Bright comets form spectacular phenomena in the night sky (see Fig. 1), and they have always been subject of attention and fascination. Today, it is generally believed that comets are the least modified bodies in our solar system, although they are certainly not unmodified. A cometary nucleus consists of a mixture of volatile ices (H2O, CO, CO2, ...) and silicate dust particles. Their icy nature indicates that comets have been preserved at cold temperatures since the early stages of our solar system. Determining the chemical composition and physical structure of cometary nuclei to better understand these early phases in solar system history is therefore a primary goal of cometary science. Several key questions need to be answered:
- How have comets been formed?
- What is the composition of cometary nuclei?
- Are all comets the same?
- Has their composition been modified since their formation?
Comets come into the inner solar system from at least two reservoirs. The Kuiper-Edgeworth belt [71, 137], or also called trans-Neptunian belt, beyond about 40 astronomical units (AU) is a ring-like reservoir of bodies concentrated near the ecliptic plane. It is believed that the Kuiper belt is the source region for most short-period comets, especially for comets belonging to the Jupiter family. These short-period comets have orbital periods around 5 years and an aphelion near Jupiter's orbit. Transition objects on orbits between Kuiper belt objects and comets in the inner solar system are called centaurs. Long-period comets, with orbital periods >200 years, come into the solar system from the Oort-cloud . This shell of objects surrounding the solar system at distances of several 104 AU has been postulated based on the orbital parameters of comets coming into the solar system for the first time on elongated, very long-period orbits. The complex structure and dynamical
H. Rauer, Comets. In: K. Altwegg et al. Advanced Courses, pp. 165-254 (2008) DOI 10.1007/978-3-540-71958-8.3
Trans-Neptunian Objects and Comets, Saas-Fee
© Springer-Verlag Berlin Heidelberg 2008
Fig. 1. Image of comet Hale-Bopp during its perihelion in 1997. The straight ion tail, blue in the light of CO+ ions, and the slightly curved and diffuse dust tail are clearly visible. The dust tail can be seen because small dust particles efficiently scatter the incoming solar radiation evolution of these reservoir regions are described in detail in the chapter of Morbidelli  in this book.
Cometary nuclei are small bodies with radii below 15 km, most of them even below 5 km. Our knowledge of the physical properties of cometary nuclei is outlined and compared to the population of Kuiper belt objects in the chapter by Dave Jewitt. Here, we concentrate on the dynamical and chemical processes of the cometary gas and dust component.
When a comet approaches the Sun along its orbit (Fig. 2), heating by absorption of sunlight leads to sublimation of its icy components (see Sect. 2). A neutral gas coma forms around the nucleus, extending typically a few 105 km nucleocentric distance. The molecules sublimating from the nucleus ices are called "parent species." Chemical destruction of the parent species in the coma leads to the formation of daughter products: neutral radicals, atoms, and ions (see Sect. 5).
Ionized molecules interact with the solar magnetic field and form the ion, or plasma, tail extending a few 107 km in lengths (Fig. 1; Sect. 3.4). In addition to the ion tail, a neutral tail can be observed in active comets consisting of atoms accelerated by solar radiation pressure. This tail is well visible in case of sodium atoms. Another example for a neutral tail is the neutral hydrogen cloud surrounding comets. The molecules, atoms, and ions in the comae and tails are visible because they emit radiation at wavelengths from the UV up to the radio range (Sect. 4) that can be observed with ground- and space-based telescopes.
The silicate dust particles embedded in the nucleus ices are lifted from the surface by the sublimating volatiles. They form the dust coma in the
nucleus vicinity and finally the cometary dust tail under the influence of solar gravity and radiation pressure (Sect. 3.3). The dust coma and tail (Fig. 1) are visible because small dust particles scatter the solar light very efficiently (Sect. 7).
A long history of ground-based observations of cometary comae, dust, and ion tails exists. However, observations from Earth or Earth-orbit are unable to resolve the small nucleus embedded in the gas and dust coma. In situ investigations by space missions have therefore been made since the mid-1980s (Table 1), with the highlight of five spacecrafts visiting comet Halley in 1986, providing the first images of a cometary nucleus and a wealth of data on the coma surrounding it. Up to today, Halley is the comet for which we have the most detailed knowledge. However, even future space missions, with the exception of landers such as ESA's Rosetta mission, do not investigate the nucleus directly, but analyze its coma on fly-bys or orbiting trajectories. Thus, a very good understanding of the dynamical and chemical processes in
Table 1. Overview of past and future space missions to comets
Year Space mission
1986 Suisei, Sakigake 1986 Vega 1, 2
2001 Deep Space 1
2005 Deep Impact 2014 Rosetta
Halley Halley Halley Borrelly Wild 2
Giacobini-Zinner plasma tail solar wind sample return impactor orbiter+lander the coma and of the nucleus surface is required for the interpretation of these measurements.
Several reviews on various aspects of cometary physics have been published in the past. A very good recent compendium of reviews is the book Comet II . Lecture books on comets are rare, only two have been published so far [79,200]. The aim of this article is to aid students and scientists newly entering the field of cometary physics to obtain an overview on the basic ideas of cometary science and guide them to sources of deeper information in the field of their specialized interest.
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