Passage to a Ringed World - NASA's History Office

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In this diagram from his book, Systema Saturnium, Christiaan Huygens explained the inclination of Saturn’s rings according to the planet’s orbital position with respect to Earth. CHAPTER 2 Saturn was the most distant planet that could be seen by an- cient astronomers. Like Mercury, Venus, Mars and Jupiter, it appeared to be a bright, star-like wanderer that moved about the fixed stars in the night sky. For this reason, these Historical Observations In 1609, Galileo Galilei built his own telescope and put it to use in making observations of the night sky. In his initial observations of Saturn, Galileo thought he was seeing three planets because of the poor quality of the lenses of his telescope. By 1614, his notes indicated that he observed the rings for the first time — though he did not identify them as such. The optical aberrations in early telescopic lenses prevented clear viewing. Because of the way Saturn’s rings The Ringed World appeared when viewed through early telescopes, observers spent the early years trying to understand what they interpreted as the planet’s odd shape and behavior. Some saw the rings as cup handles. By 1659, Christiaan Huygens had developed the concept of a planetary ring system, which helped the astronomers of the day understand what they were seeing. Because of its dense cloud cover, all we can see of Saturn is the atmosphere — that is, if we do not look at the rings! Saturn has been a mystery celestial bodies were called “planetes,” the Greek word for “wanderers.” Little was learned about Saturn, besides its apparent celestial motions, until the telescope was invented in Holland in the early 1600s. to us since Galileo’s first telescopic observations in the early 17th century. Up to the time of the visits made by Voyagers 1 and 2, Saturn appeared as a fuzzy yellow ball with some visible banding and some pole darkening. We now know that the “surface” of Saturn is gaseous and that the patterns present are due to clouds in its gaseous envelope. From Galileo’s time through the next 300 years, telescopes improved and more moons were discovered to be THE RINGED WORLD 17
18 PASSAGE TO A RINGED WORLD orbiting Saturn. But not until the first spacecraft were able to fly by the planet did we really advance our knowledge of Saturn’s atmosphere, complicated ring structure, magnetic field, magnetosphere and satellites. So far, there have been three flyby missions to Saturn. Detailed information about Saturn’s structure was obtained by these spacecraft. The first, Pioneer 11, flew by in August 1979. Pioneer’s photopolarimeter made important measurements of Saturn’s atmosphere, and the data could be assembled into low-resolution images. The atmosphere’s appearance changes so rapidly, however, that the time required to take a picture was too long to capture the details. Only subsequent flyby missions by Voyager 1 in October 1980 and Voyager 2 in August 1981 were able to show us the complex structure of the atmosphere and its rapidly changing features. In addition to taking images, the Voyagers conducted many different experiments — the data collected greatly added to our knowledge of Saturn’s interior structure, clouds and upper atmosphere. The composition of planets in the solar system is largely controlled by their temperatures, as determined by their distances from the Sun. Refractory compounds (those with high melting points) were the first to condense, at temperatures around 1500 kelvins, followed by silicates at 1400 kelvins. These substances formed the rocky cores of the planets. While hydrogen is the predominant element in the universe and in our solar system, other gases are present, including water, carbon dioxide and methane, all of which condense below 500 kelvins. As ices, these predominate in the cooler, outer solar system. This gaseous material collected as envelopes around the planets and moons. (Where conditions are right, large quantities of water in its liquid state can form, as exhibited by Earth’s oceans, Mars’ flood plains and potentially beneath the surface of Europa, a moon of Jupiter.) The large outer planets contain much of the primordial cloud’s gases that were not trapped by the Sun. Hydrogen is the most abundant material in the Sun and in all the large gaseous planets — Jupiter, Saturn, Uranus and Neptune. Each of these giant planets — known as the “gas giants” — has many moons. These moons form satellite systems, suggesting that miniature solar systems formed around the gas giants by processes similar to those that formed the solar system itself. Characteristics of Saturn Cool and Slow. Saturn is the sixth planet from the Sun. It is nine-and-ahalf times farther from the Sun than Earth. The diameter of the Sun viewed from Saturn is about onetenth the size of the Sun we see from here on Earth. Sunlight spreads as it travels through space; an area on Earth receives 90 times more sunlight than an equivalent area on Saturn. Because of this fact, the same lightdriven photoprocessing in Saturn’s atmosphere takes 90 times longer than it would on Earth. You would not have to worry about getting a sunburn on Saturn! Remembering the astronomer–mathematician Johannes Kepler’s laws of planetary motion, the farther away from the Sun, the slower a planet travels in its orbit, and the longer it takes to complete its orbit about the Sun. Saturn travels at an average velocity of only 9.64 kilometers per second, whereas Earth travels at an average velocity of 29.79 kilometers per second. Saturn’s yearly orbit about the Sun — a “Saturn year” — is equal to 29.46 Earth years. If you lived on Saturn, you would have only one birthday every 29-plus (Earth) years. Since the orbit of Saturn is not circular but is elliptical in shape, its distance from the Sun changes during its orbital revolution around the Sun. The elliptical orbit causes a small change in the amount of sunlight that reaches the surface of the planet over the Saturn year, and may affect the planet’s upper atmospheric composition over that period. Slightly Squashed. Saturn’s period of rotation around its axis depends on how it is measured. The cloud tops show a rotation period of 10 hours and 15 minutes at the equator, but 23 minutes longer at higher latitudes. A radio signal associated with Saturn’s magnetic field shows a period of 10 hours and 39.4 minutes. The high rotation rate creates a strong centrifugal force, causing an equatorial bulge and a flattening of
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 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
Page 72 and 73: patches on the surface. Although it
Page 74 and 75: 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
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National Aeronautics and Space Admi
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