The radius for gravitational capture of meteoroids by the Earth is

where R is the Earth's radius, h is the height where marked deceleration of meteor bodies begins, α0 is the average distance of the Earth from the Sun, v∞ and vg are preatmospheric and geocentric (undisturbed) velocities of the body.

The existence of terrestrial meteorite craters is determined by laws of accumulation and disappearance. Both crater lifetime and discovery probability are proportional to the area of the structure (D2). The distribution in age (T, year) and dimensions (D, metre) is defined by the inequality of . The available material on terrestrial craters allows the study of the time variation in the stream of large cosmic bodies striking the Earth. Over the last billion years the stream of crater-forming meteorites has decreased according to the law Tτ where τ ≤ 0.2. The absolute value of the flux can be determined from a complete investigation of a portion of the Earth's surface.

The problem of calculating the mass distribution of meteorites and their influx rate can be resolved into three sections; (a) the flux of meteorites hitting the surface of the Earth must be calculated (b) the mass distribution of these meteorites must be assessed, care being taken to include the effect of fragmentation in the atmosphere and (c) atmospheric ablation, which is a function of mass, velocity, composition and zenith angle must be taken into account. The final result will give the meteoritic influx to the Earth's upper atmosphere and its mass distribution.

It is generally assumed that ejections from active cometary nuclei are the major source of replenishment of the interplanetary dust complex. Conjectures against this concept are usually based on comparison of the quantitative efficiency of the dust production by comets with the efficiency of all the dissipative processes involved. The present paper discusses this problem from the dynamical point of view, tracing the evolution of swarms of cometary ejecta as they pass through different evolutionary stages. It is concluded that the contribution of the present population of active comets, of all revolution periods, is not only inadequate to explain the abundance of interplanetary particles, but also inconsistent with the distribution of their orbits. Other potential sources and their implications for the equilibrium problem are reviewed.

Observations of the scattered sunlight and thermal emission from cometary dust provide information on the composition and dominant size of the dust. The observations will be summarized here and compared to theoretical models for dielectric and absorbing materials, with emphasis on the thermal emission. The compatibility of the optical data with the size distribution derived from dynamical studies is discussed.

In this review attention is focused on those areas of the dynamical investigation of cometary dust in which significant progress has been achieved since the previous review papers (Sekanina 1976a, 1977). Much of the progress stems from work based on the model of dust comets formulated by Finson and Probstein (1968a). Their introduction of a new technique for dust-tail studies has made it possible for the first time to gain insight into such properties of cometary dust as the size distribution function of particles shortly after emission from the comet nucleus, the distribution of particle ejection velocities, and the production of dust versus time. Since the Finson-Probstein approach is of a combined dynamical/photometric type, information on particle sizes and masses is provided indirectly, through parametric functions determined from the observed distribution of light intensity in the tail. The basic limitation is that particle radius a cannot be separated from particle density ρ, as their product is related to a directly observed quantity β, the acceleration exerted on the particle by solar radiation pressure. Expressed in units of solar attraction, β for a spherical particle is given by

where c0 = 0.585 × 10−4 g/cm2 and Qpr is the integrated scattering efficiency of the particle for radiation pressure, which varies significantly with a for particles whose dimensions are smaller than the effective wavelength of solar radiation.

The Finson-Probstein (1968a) method of analysis of the distribution of light intensity in dust tails has so far been applied to only a handful of comets. Yet, the results already suggest a striking diversity in the properties of the particle-size related distribution function f(β), one of three parametric functions determined. Here β is the acceleration exerted on the particle by solar radiation pressure, measured in units of solar attraction. For a spherical particle β is a function of its radius a, density ρ, and the integrated efficiency factor for radiation pressure Qpr (i.e., the ratio of the particle's effective cross-section for radiation pressure to its geometric cross-section): where c0 = 0.585 × 10−4 g/cm2.

The dust production rates of comets on various orbits with perihelia between 0.1 and 0.5 a.u. and semimajor axes between 5 and 40 a.u. are calculated. A new model is used taking into account the screening of solar radiation by the dust and the multiple scattering of the photons. Screening and multiple scattering effects tend to compensate each other over a range of moderate optical thicknesses.

Anomalous polarization at small phase angles is confirmed as a common feature of dusty cometary atmospheres. The opposition effect is detected and interpreted as evidence of similarity between grains covering interplanetary, cometary and asteroidal surfaces. The prevailing radii of dust grains in the comet's atmosphere are estimated to be 0.15-0.19 μm.

The ultraviolet spectrum of Comet West (1976 VI) in the range 1200-3200 Å was recorded by rocket-borne instruments on March 5.5, 1976. At the time of launch, r = 0.385, Δ = 0.84 and the phase angle was 78°. Longward of 2100 Å the continuum of solar radiation scattered by cometary dust is detected and is found to closely follow the solar spectrum. Since the dust coma is completely included in the spectrometer slit, the ultraviolet albedo can be determined relative to the visible and this ratio is found to be ≈0.3 at 2700 Å. There is evidence for a further decrease in albedo near 2200 Å. Using a visible albedo of 0.2 gives a value of 0.06 for the cometary albedo at 2700 Å, a value similar to that found for the moon and lunar dust in this spectral region.

Following Sekanina's (1976a,b) suggestion that striae (also known as “synchronic” or pseudosynchronic bands) observed in the dust tails of several comets could be products of fragmentation of friable dust particles ejected from the nucleus, we have completed an investigation of these structures in Comet West 1976 VI (Sekanina and Farrell 1979, detailed paper in preparation), which shows that the dynamical solutions, provided by the fragmentation model on the assumption that solar attraction and radiation pressure are the only forces involved, are indeed in agreement with the observed motions of 16 striae through the tail over a time interval of more than three days. The match is so good that the probability of its being a coincidence of some sort must be virtually nil.

Experiments presently conducted at the Martin Marietta Denver facility to collect cometary dust during a Tempel 2 flyby are reviewed. It is shown that a number of viscid materials are quite suitable to collect particles up to 50 μm diameter at speeds up to 50 m/s. First results are presented for the more complicated setups using nonviscid metal surfaces for the collection of dust. These targets allow the use of surface analysis techniques for a more elaborate chemical analysis of the collected dust particles.

During a Rendezvous-type cometary mission it will be possible to collect dust particles on exposed surfaces. A secondary-ion-mass-spectrometer is proposed for the subsequent chemical investigation of such dust collections. An ion gun shoots inert gas or oxygen-ions onto the sample. The energy of these primary ions can be chosen between 1 and 5 keV. The positive and negative secondary ions emitted from the dust surfaces are then analysed with a quadrupole mass filter. The mass range of the instrument will be 1-150 amu. The sensor is able to measure (a) nearly all elements in this mass region, (b) molecules and organic components present in the dust, (c) isotopic ratios of interesting elements. Results of a first series of simulation studies are reported.

The relative speed between Comet Halley and the presently planned Halley Probe will be approx. 55 km/s. At such a speed the method of impact ionization mass-spectrometry is perfectly suited to analysing cometary dust particles with masses from 10−16g to 10−10g. First results are reported by Dalmann et al. If a micrometeoroid hits the sensitive target, ions from both particle and target material are produced. The total number of ions is registered with a charge-sensitive amplifier and this signal Q allows the determination of the particle mass m (Q ∼ m). By an electric potential difference of 3 kV, ions are drawn into a field-free drift-tube. The ions are separated in time in the TOF-Spectrometer due to their different masses. The mass spectrum is registered as output current of the particle multiplier at the end of the TOF-tube. It gives information about the chemical composition of the detected particle. Characteristics of such spectra recorded with the Impact-Spectrometer currently used in the laboratory are the following: a) mass resolution m/Δm > 100 at 23 amu, b) flight-time (t ∼ 14 μs at 100 amu; the corresponding Δt ∼ 70 ns between 99 and 100 amu).

The Helios 1 spacecraft was launched in December 1974 into a heliocentric orbit of 0.3 AU perihelion distance. It carries on board a micro-meteoroid experiment which contains two sensors with a total sensitive area of 121 cm2. The ecliptic sensor measures dust particles which have trajectories with elevations from −45° to +55° with respect to the ecliptic plane. The south sensor detects dust particles from −90° to −4°. The ecliptic sensor is covered by a thin film (3000 Å parylene coated with 750 Å aluminium) as protection against solar radiation. The other sensor is shielded by the spacecraft rim from direct sunlight and has an open aperture. Micrometeoroids are detected by the electric charge produced upon impact and the ions are mass analysed in a time-of-flight-spectrometer. During the first 6 orbits of Helios 1 around the sun the experiment registered a total of 168 meteoroids, 52 particles were detected by the ecliptic sensor and 116 particles by the south sensor. Most impacts on the ecliptic sensor were observed when it was pointing in the direction of motion of Helios (apex direction). In contrast to that the south sensor detected most impacts when it was facing in between the solar and antapex directions. Orbit analysis showed that the “apex” particles which are predominantly detected by the ecliptic sensor have eccentricities e < 0.4 or semimajor axes a < 0.5 AU. From comparison with corresponding data from the south sensor it is concluded that the average inclination of these particles is below 30°. The excess of impacts on the south sensor have orbit eccentricities e > 0.5 AU. β-meteoroids which leave the solar system on hyperbolic orbits are directly identified by the imbalance of outgoing (away from the sun) and ingoing particles. Mass analyses of the spectra showed that 40% of the observed spectra have the peak abundance above mass 35 amu which are preliminarily identified as iron meteoroids. 40% of the spectra have the peak abundance below mass 35 amu which correspond to chondritic composition. 20% of the spectra could not be identified in either class.

Analysis of motion of dust particles around the Sun under the combined influences of gravitational attraction, radiation pressure and Poynting-Robertson effect shows that at the stage of increasing solar luminosity (Hayashi stage) the particles spiraled away from the Sun, the smaller the faster. This effect could manifest itself near the inner edge of the gas-dust protoplanetary nebula in the zone penetrated by solar radiation. If silicate particles were more fragmented at collision than iron ones (Orowan, 1969), then the above effect could produce a composition fractionation because of a more rapid recession from the Sun of silicate particles as compared to iron ones. This effect can explain the abundance of iron in Mercury.

The radiation pressure (RP) felt by a perfectly absorbing particle is due to the momentum withdrawn each second from the beam. The Poynting-Robertson (PR) drag is produced since the particle continually absorbs mass in the form of radiation, which, upon re-emission, has the same mean momentum density as the particle itself. We find that, relative to the force felt by a perfectly absorbing particle, the RP+PR forces felt by a scattering particle must be multiplied by Qpr, the radiation pressure coefficient, which can be evaluated from Mie theory.

The classical restricted three-body problem is generalized to include the force of radiation pressure and the Poynting-Robertson effect. The positions of the Lagrangian points L4 and L5 are found as functions of β, the ratio of radiational to gravitational forces. The Poynting-Robertson effect renders the L4 and L5 points unstable on a time scale (T) long compared to the period of rotation of the two massive bodies. For the solar system, T is given by T = [(1-β)2/3/β] 544 a2 yr where a is the separation between the Sun and the planet in AU. Implications for space colonization and a mechanism for producing asymmetries in the interplanetary dust complex are discussed; testing the latter may be possible from the Zodiacal Light/Background Starlight Experiment aboard the International Solar Polar Mission spacecraft to be launched in 1983.

Assuming that the zodiacal cloud is composed of two populations, the effect of collisions among the largest grains is investigated. It is found that the population of fragments is unable to explain the observed flux of submicron grains.

Recent observations of zodiacal light have established a reliable and consistent picture of the spatial distribution of interplanetary dust in the ecliptic plane. The spatial density nr varies with heliocentric distance r according to a power law nr ∝ r−ν. From Helios observations an exponent v = 1.3 is derived for the distance interval from 0.08 A.U. to 1 A.U. (Link et al. 1976). Outside the earth's orbit the Pioneer 10 and 11 results suggest a higher exponent v = 1.5 for the distance interval from 1 A.U. to 3.3. A.U. (Hanner et al., 1976). Giese and Grün (1976) showed that the results from zodiacal light observations are compatible with the micrometeoroid fluxes derived from in situ measurements and lunar crater statistics. They found that micrometeoroids in the size range from 10 μm to 100 μm radii (corresponding roughly to 10−8g to 10−5g) contribute most to the zodiacal light brightness.

The orbital distribution of large interplanetary particles (10−6 g < m < 10−3g) is known from meteor observations. Sekanina and Southworth (1975) reported average orbital elements of these particles: ā ∼ 1.25 A.U., ē ∼ 0.4 and ī ∼ 20°. Orbital information on micrometeoroids (m < 10−8g) is obtained from in situ detectors on board the Pioneer 8 and 9 and Helios 1 spaceprobes and the HEOS-2 satellite. Characteristics of the different micrometeoroid experiments are given in Table 1. There is almost no time overlap in the data taking intervals of the experiments. Therefore one has to assume that there are no time variations of the meteoroid flux on the time scale of 1 to 10 years if one compares the results from the different experiments. This assumption may be violated for the smallest of the observed particles (m < 10−13g) due to strong electromagnetic interaction of these particles with the interplanetary magnetic field (Morfill and Grün 1979).