Interplanetary dust probably has a cometary origin. Collectible samples of this material are likely to be more representative of the meteoroid complex than are meteorites. Since 1974, NASA U-2 spacecraft has been able to collect over 200 interplanetary particles in the stratosphere. Roughly 10% have spheroidal shapes indicating previous melting. The rest seems to come from the gentle fragmentation of a single type of parent-body material: a black aggregate of grains mainly 1000 A in size, whose properties are closely similar to C1 and C2 chondritic material.

Statistical studies of the photographic data on atmospheric trajectories and orbits of meteors (from 10-4 g to hundreds of tons) point to 5 groups of bodies with different structure and composition. The following classification is proposed: Fireball group I = exceptional cases of so-called “asteroidal” bodies among Super-Schmidt (SS) and small-camera (SC) meteors 5 ordinary chondrites. Fireball group II = group A among SS and SC = carbonaceous chondrites. Group B among SS and SC = dense cometary materials with small perihelion distances (not distinguishable among fireballs). Fireball group IIIa = group C among SS and SC = regular cometary material. Fireball group IIIB = group D (above C) of SS and SC = Draconid-shower type of cometary material. Statistics of the orbits for separate groups is considered and characteristic orbits are given.

Thermal attack is the basic process that dominates the atmospheric passage of a meteorite. The severity of heating by shocked air (with temperature ranging from 10,000° K to 25,000° K) depends upon pre-atmospheric velocity and angle of entry. It can produce eventually a destructive thermal stress pulse inside the body. While gross fragmentation at higher altitudes seems to have mainly a thermal origin, at lower altitudes the destruction (with occasionally its remarkable abundance of fragments) is likely to be attributed to the combined effect of high temperature and increased pressure. If it is assumed that these conditions control both the number and size distribution of fragments, then much can be learned of the comminution process by studying the fragment distribution in existing meteorite falls.

The well-known associations of meteor streams with periodic comets and the probable cometary origin of the zodiacal dust cloud point to the importance, in the cometarg debris, of particles with masses exceeding roughly 10-6 gram. It is shown that these large particles dominate in the sunward-oriented anomalous tails of comets. Their study is essential for meaningful estimates of the mass of meteor streams and of the injection rate of the cometary debris that contributes to the zodiacal cloud. Favorable conditions for the detection of anomalous tails can be recognized in advance, as demonstrated by the successful predictions for comets Kohoutek (1973 XII) and Bradfield (1975p). To answer the question as to whether short-period comets can support the zodiacal cloud, a study of anomalous tails at future returns of these comets is considered indispensable.

In studying the evolution of meteor streams from cometary debris it is desirable to know the distribution of particles both around the orbit and in the cross section perpendicular to the orbit. Earth-based observations are limited to a once-per-year sample and also limited by the position of the observer on earth as it crosses the meteor stream. A continuous series, 11 years, of records of the Quadrantid meteor shower illustrates the problems and the results that can be achieved. International cooperation would greatly assist such studies.

The purpose of the present paper is to review all asteroid diameter measurements, current through mid-1976, and to combine them in a consistent way to give the best available estimates for a sample totalling 187 objects. From these diameters it is possible to determine the size-distributions of minor planets down to diameters of 50 km in the inner belt and 100 km in the outer belt. The associated albedos further indicate the distribution of objects of the C, S, and M classes throughout the belt.

A sample of 359 minor planets with available colorimetric, spectropnotometric, thermal-radiometric, and/or polarimetrie data are classified into broadly defined compositional types C (carbonaceous), S (silicaceous), M (metal-rich), E (metal-free; enstatite?), O (ordinary-chondritic?), T (Trojan; unidentified composition), and U (unclassifiable; none of the above).

The small asteroids in Mars- or earth-crossing orbits are almost invariably of type S or O. For the main belt between 2.2 and 3.5 AU., distributions of the various types over diameter and orbital parameters are derived with corrections for observational selection bias. For 560 main-belt asteroids with diameters >50 km, 76% are of type C, 16% of type S, 5% M, and 3% of other types. The S objects become progressively less common with distance. The shapes of the diammeter-frequency relations for primitive (C) and evolved (S + M) types are statistically indistinguishable, both showing a change of slope at 160-km diameter.

While large asteroids avoid the Kirkwood gaps more strongly than small ones, we find no significant gap-related anomalies in the relative frequencies of compositional types. Thus most of the observational basis for models in which primitive and evolved asteroids respond differently to collisional evolution has been removed.

The quality and quantity of asteroidal spectral reflectance data has increased rapidly in recent years. This data can be either (a) classified into spectral, polarization and/or albedo groups (e.g., ‘C’, ‘S’, etc.) which should tend to lump together grossly similar materials, or (b) interpreted utilizing laboratory and theoretical understanding of spectral features (e.g., absorption bands, continuum shape and slope) which are diagnostic of mineralogy or mineral chemistry. Mineral assemblages have been identified on asteroid surfaces which are comparable to most known meteorite types or which have undergone the types of processes (e.g., melting and differentiation) necessary to produce the meteoritic assemblages. The ordinary chondrites, which dominate the meteoritic flux reaching the Earth’s surface, are very rare or absent on Main Belt asteroids but appear common on the small asteroids which approach or cross the orbit of the Earth. The present interpretations of asteroidal spectra are not yet quantitative enough to permit the evaluation of specific asteroids as the sources bodies of particular meteorite specimens in terrestrial collections.

Advances in IR detector technology, the increased availability of large aperture telescopes, and the techniques of Fourier transform spectroscopy (FTS) now permit IR (λ > 1μ) spectroscopic observations of asteroid surfaces. Asteroids already observed include Ceres, Vesta, and Eros. These initial results demonstrate that such studies can contribute new data concerning asteroid compositions. Some of the most diagnostic features of mineral spectra are in the IR spectral region, and for featureless spectra characterized only by slopes the extension of the spectral reflectivity curve into the IR provides tighter constraints on possible mineralogies. A systematic study of additional asteroids should exploit even further this new observational link to problems of meteorite origin and solar system evolution.

This paper assesses the state of asteroid infrared reflectance measurements and discusses what they have contributed to our understanding of asteroidal surfaces. The data suggest that space weathering has not significantly affected the optical properties of the asteroids. Thus comparisons of asteroid reflectance spectra with unweathered meteorite and rock data is a valid procedure. The high 1.6 and 2.2 μm reflectances of a number of asteroids strongly suggest the presence of a metallic phase.

Telescopic observations of the polarization of light by asteroids are interpreted on the basis of a systematic polarimetric analysis of terrestrial, meteoritic and lunar samples. Laboratory measurements were made using samples with different surface textures, and scanning electron microscope pictures were used to investigate the influence of microtexture and crystalline structure.

It is demonstrated that asteioid surfaces do not accumulate thick regolithic layers of micro-fragments, as do the Moon and Mercury. This is because the majority of debris ejected by impacts are lost, due to the low gravitational escape velocity from these bodies. However, asteroids are not bare rocks, but are coated with a thin layer of adhesive debris. This coating apparently has the composition of the body itself. The fact that there is no indication of significant maturation by space weathering suggests that the dust which coats the surface of asteroids is frequently replaced by further impacts.

Asteroids may be classified polarimetrically in several groups: those in group C are made of very dark material and behave like carbonaceous chondrites, or very dark Fe-rich basalts; Those in group S correspond to silicates and stony meteorites. A third group represented by Asteroid 21 Lutetia and 16 Psyche may be metallic.

Physical interpretations of negative polarization and opposition effect confirm the existence of a surface layer on the asteroids of their own dust. Future work should establish whether the impacting grains are cometary or asteroidal/meteoritic.

The brightnesses of Ceres and Vesta were observed at 3 mm wavelength. For Ceres, pure rock cannot reproduce the observed values, and a dust layer is required, much similar to lunar material. For Vesta, its different thermal characteristics appear to require a more compacted layer of material on its surface.

The hypothesis that the asteroid belt is the source region for nearly all meteorite remains viable and there is no compelling reason to ascribe any meteorites to cometary origin. On the other hand, uncertainties about the true composition of the larger S type asteroids and difficulties in finding plausible main-belt source-bodies for the ordinary chondrites leave room open for further speculation on this question. The scenario for the evolution of asteroids, based on collisional models of two distinct populations of different physical properties, is being criticized and refined. It remains uncertain whether this approach will ultimately prove to be the correct interpretation of the collisional evolution of asteroids.

Inventories in sizes of large planetesimals can be estimated for different periods in the late planet-forming process by different techniques. For example, tabulations of craters on Mercury, the moon, and Mars give direct evidence of asteroid-like size distribution including bodies in excess of 100 km diameter. Large bodies exceeding, 1000 km diameter probably existed earlier. Consequences of interactions between planets and such bodies are considered.

Earth-impacting meteoroids are derived from both comets and asteroids, and some uncertainty still exists regarding with which of these bodies some stone meteorites should be identified. In contrast, the long cosmic ray exposure ages of iron meteorites strongly suggest a long-lived asteroidal source capable of providing ~108 g/yr of this material to the earth’s surface over at least much of solar system history. Spectrophotometric data show that differentiated asteroids are concentrated in the inner portion of the asteroid belt. The orbital histories of fragments of inner belt asteroids are investigated, considering the combined effects of close planetary encounters, secular perturbations, and secular resonances. Particular attention is given to the low inclination (<15°) objects with small semimajcr axis (2.1 to 2.6 A.U.), which can make fairly close approaches to Mars (<0.1 A.U.). It is found that the annual yield and dynamical lifetime of collision fragments of these asteroids is in agreement with the observed impact rate and exposure age of iron meteorites. A smaller yield of stone meteorites (-107 g/yr) is expected, because elimination of these objects by collision is probable on the long dynamical time scaTe. Achondrites could be produced in this way; the yield is probably too low to account for chondrites. Chondrites could possibly be derived indirectly from these bodies insofar as these asteroids are also sources of Apollo and Amor objects.

Zimmerman and Wetherill’s proposed mechanism for the production of meteorites from asteroids has been explored by numerous numerical computations of orbits. Starting from the vicinity of the 2/1 Kirkwood gap, 30% of our orbits soon have aphelia reaching beyond, but not much beyond 4 A.U.; (3% only exceed 4.3 A.U.). These results suggest that the proposed mechanism might work, although we have not yet checked whether Jupiter’s action will be large enough for those perihelia. Furthermore, our investigation has been extended to the 3/1, 7/3 and 5/2 Kirkwood gaps, and shows that these gaps should also be considered asa possible source of meteorites.

From discoveries made in several independent surveys of the sky, the number of Apollo asteroids to absolute visual magnitude 18 is estimated to be of the order of 103; the ratio of Mars-crossing asteroids to Apcllos is estimated to lie between 10 and 60. The loss of Apollos to magnitude 18 over the last 3 billion years, by collision with the planets and ejection from the solar system, is estimated to have been several times 104 objects. Mars-crossing asteroids appear to be an inadequate source for all of these objects. Most Apollos probably are dead short-period comets.

From the cosmic ray exposure age data, (time scale 107 - 108 years), of the lunar surface materials, we discuss the mixing process of the lunar surface layer caused by the meteoroid impact cratering. The gardening effect calculated using a crater formation rate slightly modified from the current population data is consistent with observed exposure ages of the lunar samples.

There seem to be no objection any more to regard the asteroid belt as the past and present source of initial and intermediate parent-bodies of meteorites. Initial parent bodies were of the size of the present-day largest asteroids; the irregular size of medium sized asteroids suggests intermediate parent bodies fragmented by collisions. Finally, most of the terminal parent bodies can be identified with Earth-crossing or grazing objects. In contrast, icy planetesimals of the outer zone of the primeval nebula underwent almost no thermal evolution, but only a complicated orbital evolution leading to the comets observed now. This does not exclude the possibility that some extinct comets end up on asteroid-type orbits, although their nuclei might still contain ices covered by an insulating crust that can preserve them for a very long time. These could be the parent bodies of bolides, as opposed to meteorites.