Dual-station image intensified television studies have indicated very few meteors at heights greater than 120 km, and few statistically significant hyperbolic orbits. However, the optimum intersection height for these studies was about 95 km, and the relatively small fields of view resulted in a bias against high (and therefore fast) meteors. We have developed height sensitivity correction factors, and found that short baseline television studies resulted in relatively little bias against high meteors, and the absence of meteors above 120 km appears to be real. We report preliminary results from a three-station, image-intensified video meteor detection system sensitive to apparent magnitude about +9.5 with optimum intersection heights 115-125 km. We have detected neither particularly high meteors nor meteors in clearly hyperbolic orbits. We conclude that the proportion of true hyperbolic meteors in the mass range 10-4 to 10-6 kg is less than a few percent, and that optical meteors corresponding to meteoroids in this mass range do not ablate at heights above approximately 120 km. We suggest several ways to reconcile these results with southern hemisphere radar studies.

The existence of meteoroid streams is indicated by the regular appearance of coherent meteor activity at specified times during the year. Since it is the interaction of the meteoroid with the atmosphere that is detected, the meteoroid has to be greater than about 100 micrometers in radius. Observation of these interactions gives information on individual meteoroids as well as collective phenomena. It is generally agreed that streams form through the ejection of dust particles from the surfaces of comets and asteroids at speeds considerably lower than the orbital speed. The subsequent motion of these particles is affected by gravitational perturbations from the planets and the effects of solar radiation forces. This review is intended to present an overview of the development of the subject and of our current state of knowledge.

The orbital evolution of two modelled ‘Perseid’ meteoroid streams is investigated using direct numerical integration techniques. We conclude that, in the absence of significant meteoroid velocity determination errors, the observed meteoroid orbital semi-major axis distribution is a direct consequence of the cometary ejection process and not due to subsequent orbital evolution. A high ejection-velocity (~ 0.6 km s-1) model stream succeeds in reproducing the observations. Conclusions are made concerning how the orbital stability of Earth-orbit-intersecting Perseid metecroids varies with initial orbital semi-major axis.

We have modelled the evolution in the distribution of orbits of 1.5 x 106 particles released at the 1862, 1737 and 1610 apparitions of 109P/Swift-Tuttle for a range of particle masses and for a range of ejection angles about the comet-solar line. The evolution of the stream is dominated by the influence of planetary perturbations, most notably those due to Jupiter. A periodicity of approximately 12 years in Perseid activity is noted in the material ejected during the 1862 apparition. Without Jovian perturbations, the nodes of these recently released Perseid particles would not cross the Earth's orbit. Activity from 1862 is notable in the simulations for 1991-1994 and also for the early 1980's. Ejections in 1737 and to a lesser extent in 1610 contribute material seen at the Earth in the late 1980's.

The activity profile of the new filament in the Perseid meteor stream and its 1986-1994 levels were analyzed from Ondřejov radar observations. The filament was first detected in 1986 at its minimum activity level. Its maximum flux was observed in 1993. A secondary maximum appeared in the period 1988-1989. The cross-section profile of the new cloud shows an activity peak at L⊙ = 138°.87±0°.04 (equinox 1950.0).

We have simulated the evolution of the Leonid stream via numerical integration of 3 million test particles ejected from 55P/Tempel-Tuttle during five perihelion passages of that comet. Using the Whipple ejection velocity formula and a random ejection spread in true anomaly about the parent comet orbit inside 2.3 AU, we have followed the subsequent evolution of Leonid meteoroids differing by over 5 orders of magnitude in mass under the influence of radiation pressure and planetary perturbations. By comparing the model predictions of Leonid activity on a year by year basis with the available observations we have attempted to determine roughly the time of ejection associated with each Leonid storm occurrence and model the observed mass distribution. On the basis of the demonstrated accuracy of the model we make predictions regarding times of peak activity and relative strengths for the Leonid returns for each year during the latter part of the 1990s.

The Leonids returned with higher than usual rates in November 1994. From an analysis of the available visual and radio forward m teor scatter observations, it is found that the meteor stream activity curve and the magnitude distribution index are similar to those of the outbursts in 1961 and 1965 during the previous return of the parent comet. It is possible that the outburst was due to the same dust component or dust with a similar stream evolution history.

Despite the complicated structure of old meteoroid streams in Halley-type orbits, the positions of their ecliptic-plane crossings are insensitive to the initial conditions of ejection and have a form determined by resonances due to planetary perturbations. This allows the main observational features of meteor showers such as the η-Aquarids, Orionids, Lyrids and Perseids to be understood. A proportion of the particles ejected into such orbits librate about the centres of mean-motion resonances and produce compact streams which may lead to meteor storms.

2P/Encke is the only Earth-crossing short-period comet to have a meteoroid/dust trail identified in the data collected by IRAS Such trails have been suggested by Kresák to be the cause of meteor storms, these occurring when the comet/trail node is near 1 AU and the Earth happens to pass through the trail. Here we present the results of integrations of variational orbits of 2P/Encke (the differences in the assumed initial semi-major axes representing the order of changes that could occur due to non-gravitational effects) from which we derive indications of when this comet may have produced meteor storms in the past. Pairs of sets of storms are expected about 300 yr apart, but the effects of chaotic dynamical evolution (and our ignorance of 2P/Encke's non-gravitational forces for any but the last two centuries) mean that we cannot define the epochs in which these may have occurred to better than 200 BC to AD 500 for the last pair, and 3600 to 1800 BC for the previous pair. Looking forwards in time, no meteor storm due to 2P/Encke will occur for at least 600 yr.

P/1994 P1 Machholz 2 is the first Earth-crossing shortperiod comet to have been observed to have split since 3D/Biela about 150 years ago, and that fragmentation resulted in the Andromedid meteor storms of 1872 and 1885. Thus a pertinent question is whether meteor storms might result from P/Machholz 2 in the foreseeable future. Here we integrate its orbit to ascertain when the node might next occur near a heliocentric distance of 1 AU, and find that this is not anticipated far over 1000yr, although some meteor activity in ∽300yr is possible. On such time-scales the recently-released debris will have dispersed, so that meteor storms are not expected from this comet.

We estimate the probability that specific meteoroids were produced by a specific parent body and obtain the ejection velocity and several other conditions of the ejection as a by-product. The duration of cometary activity leading to a meteor stream, or the epoch of a collision can be estimated from the ejection times of meteoroids in the same stream. The level of activity of a comet or probability of asteroid collisions can be estimated from the part of the orbit where ejection took place. We show that Phaethon ejected Geminid meteoroids over at least 3000 years lasting to as recently as ≈ AD 1600. Ejections are predominantly in the direction of the sun, i.e., from Phaethon's day side, and give insights into the ejection process which is suggestive of cometary activity. Our calculations account for the uncertainty in the observational data and include the effects of planetary perturbations, radiation pressure, Poynting-Robertson light drag and solar wind corpuscular drag.

We have reworked Whipple's (1951) theory of the ejection of meteoroids from comets to include the effects of cooling by the sublimation of the cometary ice and the adiabatic expansion of the escaping gases. We consider only those particles moving significantly slower than the gas speed and find that the inclusion of these effects does not yield results much different from Whipple's theory. We have extended the theory to include the case of an active area in the form of a spherical cap and have shown how the characteristics of the ejection process change when the cap is in the form of a pit or a depression. We present a empirical formulae which should be useful to modellers of meteor stream evolution.

Asteroids, comets and interstellar dust are possible sources of the particles that constitute the dust in the inner solar system. Each of these components gives rise to particular, characteristic features, the amplitudes of which can be used to estimate the size of the associated source. The asteroidal component feeds the dust bands and the Earth's resonant ring, while the cometary component may account for the large scale height of the zodiacal cloud observed at 1 AU Previous discussions of the observed strengths of these various features indicated that the source of about one third of the thermal flux observed, for example, in the IRAS 25μm waveband is asteroidal, while two thirds is cometary. However, a variety of assumptions go into this calculation (the size-frequency distribution of the particles is particularly significant) and we now know that the result is highly dependent on these assumptions. The zodiacal cloud is also the source of the IDPs collected on Earth. Because of strong gravitational focusing by the Earth of particles in low e and I orbits, it is probable that the majority of IDPs originate from asteroids, particularly those asteroids in the Themis and Koronis families.

The Earth's resonant ring is populated primarily by asteroidal dust particles because cometary particles have higher Poynting-Robertson drag rates and the Earth's resonances are not strong enough to trap them (Gomes, 1995). It has been shown that asteroidal particles in a limited size range from 5 — 30μm are responsible for the observed trailing/leading flux asymmetry caused by the trailing dust cloud embedded in the ring (Jayaraman and Dermott 1995). The magnitude of the flux asymmetry is a direct function of the area of dust in the ring, which in turn depends upon the number of asteroidal particles in the zodiacal cloud. Using a dynamical model of the ring and the background zodiacal cloud and estimating the surface area of dust needed in the ring to match the observed flux asymmetry in the 25 micron COBE waveband, we have calculated the fraction of asteroidal dust in the zodiacal cloud as a function of p, the slope of the size-frequency distribution of particles.

The Space Infrared Telescope Facility (SIRTF) is planned for launch by NASA in 2001 in a heliocentric orbit at 1.01 AU The spacecraft will drift away from the Earth slowly, reaching a distance of 0.3 AU behind the Earth at the end of its 2.5 year mission. This implies that SIRTF will spiral through the Earth's resonant dust ring (Wright et al., 1995) and, in particular, that it will traverse the dust cloud in the ring that trails the Earth in its orbit. We have used a dynamical model of the ring (Dermott et al., 1994) followed by simulation of the SIRTF orbit to predict the variations in the zodiacal thermal emission due to the trailing dust cloud as seen by SIRTF. Because the dust ring is inclined to the ecliptic, the latitude of peak flux of the trailing cloud will have yearly oscillations about the ecliptic. The amplitude of the oscillations will increase as SIRTF approaches the cloud, reaching a maximum of 20 during the mission. The magnitude of the flux variations can be as high as 4 – 5% or 2–3 MJy/Sr, SIRTF's measurements of these effects will allow us to model the number density and thermal characteristics of asteroidal dust particles near the Earth.

The recent discovery of the so-called Kuiper belt objects has prompted the idea that these objects produce dust grains that may contribute significantly to the interplanetary dust population at 1 AU. We have completed a numerical study of the orbital evolution of dust grains, of diameters 1 to 9 μm, that originate in the region of the Kuiper belt. Our results show that about 80% of the grains are ejected from the Solar System by the giant planets while the remaining 20% of the grains evolve all the way to the Sun. Surprisingly, these dust grains have small orbital eccentricities and inclinations when they cross the orbit of the Earth. This makes them behave more like asteroidal than cometary-type dust particles. This also enhances their chances to be captured by the Earth and makes them a possible source of the collected interplanetary dust particles (IDPs); in particular, they represent a possible source that brings primitive/organic materials from the outer Solar System to the Earth.

When collisions with interstellar dust grains are considered, however, Kuiper belt dust grains larger than about 9 μm appear likely to be collisionally shattered before they can evolve to the inner part of the Solar System. Therefore, Kuiper belt dust grains may not, as they are expected to be small, contribute significantly to the zodiacal light.

Interplanetary dust particles deposited in the Earth's atmosphere (IDPs) are an important source of extraterrestrial material. The origin of these particles, whether asteroidal or cometary, is a dispute that can be resolved into two separate questions. What are the fractions of the asteroidal and cometary components of the zodiacal cloud? And what fraction of each component is captured by the Earth? Evidence from the structure of the zodiacal cloud, particularly the dust bands and the Earth's resonant circumsolar ring, indicate that about one third of the cloud is asteroidal while two thirds is cometary. The geometry of capture is such that low eccentricity (e), low inclination (I) particles will be preferentially captured over high e, I particles. Two known abundant sources of low e, I particles are the asteroidal families Themis and Koronis, as witnessed by the associated zodiacal dust bands. We show that capture rates of particles from these two families achieve dominance over every other source. This leads to the result that particles from Themis and Koronis may account for over half of all deposited IDPs and that very few of the IDPs deposited in the Earth's atmosphere are of cometary origin. Furthermore, there are dynamical arguments which suggest that the accretion rate of Themis and Koronis dust particles varies with time. Two perodicities occur, a seasonal variation with a period of six months and a long-term variation with a period near 105 years. The amplitude and period of this long-term variation is supported by measurements of extraterrestrial 3He found in deep-sea sediments.

Two major sources are observed to contribute to the Zodiacal Cloud: main-belt asteroids and active comets. However, the discovery of 100 km size objects in trans-Neptunian orbits (Jewitt & Luu 1993) coupled with Pioneer 10 measurements showing an essentially constant flux of 10 μm dust from 4 to 18 AU with indications that dust may be in near-circular orbits (Humes 1980) suggests that collisions in the Kuiper Belt may contribute significantly to the Zodiacal Cloud and to the interplanetary dust collected from the Earth's stratosphere. Kuiper Belt dust collected at Earth could be identified by high densities of solar flare tracks, unusually thick amorphous rims, and high concentrations of spallogenic isotopes.

We examine whether the dark, orbitally-leading hemisphere of Saturn's satellite Iapetus might be coated by debris from low-albedo Phoebe, which orbits retrograde well exterior to Iapetus. Using simplified analytical models along with more complete numerical integrations, we follow the paths of various-sized particles launched gently off Phoebe following collisions with interplanetary and interstellar meteoroids. Micron grains can quickly reach Iapetus since (due to solar radiation) they trace elliptical orbits; larger grains may only hit after their more-circular orbits collapse due to Poynting-Robertson drag; few very large and very small Phoebe grains strike Iapetus. Despite some inconsistencies with observations, we conclude that Phoebe may possibly be the agent that has darkened Iapetus.

The dynamical behavior of dust particles ejected from Martian satellites is examined. On the basis of numerical simulations and analytical estimation for the orbital evolution of particles, we have found that there exists two kinds of resonance orbits around Mars. Particles ejected from Phobos and Deimos are influenced by these orbital resonances, and form dust rings with an asymmetrical structure along the orbit of the satellite, i.e., dust rings with a thin disk-like shape for Phobos and with a vertically extended structure for Deimos.