The orbital comet-asteroid criteria, their premises, dynamical reasoning, and ranges of applicability are reviewed. Mapping of all known comets and asteroids in a plane of energy intergral in the two body problem (1/a) vs. that in the restricted three-body problem (Tisserand invariant) is presented. The potential evolutionary paths from different sources of active comets into short-period orbits are delineated and interfaced with the process of reducing the perihelion distances of the asteroids. The significance of resonances with Jupiter is emphasized. Statistics of observed close approaches of individual comets and asteroids to the Earth is analyzed to estimate their relative fluxes. Active cometary nuclei are found to represent about 1/8 of the flux of objects with radii exceeding 1 km in the vicinity of the Earth, and their contribution tends to diminish significantly for still smaller bodies. However, there is no evidence against comets leaving inactive asteroid-like nuclei with considerable lifetimes which may represent a significant, though secondary, source of meteors and even meteor streams. An overwhelming majority of the Apollo and Amor objects are suggested to be of asteroidal nature; the most probable exceptions are selected and recommended for detailed observation.

The Trojans of Jupiter have the same magnitude-frequency distribution as the asteroids, but there is a puzzling asymmetry of the densities in Preceding and Following Lagrangian points. The outer satellites of Jupiter have a peculiar magnitude-frequency distribution, with large and also small ones missing; they therefore cannot be captured asteroids or Trojans unless during the capture a resisting medium was present out to ˜400 RJ for proto-Jupiter.

The correct identification of asteroid families is a prerequisite for understanding their nature, their orbital evolution and their physical origin. For this reason, a statistical investigation of asteroid families has been carried out, using a new clustering technique developed by A. I. Gavrishin. Proper elements for 2764 asteroids (1810 numbered and 954 Palomar-Leiden-Survey (PLS) asteroids) have been computed. Using these data, the Gavrishin method gives only ten significant classes. Five of them are coincident with the Hirayama families 1, 2, 3, 5, and the Flora group, that cannot be univocally subdivided. The PLS families are recognized. Furthermore many small-sized families reported by several authors lose their statistical relevance.

Several isotopie anomalies, unexplained by known nuclear or non-nuclear processes within the solar system have been attributed to the preservation of pre-solar variations. The largest of these (in number of atoms) in an 16O-excess (up to 5%) in “high-temperature condensate” minerals in primitive meteorites. Some of these same minerals have an excess of 26Mg, probably a decay product of 26Al, that could have been a major source of heat for melting and metamorphosing planetesimals. Excesses of 22Ne and of isotopes of Xe, found in carbonaceous chondrites, may have origins in presolar solid particles. Large variation in the isotopie abundances of nitrogen and carbon in meteorites may also represent isotopie heterogeneity in the solar nebula. Most of these “Isotopically anomalous” elements are found to be highly concentrated in minute phases within the meteorites, rather than being uniformly distributed. The identification and characterization of these carriers of presolar materials constitutes the principal thrust of current research in this area.

We review the evidence for the presence of short lived 26Al (τ1/2 = 0.72 × 106 years) in the early solar system. Large excesses of 26Mg of up to 5% have been found in Ca-Al rich inclusions of the Allende meteorite. The Mg excesses correlate well with 27Al/24Mg and in two cases they are found in high purity separates of coarse grained Al-rich minerals which tend to exclude Mg. The data demonstrate that 26Al was present in the early solar system. Mechanisms for addition of 26Al to the solar nebula or for production within an active solar system are required within a few million years of condensation of small (centimeter sized) objects. The 26Al abundance in Allende inclusions is high enough to provide for effective melting of kilometer size bodies or larger, if such bodies accreted early enough (a few million years) to incorporate the 26Al.

Anomalies in meteorites due to extinct 22Na and 26Al have been interpreted as due to energetic-proton irradiation of the primeval nebula. I discuss quantitative limits that can be placed on this hypothesis. I describe other abundance effects that would have to coexist if this idea were correct and energetic constraints of an astrophysical type. The existing evidence is more easily explained by presolar carriers that are needed in any case for other anomalies.

Cooling curves were determined for ordinary chondrites within the time- and temperature intervals from ~4.6 to ~4.1 × 109 years and ~1400 K to 300 K respectively. This was done by analyzing the 244-plutonium fission track record in whitlockite and adjacent mineral track detectors. The resulting cooling rates constrain the sizes of the parent asteroids to ~120 - 200 km in radius. These dimensions together with the fact that chondrites had never been molten, suggest lower 26Al/27Al initial ratios for ordinary chondritic than for Allende refractory materials.

The formation of meteoritic chondrules via condensation from the primordial solar nebula is discussed. Chondrule formation in regions where the gas/dust ratio was enhanced, and where transient high energy events heated the gas and temporarily vaporized the dust, is advocated. The observed diversity of chondrule types can be understood as resulting from local variations in the initial gas/dust proportions and other parameters.

Primitive chondrites heated for one week under conditions reasonable for early solar system objects readily lose volatile/mobile trace elements, e.g., Ag, Bi, Cs, Ga, In, Se, Te, Tl and Zn. Trace element contents decrease by 10-100x progressively with temperature up to 1000° C; apparent activation energies calculated from these data suggest bonding differences between chondrites. Comparison of data for E3-6 chondrites and heated Abee (E4) indicates that volatile/mobile trace elements in E-group chondrites reflect metamorphic loss from a parent object; prior nebular cosmochemical fractionation modified non-volatile element contents. Apparently L3-6 chondrites escaped such open-system metamorphism. Information on nebular condensation process(es) may be gained from L-group compositional data; only nonvolatile elements in E-group chondrites should be used for this purpose, however.

Recent solar abundance data confirm the relationship between the sun and carbonaceous chondrites. New results for the solar S/Ca and Na/Ca ratio discriminate between types 1, 2, and 3. These and interstellar properties pose constraints on the condensation process. Protostellar separation of gas and grains will affect stellar metal content.

Inclusions of one type of meteorite enclosed in another have been found in several gas-rich meteorites, unequilihrated chondrites and mesosiderites. The inclusions in all but one case are chondritic; a majority are mineralogically and isotopically similar to carbonaceous chondrites. These meteorite mixtures most probably resulted from collisions among asteroids.

Several meteorite types, distinct from the large chondrite groups (H, L, LL, E and carbonaceous), are known to also have chondritic compositions. These meteorites preserve information on conditions at additional formation locations in the early solar nebula. Silicate inclusions from group IAB iron meteorites are one such type. Evidence for their chondritic nature is given and their formation discussed.

The chemistry of iron meteorites is compared to predictions of the chemical fractionations that develop during the cosmic history of the metal phase, from condensation and accretion through melting, segregation and freezing. Of the 12 resolved iron meteorite groups, 3 appear to have evolved in bodies which accreted at T > 1000° K. In several cases, the core-forming process seems to have ceased prematurely, just as the metal began to melt and flow (group IAB with its silicate inclusions) or after the metal aggregated into pods but before it sank to form a core (group IVA, with groups IIAB and IIIAB being in more advanced stages). The Shaw chondrite contains residual metal from a partial melting process, as required to complement the fractional melts which refroze prematurely in the case of group IAB meteorites.

Recent progress in studying and understanding the magnetic record of meteorites is reviewed. Magnetic data are not compatible with the simple picture of a single planetary parent-body with core-mantle structure and a dynamo-generated magnetic field, as earlier envisaged by Soviet colleagues. The strong preterrestrial magnetization of iron meteorites, previously believed to have been acquired during cooling in parent-body fields of -.6 Oe, has now been shown to be probably a spontaneous moment, directionally controlled by the octahedral Ni-Fe structure. For each class of meteorites, the magnetic record is basically in accord with conclusions based on chemical-mineralogical-petrologic characteristics. For example, the complex remanence of brecciated achondrites bears no record of their primary igneous differentiation, but only of multiple brecciation events. Similarly, the unreilites show the expected magnetic imprint of shock-metamorphism at impact. Although systematic trends were found among ordinary chondrites groups, allowing for a rudimentary magnetic classification, only very few appeared to possess a primordial remanence component, which was used to estimate parent body fields in the range 0.1 = .3 oe. Most ordinary chondrites have been magnetically affected by brecciation at formation (e.g., LL), or by metamorphism within the parent body, or individually - by shock at the breakup of the parent body. Only the carbonaceous chondrites have preserved a clear magnetic record of their formation at low-temperature (T < 500° K), in strong magnetic fields (H ≥ 1 oe). The evidence is compatible with cold condensation and aggregation of component grains either in extended, enhanced solar wind fields, or in cometary magnetic fields.

The problem of thermal metamorphosis of meteorites, possibly the Moon and Mercury, and perhaps other planetary objects is reviewed. Classical mechanisms of heating include fossil nuclides (especially 26Al), accretional heating, tidal heating, chemical reaction heat, and electrical induction. These various mechanisms involve constraints on the early thermal profiles of the Moon or Mercury. In the case of the meteorites, the primary contenders for a viable mechanism currently are fossil nuclides and electromaqnetic induction, or some combination of these. But the issue of energetic mechanisms in the early solar system remains enigmatic. The fossil nuclide hypothesis leads to constraints upon nucleosynthesis while electromagnetic induction places significant constraints upon electrodynamic effects such as solar spin damping.

On the basis of their chemical and mineralogical composition, 420 iron meteorites have been classified into 12 different groups. Each group seems to have come from a separate parent body. The largest group, IIIAB, probably formed an asteroidal core in a body approximately 100 km in radius, which was largely destroyed by collision 630 My ago. Another 67 analyzed irons do not belong to these groups, and may represent samples from another 20 or more bodies.

Analyses of eleven major elements in five howardite samples and in the silicate fraction of seven mesosiderites are presented in a recalculated form and compared. The mesosiderite silicate fractions show distinct differences in chemical composition from the howardites, but the average Ca/Al determined for mesosiderites (1.05), which differs from most values previously published, is close to that typical of howardites (1.08). The inverse Ca/Mg relationship in the howardites is present also in the mesosiderites, the trend being displaced relative to that of the howardites but parallel to it. The chemical differences confirm that mesosiderites are not mixtures of howardite and metal. The Ca/Al and Ca/Mg relationships suggest that the two meteorite groups were subject to similar genetic controls, and may therefore have had a common parent body. Such a body is required by the cooling rate of the metal of mesosiderites to have been larger than any known asteroid.

Empirical data are confronted with different hypotheses on the origin of comets. The hypotheses are classified into three categories: 1) Comets were condensed from the solar nebula and ejected later into the Oort’s cloud. 2) Comets were condensed in situ, more or less recently, on their present trajectories; 3) Reversing the arrow of time in the traditional evolution of comets. Only two hypotheses, both from the first category, are found to be in agreement with all empirical data. The first hypothesis explains the origin of the Oort’s cloud by the perturbations of the giant planets (mainly Uranus and Neptune and possibly Pluto) on a ring of proto-comets, during the final accretion stages of the solar system. The second hypothesis uses the fast mass loss of the solar nebula to expell an outer ring of proto-comets into elliptic trajectories. Although no empirical evidence requests that the Oort’s cloud be older than a few million years, its matter is not likely to be from a different reservoir than solar system stuff, and no satisfactory theory explains its formation more recently than 4,5 billion years ago.

The quantitative processes of eruptive development of planets into comets and other small bodies is studied from the Physical and Orbital evolution of these minor bodies. The escape of comets (and of their products of decay) into interstellar space has been of the order of 1030 to 1031 during the lifetime of the solar system. The mean density of the planets and of their large satellites, and their specific rotational energy, serve as independent checks of the amount of lost material. As far back as 1955 and 1962, the eruption theory has predicted the high volcanic activity on Venus and Mars and on some satellites, that was actually discovered by Mariner 10 and Venus 9-10. Interstellar molecules confirm that ejection of cometary gases is widespread in the Galaxy. Six new bright short-period comets discovered in 1975 provide direct evidence for comet formation in the system of Jupiter in its 1961-1969 period of high activity. Brought together, these facts prove that planetary bodies began their existence at stellar temperatures. They cooled down from the surface, forming crusts of rocky and icy materials, that have initiated a long period of eruptive evolution, characterized by numerous cataclysms with the ejection of tremendous amounts of gas and dust, separated by more quiescent phases, like the present state of the earth. Comets, asteroids, meteoritic and meteoric material bear witness for the explosive processes on planetary bodies.

Dynamical calculations by Ovenden, indicating the former existence of a 90-Earth-mass planet in the asteroid belt, have now been supported by a study of orbital element distributions of very-long-period comets. The indicated epoch for disintegration of the planet is just 5 x 106 years ago.