[en] A major problem in meteor astronomy is why the orbits of meteoroids within particular streams are so dispersed. For streams with aphelia well within Jupiter (such as the Geminids) planetary perturbations cause insignificant dispersion but can accommodate the required motion of the nodal heliocentric distance to explain why the Geminids were not observed prior to the 1860's. The spread in the orbits would also require unreasonably large ejection velocities from the parent. Another dispersal mechanism is therefore required. By incorporating perturbations due to the Yarkovsky-Radzievskii effect into the model the Geminid dispersal can be understood; by including also the effects of the radiation pressure and Poynting-Robertson forces the main observed characteristics of the stream (shower duration variation with magnitude; skew rate profile; changes in mass distribution and radiant diffuseness as the shower progresses) are explicable. The necessary spin rates (about 3000 rev/sec for 1 mm and 1000 rev/sec for 1 cm radius meteoroids) would be attained within a thousand years of release from the parent body, due to spin-up under solar radiation pressure. It therefore appears that the Yarkovsky-Radzievskii effect is an important source of stream dispersion which has been hitherto neglected, but should be included in future models. (author). 1 fig., 37 refs

[en] Mechanisms leading to higher particle concentrations in several places along the meteor stream associated with Halley are discussed. The positions of the mass concentrations represented by the mean anomaly of the stream orbit, as determined from long series of observations of the Orionids and Eta Aquarids, are correlated with the deviations in the semi-major axis and nodes of the evolving orbit of the comet. It is shown that random deviations in the orbital elements of the comet may be responsible for the nonstable mass concentrations in the stream. (author). 4 figs., 14 refs

[en] Progress is reported in evaluating the mass spectra of cometary dust grains that have been obtained by impact mass analyzers flown onboard the Giotto and Vega spacecrafts. Statistical analysis of about 5000 spectra transmitted to the Earth in the more compressed modes 1 to 3 demonstrates that the mean elemental composition is similar to that of Cl carbonaceous chondrites with the exception of the light elements C, H, O, and N which approximate solar composition. The isotopic ratios do not differ from solar system values. Five classes of particles are suggested: 1) with dominant Mg-Si element group, 2) with dominant CHON abundances, 3) composites of classes 1 and 2, 4) grains rich in carbon, 5) grains rich in iron and sulphur. (author). 4 figs., 1 tab., 6 refs

[en] Methods of processing VEGA-TV observations of the nucleus of P/Halley and derived scientific results are summarized. The existence of lineamentary surface structures is interpreted as an indication for boundary zones between km-sized blocks, building the cometary nucleus. The consequences for cometary origin were investigated. (author). 3 figs., 5 refs

[en] All known groups of fireballs contain bodies which may reach the Earth's surface as meteorites and are of planetary origin. Since some fireballs have cometary orbits in the Solar System, then comets are probably of planetary origin. The possible explosion of a large planet is discussed again. This time on the basis of gravitationally decelerated expansion of an originally superdense embryo. (author). 2 figs., 15 refs

[en] A precise analysis of the orbit of minor planet (1221) Amor, based on all observations made in 1932 to 1980, shows that in 1956 there was an unexplained event in the motion of the asteroid. Possible interpretations of that fact as well as peculiarities in the long-term motion of Amor in 1650 to 2170 are discussed. The ephemeris of Amor for 1988 is presented. (author). 1 fig., 2 tabs., 4 refs

[en] The method for determining inclination distribution of zodiacal particle orbits according to 3D- density models of zodiacal dust presented by Giese and Kneissel (1987, this volume) is briefly discussed. The results show that models with additional bulges at the solar poles bear an istropic component of the inclination distribution amounting up to 20% of all orbits, whereas infrared models show almost no isotropic component. The existence of an isotropic component for zodiacal dust orbits is questioned by a comparison with the orbital elements of meteoroidal particles which are a source of zodiacal dust from mutual collisions. (author). 36 refs

[en] Gravitation, solar radiation pressure and Lorentz force are the dominant forces determining the motion of interplanetary particulates. The latter two becoming significant for micron- and submicron- sized particles. In situ measurements by spaceprobes, microcrater distributions and remote observations both in the IR and visible wavelength range have established the mass frequency and spatial distribution of dust particles in interplanetary space. Consequences of the Poynting-Robertson effect and mutual collisions for these distributions and the contributions of various sources (interstellar dust, asteroids and comets) are discussed. It is shown that the contribution from a distributed source of large particles in the inner solar system is most important. Collisions between these meteor sized particles (m>10^-5 g) produce large amounts of zodiacal light particles (10^-10 g to 10^-5 g) and β-meteoroids (m<10^-10 g) which leave the solar system on hyperbolic orbits. At present the Poynting-Robertson effect transports into the inner solar system less than 10% of the zodiacal light particles which are produced by collisions from bigger particles. (author). 4 figs., 41 refs

[en] Backward integrations of motion of the Halley-type comets, covering over 11,000 years, are used to investigate their librations around high-order resonances with Jupiter. These occur for comets moving in direct orbits with revolution periods between 50 and 90 years. The fact that about one half of them is librating at a time explains the concentration of revolution periods in this range. The librations are of rather uniform period and amplitude, and tend to persist over about 200 revolutions of the comet. Their presence implies that the lifetimes of these comets since their captures into periodic orbits of small perihelion distance mostly exceed 300 revolutions. Possible exceptions among the 9 comets with present periods of 50 to 90 years are the already extinct P/Westphal, and also P/Halley, for which a close approach to the orbit of Jupiter 150 revolutions ago was identified. (author). 2 figs., 1 tab., 10 refs

[en] The application is briefly described of the complete solution of the basic equations of meteoric physics, found during a study of photographic meteor theory, to the construction of theoretical Fresnel characteristics. It is shown how meteoroid deceleration can be incorporated in concepts of radar physics. The corresponding equations are derived and the possibility of using these Fresnel characteristics for the evaluation of ablation parameter σ and pre-atmospheric velocity v_∞ from the registered amplitudes is briefly discussed. (author). 11 refs