Previous discussions of the origin of the zodiacal cloud have attempted to distinguish between an asteroidal and a cometary source on the basis of collisional dynamics, that is, by calculating the rates of production and destruction of particles from the two possible sources. The uncertainties in these calculations are too large to permit a useful conclusion. The recognition that the solar system dust bands discovered by IRAS are probably produced by the gradual comminution of the asteroids in the major Hirayama asteroid families may allow us to estimate, with comparative confidence, the contribution to the zodiacal cloud of the asteroid belt as whole.

Analyses of both the zodiacal light in the visible and the zodiacal emission in the infrared have provided us with ample evidence to claim that the interplanetary dust particles are mixtures or coagulations of more than one constituents and their mixing ratios vary with the distance from the sun.

Asteroid families are considered for the most to represent fragments of collisional breakup of precursor bodies. If true, this offers the unique possibility to examine the interiors of large bodies and to study the processes of collision on a scale much larger than can be done in laboratory. Indeed, the general features of the mass distributions and of the ejection velocities of the family members can be interpreted in terms of collisional disruption of a parent body followed by self-gravitational reaccumulation on the largest remnant. However, several problems remain open: a) the degree of fragmentation in real families is generally lower than that observed for experimental targets; b) the relative velocities computed including also proper eccentricity and inclination differences are higher by about a factor 4 than those derived from semiaxes differences only; c) only very few of the presently proposed families have distributions of inferred mineralogies consistent with cosmochemistry. Further studies are needed, including better proper elements computation, classification methods, and new investigations on the physics of hypervelocity impacts.

Cometary debris of all sizes impacts the Earth but it is likely that only particles the size of dust survive atmospheric entry and are collected as meteoritic samples. Conventional meteorites and a substantial fraction of collected interplanetary dust particles appear to be asteroidal debris. Nearly half of the collected interplanetary particles have properties consistent with cometary material and resemble Halley dust that has lost the maiority of its carbon and nitrogen. These particles might be aggregates of presolar grains ana they provide some insight into the properties of interstellar grains.

At the early stage of evolution the meteoroid streams may be considered as elliptical rings of relatively small thickness. The influence of planetary perturbations can essentially increase the stream width and its thickness. As a result one stream may produce several couples of meteor showers active in different seasons of the year. 22 short-period meteoroid streams under review may theoretically produce 104 meteor showers. The existence of 67 is confirmed by observations.

Photopolarimetry of P/Halley coma characterizes, in the cloud of dust grains released by the nucleus, the presence of a significant population of large flakes, made of aggregated small and very dark grains. These globules are subsequently transported toward the Sun, with some modifications in texture, and concentrate in the inner part of the Solar System. Comets appear to be the source for the fluffy grains which are observed in the zodiacal light.

We have found a similarity between the size spectra of the observed interplanetary dust and the survived cometary dust, referring to the dynamical behaviour of the dust leaving the comet. As a result, we can suggest that short-period comets with relatively higher eccentricities are major source of the interplanetary dust, especially those with radii less than 10 µm. It is predicted, furthermore, that a supply rate of the dust, which move on bound orbits after leaving the comet, becomes about 8×10 g/s from 85 short-period comets, and nearly 3×104 g/s from 101 long-period comets.

Recent investigations of the terrestrial influx of small meteoroids (sizes l00µm–1 cm) are reviewed, and it is shown that (i) Previous radar measurements have underestimated the influx of these bodies by at least an order of magnitude; (ii) The true height distribution of meteors in the atmosphere indicates a very low bulk density, in general of order 0.01-0.10 gm cm-3; (iii) Several Apollo asteroids have associated meteoroid streams, implying a genetic relationship and hence the possibility that these asteroids are in fact extinct or dormant cometary nuclei; and (iv) Along with the other three retrograde intermediate-period comets, P/Halley most likely originated in the Kuiper Cloud of comets just beyond the planetary region, and not in the distant Oort Cloud as is usually assumed.

A brief overview of the procedure for modeling diffuse interstellar clouds is given, and specific needs for atomic and molecular data are pointed out.

Absorption line spectroscopy in the ultraviolet, visible, and infrared regions can provide important probes of interstellar chemistry. Significant recent developments include the application of optical absorption line techniques to the study of thick molecular clouds and the improvements in infrared detectors that will eventually lead to effective interstellar spectroscopy in the infrared. Demands for basic molecular data will grow in scope and in level of precision.

Recent detections of new molecules in dense interstellar clouds, first detections of certain chemical elements in interstellar molecules, and new information on isotopic fractionation of hydrogen in the interstellar medium are discussed in the context of the need for new laboratory data on transition rest frequencies, reaction rates, and branching ratios.

Difficulties faced by various models of interstellar clouds have been discussed. A new evolutionary model which uses energy equation instead of empirical temperature-density relation used in earlier models has been presented. This calculation shows that for a given initial density, the collapsing cloud has a minimum mass which is significantly smaller than the Jean's mass. The clouds with larger mass than the critical mass continue collapsing and physical and chemical evolution remain similar to earlier evolving models. Clouds with mass smaller than the critical mass initially collapse but ultimately bounce back, producing physically similar clouds in collapsing and expanding phases. The chemical evolution in these two physically similar clouds is different mainly due to differences in their lifetime. The lifetime of this oscillating cloud is also longer than the collapsing cloud.

The chemistry of circumstellar envelopes around evolved stars is extremely rich, especially for C-rich ones. 34 molecules have been identified in IRC+10216. Recent observations and modeling prove that most of the unstable species are formed by photochemistry in the outer layers of the envelope.

This paper discusses a number of problems in the field of stellar astronomy which are caused by inadequate kinowledge of atomic and molecular data.

Chemical models of dense interstellar clouds are reviewed with particular emphasis on recent results. The need for theoretical and experimental data on rate coefficients is pointed out and some observational studies are suggested.

The column densities of interstellar CH+, first detected about fifty years ago, cannot be explained with models of the chemistry in low temperature gas. The resolution of this classic problem is necessary for us to have confidence in our understanding of interstellar chemistry and its role in determining the physical conditions in interstellar clouds and in the utility of molecular abundance measurements as diagnostics. The possibility that the observed CH+ is formed primarily in shocks in diffuse clouds is addressed. The way in which the chemistry affects the structure of such a diffuse cloud shock is also discussed. The analogous chemical influence on the structures of shocks in dense molecular clouds is also considered as is the possibility that gas in some dense molecular clouds passes repeatedly through dynamical cycles and is shocked frequently enough to influence the global chemical structures in those clouds. Some atomic and molecular data needs are mentioned.

The chemical effects of interstellar grains are briefly reviewed. Their dominant chemical role is to catalyze the formation of H2 which is the seminal molecule for efficient gas phase chemistry. In regions of at least moderate extinction grains accumulate molecular mantles of CO, H2O, etc. Solid state chemistry in such mantles may produce molecules of a type or in an abundance not achievable in the interstellar gas. Return of mantle material to the gas can – at least transiently – dominate gas phase chemistry. It is argued that the freeze-out of heavy atomic and molecular species on to grain surfaces limits the time available for chemistry, restricts molecular cloud chemistry to a “young” character, and suggests that chemical models of molecular clouds must have cyclic dynamics. Such models are briefly described.

Comets may be our best probes of the physical and chemical conditions in the outer regions of the solar nebula during that crucial period when the planets formed. The volatile composition of cometary nuclei, in particular, can be used to decide whether comets are the product of a condensation sequence similar to that invoked to explain the compositions of the planets and asteroids, or if comets are simply agglomerations of interstellar grains which have been insignificantly modified by the events that shaped the other bodies in the solar system. Although cometary nuclei are not generally accessible to observation, observations of cometary comae can illuminate at least some of the mysteries of the nuclei provided one has a detailed knowledge of the excitation conditions in the coma and also has access to basic atomic and molecular data on the many species present in comets. This paper examines the status of our knowledge of the volatile composition of cometary nuclei and discusses how these data are obtained.

A general review of the chemistry of planetary atmospheres was given with the emphasis on the outer planets and their satellites. The impact of the scientific findings of the Voyager Mission on our understanding of the chemistry and composition of the Jovian, Saturnian, and Uranian atmospheres was highlighted. Hydrocarbon photochemistry will be treated in the context of comparative planetary atmospheres and illustrated by important similarities and differences. The interesting chemistry of satellites with significant atmospheres, Titan, Triton, and Io was explored in the evolutionary context and the impact on the magnetospheres of their parent planet.

From an analysis of the IRAS data of Roberts 22,Ml-92,M2-9,OH 231.8+4.2,M1-91,MWC 922,Hen 401,Mz-3,OH 19.2-1.0 and OH 26.5+0.6 it is found that the characteristics of the dust shells or disks around these objects are similar to that observed in planetary nebulae. These ten objects may be described as transition objects evolving from the tip of AGB towards left in the HR diagram. The bipolar and disk geometry of the dust envelopes around these objects may be the result of large angular momentum of the progenitor star or the central objects may be evolved binary systems embedded in thick disks formed from severe mass loss.