Passage to a Ringed World - NASA's History Office

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the tour cannot continue. Of course, the large number of Titan flybys will produce extensive coverage of Titan. Tour Terminology “Rev” numbers ranging from 1 to 74 are assigned to each tour orbit, assuming each orbit begins at apoapsis. The partial orbit from SOI to the first apoapsis is Rev 0. The rev number is incremented at each succeeding apoapsis. Two or more revs about Saturn may occur between successive Titan flybys; therefore, there is no correspondence between rev number and flyby number. Titan flybys are numbered consecutively, as are the targeted flybys of icy satellites. For example, the first flyby of Enceladus is Enceladus 1; the first of Rhea is Rhea 1. “Orbit orientation” is the angle measured clockwise at Saturn from the Saturn–Sun line to the apoapsis. This is an important consideration for observations of Saturn’s magnetosphere and atmosphere. The time available for observations of Saturn’s lit side decreases as the orbit rotates toward the anti-Sun direction. Arrival conditions at Saturn fix the initial orientation at about 90 degrees. Due to the motion of Saturn around the Sun, the orbit orientation increases with time, at a rate of orientation of about one degree per month, which over the four-year tour results in a total rotation of about 48 degrees clockwise (as seen from above Saturn’s north pole). Period-changing targeted flybys that rotate the line of apsides (orbital points nearest or farthest from the center of Saturn) may be used to add to or subtract from this drift in orbit orientation. The “petal” plot on the facing page shows how targeted flybys combine with orbit drift to rotate the orbit from the initial orientation clockwise most of the way around Saturn to near the Sun line. In the coordinate system used in this diagram, the direction to the Sun is fixed. Encounters of satellites occur either inbound (before Saturn-centered periapsis) or outbound (after Saturn-centered periapsis). A “targeted flyby” is one where the Orbiter’s trajectory has been designed to pass through a specified aimpoint (latitude, longitude, altitude) at closest approach. At Titan, the aimpoint is selected to produce a desired change in the trajectory using the satellite’s gravitational influence. At targeted flybys of icy satellites, the aimpoint is generally selected to optimize the opportunities for scientific observations, since the gravitational influence of those satellites is small. However, in some cases the satellite’s gravitational influence is great enough to cause unacceptably large velocity-change penalties for a range of aimpoints, which makes it necessary to constrain the range of allowable aimpoints to avoid the penalty. If the closest approach point during a flyby is far from the satellite, or if the satellite is small, the gravitational effect of the flyby can be small enough that the aimpoint at the flyby need not be tightly controlled. Such flybys are called “nontargeted.” Flybys of Titan at distances greater than 25,000 kilometers — as well as flybys of satellites other than Titan at distances of greater than a few thousand kilometers — are considered nontargeted flybys. Flybys of satellites other than Titan at distances up to a few thousand kilometers must be treated as targeted flybys to achieve science objectives, even though the satellite’s gravitational influence is small. Opportunities to achieve nontargeted flybys of smaller satellites will occur frequently during the tour and are important for global imaging. If the transfer angle between two flybys is 360 degrees (that is, the two flybys occur with the same satellite at the same place), the orbit connecting the two flybys is called a “resonant orbit.” The period of a Titan-resonant orbit is an integer multiple of Titan’s orbital period. The plane of the transfer orbit between any two flybys is formed by the position vectors of the flybys from Saturn. If the transfer angle is either 360 degrees (that is, the two flybys occur with the same satellite at the same place) or 180 degrees, an infinite number of orbital planes connects the flybys. In this case, the plane of the transfer orbit can be inclined significantly to the planet’s equator. Any inclination can be chosen for the transfer orbit, as long as sufficient bending is available from the flyby to get to that inclination. If a spacecraft’s orbital plane is significantly inclined to the equator, the transfer angle between any two flybys form- PLANNING A CELESTIAL TOUR 85
86 PASSAGE TO A RINGED WORLD ing this orbital plane must be nearly 180 or 360 degrees. If the angle between the position vectors is other than 180 or 360 degrees — as is usually the case — the orbital plane formed by the position vectors of the two flybys is unique and lies close to the satellites’ orbital planes (except for Iapetus), which are close to Saturn’s equator. In this case, the orbit is “nonresonant.” Nonresonant orbits have orbital periods that are not integer multiples of Titan’s period. Nonresonant Titan–Titan transfer orbits connect inbound Titan flybys to outbound Titan flybys, or vice versa. General Tour Strategy This section contains specific descriptions of the segments in the reference tour T18-3. Titan 1–Titan 2. The first three Titan flybys reduce orbital period and inclination. The Orbiter’s inclination is reduced to near zero with respect to Saturn’s equator only after the third flyby; so, these three flybys must all take place at the same place in Titan’s orbit. The period-reducing flybys were designed to be inbound, rather than outbound, to accomplish the additional goal of rotating the line of apsides counterclockwise. This moves the apoapsis toward the Sun line in order to provide time for observations of Saturn’s atmosphere, and to allow Saturn occultations on subsequent orbits to occur at distances closer to Saturn. Titan 3. After the inclination has been reduced to near Saturn’s equator, a targeted inbound flyby of Enceladus is achieved on the fourth orbit on the MAIN CHARACTERISTICS OF TOUR SEGMENTS Titan Flyby Comments T1–T2 Reduce period and inclination, target for Probe mission. T3 Rotate counterclockwise and transfer from inbound to outbound. T4–T7 Raise inclination for eight equatorial Saturn–ring occultations and lower again to equator. T8–T15 Rotate clockwise toward anti-Sun direction. T16 Increase inclination and rotate for magnetotail passage. T17–T31 180-degree transfer sequence (including several revolutions for ring observations). T32–T34 Target to close icy satellite flybys of Enceladus, Rhea, Dione and Iapetus. T35–T43 Increase inclination to 71 degrees (maximum value in tour). way to an outbound flyby of Titan on orbit five on March 31, 2005. Changing from an inbound to an outbound Titan flyby here orients the line of nodes nearly normal to the Earth line. This minimizes the inclination required to achieve an occultation of Saturn, preparing for the series of near-equatorial Saturn and ring occultations that follows. A Titan flyby occurring normal to the Earth line can be inbound or outbound (like any Titan flyby). For Titan flybys occurring nearly over the dawn terminator as in the reference tour, the spacecraft is closer to Saturn during the occultation if the Titan flyby is outbound than if it is inbound. The science return from the occultation is much greater if the spacecraft is close to Saturn than if it is far away. In particular, the antenna “footprint” projected on the rings is smaller when the occultation occurs closer to Saturn, improving the spatial resolution of the “scattered” radio signal observations. This is an important influence on the design of the tour. Lowering inclination to the equator, switching from inbound to outbound Titan flybys and rotating the orbit counterclockwise near the start of the tour all help keep the spacecraft close to Saturn during the subsequent series of equatorial occultations. Titan 4–Titan 7. Here, the minimum inclination required to achieve equatorial occultations is about 22 degrees. The two outbound flybys on March 31 and April 16, 2005, in-
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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
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This spectacular view of the edge o
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the interaction was described as a
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have a signature of a tone decreasi
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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
Page 92 and 93: (SOI) propulsive maneuver to initia
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
Page 106 and 107: management subsystem and the Probe
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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
Page 134 and 135: two low-gain antennas 1 and will pr
Page 136 and 137: During much of Cassini’s orbital
Page 138 and 139: articulation control subsystem’s
Page 140 and 141: AFTERWORD It has been said that in
Page 142 and 143: 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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