Determination of the meteoroid velocity distribution at the Earth using high-gain radar

Plasma formed in the immediate vicinity of a meteoroid as it descends through Earth's atmosphere enables high-gain radars such as those found at Kwajalein, Arecibo, and Jicamarca to detect ablating meteoroids. In the work presented here, we show that these head echo measurements preferentially detect more energetic meteoroids over less energetic ones and present a method of estimating the effects of this bias when measuring the velocity distributions. To do this, we apply ablation and ionization models to estimate a meteoroid's plasma production rate based on its initial kinetic energy and ionization efficiency. This analysis demonstrates that, almost regardless of the assumptions made, high-gain radars will preferentially detect faster and more massive meteoroids. Following the model used by Taylor (1995, Icarus 116, 154-158), we estimate the biases and then apply them to observed meteoroid velocity distributions. We apply this technique to observations of the North Apex meteoroid source made by the Advanced Research Project Agency Long Range Tracking and Instrumentation Radar (ALTAIR) at two frequencies (160 and 422 MHz) and compare results from the Harvard Radio Meteor Project (HRMP) at High Frequency (HF, 40.9 MHz). Both studies observe a peak in the distribution of North Apex meteoroids at approximately 56 km s⁻¹. After correcting for biases using Taylor's method, the results suggest that the mass-weighted peak of the distribution lies near 20 km s⁻¹ for both studies. We attribute these similarities to the fact that both radar systems depend upon similar ablation and ionization processes and thus have a common mass scale.     

Deconvoluting measurement uncertainty from the meteor speed distribution

Debiasing the velocity distribution of meteors observed by the Canadian Meteor Orbit Radar (CMOR) yields a distribution with large numbers of slow meteors. The distribution also contains significant numbers of hyperbolic meteors, in conflict with the expectation that interstellar meteors should be rare. In Moorhead et al. (2017a), we noted that measurement uncertainties were possibly smoothing the speed distribution and redistributing meteors to the extreme ends of the speed distribution. In this report, we use techniques analogous to image sharpening to remove the blurring caused by measurement uncertainties. The deconvolved speed distribution appears to have no meteors slower than 14 km s⁻¹ and none faster than 74 km s⁻¹. The result is to substantially raise the characteristic velocity of incoming meteoroids from 12.9 to 20.0 km s⁻¹.     

Velocity distribution of meteoroids in the vicinity of planets and satellites

Collisions in the Solar System play an important role in its history. Impact processes depend essentially on the velocity distribution of meteoroids colliding with a chosen planet. According to Carleman's theorem it is sufficient to find the set of M _k = mathematical expectation of v ^k , v being the collisional velocity. We suppose that M _k for meteoroids of asteroidal nature differs slightly from that for asteroids themselves. So among all numbered minor planets we select those which may potentially collide with the chosen major planet. Then we calculate v at intersection points and count the average over all such points and all selected asteroids. The gravitation of a body-target may be taken into account or not. Numerical results are collected in four Tables.St.Petersburg University     

Meteorites from meteor showers: A case study of the Taurids

We propose that the Taurid meteor shower may contain bodies able to survive and be recovered as meteorites. We review the expected properties of meteorite‐producing fireballs, and suggest that end heights below 35 km and terminal speeds below 10 km s^?1 are necessary conditions for fireballs expected to produce meteorites. Applying the meteoroid strength index (PE criteria) of Ceplecha and McCrosky (1976) to a suite of 33 photographically recorded Taurid fireballs, we find a large spread in the apparent meteoroid strengths within the stream, including some very strong meteoroids. We also examine in detail the flight behavior of a Taurid fireball (SOMN 101031) and show that it has the potential to be a (small) meteorite‐producing event. Similarly, photographic observations of a bright, potential Taurid fireball recorded in November of 1995 in Spain show that it also had meteorite‐producing characteristics, despite a very high entry velocity (33 km s^?1). Finally, we note that the recent Maribo meteorite fall may have had a very high entry velocity (28 km s^?1), further suggesting that survival of meteorites at Taurid‐like velocities is possible. Application of a numerical entry model also shows plausible survival of meteorites at Taurid‐like velocities, provided the initial meteoroids are fairly strong and large, both of which are characteristics found in the Taurid stream.     

Velocity distribution of meteoroids in the vicinity of planets and satellites

Collisions in the Solar System play an important role in its history. Impact processes depend essentially on the velocity distribution of meteoroids colliding with a chosen planet. According to Carleman's theorem it is sufficient to find the set of M _k = mathematical expectation of v ^k , v being the collisional velocity. We suppose that M _k for meteoroids of asteroidal nature differs slightly from that for asteroids themselves. So among all numbered minor planets we select those which may potentially collide with the chosen major planet. Then we calculate v at intersection points and count the average over all such points and all selected asteroids. The gravitation of a body-target may be taken into account or not. Numerical results are collected in four Tables.     

Albedos and size distribution of meteoroids from 0.3 to 4.8 AU

Comparison is made between the run of number density of meteoroids from penetration detectors aboard Helios A (masses below 10^?8 g) and Pioneer 10 (masses near and above 3 × 10^?9 g), the source function of the zodiacal light deduced from photometric observations aboard Helios A and Pioneer 10, counts versus brightness of objects passing by Pioneer 10 from the Sisyphus experiment and the distribution of meteoroids deduced from radar and optical meteors at the Earth. The Sisyphus experiment on Pioneer 10 observed reflecting glints on meteoroids rather than the meteoroids themselves and the counting statistics refer not to the effective radii of the meteoroids but to the effective radii of curvature of the reflecting glints on the meteoroids. The penetration detectors appear to find some increase in number density toward the Sun and a flat distribution outward to 5.2 AU. The overall behavior of the zodiacal light is that the relative distribution over direction is unchanged while the source scattering function diminishes as the inverse 1.4 power of distance from the Sun. The fit to the brightness of the zodiacal light obtained from these statistics can be combined with the mass distribution results from the optical meteors to deduce a mean geometric albedo of meteoroids of 0.006 at 1 AU from the Sun. Combination of the space distribution from radar meteors with the scattering source function of the zodiacal light yields geometric albedos for meteoroids running from 0.07 at 0.1 AU, from the Sun through 0.006 at 1 AU down to about 0.0001 at 3.3 AU which may run flat thence outward. This result is imposed by the indicated modest increase in density of meteoroids very near the Sun, a minimum between the Sun and the Earth near 0.4 AU and rising density outward to somewhere beyond 3.3 AU which is very different from the inverse 1.4 power of the distance shown for scatterers (product of number density and albedo) by the zodiacal light. A check on the distribution at very large sizes is possible if a search is made for fireballs in Jupiter's atmosphere by the Mariner Jupiter Saturn 1977 television cameras during the two encounters with Jupiter in 1979. An easy detection of such activity would put the maximum in the meteoroid distribution out near Jupiter and lend further confirmation to the indicated drop in albedo.     

Collisional balance of the meteoritic complex

Taking into account meteoroid measurements by in situ experiments, zodiacal light observations, and oblique angle hypervelocity impact studies, it is found that the observed size distributions of lunar microcraters usually do not represent the interplanetary meteoroid flux for particles with masses ?10^?10g. From the steepest observed lunar crater size distribution a “lunar flux” is derived which is up to 2 orders of magnitude higher than the interplanetary flux at the smallest particle masses. New models of the “lunar” and “interplanetary” meteoroid fluxes are presented. The spatial mass density of interplanetary meteoritic material at 1 AU is ～10^?16g/m³. A large fraction of this mass is in particles of 10^?6 to 10^?4 g. A detailed analysis of the effects of mutual collisions (i.e., destruction of meteoroids and production of fragment particles) and of radiation pressure has been performed which yielded a new picture of the balance of the meteoritic complex. It has been found that the collisional lifetime at 1 AU is shortest (～10⁴years) for meteoroids of 10^?4 to 1 g mass. For particles with masses m > 10^?5g, Poynting-Robertson lifetimes are considerably larger than collisional lifetimes. The collisional destruction rate of meteoroids with masses $\text{m ? 10}{}^{\mathrm{?3}}\text{g}$ is about 10 times larger than the rate of collisional production of fragment particles in the same mass range. About 9 tons/sec of these “meteor-sized” (m > 10^?5g) particles are lost inside 1 AU due to collisions and have to be replenished by other sources, e.g., comets. Under steady-state conditions, most of these large particles are “young”; i.e., they have not been fragmented by collisions and their initial orbits are not altered much by radiation pressure drag. Many more micrometeoroids of masses m ? 10^?5g are generated by collisions from more massive particles than are destroyed by collisions. The net collisional production rate of intermediate-sized particles 10^?10g ? m ? 10^?5g is found to be about 16 times larger at 1 AU than the Poynting-Robertson loss rate. The total Poynting-Robertson loss rate inside 1 AU is only about 0.26 tons/sec. The smallest fragment particles (m ? 10^?10g) will be largely injected into hyperbolic trajectories under the influence of radiation pressure (β meteoroids). These particles provide the most effecient loss mechanism from the meteoritic complex. When it is assumed that meteoroids fragment similarly to experimental impact studies with basalt, then it is found that interplanetary meteoroids in the mass range 10^?10g ? m ? 10^?5g cannot be in temporal balance under collisions and Poynting-Robertson drag but their spatial density is presently increasing with time.     

Image‐intensified video results from the 1998 Leonid shower: I. Atmospheric trajectories and physical structure

Abstract— Two‐station electro‐optical observations of the 1998 Leonid shower are presented. Precise heights and light curves were obtained for 79 Leonid meteors that ranged in brightness (at maximum luminosity) from +0.3 to +6.1 astronomical magnitude. The mean photometric mass of the data sample was 1.4 × 10^?6 kg. The dependence of astronomical magnitude at peak luminosity on photometric mass and zenith angle was consistent with earlier studies of faint sporadic meteors. For example, a Leonid meteoroid with a photometric mass of ～1.0 × 10‐⁷ kg corresponds to a peak meteor luminosity of about +4.5 astronomical magnitudes. The mean beginning height of the Leonid meteors in this sample was 112.6 km and the mean ending height was 95.3 km. The highest beginning height observed was 144.3 km. There is relatively little dependence of either the first or last heights on mass, which is indicative of meteoroids that have clustered into constituent grains prior to the onset of intensive grain ablation. The height distribution, combined with numerical modelling of the ablation of the meteoroids, suggests that silicate‐like materials are not the principal component of Leonid meteoroids and hints at the presence of a more volatile component. Light curves of many Leonid meteors were examined for evidence of the physical structure of the associated meteoroids: similar to the 1997 Leonid meteors, the narrow, nearly symmetric curves imply that the meteoroids are not solid objects. The light curves are consistent with a dustball structure.     

A search for interstellar meteoroids using the Canadian Meteor Orbit Radar (CMOR)

Using the CMOR system, a search was conducted through 2.5 years (more than 1.5 million orbits) of archived data for meteoroids having unbound hyperbolic orbits around the Sun. Making use of the fact that each echo has an individually measured error, we were able to apply a cut-off for heliocentric speeds both more than two, and three standard deviations above the parabolic limit as our main selection criterion. CMOR has a minimum detectable particle radius near 100 μm for interstellar meteoroids. While these sizes are much larger than reported by the radar detections of extrasolar meteoroids by AMOR or Arecibo, the interstellar meteoroid population at these sizes would be of great astrophysical interest as such particles are more likely to remain unperturbed by external forces found in the interstellar medium, and thus, more likely to be traceable to their original source regions. It was found that a lower limit of approximately 0.0008% of the echoes (for the 3σ case) were of possible interstellar origin. For our effective limiting mass of 1×10⁻⁸ kg, this represents a flux of meteoroids arriving at the Earth of 6×10⁻⁶ meteoroids/km²/h. For our 2σ results, the lower limit was 0.003%, with a flux of 2×10⁻⁵ meteoroids/km²/h. The total number of events was too low to be statistically meaningful in determining any temporal or directional variations.     

Mechanisms accounting for variations in the proportions of carbonaceous and ordinary chondrites in different mass ranges

The number ratio of carbonaceous to ordinary chondrites (the CC/OC ratio) varies with mass. It is very high (≳90) in small mass ranges (10⁻⁸ to 10⁻¹² kg) among interplanetary dust particles and micrometeorites; it is moderately high (~5 to 30) for 1 to 10 m size fireball meteoroids (with estimated masses between ~10³ and ~10⁶ kg). In the range of most normal-sized meteorite falls (0.01–20 kg), the ratio is low (0.04–0.05); the ratio increases at greater mass ranges: at ≥200 kg, the ratio is 0.09; at ≥500 kg, the ratio is 0.20. The CC/OC ratio also increases from 0.05 to 0.16 for small meteorite finds (10⁻³ to 10⁻⁴ kg). High CC/OC ratios at low and high mass ranges are due to the predominance of CC material in the outer solar system. Small particles from this region spiral into the inner solar system typically in ≤10⁶ years due to Poynting–Robertson drag. Meter-sized meteoroids in this region are affected by Yarkovsky forces, pushing them into resonances where they are efficiently transferred to the inner solar system. Normal-sized meteorites are derived from centimeter-to-decimeter-sized meteoroids that have sluggish drift rates (i.e., they are less affected by the seasonal Yarkovsky effect) compared to larger bodies. Consequently, the centimeter-to-decimeter-sized meteoroids spend more time in interplanetary space (where they are subject to collisions) than larger objects. The greater friability of carbonaceous chondrites relative to ordinary chondrites tends to winnow the carbonaceous chondrites out in this size/mass range during their long interplanetary sojourn, thereby decreasing the CC/OC ratio.     

Release of neutral sodium atoms from the surface of Mercury induced by meteoroid impacts

Meteoroid impact has been shown to be a source of sodium, and most likely of other elements, on the Moon. The same process could be also relevant for Mercury. In this work we calculate the vapor and neutral Na production rates on Mercury due to the impacts of meteoroids in the radius range of 10⁻⁸-10⁻¹ m. We limit our calculations to this size range, because meteoroids with radius larger than 10⁻¹ m have not to be found important for the daily production of the exosphere. This work is based on a new dynamical model of the meteoroid flux at the heliocentric distance of Mercury, regarding objects in the size range 10⁻²-10⁻¹ m. This size range, never investigated before, is not affected by nongravitational forces, such as the Poynting-Robertson effect, which is dominant for particles smaller than 10⁻² m. In order to evaluate the release of neutral sodium atoms also for smaller meteoroids we have used the distribution reported by M.J. Cintala [1992. Impact-induced thermal effects in the lunar and mercurian regoliths. J. Geophys. Res. 97, 947-973] calculated for particle size range 10⁻⁸-10⁻³ m. We have extrapolated this distribution up to 10⁻² m and we have based the impact calculations on a new surface composition assuming 90% plagioclase and 10% pyroxene. The results of our model are that (i) the total mass of vapor produced by the impact of meteoroids in the size range 10⁻⁸-10⁻¹ m is 4.752×¹⁰⁸ g per year, and (ii) the production rate of neutral sodium atoms is 1.5×¹⁰²² s⁻¹.     

An explanation for lapetus' asymmetric reflectance

Cook and Franklin (1970, Icarus 13, 282) consider Iapetus originally to have been coated with about a meter of ice. They suggest that Iapetus' orbital velocity about Saturn has caused an asymmetric erosion of this ice layer which has now nearly laid bare its “leading” hemisphere, but not as yet the entire “trailing” hemisphere. Rather than an erosion process which operates more actively on the leading side, this paper considers an ice deposition mechanism operating more actively on the trailing side. The two main assumptions used are (1) that there are more icy than rocky meteoroids in Saturn's environment, and (2) that some portion of each icy meteoroid will stick to a surface at collision velocities less than 2.4kmsec^?1, but will completely vaporize itself at greater velocities. A meteoroid can have the minimum collision velocity of about 1.7kmsec^?1 with Iapetus only if their velocity vectors are nearly parallel, and under these conditions such collisions would tend to be with the trailing hemisphere. Collisions with the leading hemisphere will tend to be at a much higher velocity.     

A meteoroid stream survey using the Canadian Meteor Orbit Radar: I. Methodology and radiant catalogue

Using a meteor orbit radar, a total of more than 2.5 million meteoroids with masses ∼10⁻⁷ kg have had orbits measured in the interval 2002-2006. From these data, a total of 45 meteoroid streams have been identified using a wavelet transform approach to isolate enhancements in radiant density in geocentric coordinates. Of the recorded streams, 12 are previously unreported or unrecognized. The survey finds >90% of all meteoroids at this size range are part of the sporadic meteoroid background. A large fraction of the radar detected streams have q<0.15 AU suggestive of a strong contribution from sungrazing comets to the meteoroid stream population currently intersecting the Earth. We find a remarkably long period of activity for the Taurid shower (almost half the year as a clearly definable radiant) and several streams notable for a high proportion of small meteoroids only, among these a strong new shower in January at the time of the Quadrantids (January Leonids). A new shower (Epsilon Perseids) has also been identified with orbital elements almost identical to Comet 96P/Machholz.     

A dynamical model of the sporadic meteoroid complex

Sporadic meteoroids are the most abundant yet least understood component of the Earth's meteoroid complex. This paper aims to build a physics-based model of this complex calibrated with five years of radar observations. The model of the sporadic meteoroid complex presented here includes the effects of the Sun and all eight planets, radiation forces and collisions. The model uses the observed meteor patrol radar strengths of the sporadic meteors to solve for the dust production rates of the populations of comets modeled, as well as the mass index. The model can explain some of the differences between the meteor velocity distributions seen by transverse versus radial scatter radars. The different ionization limits of the two techniques result in their looking at different populations with different velocity distributions. Radial scatter radars see primarily meteors from 55P/Tempel-Tuttle (or an orbitally similar lost comet), while transverse scatter radars are dominated by larger meteoroids from the Jupiter-family comets. In fact, our results suggest that the sporadic complex is better understood as originating from a small number of comets which transfer material to near-Earth space quite efficiently, rather than as a product of the cometary population as a whole. The model also sheds light on variations in the mass index reported by different radars, revealing it to be a result of their sampling different portions of the meteoroid population. In addition, we find that a mass index of s=2.34 as observed at Earth requires a shallower index (s=2.2) at the time of meteoroid production because of size-dependent processes in the evolution of meteoroids. The model also reveals the origin of the 55° radius ring seen centered on the Earth's apex (a result of high-inclination meteoroids undergoing Kozai oscillation) and the central condensations seen in the apex sources, as well as providing insight into the strength asymmetry of the helion and anti-helion sources.     

Frequency of hyperbolic and interstellar meteoroids

Hyperbolic meteor orbits from the catalog of 64,650 meteors observed by the multistation video meteor network located in Japan (SonotaCo 2009) have been investigated with the aim of determining the relation between the frequency of hyperbolic and interstellar meteors. The proportion of hyperbolic meteors in the data decreased significantly (from 11.58% to 3.28%) after a selection of quality orbits, which shows its dependence on the quality of observations. Initially, the hyperbolic orbits were searched for meteors unbound due to planetary perturbation. It was determined that 22 meteors from the 7489 hyperbolic orbits in the catalog (and 2 from the selection of the orbits with the highest quality) had had a close encounter with a planet, none of which, however, produced essential changes in their orbits. Similarly, the fraction of hyperbolic orbits in the data, which could be hyperbolic by reason of a meteor's interstellar origin, was determined to be at most 3.9 × 10^?2. From the statistical point of view, the vast majority of hyperbolic meteors in the database have definitely been caused by inaccuracy in the velocity determination. This fact does not necessarily assume great measurement errors, since, especially near the parabolic limit, a small error in the value of the heliocentric velocity of a meteor can create an artificial hyperbolic orbit that does not really exist. The results show that the remaining 96% of meteoroids with apparent hyperbolic orbits belong to the solar system meteoroid population. This is also supported by their high abundance (about 50%) among the meteor showers.     

OPTICAL PREDICTIONS FOR HIGH GEOCENTRIC VELOCITY METEORS

In this study we numerically modelled the atmospheric ablation and luminosity of cometary structure meteoroids with geocentric velocities from 71 to 200 km/s. We considered meteoroid masses ranging from 10⁻¹³ to 10⁻⁶ kg. Expected heights of ablation and maximum luminosity absolute magnitudes are determined. Height and trail length values are used to calculate the angle traversed in a single video frame. It is found that for pre-atmospheric meteoroid masses of greater than 10⁻⁸ kg, high geocentric velocity meteors should be detectable with current electro-optical technology if properly optimised.     

Cosmogenic radionuclides and noble gases in Antarctic H chondrites with high and normal natural thermoluminescence levels

Abstract— We report noble gas data for 37 H chondrites collected from the Allan Hills by EUROMET in the 1988–1989 field season. Among these are 16 specimens with high levels (>100 krad) of natural thermoluminescence (NTL), originally interpreted as signaling their derivation from a single meteoroid with an orbit that became Earth‐crossin‐100 ka ago. One of these 16 is an H3 chondrite with a cosmic‐ray exposure age of ～33 Ma and clearly represents a separate fall. The other 15 H4–6 chondrites derive from three separate meteoroids, each of which is represented by a five or six member group. These groups have mean exposure ages of 3.7, 4.1, and 6.6 Ma: the middle‐group members all contain solar Ne. The two younger groups also seem to each include a few H chondrites with normal NTL levels. Measurements of cosmogenic ¹⁰Be (1.5 Ma), ²⁶AI (710 ka), and ³⁶CI (301 ka) in 14 of the high‐NTL chondrites indicate that all reflect a simple irradiation history. In contrast, many of a different (38 member) randomly selected suite of Antarctic H chondrites seem to have different cosmic‐ray irradiation histories. The 3.7 and 6.6 Ma groups from the 37 member Allan Hills suite come from about 5–30 and about 5–10 cm depths in 80–125 and 60–125 cm radius meteoroids, respectively.     

The effects of meteoroid stream enhanced activity on human space flight: an overview

The origin, dispersion mechanisms and evolution of particle streams producing enhanced activity (outburst or storm) of meteoroids are discussed in relation to their effects on artificial satellites and space platforms. A review of the active meteoroids suggests that at least five streams may undergo outburst or storm activity in the next few years. Modern radio techniques not affected by illumination conditions and cloud coverage, improve significantly the detectability of meteor streams. The impact probabilities of storm meteoroids on space platforms in Earth orbit can increase by factors in excess of 10²–10⁴ over the sporadic background.     

Neutral sodium atoms release from the surfaces of the Moon and Mercury induced by meteoroid impacts

In this work, we calculate the neutral Na production rates on the Moon and Mercury, as due to the impacts of meteoroids having an impact probability on the surface that can influence the daily observations of the exosphere: the meteoroids radius range considered for the Moon and Mercury are 10⁻⁸–0.15 and 10⁻⁸–0.10 m, respectively. We also estimate the mass of meteoroids that has impacted the surfaces of the Moon and Mercury in the last 3.8 Gy (after the end of the Late Heavy Bombardment).The results of our model are that (i) the Na production rates are ∼(3–4.9)×10⁴ and ∼(1.8–2.3)×10⁶ atoms cm⁻² s⁻¹, for Moon and Mercury, respectively, and (ii) in the last 3.8 Gy, the mass of meteoroids that has impacted the whole surface of the Moon and Mercury has been 8.86×10¹⁸ and 2.66×10¹⁹ g, respectively.     

A search for large meteoroids in the Perseid stream

Abstract— We report on two surveys conducted during the times of Perseid shower maximum in 1997 and 1998. The first survey entailed the video monitoring of the Moon's disk with the intent of recording the optical flashes that should result when large meteoroids strike the lunar surface. The second survey consisted of a combination video camera and very low frequency (VLF) radiowave receiver system capable of detecting electrophonic meteors during their ablation in the Earth's atmosphere. Using standard ablation theory, we find that for a Perseid meteoroid to be capable of generating electrophonic sounds, it must have an initial mass in excess of 495 kg. We also find, as a result of the surveys, an upper limit of 2 × 10^?17 m^?2 s^?1 to the flux of electrophonic Perseid meteors entering the Earth's atmosphere. Although our study indicates that large, meter-sized meteoroids must, at best, be sparsely distributed within the Perseid stream, we briefly discuss some tantalizing lines of evidence, found from within the astronomical literature, that hint at their true existence.