As this excellent book demonstrates, the study of comets has now reached the fas­ cinating stage where we understand comets in general simple tenns while, at the same time, we are uncertain about practically all the details of cometary nature, structure, processes, and origin. In every aspect, even including dynamics, a choice among several or many competing theories is made impossible simply by the lack of detailed knowledge. The space missions, snapshot studies of two comets, partic­ ularly the one that immortalizes the name of Sir Edmund Halley, have produced a huge mass of valuable new infonnation and a number of surprises. Nonetheless, we face the tantalizing realization that we have obtained only a fleeting glance at two of perhaps a hundred billion (lOll) or more comets with possibly differing natures, origins, and physical histories. To my personal satisfaction, comets seem to have discrete nuclei made up of dirty snowballs, as I concluded four decades ago, but perhaps they are more like frozen rubbish piles.

Inhalt

1. Introduction.- 1.1 Plan of the Book.- 1.2 General Concepts.- 1.3 Orbits of Some Comets for Spacecraft Missions.- 1.4 The Oort Cloud Compared to the Planetary Orbits.- 1.5 Summary of Spacecraft Data from Comets.- 1.6 Summary of New Data from Remote Sensing.- 1.7 Reasons for Planned and Proposed Comet Missions.- 1.8 Laboratory and Space Experiments and Simulations.- 2. The Nucleus.- 2.1 Introduction.- 2.1.1 Early History.- 2.1.2 The Nucleus as Source of the Ephemeral Activity.- 2.1.3 Space Missions.- 2.2 Observational Techniques.- 2.2.1 Size Determination from Photometry at Large Heliocentric Distances.- 2.2.2 Gas and Dust Production.- 2.2.3 Nongravitational Forces.- 2.2.4 Rotation Parameters from Variability of Brightness.- 2.2.5 Infrared Observations.- 2.2.6 Radar Observations.- 2.2.7 Direct Observations.- 2.3 Models of the Nucleus.- 2.3.1 The Sand Bank Model.- 2.3.2 Nongravitational Forces.- 2.3.3 Icy Conglomerate Nucleus.- 2.3.4 Nucleus Formation.- 2.4 The Nucleus of Comet P/Halley.- 2.4.1 Expectations.- 2.4.2 The Giotto Encounter.- 2.4.3 Imaging Properties and Orientation of the Nucleus.- 2.4.4 Shape and Size.- 2.4.5 Spin of the Nucleus.- 2.4.6 Surface.- 2.4.7 Surface Features and Morphology.- 2.4.8 Activity.- 2.4.9 The Near Source Region.- 2.4.10 Global Dust Distribution.- 2.5 Modeling and Laboratory Experiments.- 2.5.1 Sublimation of an Icy Surface.- 2.5.2 Sublimation Through an Inert Layer.- 2.5.3 Implications from the Restricted Activity.- 2.5.4 The Outflow of Dust.- 2.5.5 Two-Dimensional Gas Dynamics.- 2.5.6 The Extended Surface Source Model.- 2.5.7 Comparison of the HMC Observations with Models.- 2.5.8 Laboratory Experiments to Simulate Surface Activity.- 2.5.9 Laboratory Experiments for Nucleus Formation.- 2.5.10 Evolution of the Core.- 2.5.11 Computer Experiments.- 2.6 Physical Parameters of the Nucleus.- 2.6.1 Temperature Distribution within the Nucleus.- 2.6.2 Surface Temperature.- 2.6.3 Perihelion Asymmetry.- 2.6.4 Mass.- 2.6.5 Comets, Meteoroids, and Meteor Showers.- 2.6.6 Density.- 2.6.7 Splitting and Decay.- 2.7 Summary.- 2.7.1 A New Model of Comet Nuclei.- 2.7.2 Composition and Structure.- 2.7.3 Formation.- 2.7.4 Sublimation.- 3. The Neutral Coma.- 3.1 Introduction.- 3.2 The Coma: A Transient Phenomenon.- 3.2.1 Cometary Forms.- 3.2.2 The Nucleus as the Source of Coma Gas.- 3.2.3 Grains as Distributed Sources of Coma Gas.- 3.2.4 Hydrodynamic Flow.- 3.2.5 Photolytic Processes in the Coma.- 3.2.6 Chemical Processes in the Coma.- 3.2.7 The Exosphere: Escape from the Coma.- 3.3 Excitation and Emission Mechanisms.- 3.3.1 Fluorescence.- 3.3.2 Prompt Emission.- 3.3.3 Collisional Effects.- 3.3.4 Some Specific Examples.- 3.4 Composition of the Coma.- 3.4.1 Identification of Species.- 3.4.2 Abundances in the Coma.- 3.4.3 Isotope Ratios.- 3.5 Summary and Prospectus.- 4. Dust.- 4.1 Introduction.- 4.2 What is Comet Dust?.- 4.3 Dust Release from the Nucleus.- 4.4 Dust Emission.- 4.5 Dust Coma Structure.- 4.5.1 Simple Models.- 4.5.2 Advanced Models.- 4.5.3 Observations and Measurements.- 4.6 Dust Tails.- 4.6.1 General Features.- 4.6.2 Fine Structures.- 4.6.3 Dust Trails.- 4.7 Mass Distribution.- 4.8 Optical and Infrared Properties.- 4.8.1 Scattered Light.- 4.8.2 Polarization.- 4.8.3 Absorption and Thermal Emission.- 4.9 Dust Composition.- 4.9.1 Introduction.- 4.9.2 Cometary Dust Composition Before the Halley Encounters.- 4.9.3 The PIA and PUMA Experiments.- 4.9.4 Principal Particle Types.- 4.9.5 Sizes and Densities of Grains.- 4.9.6 The Organic Component.- 4.9.7 The Isotopes.- 4.9.8 The Average Chemical Composition.- 4.9.9 Comparison to Carbonaceous Chondrites and the Sun.- 4.9.10 Variability of Mg, Si, and Fe.- 4.10 Dust Production Rate and the Dust to Gas Ratio.- 4.11 Summary.- 5. The Plasma.- 5.1 Introduction.- 5.2 Large-Scale Processes of the Solar Wind-Comet Interaction.- 5.3 Ion Composition.- 5.4 Ionospheric Processes.- 5.4.1 Ion Dynamics.- 5.4.2 Thermal Structure of the Ionosphere.- 5.4.3 Stability of the Magnetic Cavity.- 5.5 Plasma Wave Turbulence.- 5.5.1 Ultra Low Frequency Waves.- 5.5.2 High-Frequency Waves.- 5.6 Ion Acceleration.- 5.6.1 Energetic Ions in the Distant Coma.- 5.6.2 Energetic Electron Acceleration.- 5.6.3 Energetic Ions in the Inner Coma.- 5.6.4 Structure of the Bow Shock.- 6. Orbital Distribution of Comets.- 6.1 Introduction.- 6.2 Origin and Dynamical Evolution.- 6.3 Time Scales.- 6.4 The Outer Part of the Presolar Nebula.- 6.5 The Distribution of the Reciprocal Semimajor Axes.- 6.6 Other Theories for the Origin and Evolution.- 6.7 Short-Period Comets.- 6.8 Differences Between Long- and Short-Period Comets.- 7. Comet Formation and Evolution.- 7.1 Introduction.- 7.1.1 The Nucleus.- 7.1.2 The Comet Cloud.- 7.1.3 Comet Evolution.- 7.1.4 Recent Advances.- 7.2 Chemical and Physical Clues from Observations.- 7.2.1 Chemistry.- 7.2.2 Shape of the Nucleus.- 7.2.3 Structure and Density of the Nucleus.- 7.3 Formation of Comet Nuclei.- 7.3.1 Models for Giant Planet Subnebulae and the Solar Nebula.- 7.3.2 Presolar Nebula.- 7.3.3 Formation Mechanisms.- 7.3.4 Long-Term Processes.- 7.4 Dynamical Evolution of Comets.- 7.4.1 Infeed into the Planetary Region.- 7.4.2 Planetary Perturbations of Comet Orbits.- 7.5 Physical Evolution of Comet Nuclei.- 7.5.1 Some Scenarios for Comet Evolution.- 7.5.2 Observed Evolutionary Effects.- 7.6 Summary.- 8. Implications of Comet Research.- 8.1 Introduction.- 8.2 Progress in Physics and Chemistry of Comets.- 8.2.1 Composition of the Nucleus.- 8.2.2 Structure of the Nucleus.- 8.2.3 Plasma Structures in Coma and Tail.- 8.2.4 Orbital Theories.- 8.3 Progress on the History of the Early Solar System.- 8.4 Comets and the Origins of Life.- 8.4.1 The Origins of Life.- 8.4.2 Organic Materials in Comets.- 8.4.3 Comet Transport of Volatiles.- 8.4.4 The Role of Comets.- 8.5 Unsolved and New Problems.- 8.5.1 Nucleus Rotation and Surface.- 8.5.2 Chemical Abundances in the Coma and the Nucleus.- 8.5.3 Dust Tail.- 8.5.4 Coma and Tail Plasma.- References.