Near-Earth Asteroids And Meteor Showers

26.41> =(aeffir 0:75 )m=s (1) where R c is the radius of a cometary nucleus in km, ae and ffi are particle's radius and density both in CGS units. The maximumdifferences between orbital semi-major axis \Deltaa and the eccentricity \Deltae of the ejected meteoroid from those of the comet will be (Plavec 1955): \Deltaa = \Sigma6:72 \Theta 10 \Gamma5 a 2 c v p 2=r \Gamma 1=a c AU \Deltae = \Sigma6:72 \Theta 10 \Gamma5 re c v p 2=r \Gamma 1=a c (2) where a c and e c are semi-major a

Meteor showers associated with the near-Earth asteroid (2101) Adonis

The orbital evolution of the near-Earth asteroid (2101) Adonis under gravitational action of six planet (Mercury to Saturn) is investigated by the Halphen-Goryachev method. The theoretical geocentric coordinates and velocities of four possible meteor showers associated with this asteroid are determined. Using published data, the theoretically predicted showers are identified with the observed ones, namely, night-time σ-Capricornids and χ-Sagittariids, and day-time χ-Capricornids and Capricornids-Sagittariids. The existence of meteor showers associated with Adonis provides evidence supporting the conjecture that this asteroid may be of a cometary nature. The small 50-m near-Earth asteroid 1995 CS probably represents a large Adonis fragment and belongs to a part of the Adonis meteoroid stream, which produces the day-time χ-Capricornids meteor shower

Fragmentation and densities of meteoroids

The phenomenon of meteoroid fragmentation in the Earth's atmosphere was recorded repeatedly by means of different methods and especially using the photographic technique of instantaneous exposure. Among the four principal forms of fragmentation, the quasi-continuous fragmentation, i.e. a gradual release of the smallest fragments from the surface of a parent meteoroid and their subsequent evaporation, is most common. The analysis of photographic observations shows that a substantial fraction of meteoroids is exposed to this type of fragmentation. According to the theory of quasi-continuous fragmentation and on the basis of light curves of meteors photographed in Dushanbe (Tajikistan), the mean bulk densities of meteoroids belonging to six meteoroid streams and the sporadic background have been determined, which vary in the range from 0.4 g cm-3 (Leonids) to 2.9 g cm-3 (Geminids)

Search for meteor showers associated with Near-Earth Asteroids

Observed meteor showers associated with some Near-Earth asteroids (NEAs) is one of the few criteria that such asteroids may be considered to be candidate extinct cometary nuclei. In order to reveal new NEA-meteor shower associations, we calculated the secular variations of the orbital elements of 17 Taurid Complex asteroids with allowance for perturbations from six planets (Mercury-Saturn) over one cycle of variation of perihelia arguments. The Earth-crossing class of these NEAs and theoretical geocentric radiants and velocities of their meteor showers were determined and compared with available observational data. It turns out that each Taurid Complex asteroid is associated with four meteor showers. This is evidence for the cometary origin of these asteroids

Some features of Geminid meteoroid disintegration in the Earth's atmosphere

Among 11 bright Geminid meteors photographed at the Institute of Astrophysics, Tajik Academy of Sciences (Dushanbe, Tajikistan), 3 displayed distinct high-frequency (more than 100 Hz) pulsations of brightness, or flickering. As a rule, for these Geminids, pulsations of brightness begin at the middle of their photographed path, at the height of about 75 km, and last until the end of the visible trajectory. Different possible meteoroid ablation mechanisms causing the flickering of the bright Geminids are discussed. The obtained results allow us to conclude that the observed high-frequency flickering of Geminid fireballs may be explained by an autofluctuating mechanism of the meteoroid ablation, i.e. by melting and cyclic ablation of the surface-layer of meteoric matter with the period corresponding to the observed period of the flickering.

The concentration of calcium ions and of free electrons in a meteor coma

Prism spectra are studied of eight bright meteors, belonging to different showers and photographed by small cameras in the Gissar observatory of the Institute of Astrophysics, Tajik Academy of Sciences. Monochromatic light curves for the H and K spectral lines of the ionized calcium doublet are constructed and from the ratio of their intensities at selected points along the meteor trajectories the column  NCaII and the volume nCaII concentrations of calcium ions are determined. The obtained values $N_{\rm Ca II}= 10^{16}{-}10^{18}$ cm-2 and $n_{\rm Ca II}= 10^{13}{-}10^{15}$ cm-3 are in good agreement with the estimates of calcium abundance obtained by other methods and confirm that there is almost full ionization of calcium atoms in meteor comae. The volume concentration of free electrons $n_{\rm e}=4\times 10^{13}$ cm-3 is defined, and the excitation temperature is calculated along the meteor paths

Densities and porosities of meteoroids

Using a physical theory of meteors and on the basis of the results of double-station photographic observations of meteors in Dushanbe (Tajikistan), Kiev, and Odessa (Ukraine), the mean mineralogical and bulk densities of meteoroids belonging to nine meteoroid streams and sporadic background are determined. The mean mineralogical densities $\delta_{\rm m}$ of meteoroids range from 2.2 g cm-3 (Perseids) to 3.4 g cm-3 (Quadrantids, δ-Aquarids, and α-Capricornids). The meteoroid bulk densities δ, which were determined according to the theory of quasi-continuous fragmentation of meteoroids in the Earth's atmosphere, vary from 0.4 g cm-3 (Leonids) to 2.9 g cm-3 (Geminids). Using the relation between bulk density and mineralogical density the porosity of meteoroids was estimated. The Geminid meteoroids are found to have the lowest porosity, while the Leonid and Draconid meteoroids have the most porous structure (83%). These results confirm the porous-structure nature of meteoroids' parent bodies i.e. comets and asteroids