In a remarkable 1750 publication, Thomas Wright of Durham speculated that all cometary orbits enclose the same physical area. Here we investigate Wright's intriguing claim, and while it is easily shown that no such universal area exists, we do find that there is some utility in exploring the relationship between orbital period and orbital area for periodic cometary bodies. Indeed, by considering orbital areas, a useful diagram can be constructed in which the distinction between Jupiter-family, Halley-type, and Centaur/Chiron-type objects is made clear to the eye.

We present a series of classical meteor light curve profiles based upon a set of simplified analytic atmospheric models. The model atmospheres specifically express the density variation as a power law in atmospheric height, and are derived under a variety of assumptions relating to the atmospheric temperature profile and the variation of the acceleration due to gravity. We find that the light curve profiles show only small differences with respect to any variation in the temperature profile and the geometry imposed upon the atmospheres.

Model predictions based upon the existence of a 7:2 mean-motion resonance with Jupiter suggest that in about 20% Of Taurid meteoroid-stream encounters enhanced numbers of meteors and fireballs should be seen. Here we examine the veracity of the predictions against a set of six temporally overlapping fireball surveys. It is found that in the time interval from 1962 to 2002, enhanced, or at least conspicuous, numbers of Taurid fireballs were recorded on all eight occasions predicted by the resonance-controlled, swarm-encounter model. We find evidence for elevated Taurid activity in 1974 and possibly in 1985 as well, indicative, perhaps, of additional stream structure.

Abstract— In the early morning hours of December 13, 2002, a bright Geminid fireball with an absolute magnitude of −9.2 ± 0.5 was observed from Southern Saskatchewan, Canada. The fireball displayed distinct small‐scale oscillations in brightness, or flickering, indicative of the parent meteoroid being both non‐spherical and rotating. Using the light curve derived from a calibrated radiometer, we determine a photometric mass of 0.429 ± 0.15 kg for the meteoroid, and we estimate from its initial rotation rate of some 6 Hz that the meteoroid was ejected from the parent body (3200) Phaethon some 2500 ± 500 years ago. We find that 70% of Geminid fireballs brighter than magnitude −3 display distinct flickering effects, a value that is in stark contrast to the 18% flickering rate exhibited by sporadic fireballs. The high coincidence of flickering and the deep atmospheric penetration of Geminid fireballs are suggestive of Geminid meteoroids having a highly resilient structure, a consequence, we suggest, of their having suffered a high degree of thermal processing. The possibility of Gemind material surviving atmospheric ablation and being sampled is briefly discussed, but the likelihood of collecting and identifying any such material is admittedly very small.

An analytic model of dustball meteoroid ablation is developed and used to generate synthesized Leonid meteor light curves. The model light curves are compared against observational data collected during the 1998, 1999, 2000 and 2001 Leonid outbursts. A power- law distribution of the form m(-alpha) is assumed for the fundamental grain mass distribution, and we find that alpha = 1.6 +/- 0.1 provides a good description to the typical Leonid meteor light- curve morphology, although the range 1.0 less than or equal to alpha less than or equal to 2.0 is required to explain the entire gamut of observed light curves. We find an interesting discordance between the light- curve morphologies derived for the 1998 and 1999 Leonid returns; the former light curves being best described by 1.6, the latter having 1.0 and being noticeably more rotund in shape. We suggest that the 1999 Leonid meteoroids were relatively rich in larger- mass fundamental grains. Since, however, both the 1998 and 1999 returns were composed of material ejected from comet 55P/ Tempel- Tuttle in 1899, it is suggested that some form of orbital 'sifting', based upon meteoroid structure, has occurred. In addition, we find evidence for the existence of dustball meteoroids that are much richer in larger- mass grains than a simple power- law model would predict. Such dustballs may be clustered assemblages produced by accretion in the near- cometary nucleus environment. We find no clear correlation between Leonid meteor light- curve morphology and streamlet age, indicating that fragmentation, 'weathering' and thermal cycling effects are apparently not important for modifying Leonid meteoroid structure on time- scales of the order of at least several hundreds of years.

Interpreting fireball flickering in terms of the rotational modulation of the ablation process, the time since ejection into space of three Geminid meteoroids is determined. By estimating the time required to spin-up a meteoroid through non-isotropic photon scattering interactions with the solar radiation field, we find meteoroid ages consistent with ejection times some 1000-4000 yr ago. There appears to be some indication that the stream formation process lasted for at least similar to1000 yr. We also estimate the rotational bursting conditions for the three Geminid meteoroids and find tensile strengths of some 3 x 10(5) Pa. We interpret our results as being supportive of the argument that the parent body to the Geminid stream, (3200) Phaethon, is an aged cometary nucleus.

The remarkable fact about the Mazapil meteorite is that it fell on the same night, in 1885, that the Andromedid meteor shower under-went a spectacular outburst. The simultaneity of these two events has driven speculation ever since. From similar to1886 to similar to1950 the circumstances of the Mazapil fall were taken, by a number of researchers, as the paradigm that demonstrated the fact that comets were actually swarms of meteoritic boulders. Beginning similar to1950, however, most researchers began to adopted the stance that the timing of the Mazapil fall was nothing more than pure coincidence. The reason behind this change in interpretation stemmed from, amongst other factors, the fact that none of the prominent annual meteor showers could be clearly shown to deliver meteorites. Also, with the introduction of the icy-conglomerate model for cometary nuclei, by F. Whipple in the early 1950s, it became increasingly clear that only exceptional circumstances would allow for the presence of large meteoritic bodies in cometary streams. Further, by the mid 1960s it had been shown that meteorites could, in fact, be delivered to the Earth from the main belt asteroid region via gravitational resonances. With the removal of the dynamical "barrier" against the delivery of meteorites from the asteroid region, the idea that the Mazapil meteorite could have been part of the Andromedid stream fell into complete disfavor. This being said, we nonetheless present the results of a study concerning the possible properties of the parent object to the Mazapil meteorite based upon the assumption that it was a member of the Andromedid stream. This study is presented to illustrate the point that while cometary showers do not yield meteorites on the ground, this does not, in fact, substantiate the argument that no meteoritic bodies reside in cometary streams. Indeed, we find no good reason to suppose that an object with the characteristics of the Mazapil meteorite could not have been delivered from the Andromedid stream. However, we argue that upon the basis of the actual reported observations and upon the scientific maxim of minimized hypothesis and least assumption it must be concluded that the timing of the fall of the Mazapil meteorite and the occurrence of the Andromedid outburst were purely coincidental.

We investigate the conditions under which Leonid meteoroids might generate short duration (burster) electrophonic sounds. A "first order" theory is employed to estimate the approximate electron number density in the meteoroid ablation column as a function of time. Using the threshold conditions discussed in an earlier communication (Beech & Foschini 1999) we find that Leonid meteoroids more massive than about 0.1 kg can potentially generate short duration electrophonic bursters.

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.

It is argued that Comet 72P/Denning-Fujikawa is an old and intermittently active comet evolving, at least observationally, towards a transitional minor planet status. We have studied the fate of hypothetical meteoroids ejected from the comet during its two known periods of activity (1881 and 1978). A complex history of orbital evolution is found. Meteoroids ejected in 1881 first become Earth-orbit-crossing in 1960, while meteoroids ejected in 1978 appear to hold stable, non-Earth-orbit-crossing orbits until at least 2110. If copious amounts of meteoroids were ejected in 1881 we find some indication that the Earth may encounter a populous, coherent subgroup, or 'streamlet', of them in 2009 and 2010, leading to the possibility of outburst activity in those years. We have investigated the possibility that the activity of Comet 72P/Denning-Fujikawa, over the past similar to 200 years, has been governed by impacts suffered by the comet as it moves through the main-belt asteroid region. While encounters with centimetre-sized objects will take place each time the comet orbits the Sun, the likelihood of the comet encountering a large metre-sized asteroid is essentially zero on the time-scales considered. The outburst activity of the comet may be impact-modulated in the sense that small-object impacts might trigger the explosive release of gases trapped in subsurface cavities.

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 arg 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 took 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.

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 Earth's 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.

The sounds accompanying electrophonic burster meteors are characteristically described as being akin to short duration pops and staccato-like clicks. As a phenomenon distinct from the enduring electrophonic sounds that occasion- ally accompany the passage and ablation of large meteoroids in the Earth's lower atmosphere, the bursters have proved stub- bornly difficult to explain. A straightforward calculation demon- strates that in contradistinction to the enduring electrophonic sounds, the electrophonic bursters are not generated as a conse- quence of interactions between the meteoroid ablation plasma and the Earth's geomagnetic field. Here we present a novel and hitherto unrecorded model for the generation of short-duration pulses in an observer's local electrostatic field. Our model is developed according to the generation of a strong electric field across a shock wave propagating in a plasma. In this sense, the electrophonic bursters are associated with the catastrophic disruption of large meteoroids in the Earth's atmosphere. We de- velop an equation for the description of the electric field strength in terms of the electron temperature and the electron volume density. Also, by linking the electron line density to a meteor's absolute visual magnitude, we obtain a lower limit to the visual magnitude of electrophonic burster meteors of Mv 6:6 ,i n good agreement with the available observations.

The outburst of the Lyrid meteor shower in 1803 was remarkable for being rich in bright fireballs and the generation of electrophonic sounds. The implications implicit to the detection of electrophonic sounds are studied in this paper. We present estimates for the minimum-sized Lyrid meteoroid capable of generating electrophonic sounds, and compare these lower limits with the largest meteoroid that might reasonably be ejected from a cometary nucleus through coupling with the sublimation gas outflow. A difference of a factor of order 30 is found between the two limiting sizes. A minimum diameter of order Im is required for a Lyrid meteoroid to satisfy the conditions necessary for generating electrophonic sounds. The mechanisms responsible for the placement of large, metre-sized meteoroids into the Lyrid stream are not well defined, but they possibly relate to surface ageing effects of the parent comet, Comet C/1861 G1 Thatcher, and to a history of nuclear fragmentation.

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 x 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.

The stream meteoroid impact probability for space platforms is reviewed and found to be very low under normal circumstances. While the literature contains numerous accounts of spacecraft apparently suffering damage and/or interference during meteoroid stream encounters, we find that there is, in fact, very little evidence to support such claims. This conclusion may not be valid, however, during meteor storms, when the flux of visual meteors can increase by factors in excess of 103 to 104 of that from the sporadic background. Special attention is directed towards the Leonid meteor storms of 1965 and 1966—the only meteor storms since the dawn of the space age. The space platform impact probabilities during the 1966 storm were small but none negligible, being of order 1% for an exposed surface area of 2m2 at a limiting meteoroid mass of 10−7 g (and assuming a stream mass index s=2.0). The circumstances surrounding the possible encounters of the Pegasus II and III, and Mariner 4 spacecraft with Leonid stream meteoroids are discussed in some detail. While the 1966 Leonid meteor storm is the strongest on record (in the sense of the highest visual meteor rates) no apparent meteoroid inflicted damage to a spacecraft can be unambiguously linked to it. This result is mostly a consequence of the small number and small size of spacecraft in Earth-orbit at the time of the 1966 storm.

The activity of the 1996 Leonid shower from two radars, global visual and single-station low-light-level TV (LLTV) observations is presented and summarized. Radar observations from Ondrejov in the Czech. Republic indicate a peak rate of (>+1) Leonids near lambda. = 235.degrees 2+/-0.1 (Equinox 2000). As observed by this radar, this peak interval was characterized by a significant increase in the number of bright Leonids as demonstrated by a noticeable lowering of the mass index near the peak. From radar observations in Ontario, Canada (using the CLOVAR system), a raw peak flux of 1.3+/-0.3x10(-2) meteoroid km(-2) h(-1) brighter than radio magnitude +7.7 was reached at lambda. = 235.degrees 3+/-0.1, uncorrected for initial train radius effects. Single-station LLTV observations suggest a peak shower flux of 1.8+/-0.4x10(-2) meteoroid km(-2) h(-1) brighter than absolute magnitude +5+/-0.5 between 235.degrees 3 and 235.degrees 39. The position of the radiant on the night of maximum of the shower is found to be alpha=152.degrees 9+/-1 degrees.0 and delta=22 degrees.1+/-1.degrees 0 from CLOVAR observations and alpha=153.degrees 3+/-1.degrees 7 and delta=22.degrees 1+/-1.degrees 7 from LLTV observations, Visual observations of the shower yield a peak zenithal hourly rate (ZHR) of 86+/-22 at 235.degrees 17+/-0.degrees 07 or an equivalent flux of 1.2+/-0.4x10(-2) meteoroid km(-2) h(-1) brighter than absolute visual magnitude +6.5. The visual peak was short-lived (1.5+/-0.5 h HWHM) and richer in fainter meteors than neighbouring intervals. Discrepancies in the estimated absolute Leonid flux found using differing methods are noted and possible reasons for the differences discussed. The stream in 1996 showed two distinct meteoroid populations: a population of recently ejected meteoroids rich in smaller particles near 235.degrees 17 which is very narrow in nodal extent (HWHM 0.degrees 07+/-0.degrees 02), and an older component (of order IO revolutions in age) peaking near 235.degrees 4 which is rich in larger stream meteoroids, of long duration (FWHM 1.degrees 2+/-0.degrees 4), which contributed most to the total mass flux at Earth from the stream in 1996.

This study is motivated by the possibility of determining the large-body meteoroid flux at the orbit of Venus. Towards this end, we attempt to estimate the times at which enhanced meteoric activity might be observed in the planet's atmosphere. While a number of meteoroid streams are identified as satisfying common Earth and Venus intercept conditions, it is not clear from the Earth-observed data if these streams contain large-body meteoroids. A subset of the Taurid Complex objects may produce fireball-rich meteor showers on Venus. A total of 11 short-period, periodic comets and 46 near-Earth asteroids approach the orbit of Venus to within 0.1 au, and these objects may have associated meteoroid streams. Comets 27P/Crommelin and 7P/Pons—Winnecke are identified as candidate parents to possible periodic meteor showers at the orbit of Venus.