In a cometary coma, various hydrogen compounds, especially water, that are photodissociated by solar ultraviolet radiation form a gigantic hydrogen cloud with a radius of several million kilometers around the cometary nucleus. These cometary hydrogen clouds have been observed by a number of spacecraft outside the terrestrial atmosphere and in interplanetary space. For instance, the hydrogen cloud of Comet Halley was a good target for the ultraviolet photometers and spectrometers on board the International Ultraviolet Explorer (IUE), Suisei, Pioneer Venus, DE-1, and sounding rockets. Hydrogen cloud activity could be used as a good measure of cometary activity, but some problems remain to be explored in the future.

This paper reviews the role of past and future visual observations in cometary research. The strengths and limitations of visual observations are explored for the benefit of both investigators who might have to use them and observers who wish to make real contributions to the field. We consider the characteristics of the eye-brain combination as a detector and compare them with those of modern detectors. We specifically evaluate visual discoveries, magnitude estimates, and drawings.

The structural and thermodynamical properties of water ice and ice mixtures containing CO, CO2, CH4, and NH3 are thought to be important for the evolution of cometary nuclei. Based on recent laboratory studies performed by several groups, an overview is given of the properties of various ices condensed at low temperatures and of their evolution during heating up to a temperature of about 200 K, typical of the perihelion temperature of a comet such as P/Halley. It is shown that the porous surface of amorphous water ice plays an important role in the retention of other volatiles. The kinetics of formation and of decomposition of clathrate hydrates are discussed. The molecular hydrates formed by NH3 are briefly presented, and the possibility of their occurrence in comet nuclei is discussed. With special attention drawn to amorphous ices and clathrate hydrates, a qualitative discussion of the influence of the physical properties of various types of ices on the thermal behavior of comet nuclei and on gas production rates of comets is presented.

Much has been learned about the physics and chemistry of comets from the successful spacecraft encounters and intensive remote observing programs of Comets Halley and Giacobini-Zinner. Instead of being the panacea for our comet curiosity, these tantalizing “snapshots” have raised new questions, and many fundamental problems remain unsolved. To reap fuller benefits, extensive modeling is necessary to characterize the physical structure and chemical properties of the coma and to infer the composition and structure of the nucleus.

For the last 10 years, many experimental results have been obtained on the chemical and physical changes induced by ion and electron irradiation of materials relevant to comets. Those results are reviewed here, together with their physical interpretation and their relevance for cometary astrophysics. Cometary material is, from the time of its origin, altered by the large amount of energy deposited by energetic ions. Four phases of the irradiation history are considered: the pre-cometary phase, during which interstellar dust is bombarded by cosmic-ray ions; the accretion phase, during which comets are built up, possibly in an environment rich in fast ions (T-Tau phase); the cometary phase, during which the outer layers of the comets are irradiated by galactic ions in the Oort cloud; and the post-cometary phase, during which dust, lost from the comet, is bombarded by solar ions.

The relevant applications of laboratory results are reviewed. In particular, the ability of ion irradiation of simple carbon-containing ices to produce complex refractory organic materials is discussed. In the Oort cloud, this process can occur several meters into the surface, so that the buildup of a stable organic crust may occur. A comparison of ion irradiation at various stages is also made with other models for the production of organics.

The hydrodynamic approach to the modeling of the collisional regions of the cometary atmospheres is reviewed. A concise account of the existing Monte Carlo simulations is also incorporated. Emphasis is placed on works stimulated by the Halley flyby missions. The basic physical processes are described, based on the analogy with the physics of underexpanded jets. The general structure of the governing equations is presented and their legacy discussed. A detailed intercomparison is made of the application of these methods to the modeling of the comas. The reliability of the conclusions brought forward by existing models is discussed.

In situ observations of comet Halley provided the first photographs of a cometary nucleus and yielded information about its environment, including the emitted gas and dust. The relation between these measurements and properties of and processes on the nucleus is established by theoretical modelling, while laboratory experiments may provide some of the physical parameters needed. In addition, laboratory tests can stimulate new ideas for processes that may be relevant to cometary physics. Processes to be studied in detail by large-scale laboratory experiments may include: (1) heat transport phenomena during sublimation of porous ice-dust mixtures, (2) material modification and chemical fractionation caused by the sublimation processes, (3) buildup and destruction of dust mantles, (4) detailed studies of gas release from mixtures of volatile ices, and (S) the investigation of ice and dust particle release mechanisms. The KOSI-team (Kometensimulation) carried out sublimation experiments with ice-mineral mixtures in a large Space Simulator. During initial experiments, cylindrical samples of 30-cm diameter and 15-cm thickness were irradiated with up to 2700–W/m2 light energy. The samples consisted of water-ice or water- and CO2-ice mineral mixtures. The experiments showed the importance of advection for heat transport into the interior. It was found that the sublimation of CO2 advances into the sample at a higher speed than that of water vapor release. Therefore, emission of volatile gases responded to insolation changes with a time lag of several hours. The ratio of the emitted gas species, as well as the dust-to-gas mass ratio, differs significantly from the values within the sample. A partly permeable refractory mantle of minerals and carbonaceous material developed with time. Dust and ice particle emission has been observed to occur from irradiated dirty ices as well as from dust mantles.

The presently available multidimensional dusty gasdynamic cometary atmosphere models are reviewed. This paper examines the simplifying assumptions and their potential consequences. Special attention is paid to the question of potential nightside dust accumulation presently predicted by Korosmezey and Gombosi (1989).

The properties of a porous mineral crust on the surface of an icy cometary nucleus and the crust's influence on the thermal regime and gas production in the nucleus have been studied by laboratory simulation experiments. A nucleus model of H2O ice with the impurity of graphite particles has been shown to display the same temperature and surface albedo as those determined for Comet Halley’s nucleus by the VEGA 1, VEGA 2, and Giotto spacecraft. The effective thermal conductivity of a crust with a density of 0.5 × 102 kg m−3 to 0.7 × 102 kg m−3 is less than 10−1 W m−1 K−1, while the crust’s strength (103 to 104 Pa) is not sufficient to withstand its erosion by the sublimating gases. A crust that is 1 cm thick lowers the gas production of the nucleus model by one order of magnitude. The destruction of the crust, and the gas and dust production of Comet Halley’s nucleus can be explained either by a spotty surface on the nucleus or, more likely, by the presence of volatile impurities such as CO2 with concentrations of 1 × 10−2 to 3 × 10−2 in the H2O ice under the crust.

Cometary dust is observed in a variety of ways: scattered light, thermal emission, stellar occultations, coma and tail morphology, radar echoes, and meteors associated with comets. To interpret these observations with respect to the physical parameters of the dust (size, shape, and composition) and the properties of the dust-emitting region, the physics of how electromagnetic radiation interacts with small particles must be understood.

The interaction between electromagnetic radiation and a small particle depends on the size, shape, and composition of the particle. The composition determines the optical constants of the material. Classically, a solid can be approximated by a collection of bound charges with a number of resonant frequencies. The optical constants (the index of refraction or dielectric constant) are a measure of the ability of the material to vibrate in response to an incoming electromagnetic wave. The laboratory measurement of optical constants is very difficult, and many published optical constants may be in error. Care must also be taken in choosing the optical constants that best represent the assumed composition of the cometary dust.

Scattering theory is usually synonymous with Mie theory, although Mie theory pertains only to spherical particles. In many cases, homogeneous spheres may be good approximations of the dust in the coma, but an understanding of the effects of non-sphericity and heterogeneity is essential to determine the limitations of the spherical approximation. A variety of methods exist that, although computationally intensive, do provide insight into shape effects.

Experimental laboratory techniques used in studying the photochemistry of stable and unstable molecules are discussed. The laboratory evidence for the photochemical formation of C2 from C2H, C3 from C3H2, and NH from NH2 is presented. Other recent results obtained in laboratory studies of H2O, H2S, NH3, and HCN are reported.

Prior to the 1986 apparition of comet Halley, all attempts to determine the physical properties of cometary dust were limited to remote observations and the analysis of various particles captured by the Earth’s atmosphere. The in situ measurements made by the three spacecraft that passed within 10,000 km of the nucleus provided the first opportunity to investigate both the full size-range of particles and the complete process of dust production. Information on composition is derived through mass spectra and the scattering and emission of light from the grains, whilst the dynamics of the dust coma can be modeled from the three separate sets of measurements made over a period of eight days.

On board the space probes Giotto and VEGAs and 2, which were sent through the coma of comet Halley, were the mass spectrometers PIA and PUMAs 1 and 2 for the in situ analysis of cometary dust. This paper summarizes the results obtained up to now, about four years after the flybys. It is sought to make the reader aware of the significance, but also of the limitations, of these unprecedented data gathered within a few hundred seconds with an innovative instrument about 1.5 x 108 km away from Earth. The first part of this paper shows how the bulk properties of the dust are derived—the main constituents CHON and silicates, the core-mantle structure, the molecular composition of the organic component, the average composition of the dust and of the whole comet, and the gas/dust ratio. The second part reviews what can safely be said about the properties of the individual grains—their similarity to a certain class of interplanetary dust particles and their distinction from others, and their density and masses. The final part of this paper shows that some cometary grains contain isotopically ultra-light carbon (12C/13C up to 5000), a finding that is significantly substantiated by reports of similar carbon isotopic composition in meteoritic graphite.

The evidence for the accumulation of the terrestrial planets and comets from solid grains is reviewed briefly. The various proposals for formation of cometary nuclei are described and commented upon. With three exceptions, all hypotheses conclude or imply that a single compact object forms. It has been almost universally assumed that grain accretion produced compact aggregates as is the case with liquid drops, and that this process continued. Several hypotheses start with Goldreich-Ward-type gravitational instabilities. The collapse for this case also occurs at low velocities in the cm·s−1 to m·s−1 range. Experiment and theory show that under these conditions, low-density, filamentary clusters form that are fractal aggregates with a fractal dimension approximately equal to two. Agglomeration of these clusters produces larger, compressible planetesimals or cometesimals, which efficiently combine upon colliding. In order to form cometary nuclei, the initial temperature must be about 50 K and not undergo a significant temperature rise during the accumulation process. The collision process can be analyzed with some simplifying assumptions using the limited experimental data available for the compaction of low-density powders. The calculations show that accumulation will occur at low temperatures. For a more refined analysis, experiments to study impacts on low-density powders are required. Models of cometary nuclei are reviewed, and a simple model of the structure that results from the accumulation of fluffy aggregates is described.

Dust in the plasma and radiative environment of a comet is necessarily electrically charged. This charging has both physical and dynamical effects on the dust, being particularly important on the smallest particles observed in the dust size spectrum. In this paper, these dynamical effects are reviewed and the pertinent observations are discussed.

Firstly, observational data available at present to infer physical conditions of the formation environment of cometary matter are briefly surveyed. These include the chemical and isotopic composition of cometary matter, and the nuclear spin temperature derived from the ortho/para abundance ratio of H2O molecules. Secondly, theories on the origin of comets—theories based upon the chemical composition of the volatile component of cometary matter—are reviewed. The theories are classified into two types, distinguished by whether cometary volatiles originate as solar nebula condensates or as the sublimation residue of interstellar ices. Observational items helpful to test the theories are pointed out. Thirdly, discussion is given on the physical properties of ices relevant to the chemical theory of the origin of comets.

The plasma surrounding a comet has the interplanetary magnetic field frozen in. The geometric interpretation of this property is considered. The frozen-in character of the magnetic field leads to the draping of magnetic field lines around the inner coma, where, by exclusion of the inner purely cometary ionosphere, a magnetic cavity is formed inside a region of magnetic field pile-up. The consequences of these physical processes can nicely be diagnosed and tested by interplanetary tangential discontinuities serving as tracers of the magnetoplasma flow. The topology of the magnetic field around the cavity and the shape of the ionopause, as well as the formation of the magnetic tail, are discussed. Particularly in the outer regions, the magnetic field is disturbed by strong magnetic turbulence. This turbulence plays a role in accelerating cometary and also solar wind ions to high energies.

The chemical similarities between comets, carbonaceous chondrites, and interstellar molecules and grains are reviewed first. The evolution of frosty interstellar grains is then followed during the collapse of a molecular cloud fragment and the subsequent formation of the Solar System. The paradigm clarifies the probable origin of the two populations of comets of different symmetry (the Oort Cloud and the Kuiper Belt) and implies an exogenous origin for all carbon and water on Earth. This origin is explained by the orbital diffusion of planetesimals that is required by the growth of protoplanets.

The global interaction of the solar wind with a comet as it orbits the Sun is reviewed. After a brief survey of the flow transition regions observed at comet Halley is presented, theoretical models are given for the cometocentric distance of the bow shock, the cometopause, and the ionopause. In addition, predictions are made as to what heliocentric distance these boundaries should form at. The results of these models are compared with the in situ observations at comet Halley.

The possible role of comets in bringing volatiles to the inner planets is investigated by means of laboratory studies of the ability of ice to trap gases at low temperatures. The pattern of the heavy noble gases formed in the atmosphere of Venus can be explained by the impact of a planetesimal composed of ices formed in the range of 20 to 30 K. The noble gas patterns on Mars and Earth are less explicable by cometary bombardment alone.