORLD Voyager’s solar and Earth occultation data, acquired as the spacecraft passed through Titan’s shadow, revealed that the dominant atmospheric constituent is nitrogen. Methane, the atmospheric gas detected from Earth, represents several percent of the composition of the atmosphere. Titan’s surface pressure, one and a half bars, is 50 percent greater than Earth’s — in spite of Titan’s smaller size. The surface temperature was found to be 94 kelvins, indicating that there is little greenhouse warming. The temperature profile in Titan’s atmosphere has a shape similar to that of Earth: warmer at the surface, cooling with increasing altitude up to the tropopause at 42 kilometers (70 kelvins), then increasing again in the stratosphere. Pressure, millibars 0.001 00.1 0.1 1 10 100 1000 Shallow Ethane–Methane Sea Ultraviolet Light High-Molecular-Weight Haze The opacity of Titan’s atmosphere turned out to be caused by photochemical smog: Voyager’s infrared spectrometer detected many minor constituents generated primarily by photochemistry of methane, which produces hydrocarbons such as ethane, acetylene, and propane. Methane also interacts with nitrogen atoms created by the break up of nitrogen to form “nitriles” such as hydrogen cyanide. Titan may well deserve the title “smoggiest world in the solar system.” A person standing on Titan’s surface in the daytime would experience a level of daylight equivalent to perhaps 1/1000 the daylight at Earth’s surface, given Titan’s greater dis- Ethane Haze Layer Temperature Profile Stratosphere Troposphere Infrared Light Blue Light Red Light Methane–Nitrogen Cloud tance from the Sun and the haze and gases blocking the Sun. The light will still be 350 times brighter than moonlight on an Earth night with a full Moon. In the years following the Voyager mission, a stellar occultation observed from Earth in 1989 provided more data at a Titan season different from the season that Voyager observed. Extensive theoretical and laboratory experiments have also increased our understanding of Titan’s complex atmosphere. For instance, this work has helped us to understand complex atmospheric photochemistry and the composition of the haze particles. 60 80 100 120 140 160 180 200 Temperature, kelvins 525 425 325 225 125 50 0 Altitude, kilometers
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 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 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 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
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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
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National Aeronautics and Space Admi
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