As Part of our Program of observation of comet Kohoutek at Pic-du-Midi Observatory, we obtained, on January 17.8 UT, 1974 a Photograph in polarized light showing dramatically the (dust) anti-tail extending for almost 1° from the Comet’s head (reported in sky and telescope, June 1974); indeed the comet is visible in polarized light further away than in total light as noticed by weinberg and Beeson (IAU collloquim No. 25, 1974) for Comet Ikeya-Seki. A photometric and polorimetric study was performed (Bücher, A., Robley, R., and Koutchmy, S., 1975, Astron. Astrophys. 39, 289) showing that the anti-tail is strongly polarized (up to 50%). These large degrees of polarisation are of the same order of magnitude as those reported for the tail of comet Ikeya-Seki by matjagin sabitov and kharitonov (1967, Astron. Zh. 44, 1075) and by Weinberg and Beeson (op. cit.). As discussed by these latter authors, particle alignment is precluded as a significant contributor to polarization in the tail of comets. Polarization by large spheres as obtained from the Fresnal reflection coefficients applies only in the case of perfect surface, a circumstance very unlikelyn in the interplanetary space; the scattering is infact controlled by the surface microstructures (Van de Hulst, private communication). Therefore we hypothesized that submicronic grains should play an important role in the anti-tail. The classical method of Finson and Probstein (1968, Astrophys. J. 154, 327, 33) was used to draw the sky plane view of the syndneys for the day of observation. Since the ratio ß of the radiation pressure force to the gravitational attraction is propotional to the third power of the grains’ radius s for s ≲ 0.1 µ, submicronic grains with typical radii of 0.02 µ may indeed be present in the anti-tail and provide a straight-forward explanation observed polarization. This size is of the same order of magnitude as that inferred for interstellar grains which may well be embedded in the comet’s nucleus as well as meteorites for which there exists good evidence. Our conclusion does not rule out the presence of millimeter-size grains as proposed by Sekanina and Gary and O’Dell in their preliminary investigations (1974, Icarus 23, 502, 519) which did not take into account the polarimetric result; such grains may well coexist with the submicronic ones. Finally, the line of maximum intensity is close to a synchrone corresponding to a time of emission 100 days before perihelion passage. This supports the synchronic formation of anomalous tails and possibly of tails as proposed by Vsekhsvyatsky (1932, Astron. Zh. 9, 166).

Neither thermodynamic calculations nor standard nucleation theory describe condensation processes in cosmic clouds, but rather are a first approximation. Condensation is an irreversible kinetic, not equilibrium, process. The nominal molecules of condensed minerals (e.g. Al2O3, CaTiO3, MgAl2O4) occur only in the bulk lattice not in cosmic gases. Reactions among gas molecules must be an intrinsic part of condensation. Molecules yielding a particular condensate are a small fraction, less than 10−3, of condensible and reactive (e.g. H, H2, H2O) molecules. No theoretical or experimental results are available for condensation under such conditions. There is no basis for expecting nearly pure, known minerals to form in cosmic clouds. A generalization of nucleation theory following Hirschfelder (JCP, 61, 2690, 1974 can be made, in principle. No thermodynamic data is known for most of the possible compositions of nucleating clusters and no calculations are possible. Grain compositions should fall between thermodynamic and kinetic calculations. An experimental study appears feasible and is planned. The following conclusions are drawn from the theoretical analysis. (1) An interpretation of interstellar grains may not be made using only terrestrial minerals. (2) Condensation temperatures and compositions of interstellar grains or primordial solar system condensates may not be deduced from equilibrium calculations.

The planetary program has always been conducted with the hope that the results would reveal great insight into the early period of solar system history and perhaps into the actual formation processes themselves. However, little knowledge has been gained of this very early stage for several reasons: The intense surface bombardment of all larger bodies, particularly in the inner solar system during that period; the subsequent differentiation of large bodies; and atmospheric effects and continued bombardment of the surface. The most promising approach to acquisition of knowledge pertaining to the early state of the solar system, its origin and evolution, therefore seems to be in the study of small bodies, e. g., comets and asteroids.

The contribution of meteor observations to our knowledge of meteoroids and interplanetary dust is reviewed under four headings – flux, mass distribution, physical structure and chemical composition. For lower limits of particle mass ranging from 1 g to 10−5 g the mean cumulative flux into the earth’s atmosphere varies from 2 × 10−15 to 6 × 10−9 particles m−2 s−1 (2Πster)−1, and the mean size distribution of these particles is given by log N = C – 1.3 log M, where N is the cumulative number of particles counted down to a lower mass limit M, and C is a constant. The physical structure of meteoroids in the above range is essentially fragile, with generally low mean bulk densities that tend to increase with decrease in mass. A minor fraction, about 10 or 15 per cent, with orbits lying inside that of Jupiter, have densities several times the average densities, approaching those of the carbonaceous chondrites. The mean chemical composition of meteoroids seems to be similar to the bronzite chondrites for the elements heavier than number 10, but with the probable addition of extra quantities of the light volatiles H, C and O.

The composition and density of meteoroids has been the subject of a number of investigations in recent years. Analysis of photographic meteor data have been presented by Jacchia et al (1967). Verniani (1964, 1967, 1969), Benyuch (1968, 1973, 1974), McCrosky and Ceplecha (1969) and others. The density of radio meteors have been studied by Verniani and Hawkins (1965) and Verniani (1966). in the present paper the dependence of meteoroid density on the type of the orbit is discussed.

The clustering of micrometeoroid impact events on satellite--borne detectors (see e.g. Hoffmann et al., 1975) poses a question whether similar phenomena can be identified as well among larger particles observable as meteors. In fact, there is a strong conviction maintained by many observers that meteors within the streams are observed to be clustered in pairs or larger groups more frequently than one could expect from chance distributions. The rate of the dispersive effects indicates that the lifetime of any group of meteor particles is very limited. If real, the groups must be due to recent fragmentation of larger meteoroids.

A determination of velocity distributions of sporadic meteor bodies and distributions of their radiants over the celestial sphere corrected for the motion and attraction of the earth is one of the main problem of the meteor astronomy. An analytical model of these distributions has certain interest.

This note emphasizes the importance of very bright fireballs as a powerful source of meteoritic dust in the upper atmosphere and on the Earth’s surface. Recently we obtained a direct observational evidence when we photographed the Šumava Fireball, an object of −22 absolute stellar magnitude, on Dec. 4, 1974 at 17h57.5m UT. This is the brightest object on the European Network photographic records during almost 12 years of systematic fireball observations. We obtained 4. all-sky-camera records of the Šumava Fireball from two Czech and one German station of the Network. The most important combination for the trajectory computations proved to be station No. 5 at Ondrejov (Astronomical Institute of the Czechoslovak Academy of Sciences) and station No. 65 at Bernau (Max-Planck-Institut für Kernphysik, Heidelberg). The results are given in Table 1. At Ondřejov we also obtained 4 spectral records of the Šumava Fireball; the most detailed record has a dispersion of 5 Å/mm.

The characteristics of the flux density of the interplanetary small particles in the vicinity of the Earth are considered. From the results of the analysis of the characteristics of the suspended dusty particles in the turbulent upper atmosphere the density of flux is obtained. The radar meteor measurements of the turbulent intensity in the upper atmosphere allow to evaluate the flux density of the interplanetary dust as a function of the magnitude of the particles at the intervals of the sizes from ~ 1µ. to ~ 0.05µ. The comparison with the results of different experiments shows a good agreement.

Long-term variations in meteor radar rates, echo amplitudes, and meteor end-point heights have been observed. These variations appear to be controlled to a large extent by the solar cycle of activity. Exceptionally high echo rates in 1953, 1963 and 1972 coincide approximately with solar cycle minima. The long-term variations are explained in terms of a solar controlled change in the atmospheric density gradient at the meteor ablation level.

Sudden enhancements in responses recorded by micrometeoroid detectors flown on spacecrafts have been repeatedly attributed to encounters with streams of cometary debris similar to, or identical with, the meteor streams known from ground-based observations. Por measurements made in the Earth-Moon environment, spacecraft effects, atmospheric fragmentation of larger particles, and possibly lunar ejecta can be misinterpreted as interplanetary streams. For deep space observations it is necessary to inquire whether a compact dust stream can persist under the dispersive and destructive effects which increase rapidly with decreasing particle size.

The only type of concentration of cometary dust with a reasonable probability of being detected by deep-space probes, are the dust tails emanating from passing comets. Essentially all the dust released from long-period comets leaves the solar system on hyperbolic orbits, because the radiation pressure limit is high (corresponding to centimetre-sized grains emitted in the vicinity of the Earth’s orbit by comets of the Oort’s Cloud). For the short-period comets the dynamical conditions for retention of emitted particles within the solar system are much more favourable, but those which remain in circumsolar orbits tend to disperse rather rapidly (Kresák, 1976a).

A two-dimensions distribution p(e,q) of eccentricities e and perihelion distances q can be derived by means of the formula for the probability transformation:

Attention is centered on cosmic dust measures made by sensors on Pioneers 8 and 9 in Earth-like orbits. The conclusion follows Zook and Berg that the particles are largely “β-meteoroids,” interplanetary impact debris expelled by solar radiation pressure. An analysis of periodic comet orbits and comets observed during the missions failed to yield correlations, except possibly for debris from Comet Encke. A treatment of β-meteoroids from this stream is presented.

The problem of the effects of mutual collisions for the dynamics of interplanetary dust particles and grains is reviewed. Collisions are shown to give a rather characteristic dynamical signature, the importance of these effects depending mainly on the mean free collision time and the degree of inelasticity. Although a few attempts to look for collisional effects in the solar system have been made, rather much work remains to be done before the problem is fully understood.

The Poynting-Robertson (P-R) effect (Robertson, 1937, Wyatt and Whipple, 1950), assisted by a pseudo P-R effect due to the sputtering (Whipple, 1955, 1967), is known to cause small dust particles in interplanetary space to spiral toward the sun. Evaporation from the surface of such particles thus increases progressively with time and their size is being reduced accordingly. When the rate of evaporation is no longer negligibly low, it induces on the particle a measurable dynamical effect, which is associated with the implied variations in the magnitude of solar radiation pressure relative to solar attraction. By gradually reducing solar attraction, the particle evaporation tends to increase the orbit dimensions, thus acting against P-R. The P-R inward spiraling, far exceeding the dynamical effect from evaporation at larger heliocentric distances, slows gradually down as the particle approaches the sun, and virtually ceases when the critical distance is reached, where the two forces approximately balance each other. Then, typically, the perihelion distance stabilizes, while the eccentricity starts increasing very rapidly until the particle is swept out of the solar system. This, in brief, is the orbital evolution of a vaporizing particle in the absence of other potentially important but rather poorly known processes, such as particle collisions, rotational bursting, electric charging and interactions with the solar wind and with the interplanetary magnetic field.

The orbital evolution of circum-solar dust grains is obtained by numerical integration of the equations of motion which includes the grains’ interactions with the solar radiation field and the solar wind. Our past solution (Lamy, 1974) is improved by avoiding a classical approximation for the Poynting-Robertson term and leads to an important revision of the orbital behaviour. Results are presented for obsidian grains whose inward spiraling is stopped by the effect of sublimation.

An improved solution of the temperature distribution of interplanetary ice grains is presented using the refractive index measured at 100° K. The efficiency factors for absorption are obtained from Mie theory and the calculation is carried out for micronic and submicronic grains at 50, 100 and 150° K; corresponding lifetimes are given.

An attempt has been made to derive heliocentric distribution of meteoric particles in interplanetary space. Poynting-Robertson effect, collision process, and cometary dust injection have been included in this study. Three different radial distribution functions for different sizes have been derived. The number density appears to depend on particle size.

Solar radiation pressure can cause rotational bursting and eventual elimination from the solar system of small asymmetric interplanetary particles by a windmill effect. The life span determined by this process for stony meteoritic material or tektite glass with radii of 1 cm is on the order of 105 years. Same size material which contains iron, nickel or aluminum, with properties such that it is subject to 5 percent of the amount of spin damping as pure metals, can be removed from the solar system on the order of 106 years by this process. Ordinary chondritic material, despite its high resistivity, is subject to a type of magnetic spin damping, in addition to the normal spin damping, with the consequent result that this type material cannot be removed from the solar system by this process. This depletion mechanism appears to work faster than the traditional Poynting-Robertson effect by approximately two orders of magnitude for the nonmetallic particles and one order of magnitude for the metallic particles.