Martin Beech

Are lynds dark clouds fractals

We have used the catalogue of dark nebulae compiled by Lynds (1962) to investigate the geometrical nature of the darkest, opacity class 5 to 6, clouds. There would appear to be some evidence that these objects are fractals of dimensionD=1.4.

Fireball flickering: the case for indirect measurement of meteoroid rotation rates

Abstract Data collected during the Meteorite Observation and Recovery Program (MORP) indicate that 4% of bright fireballs show a periodic variation or flickering in brightness. The observed flickering frequencies vary from a few Hz to as high as 500 Hz. We interpret the flickering phenomenon in terms of meteoroid rotation. The MORP data does not reveal any apparent correlation between the flickering frequency and the properties of the meteoroid or the atmospheric flow conditions under which ablation is taking place. It is argued that the most likely cause of the flickering phenomenon is the rotational modulation of the cross-section area presented by the meteoroid to the on-coming airflow. A study is made of the Peekskill fireball and it is concluded that the meteoroid was spun-up during its long flight through the Earths atmosphere, and that its initial brake up was due to rotational bursting. We also argue that the Peekskill event provides the best observational evidence that the flickering phenomenon is truly related to the rotation rate of the impinging meteoroid. We find that the observed rotation rates of the MORP fireballs are clustered just below the allowed limit set by rotational bursting, but argue that this is due to an observational selection effect that mitigates against the detection of low-frequency flickering.

VLF detection of fireballs

The results from 80 hours of simultaneous visual/video and VLF recordings made during the Lyrids, Perseids, Orionids, Leonids and Geminids are presented. All meteor magnitudes from −11 to +4 have been sampled at least once during these observations. The only positive VLF fireball detection was made at 19:57:32 UT on August 11, 1993 from the South of France. We present a Fourier transform analysis of this event and we also derive a lower limit to the electrical field strength produced by the fireball at the antenna. Our present observations suggest a lower limit of Mv ≈ −10 ± 1 for a fireball to produce a VLF signal.

The danger to satellites from meteor storms

Abstract During past meteor storms impact probabilities of between 1 and 0.01 percent have be realized per 50m 2 of exposed surface area at altitudes corresponding to both GEO and LEO. The most likely meteoroid stream to yield a storm in the near future is that of the Leonids. Numerical simulations of the orbital evolution of hypothetical Leonid stream meteoroids suggest that storms may occur in the years 1999 and 2000.

Large-Body Meteoroids in the Leonid Stream

There are presently no direct observational constraints upon the size of the largest meteoroids within the Leonid stream. An upper lower bound may, however, be placed through secondary observations relating to the occurrence of electrophonic sounds during the 1833 Leonid storm. By considering the ablation and very low frequency radio wave emission conditions necessary to produce electrophonic sounds, a preatmospheric lower bound of 1.24 ± 0.1 m is placed upon the diameter of the largest meteoroids within the Leonid stream. The presence of meter-sized meteoroids within the Leonid stream indicates that mechanisms beyond that of surface ice sublimation are active in adding meteoroids to cometary streams.

The projection of fractal objects

Motivated by several recent studies on the geometry of molecular cloud structures, we perform two experiments that consider the projection of fractal images onto a two-dimensional screen. Evidence is presented to support the proposition that the dimensionDP of the contours surrounding the projected images are given byDs−1, whereDs>2 is the fractal dimension of the object itself. A brief survey of the geometry of astrophysical cloud structures is also presented.

Video observations, atmospheric path, orbit and fragmentation record of the fall of the Peekskill meteorite

Large Near-Earth-Asteroids have played a role in modifying the character of the surface geology of the Earth over long time scales through impacts. Recent modeling of the disruption of large meteoroids during atmospheric flight has emphasized the dramatic effects that smaller objects may also have on the Earths surface. However, comparison of these models with observations has not been possible until now. Peekskill is only the fourth meteorite to have been recovered for which detailed and precise data exist on the meteoroid atmospheric trajectory and orbit. Consequently, there are few constraints on the position of meteorites in the solar system before impact on Earth. In this paper, the preliminary analysis based on 4 from all 15 video recordings of the fireball of October 9, 1992 which resulted in the fall of a 12.4 kg ordinary chondrite (H6 monomict breccia) in Peekskill, New York, will be given. Preliminary computations revealed that the Peekskill fireball was an Earth-grazing event, the third such case with precise data available. The body with an initial mass of the order of 10(4) kg was in a pre-collision orbit with a = 1.5 AU, an aphelion of slightly over 2 AU and an inclination of 5 degrees. The no-atmosphere geocentric trajectory would have lead to a perigee of 22 km above the Earths surface, but the body never reached this point due to tremendous fragmentation and other forms of ablation. The dark flight of the recovered meteorite started from a height of 30 km, when the velocity dropped below 3 km/s, and the body continued 50 km more without ablation, until it hit a parked car in Peekskill, New York with a velocity of about 80 m/s. Our observations are the first video records of a bright fireball and the first motion pictures of a fireball with an associated meteorite fall.

The fall of the Peekskill meteorite: Video observations, atmospheric path, fragmentation record and orbit.

A general overview of the events surrounding the fall of the Peekskill meteorite is presented.

In the Right Place at the Right Time

The coming together of two hydrogen (H) atoms and one oxygen (O) atom produces something that is both marvelous and altogether greater than the sum of its parts: water (Figure 5.1). Symbolically written as H2O, water is the medium of life as we know it.

The endurance lifetime of ice fragments in cometary streams

Abstract The endurance lifetime against sublimation of meter- to decameter-sized ice fragments are calculated for typical cometary orbits. It is found that such bodies can survive for multiple perihelion passages. For fragments traveling along orbits similar to those of typical meteor shower producing comets, the sublimation mass loss rate drives radial variations equivalent to 1–0.5 m per orbit. We review the available data with respect to the possible presence of large objects within the Perseid, Lyrid, Leonid and α-Capricornid streams. Invoking cometary aging and surface fragmentation events as the mechanism for placing large meteoroids within cometary streams, we find no compelling reasons to doubt that large meteoroids are intermittently present in most, if not all cometary-derived meteoroid assemblages.