Meteoroid fragments dynamics: Collimation effect

There is significant evidence that some fraction of meteoric bodies is destroyed in the atmosphere. The evolution of the fragment cloud depends on a large number of factors, amound them: the meteoroid’s altitude and velocity at the moment of greakup, fragment sizes and properties of a body material. The interaction of shock waves forming in front of the fragments may lead to both an increase and decrease of the midsection area of the fragment cloud (Artem’eva & Shuvalov, 1996; Laurence et al., 2007). In this work, we consider the interaction of the fragments in a supersonic flow. The configuration properties of two spherical bodies of different radii are considered. Via numerical simulations, we calculate the pressure distribution in the flow around the two bodies for different relative positions. We construct the functions of the coefficients of transverse and drag forces from the angle between the central line of the two bodies and the flow direction for different distances between the two fragments. We find the conditions for the collimation effect, i.e., fragment involving into the wake of the leading (usually, the largest) fragment. We systematize the simulation results for drag and transverse forces and infer the basic aerodynamic properties of the meteoroid fragments.     

A New Model for the Separation of Meteoroid Fragments in the Atmosphere

This work is devoted to modeling of the transverse scattering of meteoroid fragments in the atmosphere by adopting supersonic gas dynamics around a system of bodies. Artem’eva and Shuvalov (1996, Shock Waves, 367) and Zhdan et al. (2004, Dokl. Phys., 315–317) found that the transverse force decreases with the increase of the distance between fragments, that is, fragments do not separate in a transverse direction under the action of constant repulsion force. This work on the decreasing transverse force uses the values of the transverse force coefficient by Zhdan et al. (2004, Dokl. Phys., 315–317) obtained from numerical modeling for spheres in a supersonic flow to derive the analytical solution of the dynamic equation for a fragment. The new model of layer-by-layer scattering of meteoroid fragments moving as a system of bodies is constructed on the basis of the analytical solutions derived in this work and the numerical data by Zhdan et al. (2005, Dokl. Phys., 514–518).     

The Influence of Geometric Shape on the Motion of Fragments of a Dissociated Meteoroid

The scatter of the fragments of a dissociated meteoroid, which is caused by their deviation from the original flight-path due to distinctive features of their aerodynamic shape, is investigated. For fragments of the simplest geometric shape, it is shown by numerical simulations that this mechanism contributes to expanding the debris cloud no less than the interaction between the fragments immediately following breakup, while they are surrounded by a common shock wave. An approximate formula is proposed for assessing the debris scatter of the disrupted body, and this formula may be used to evaluate the parameters of crater fields.     

Effects of atmospheric breakup on crater field formation

This paper investigates the physics of meteoroid breakup in the atmosphere and its implications for the observed features of strewn fields. There are several effects which cause dispersion of the meteoroid fragments: gravity, differential lift of the fragments, bow shock interaction just after breakup, centripetal separation by a rotating meteoroid, and possibly a dynamical transverse separation resulting from the crushing deceleration in the atmosphere. Of these, we show that gravity alone can produce the common pattern in which the largest crater occurs at the downrange end of the scatter ellipse. The average lift-to-drag ratio of the tumbling fragments must be less than about 10^?3, otherwise small fragments would produce small craters downrange of the main crater, and this is not generally observed. The cross-range dispersion is probably due to the combined effects of bow shock interaction, crushing deceleration, and possibly spinning of the meteoroid. A number of terrestrial strewn fields are discussed in the light of these ideas, which are formulated quantitatively for a range of meteoroid velocities, entry angles, and crushing strengths. It is found that when the crater size exceeds about 1 km, the separation between the fragments upon landing is a fraction of their own diameter, so that the crater formed by such a fragmented meteoroid is almost indistinguishable from that formed by a solid body of the same total mass and velocity.     

Meteoroid ablation models

The fate of entering meteoroids in atmosphere is determined by their size, velocity and substance properties. Material from ablation of small-sized meteors (roughly R≤0.01–1 cm) is mostly deposited between 120 and 80 km altitudes. Larger bodies (up to meter sizes) penetrate deeper into the atmosphere (down to 20 km altitude). Meteoroids of cometary origin typically have higher termination altitude due to substance properties and higher entry velocity. Fast meteoroids (V>30–40 km/s) may lose a part of their material at higher altitudes due to sputtering. Local flow regime realized around the falling body determines the heat transfer and mass loss processes. Classic approach to meteor interaction with atmosphere allows describing two limiting cases: – large meteoroid at relatively low altitude, where shock wave is formed (hydrodynamical models); – small meteoroid/or high altitudes – free molecule regime of interaction, which assumes no collisions between evaporated meteoroid particles. These evaporated particles form initial train, which then spreads into an ambient air due to diffusion. Ablation models should make it possible to describe physical conditions that occur around meteor body. Several self-consistent hydrodynamical models are developed, but similar models for transition and free molecule regimes are still under study. This paper reviews existing ablation models and discusses model boundaries.     

Constraining the luminous efficiency of meteors

We propose a new approach for studying the radiation of a fireball, one of the main processes which occur when the meteor body enters the planetary atmosphere. The only quantities which directly follow from the available observations are the fireball brightness, its height above sea level, the length along the trajectory, and as a consequence its velocity as a function of time. Other important parameters like meteoroid’s mass, its shape, bulk and grain density, temperature remain unknown. The present study takes recent results in fireball aerodynamics and considers them together with the classical postulate that a fraction of the meteoroid kinetic energy is transformed into radiation during its flight. This gives us a new analytical dependence, which in particular shows that the fireball luminosity in general is proportional to the body pre-entry mass value, its initial velocity to the power of 3, and the sine of the slope between horizon and trajectory. Research helps in finding an answer to the general important question: Which fraction of the fireball kinetic energy is transformed into light during meteoroid drag and ablation in the atmosphere?     

Extra-atmospheric masses of the Canadian Network bolides

The extra-atmospheric masses of meteoric bodies have previously been determined using the so-called photometric formula, by integrating the luminosity along the visible portion of the trajectory. On the other hand, the mass of a meteoroid characterizes the braking height and intensity of the meteoroid in the atmosphere. Some studies note a substantial disagreement between the masses obtained in these two ways, using bolides of the European Bolide Network and of the US Prairie Network as examples. In nearly all cases, the photometric mass exceeds the mass determined from the braking intensity by an order of magnitude or more. Two explanations were suggested for this fact. According to one of them, a swarm of fragments, similar in size, rather than a single body is moving. This swarm brakes as an individual fragment, while it glows as a collection of fragments; i.e., it is much brighter than an individual fragment. The extra-atmospheric mass is determined here by properly fitting the parameters describing the braking of the meteor along the entire visible section of the trajectory. The results obtained for the bolides of the Canadian Network confirm again that the photometric approach is not tenable.     

The Morávka meteorite fall: 4. Meteoroid dynamics and fragmentation in the atmosphere

Abstract— Detailed analysis of the fragmentation of the Morávka meteoroid during the atmospheric entry is presented. The analysis is based on the measurement of trajectories and decelerations of fragments seen in a video and at the locations of energetic fragmentation events from seismic data obtained at several stations in the vicinity of the fireball trajectory. About 100 individual fragments are seen on video frames. Significant deceleration of the fireball at heights of ?45 km revealed that the meteoroid had already fragmented into ?10 pieces with masses of 100–200 kg, though the fireball still appeared as a single object. At heights of 37–29 km, all primary fragments broke‐up again under dynamic pressures up to 5 MPa. The cascade fragmentation then continued, even though smaller pieces breaking off from the larger masses were increasingly decelerated and the dynamic pressure acting upon them decreased. At each fragmentation, a significant part of the mass was lost in the form of dust or tiny particles. This was the dominant process of mass loss. The continuous ablation due to melting and evaporation of the meteoroid surface was less efficient with a corresponding ablation coefficient of only 0.003 s² km‐². During fragmentation, some pieces achieved lateral velocities up to 300 m/s, about an order of magnitude more than can be explained by aerodynamic loading. The fragmentation continued even after ablation ceased, as demonstrated by the incomplete fusion crust covering all recovered fragments. We estimate that several hundreds of meteorites of a total mass of ?100 kg landed, mostly in a mountainous area not suitable for systematic meteorite searches. Six meteorites with a total mass of 1.4 kg were recovered up to the end of May 2003. Their positions are consistent with the calculated strewn field.     

Meteoroid structure from radar head echoes

The Advanced Research Project Agency (ARPA) Long Range Tracking and Instrumentation Radar has recorded thousands of head echoes from small meteoroids, which include detailed trajectory information as well as ionization measurements. In total, 25 complete ionization curves have been matched using a detailed model of meteoroid ablation, though the solutions are not necessarily unique. While measurements of the spread along the trajectory of the echoes indicate that most meteors in this size range do not have large separations among fragments, the ionization curves are consistent with fragmenting bodies in the most cases. Very precise radar measurements of meteors can be a valuable source of data on the chemical and physical properties of small meteoroids.     

Analysis of a Low Density Meteoroid with Enhanced Sodium

We present an analysis of sporadic meteor number 07406018, observed by image intensified video cameras at two stations, which showed a pronounced deceleration along its trajectory. We have applied the erosion model to analyze simultaneously the deceleration and light curve. We have found that the meteoroid had a low density of about 500 kg m⁻³, consistent with its cometary orbit. The meteoroid structure was, nevertheless, markedly different from the Draconid meteoroids, studied recently with the same model. The size of the constituent grains was larger and the erosion energy was higher than in Draconids. The meteor spectrum was also different from Draconid spectra and showed very bright Na lines. The meteoroid composition was probably different from normal cometary composition.     

Spectral Investigation of Two Asteroidal Fireballs

High resolution photographic spectra of two fireballs have been analyzed. The fireballs were produced by meteoroids of asteroidal origin of the mass of the order of 1 kg. Temperature, size, and mass of the vapor cloud around the meteoroid was derived at selected points along the trajectory. Abundances of 11 elements, including lithium, were determined. The abundances of refractory elements in the vapors of the first meteoroid indicate that only 90–95% of the ablated material was vaporized. The meteoroid was likely a chondritic body. Relative stability of the vapor cloud was disturbed for 0.1 s after a major fragmentation of the meteoroid at the height of 42 km. Size and mass of the cloud decreased after the fragmentation and this enabled more intensive heat transfer from the incoming airflow. Both the vapor temperature and the vaporization temperature of the ablated melt increased. A brief millisecond flare of the fireball was produced under these conditions by a violent vaporization of small amount of material. The composition of the vapors of the second meteoroid can be explained either by an anomalous meteoroid composition with severely depleted Al, Ca, and Mg or by highly incomplete evaporation of the ablated material reaching only about 50%.     

Motion of a Meteoroid Released from an Asteroid

Evidence of asteroid surface features as regolith grains and larger boulders implies resurfacing possibility due to external forces such as gravitational tidal force during close planet encounters. Motion of a meteoroid released from an asteroid in the gravitational fields of the asteroid and the Earth is modeled. We are interested mainly in a distance between the meteoroid and the asteroid as a function of the time. Applications to Itokawa and some close approaching NEAs are presented.     

Meteoroid Engineering Model (MEM): A Meteoroid Model For The Inner Solar System

In an attempt to overcome some of the deficiencies of existing meteoroid models, NASA’s Space Environments and Effects (SEE) Program sponsored a 3 year research effort at the University of Western Ontario. The resulting understanding of the sporadic meteoroid environment – particularly the nature and distribution of the sporadic sources – were then incorporated into a new Meteoroid Engineering Model (MEM) by members of the Space Environments Team at NASA’s Marshall Space Flight Center. This paper discusses some of the revolutionary aspects of MEM which include (a) identification of the sporadic radiants with real sources of meteoroids, such as comets, (b) a physics-based approach which yields accurate fluxes and directionality for interplanetary spacecraft anywhere from 0.2 to 2.0 astronomical units (AU), and (c) velocity distributions obtained from theory and validated against observation. Use of the model, which gives penetrating fluxes and average impact speeds on the surfaces of a cube-like structure, is also described along with its current limitations and plans for future improvements.     

The nature of the Tunguska meteorite

Abstract— Arguments in favor of the cometary origin of the Tunguska meteorite are adduced along with reasons against the asteroidal hypothesis. A critical analysis is given for the hypotheses by Sekanina (1983) and Chyba et al. (1993). On the basis of the azimuth and inclination of the trajectory of the Tunguska body with plausible values of the geocentric velocity, the semimajor axis of the orbit and its inclination to the ecliptic plane are calculated for this body. It is noted that the theory of the disintegration of large bodies in the atmosphere put forward by Chyba et al. (1993) is crude. Applying more accurate theories (Grigoryan, 1979; Hills and Goda, 1993) as well as taking into account the realistic shape of the body yield for the cometary body lower disruption heights than obtained by Chyba et al. Numerical simulations carried out by Svettsov et al. agree well with the cometary hypothesis and the analytical calculations based on Grigoryan's theory. The asteroidal hypothesis is shown not to be tenable: the complete lack of stony fragments in the region of the catastrophe, cosmochemical data (in particular, the results of an isotope analysis), and some other information contradict this hypothesis. It is shown that stony fragments that would have originated in the explosive disruption of the Tunguska body would not be vaporized by the radiation of the vapor cloud nor as a result of their fall to the Earth's surface.     

Excitation of plasma waves by bodies moving in ionized atmosphere

A body moving in an ionized atmosphere acquires an electric charge through the processes of accretion of charged particles and emission of electrons by high energy photons. The moving charged body may then interact with the charged particles of the atmosphere and any pervading magnetic field to excite plasma waves. Of particular interest is the situation in which the body collects an ionized cloud in front of it. The motion of this ionized cloud in the atmosphere induces an electrostatic instability and causes a column of ionized gas to move ahead of the body. The electrostatic instability is conducive to the excitation of electrostatic oscillations which if already present are further enhanced. A magnetic field along the direction of motion assists in the formation of the ionized cloud. If the pervading magnetic field is of suitable weak strength, it may excite extraordinary electromagnetic waves. A pervading transverse magnetic field of suitable strength may cause the excitation of magnetohydrodynamic waves.     

Meteoroid rotation and fireball flickering: a case study of the Innisfree fireball

Some 5 per cent of bright meteors show rapid, quasi-periodic brightness variations. It is argued that this effect, observationally known as flickering, is a manifestation of the rotational modulation of surface mass loss through ablation of a non-spherical meteoroid. We develop a set of time-dependent, single-body ablation equations that include the effect of cross-section area modulation. We present a discussion of the effects that the rotation of a non-spherical meteoroid has on the resultant meteor light curve, and we look in depth at the data related to the fireball associated with the fall of the Innisfree meteorite. We find that the parent object to the Innisfree meteorite was spinning at a rotation frequency of 2.5 Hz when it encountered the Earth's upper atmosphere. We also find that the Innisfree parent body had an initial mass of about 20 kg and that the ratio of its semiminor and semimajor axes was about 0.5.     

On the shape of meteor light curves, and a fully analytic solution to the equations of meteoroid ablation

The shape and characteristics (beginning and end heights, and height of maximum brightness) of meteor light curves are investigated under the constraint that the surface area S that a meteoroid presents to the oncoming air flow varies as a power law in the meteoroid mass m such that   S ∼ m ^α  . We investigate the meteoroid ablation for a range of values of α, and find that the  α= 1  condition allows for a fully analytic solution to the coupled differential equations of meteoroid ablation when the density profile is that of an isothermal atmosphere. The possible geometrical properties of Geminid meteoroids are discussed in terms of the  α= 1  ablation model and it is shown that they are consistent with being derived from an asteroidal, rather than cometary, parent body.     

Searching for Light Curve Evidence of Meteoroid Structure and Fragmentation

We used light curve analysis to search for evidence of the dustball meteoroid model. Leonid, Taurid, Alpha Monocerotid and sporadic meteors from November 2003 were observed and analyzed using uniform methodology. Meteors from these four sources were examined for evidence of fragmentation by examining light curve shape and searching for light curve irregularities. Differences in meteoroid structure should be reflected by differences in meteor light curves. The resulting meteor light curve F-parameter values showed no statistically significant differences between the meteors from the various cometary showers or the sporadic meteors. The F-parameter values also suggest that the meteoroids from these sources do not follow a single body ablation model, which suggests that all four sources produce meteoroids with a dustball structure.     

A novel approach to fireball modeling: The observable and the calculated

Estimating the mass of a meteoroid passing through the Earth's atmosphere is essential to determining potential meteorite fall positions. High‐resolution fireball images from dedicated camera networks provide the position and timing for fireball bright flight trajectories. There are two established mass determination methods: the photometric and the dynamic. A new approach is proposed, based on the dynamic method. A dynamic optimization initially constrains unknown meteoroid characteristics which are then used in a parametric model for an extended Kalman filter. The extended Kalman filter estimates the position, velocity, and mass of the meteoroid body throughout its flight, and quantitatively models uncertainties. Uncertainties have not previously been modeled so explicitly and are essential for determining fall distributions for potential meteorites. This two‐step method aims to automate the process of mass determination for application to any trajectory data set and has been applied to observations of the Bunburra Rockhole fireball. The new method naturally handles noisy raw data. Initial and terminal bright flight mass results are consistent with other works based on the established photometric method and cosmic ray analysis. A full analysis of fragmentation and the variability in the heat‐transfer coefficient will be explored in future versions of the model.     

Mostly Dormant Comets and their Disintegration into Meteoroid Streams: A Review

The history of associating meteor showers with asteroidal-looking objects is long, dating to before the 1983 discovery that 3200 Phaethon moves among the Geminids. Only since the more recent recognition that 2003 EH1 moves among the Quadrantids are we certain that dormant comets are associated with meteoroid streams. Since that time, many orphan streams have found parent bodies among the newly discovered Near Earth Objects. The seven established associations pertain mostly to showers in eccentric or highly inclined orbits. At least 35 other objects are tentatively linked to streams in less inclined orbits that are more difficult to distinguish from those of asteroids. There is mounting evidence that the streams originated from discrete breakup events, rather than long episodes of gradual water vapor outgassing. If all these associations can be confirmed, they represent a significant fraction of all dormant comets that are in near-Earth orbits, suggesting that dormant comets break at least as frequently as the lifetime of the streams. I find that most pertain to NEOs that have not yet fully decoupled from Jupiter. The picture that is emerging is one of rapid disintegration of comets after being captured by Jupiter, and consequently, that objects such as 3200 Phaethon most likely originated from among the most primitive asteroids in the main belt, instead. They too decay mostly by disintegration into comet fragments and meteoroid streams. The disintegration of dormant comets is likely the main source of our meteor showers and the main supply of dust to the zodiacal cloud. Editorial handling: Frans Rietmeijer.