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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.09.007 Byline: S. Close (a), P. Brown (b), M. Campbell-Brown (b), M. Oppenheim (c), P. Colestock (a) Keywords: Meteors; Radio observations Abstract: High-power, large-aperture (HPLA) radars detect the plasma that forms in the vicinity of a meteoroid and moves approximately at its velocity; reflections from these plasmas are called head echoes. For over a decade, HPLA radars have been detecting head echoes with peak velocity distributions >50 km/s. These results have created some controversy within the field of meteor physics because previous data, including spacecraft impact cratering studies, optical and specular meteor data, indicate that the peak of the velocity distribution to a set limiting mass should be Author Affiliation: (a) Los Alamos National Laboratory, Space and Remote Sensing Sciences, Mail Stop D436, Los Alamos, NM 87545, USA (b) Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada (c) Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA Article History: Received 8 June 2006; Revised 12 September 2006

Positions of 17 filaments found inside the Perseid meteoroid stream by method of indices are compared with those of low-order mean-motion resonances with Jupiter and Saturn. By this comparing, the Jupiter and Saturn branches of the Perseid stream were identified. The existence of gaps in the distribution of the semi-major axes of the Perseids is confirmed using the more numerous material of a new version of the IAU Meteor Data Center Catalogue. Our integrations of the motion of particles in the Perseid stream lead to an extraordinary important fact. The found filaments are located in close proximity of strong resonance's. They represent, with a high probability, increased numbers of particles gravitationally expelled from a resonant gap and (temporary) settled down in its close proximity. Keywords: Meteoroids; Orbits; Resonances

Zagami, a well characterized SNC meteorite, represents a reference sample to verify the feasibility of the non-destructive infrared micro-spectroscopy technique to extract spectral signatures from individual mineral phases in a meteorite sample. For the first time individual infrared spectra of the major mineral phases, in the 6000-600 [cm.sup.-1] (1.67-16.7 [micro]n) spectral interval, whose identification is confirmed by energy dispersive X-ray analysis and backscattered imaging, are measured. The signatures of the main mineral phases we identified in the Zagami chip are: (1) maskelynite characterized by broad and smooth Si-O vibrational bands in the 1000 [cm.sup.-1] spectral region; (2) crystalline pyroxenes showing well defined fine structures; and (3) an oxide mineral phase with an almost featureless and flat spectrum. In the part of the spectrum centered around 2 [micro]m, by analyzing the different positions of the [Fe.sup.2+] bands, we were able to discern the high-Ca from the low-Ca pyroxene phases. This result demonstrates that by means of the infrared micro-spectroscopy technique it is possible to retrieve directly the composition of pyroxenes in the En-Fs-Wo system, without relying on the use of deconvolution techniques. In addition IR signatures due to water and aliphatic hydrocarbons were observed to be more abundant in the pyroxenes than in maskelynite. This could be an indication that the organic and water signatures are due to indigenous compounds in Zagami rather than laboratory contamination, however, further investigations are necessary before this conclusion can be confirmed. Keywords: Mars; Meteorites; Spectroscopy; Mineralogy; Experimental techniques

We present a survey of 97 spectra of mainly sporadic meteors in the magnitude range +3 to -1, corresponding to meteoroid sizes 1-10 mm. For the majority of the meteors, heliocentric orbits are known as well. We classified the spectra according to relative intensities of the lines of Mg, Na, and Fe. Theoretical intensities of these lines for a chondritic composition of the meteoroid and a wide range of excitation and ionization conditions were computed. We found that only a minority of the meteoroids show chondritic composition. Three distinct populations of Na-free meteoroids, each comprising ~ 10% of sporadic meteoroids in the studied size range, were identified. The first population are meteoroids on asteroidal orbits containing only Fe lines in their spectra and possibly related to iron-nickel meteorites. The second population are meteoroids on orbits with small perihelia (q [less than or equal to] 0.2 AU), where Na was lost by thermal desorption. The third population of Na-free meteoroids resides on Halley type cometary orbits. This material was possibly formed by irradiation of cometary surfaces by cosmic rays in the Oort cloud. The composition of meteoroids on Halley type orbits is diverse, probably reflecting internal inhomogeneity of comets. On average, cometary dust has lower than chondritic Fe/Mg ratio. Surprisingly, iron meteoroids prevail among millimeter-sized meteoroids on typical Apollo-asteroid orbits. We have also found varying content of Na in the members of the Geminid meteoroid stream, suggesting that Geminid meteoroids were not released from their parent body at the same time. Keywords: Meteors; Meteoroids; Spectroscopy; Comets, composition; Asteroids, composition

We report here the observation of the first peak belonging to the 2002 Leonid meteor storm made during the night of November 18-19, 2002. This feature, produced by a 7-revolution dust trail, was observed from several photographic stations of the Spanish Photographic Meteor Network (SPMN) and one video station operated from the Instituto de Astrofisica de Andalucia (IAA) in an intensive campaign from the ground working in collaboration with the 2002 Leonid MAC mission. We used photography, slow-scan Charge Coupled Devices (CCD) and video CCD-imaging techniques to deduce the meteoroid flax density profiles in different ranges of masses. Additionally. we present multi-station work, developed during the storm, that allows us to deduce the orbital elements of ten meteoroids associated with this dust trail. We have found a clear similarity between their orbits and the one belonging to a theoretical orbit for particles ejected from 55P/Tempel-Tuttle in 1767. Keywords: Meteors; Meteoroids; Orbits; Comets: Leonids; 55P/TempeI-Tuttle

We present measured and inferred properties of a possible new class of 'sporadic' micrometeors discovered during 18 January 1995 observations made using the very sensitive 430 MHz radar system located at Arecibo Observatory in Puerto Rico. Over 200 of these objects were observed in a 2-hr period near sunrise. The average speed was about 55 km/sec in a range of 45-63 km/sec. Approximately two-thirds of the observed trajectories were apparently nearly parallel with the vertical beam and occurred on the 93-102 km height interval. The observed occurrence rate of these meteor returns combined with the size the Arecibo beam points to a meteor flux corresponding - in the 'classical' view - to [approximately]15th magnitude micrometeors. This information along with observed deceleration rates and radar scattering cross-sections of order [10.sup.-8] [m.sup.2], leads us to conclude that the majority of the meteors observed appear to be of order 1 [[micro]gram] in mass. The depth of atmospheric penetration and inferred perihelia, the majority of which lie mostly within the orbits of Mercury and Venus, point to compositions of dense refractory material. Retrograde orbits that lie well out of the plane of the ecliptic combined with the modeled effects of radiation pressure induced orbit decay suggest that these particles - with no obvious parent body - originated in the outer reaches of the solar system and that they may even be primordial in origin. It is suggested that most if not all of these particles are associated with the North Apex 'source' of sporadic meteors reported by Jones and Brown (1993, Mon. Not. R. Astron. Soc. 265, 524-532). Additionally, several possible radar scattering mechanisms are discussed, none of which seem completely satisfactory.

Strong collisional interactions occur between interplanetary meteoroids and Saturn's rings. Mass and angular momentum transport of the ring system occurs mostly through a continuous process of impact fragmentation and erosion. Laboratory experiments show that hypervelocity impacts on ice-silicate mixtures result in a tenuous dust halo above and below the ring plane. Half thickness of the dust halo could be about 1500 km. Good observational data can be obtained during edge-on viewing condition in 1995 and 1996.

Spectral analysis studies, conducted during the Arecibo Initiative in Dynamics of the Atmosphere 1989 campaign, of a near-Earth object show that it could be a meteoroid. It is a stony object and there are two Fe I emissions in the near IR region with an excitation temperature of 3100 K. Both Na and Fe are present in equal quantities and ionized calcium, manganese (Mn) and cobalt are almost absent. The ratio of Mn to Na and Fe shows that the meteoroid is of aubrite composition with an enstatite nature based on spectrum analysis.

A 'radar micrometeor' is the radar-scattering signature from the free electrons in the plasma generated by entry of a dust-sized meteoroid into the atmosphere. We report the first direct Doppler measurements, made using the Arecibo Observatory 430-MHz radar, of the so-called meteor head echo. Our observations demonstrate that this region is moving with the speed of the meteoroid as determined from the meteor head-echo altitude-time trajectory and that this radar return is distinct spatially and in velocity from the much more commonly observed trail echo. We also report the first observations of near-antapex micrometeors which are characterized by the very slow atmospheric speeds expected from low-ecliptic-inclination objects entering the atmosphere from behind Earth's orbital path. Of the 32 meteors observed during four early evening hours of observations on 10 January 1997, velocities were determined for 18 of the meteors of which 7 were at or just below Earth escape velocity (11.2 km/s). We give heliocentric orbits for the 11 meteor events with speeds greater than the escape velocity and present a detailed analysis of these orbital parameters and their possible origins. One particle was determined to be interstellar: a preliminary analysis indicates that the ecliptic coordinates of the radiant relative to the local standard of rest (LSR) (with the solar motion relative to the nearby stars removed) are [Lambda] = 43.02 [degrees], [Beta] = -43.2 [degrees], V= -25.11 km/s or, in system II galactic coordinates [l.sub.II] = 219.8 [degrees], [b.sub.II] = -52.4 [degrees], V= -25.1 km/s. Key Words: meteors; meteoroids; radar; orbits.

The discovery of cosmic spherules in sandstone over 1.4 billion years old from Finland has been reported by Deutsch, Greshake, Pesonen and Pihlaja. Cosmic spherules are an abundant form of extraterrestrial material and were first described in 1876. The new discovery provides a new way of tracking long term changes in atmospheric oxygen content, along with changes in the meteroroid population of the Solar System.

Meteoroids are produced by interstellar dust particles continually hitting the earth. A meteoroid loses mass and velocity rapidly and leaves a train of ionized gas. The train could reflect radar pulses. Meteor scientists have often wondered whether there were any hyperbolic orbits among incident meteoroids. It has been detected that the earth's solar system is a source of hyperbolic meteroids for other planetary systems.