Local intensity and emissivity, and consequently local polarization degree, temperature and albedo, can be retrieved from optical and thermal observations of zodiacal light. The local polarization degree (normalized at constant solar distance and phase angle) is found to decrease with elevation above the symmetry plane of the zodiacal cloud. The heterogeneity of the cloud, established towards the symmetry pole, is here demonstrated in the tangential plane (almost perpendicular to the ecliptic plane at 1 AU). We present a map of the local polarization degree in this plane.

The Fredholm integral equation method (FIM) is one of the solutions to the soattering of electromagnetic radiation by homogeneous and isotropio ellipsoidal particles. Some numerical calculations are performed with the FIM. The results for spherical particles are compared with those by the Mie theory. It is confirmed that the agreement between them is satisfactory for all the models calculated.

On the basis of the present method, we examine profiles of the absorption band around »= 10 μm for spherical and ellipsoidal particles composed of crystalline olivine. It is found that the profile strongly depends on the shape of the particle. Even when the particle is moderately elongated (axial ratios are 2: √2:1), the profile is Signifioantly different from that for a sphere.

A method is outlined to compute the scattering matrix of an ensemble of infinite cylinders in random orientation, as an approximation to the scattering matrix of an ensemble of very long but finite cylinders. Numerical checks are presented, which show that the results for infinite cylinders agree with theoretical results for very thick and very thin cylinders, as well as with numerical data for prolate spheroids and short finite cylinders.

The contribution to the brightness of the infrared background by asteroidal dust, distinguished both by lower color temperature and ‘band-pair’ morphology, is determined using IRAS observations. Dust band pairs are associated with at least 7 asteroid families and groups, but very little is detected from the remainder of the asteroid belt, indicating that asteroid families and groups are the source of asteroidal dust.

New spectroscopic arguments supporting the probable presence of organic molecules in the material released by comet Halley are deduced from the data obtained by the Vega three-channel spectrometer. Two excesses of emission on the UV side of the OH and CN bands at 305 and 383 nm are interpreted as being due to “prompt” radiation emitted by electronically excited OH and CN radicals directly produced by the photolysis of water vapor and an organic X-CN molecule. A broad-band emission progressively appears between 342 and 375 nm when the solar scattered continuum has been substracted. This emission increases approximately as the inverse of the projected distance to the nucleus. It is interpreted as a fluorescence emission of organic molecules, possibly condensed polycyclic hydrocarbons. Present observations support the hypothesis of grains coated with organic material and give arguments in favor of a probable interstellar origin for cometary dust.

We developed a 1-D hydrodynamic model of dusty gas flow with dust fragmentation in a cometary atmosphere and performed calculations for a dust-size distribution with radii a = 10−4 — 10 cm and densities variable with dust size. A comparison was made with Giotto observations of dust jet intensities within 100 km of the nucleus of Comet Halley. We found that dust fragmentation cannot be solely responsible for the flattening of the dust intensity near the nucleus with respect to the 1/R law. We conclude that a combination of geometric effects and grain fragmentation may explain the observed intensity profiles.

The numerical analysis of cometary dust tails (Fulle 1989) allows to obtain the mass loss rate, the size distribution, the ejection velocity and the orbital eccentricity of the dust grains ejected by the parent comets. All these physical quantities are necessary and sufficient to compute the comet mass contribution to the interplanetary dust cloud. We apply this method to three long period comets, namely C∕Bennett 1970II, C∕Bradfield 1987XXIX and C∕Liller 1988V, and obtain that each long period comet injects in bound orbits at least half of the total produced mass. When we consider that a typical long period comet can produce more than 1014 g of dust along each perihelion passage, we obtain that the considered long period comets alone injected a input mass rate of about 106 g s−1 of meteoroids in bound orbits during the last 20 years, a contribution which is very close to that from all short period comets.

Comets constitute an important source for the zodiacal dust cloud. Mainly large particles are contributed because the smaller particles are emitted into hyperbolic orbits relative to the sun. Radiation pressure force reduces the effective solar gravitational attraction. Information about large cometary particles can be derived from a variety of sources requiring quite different observational techniques. Many distinct meteor streams are connected to orbits of short period comets. These streams contain large dust particles that are very little influenced by radiation pressure force. In some cases such as the η Aquarids and Orionids connected to comet Halley the total mass and the age of the meteors have been derived (Hughes, 1987; Hajduk, 1987). The mass of the streams is 5 to 10 times larger than the present mass of the nucleus and their lifetime corresponds to 2000 to 3000 orbital periods. Visible meteors are typically 10−2 g and more of centimetre size.

Properties of cometary dust with regard to bulk density, optical characteristics and sizes, derived from recent observations, are used to model scattering properties of cometary and interstellar grains. A wide range of astronomical observations are shown to be explained if cometary objects are hypothesised as a major source of dust grains in the galaxy.

Recent high resolution observations of comets revealed a detailed spectral shape of the 3.4 μ feature. We measured IR spectra of simple 14 hydrocarbon molecules and made ”synthesized comet spectrum”. Peak wavelength and spectral shape of the synthesized spectrum are well in agreement with the observed comet features.

Spectropolarimetric observations from 3800 to 7000 Å were obtained for the nucleus of comet Halley for nine nights during 1985-86. The observations were spaced over phase angle of 2 to 66 °. The continuum polarization without molecular-line contamination as well as the polarization of the molecular lines were evaluated. The plot of polarization versus the phase angle shows small negative polarization for angles less than 20°. The highest polarization measured for phase angle ≤ 66° is 19.6% .The lowest polarization of −2.0% has been measured at the phase angle ~ 10°. The variation of polarization with phase angle is nearly linear in the range 30 to 60° giving a slope of 0.4316% per degree. There is small dependence of polarization with the wavelength in the region 3800 to 6500Å. The polarization of the molecular bands CN, C3, C2(0,0) and C2 (1,0) have also been evaluated for the spectra of 15th December 1985, 18th and 19th March 1986 and 8th April 1986.

Referring to the dust model in Mukai and Mukai(1990), where the scattering by large rough particles and Mie scattering by small particles are taken into account, a phase function of linear polarization of several comets is examined, especially in a region of phase angles α near a maximum polarization. A lower maximum polarization observed in comet Austin(1989c1) than those in comets West(1975n) and P/Halley leads a speculation that a mixing ratio of rough scattering to Mie scattering in comet Austin increases from a sun-comet distance r of 0.6 AU to 1.2 AU. This implies that a shortage of large particles in comet Austin occured in r <1 AU.

The unique morphology of the synchronic band in the cometary dust tail is explained by finite-lifetime fragment model (Nishioka and Watanabe, 1990). However, this model needs a severe restriction on the lifetime of the dust fragments;25-70 days (r = 1 A.U.). This implies that detailed analysis of the synchronic band may reveal physical properties of the cometary dust particles. In this paper, we suggest that the fragments in the synchronic band are relatively pure ice if they are not organic grains.

Dust particles originating from comets are an important constituent of the interplanetary dust regime. In order to study the ejection mechanisms from the cometary nucleus surface simulation experiments in the laboratory have been performed. Samples consisting of water ice, carbon dioxide ice and dust grains have been studied when they are irradiated by artificial sunlight within a cooled vacuum system. It has been shown that particle emission is extremely dependent on the initial composition of the samples. For samples with a distinct amount of non-volatile, mineral particles the formation of a dust mantle and, as a consequence, rapid decrease of particle ejection has been observed.

The interstellar dust model of comets is numerically worked out to satisfy several basic constraints provided by observations of comet Halley and to derive the porosity of coma dust. The observational constraints are: (1) the strengths of the 3.4 μm and 9.7 μm emission bands; (2) the relative amount of silicates to organic materials; (3) the mass distribution of the dust. The results indicate that coma dust has a porosity in the range 0.93 < P < 0.975. Preliminary calculations concerning the observed linear polarization of comet Halley are presented.

We have investigated the stability of gas-phase formaldehyde oligomers and its implications for cometary science. Our experiments indicate that when a formaldehyde-methanol solution is vaporized in the mass spectrometer, high molecular mass POM species (45, 61, 75, 91, 105, 121, 135, 151, 165 amu) exist in the gas phase in the temperature range 473 K to 773 K. These laboratory results complement our previous experiments using a formaldehyde-water solution and indicate that formaldehyde oligomers are stable in the gas phase up to at least 6 monomeric units in length. Methanol is important in the end-capping process of the oligomers, leading to increased stability and a richer mass spectrum when compared to the formaldehyde-water solution. The results are consistent with mass spectra obtained by the Giotto PICCA instrument exhibiting alternating 14-16 amu mass peaks.

The anti-tail of comet Bradfield 1987XXIX (= 1987s) was observed when the earth passed through the orbital plane of this comet on December 20, 1987. The time variation of this phenomenon was monitored continuously by Japanese amateur astronomers. Analyzing these photographs by using the Bessel-Bredkhin theory, we deal with the dust particles in the anti-tail.

Comet Bradfield 1987XXIX showed a well-developed split tail, which is a dust tail feature divided into two branches by a dark gap. Similar structures have been observed in several comets. The split tail of comet Seki-Lines is one of them; some authors tried to explain it by rapid dust vaporization or self-regulation model. However, these models can not explain in the case of comet Bradfield 1987s. It was suggested in our previous works that the trend of variation of position angle of the dark gap coincides well with that of the plasma tail or projected radial vector. We discuss the formation mechanisms of the split tail from the view point of dust-plasma interactions.

Composite images of the intensity of solar radiation scattered by dust in Halley’s coma are constructed by using the three-channel spectra obtained during the approach phase of the Vega 2 spacecraft. They cover a sector centered on the nucleus that has a radius of 40000 km and an angular extent of 50°. A radial plot of dust-scattered intensity shows that it varies as the inverse of impact parameter p where p is smaller than 3200 km or higher than 7000 km. In the intermediate 3200-7000 km distance range, the intensity varies as p−1.52. At longer distances, two jets are present with a contrast comparable to the gas jets which appear in the OH and CN images.

The color of dust shows a slight excess of near-UV radiation in a diffuse region between 10000 and 30000 km which appears to be connected with the two jets. In the region called “valley”, between the jets at distances p > 25000 km, the dust-scattered intensity shows an excess of red. The color, expressed as the ratio of intensities at 377,482 and 607 nm is interpreted in terms of Mie theory. It is suggested that the dust particles progressively differentiate. A proportionally more important population of small submicronic grains appears at p > 8000 km. This population seems to correlate with the jets.

Spherical nylon projectiles of 7mm diameter and up-to 4km/s velocities were penetrated into three types of targets; aluminum multisheet stacks, foamed polystyrene, and 1-atm air. Penetration depth, recovery rate of the projectiles were determined as a function of projectile velocity and target density, and a new type of dust collector is proposed.