Abundance considerations suggest that comets are likely to be the most pristine minor bodies in the solar system. In proportion to solar abundances, the present scanty data suggest that cometary oxygen is not depleted, whereas carbon is by a factor of 4 and hydrogen, by a factor of 2000. This implies that comets are less depleted in H, C, N, O than CI chondrites, namely 10:1 in hydrogen, 4:1 in carbon and 3:1 in oxygen. These results have been obtained by using dust-to-gas ratios in comets, to measure the relative abundance of silicon and metals to volatile material, and the spectra of atomic lines, mainly from the vacuum ultraviolet, to determine the H/O and C/O ratios of the mixture of volatile molecules.

The intensity ratio of the continuum to the molecular emissions was estimated in the spectra of eight-five comets. These consisted of 34 new, 1/a (orig) < 100 x 10-6 AU-1, 31 more evolved with 102 < 1/a < 104, and 20 periodic comets, P < 103 years. In each age group the comets were divided into two distance intervals, the first observed at less than 1 AU from the sun and the second category observed at more than 1 AU. No significant difference among the patterns of relative intensity distribution among four age groups was found. No general evidence for a difference between pre- and post-perihelion observations was found although in a few cases a pronounced effect occurs. Four conclusions are drawn. (1) There is no readily apparent difference in continuum to emission intensity ratio between new and more evolved comets. (2) An intrinsic distribution of this characteristic does occur. (3) Periodic comets with weak continua derived from new comets with the same property. (4) No weakening of the continuum in general occurs following perihelion passage. The infrared evidence for Comet Encke suggests that the faintness of its continuum may be caused by a size distribution containing only particles larger than about 10 μm.

For many comets the accelerations radially from the Sun have been determined by deviations from gravitational orbits. The radial force per unit area can be calculated on the basis of certain assumptions concerning the nature of the vaporizing material, the physical circumstances, and the geometry of the cometary nucleus. The observed accelerations combined with the calculated forces yield numerical values for (1-A)/R where A is the albedo, and R the radius of the nucleus. At extreme solar distances photometry provides the well-known quantity area-times-albedo or RA1/2. Solutions are then possible for R and A for the nuclei of the comets. The derived quantity, (1-A)A1/2, provides a limiting check on the basic assumptions and, therefore, on the basic physical properties of the nuclei. For ten short-period comets with q < 1.5 AU, the observations are satisfied by H2O ice. About half show “spotty” surfaces.

For comets of a single apparition H2O ice is generally not volatile enough to produce the observed radial accelerations. The resulting problems are discussed including the possibility that in some cases displacements of the photometric from the gravitational nucleus may produce spurious non-gravitational accelerations.

The physical characteristics of comets vary with their orbits and with their age. New comets on their first near solar passage from the Oort cloud are extremely active. The activity falls statistically with increasing age. This sequence must represent a corresponding sequence or layering of structure from the surface of a new comet inwards and is described qualitatively in this paper.

The excessive activity of new comets is ascribed to cumulative cosmic-ray damage that activates the outer few hundred gm cm-2 from the surface. The total energy input in 4.6 x 109 yr reaches 50,000 cal gm-1 near the surface so that both crystalline structures and molecules are severely damaged if not completely destroyed. Annealing at T - 10 K must be very small. Hence significant exothermic energy in the form of defects, vacancies and radicals is added to produce the extraordinary activity observed in new comets.

Other aspects and problems of cometary activity are discussed.

The sublimation or artificial icy cometary nuclei with different inclusions is investigated. Formation of meteoroids is discussed in connection with new polarizational observations of comet West. The possibility of chemical reactions on the sublimating surface is noted.

Certain astrophysically interesting properties of ice grains depend upon their crystallinity. Lower temperatures such as those in the outer layers of a dust cloud favor amorphous ice, the center of the cloud may favor crystalline ice. Amorphous grains have different binding energies than crystalline grains resulting in different evaporation rates and affecting the distance from the sun at which dust is released in cometary tails. They have also different sticking and accommodation coefficients which affect the formation of H2 and of other molecules on their surfaces. In contrast to crystalline grains, amorphous grains can be easily ionized by the stellar ultraviolet photons with important consequences for surface reactions, heating, etc.

The new model, which has recently been proposed for the motions of fragments of the split comets, is critically examined through a comparison with the traditional approach. It is concluded that the new model, based on the premise that the rate of separation of any two fragments of a split comet is determined primarily by the differential nongravitational forces in their motions, is preferable in many a respect to the traditional hypothesis, built on the assumption that significant impulses are exerted on the fragments at the time of splitting. The new model appears to have interesting implications for the physics of cometary splitting, suggesting that the breakup mechanism may essentially be non-violent or of a low degree of violence, and for estimates of cometary masses.

The composition of the inner coma is modeled assuming that about 30 chemical species composed of H, C, and O undergo reactions. Ionization and dissociation by solar radiation and over 100 forward and reverse reactions between atoms, molecules, and ions are considered in the kinetics. Vaporization from a simple H20 - C02 nucleus provides the initial composition of the gas near the surface.

The motion of Comet Halley is investigated over the 1607-1911 interval. The required nongravitational force model was found to be most consistent with a rocket-type thrust from the vaporization of water-ice in the comet’s nucleus. The nongravitational effects are time-independent over the investigated interval.

The radio observations of the 18 cm wavelength OH main lines in comets Kobayashi-Berger-Milon (1975 h) and West (1975 n) confirm the model of ultraviolet pumping by the sun, that we proposed to explain our previous observations of comet Kohoutek (1973 f). The OH scale length in (1975 n) is at least 106 km at a heliocentric distance of 1 AU, in agreement with that for (1973 f).

An analysis of the 2796 visual observations as well as of 282 V, B or U photoelectric observations was carried out using a two-parametric model for the light curve of the comet. After having applied an aperture correction to the visual observations, the following photometric parameters were derived: before perihelion n = 2.5, after perihelion n = 3.6 and a drop of m0 by 1.5 -1.9 after the perihelion passage. From the UBV photoelectric data the coma was found to be more gaseous before the perihelion passage.

Oort’s work on the cometary cloud is reviewed and extended using new data from 99 comets with high-quality orbits. These data clearly show the pronounced pile-up of the “original” reciprocals of the semi-major axes at values of less than 0.000100 AU-1. This concentration is found to be even more striking for comets of large perihelion distance, and the possible significance of this is discussed. Lyttleton’s criticisms of the concept of the Oort cloud (or, as only he calls it, “shell”) are reviewed and dismissed as largely irrelevant. A set of data on 96 comets with orbits of second-class quality is also considered.

A Monte Carlo model of stellar perturbations of the Oort cloud is used to study the distributions in energy and perihelion of comets entering the planetary region for the first time. The model is run for a variety of initial states and a range of velocity perturbations. In all cases the resulting orbits are uniformly distributed in perihelion distance in the planetary region, q < 20 AU. Most orbits are confined to a fairly narrow range in 1/a and hyperbolic orbits are rare.

Some problems of the current interpretation of the Oort Cloud are discussed. If observational selection is taken into account, no significant difference in physical characteristics of old and new comets is apparent, in spite of the required change in the radiation mechanism after the first passage near the Sun. The abundance of new comets puts special requirements on the relative size of the perihelion distances at which they lose their orbital characteristics and original surface properties, respectively. Stellar perturbations do not seem to be effective enough to displace the perihelia of new comets in a single revolution from the zone of insignificant planetary perturbations into the zone of detectability. Therefore, many physically new comets should appear as dynamically old, which is at variance with observational evidence. It is speculated whether the Oort Cloud really represents a reservoir of comets passing near the Sun for the first time or, alternatively, a region where the capability of building up extensive comas is being restored within periods of the order of 106 to 107 years.

When the perturbing planets are Uranus and Neptune, the perturbations on comets are so much weaker than with Jupiter and Saturn that a study of the comets’ orbital evolution, using exact numerical integration, would require 200 times more revolutions. This is hardly practical with present computers. Here we describe results with a simulation approach, the “Monte Carlo (random walk) method.” The proper distribution shape for the perturbations in energy are found from a few thousand numerical integrations, then this distribution of perturbations is applied to millions of simulated orbit-revolutions. This method reproduces earlier Jupiter results in 1/500 the former computation time. We find that Neptune can capture near-parabolic comets with perihelia in the range of 30 to 34 AU, increasing their 1/a-values and decreasing their perihelia until they reach a region where Uranus can interact. Uranus in turn passes some of these on to Saturn, who passes some to Jupiter. Ultimately a few reach the orbits of the visible short-period comets. The process requires about 200,000 comet orbit-revolutions, 4 × 108 years, and the efficiency is one in 6000. The rest of the comets are ejected on hyperbolic orbits.

The gravitational perturbations by Jupiter per orbital revolution are calculated for six selected groups of 1000 hypothetical random comets of low inclinations, perihelia units 6 a.u. and aphelia in the range (4 a.u., 13 a.u.). The results are organized in the form of empirical distributions and their statistical properties are investigated in detail. They provide a rich material for the study of problems such as the orbital evolution of short-period comets and the origin of the Jupiter family.

Jupiter, Saturn, and Uranus have 15 (or possibly up to 18) regular satellites (i.e., with eccentricities and inclinations near zero) which are generally assumed to have been formed along with the planets and near their present orbits. We present evidence for their having been formed much later in the history of the solar system and in initial orbits very close to their respective planets. They then evolved to their present orbits, principally by tidal friction. Their source may be captured material, possibly of cometary origin.

A new heuristic derivation of the radiation pressure and Poynting-Robertson drag forces is presented for particles with general optical properties; previous derivations considered only perfectly absorbing materials. The equation of motion for a particle of mass m and geometrical cross-section A, moving with velocity v through a radiation field of energy flux F, is (to terms of order v/c)

where r̂ is the radial unit vector, ṙ is the radial velocity, and c is the speed of light. The radiation pressure efficiency factor Qpr includes both scattering and absorption, it is evaluated using Mie theory for small spherical particles with measured optical properties that are irradiated by the actual solar spectrum. Very small particles (<0.01 μm) are not substantially affected by radiation forces.

Evidence for the chemical composition of cometary meteoroids is available from the spectra of shower meteors, from the analysis of extra-terrestrial dust particles, from a study of residues in the bottom of microcraters on plates exposed to the interplanetary environment, and from measures of the relative abundances of non-atmospheric ions in the E-region of the earth’s upper atmosphere. Quantitative measures of chemical abundances in meteoroids, based on the four techniques listed, show that in general the cometary meteoroids encountered by the earth conform to the carbonaceous chondrites type 1 in the case of the commonest metallic elements. There is also qualitative evidence of the presence of significant quantities of some of the light volatiles.

A comparison has been made of the data of Millman’s (1972) work on the relative content of Na, Mg, Ca and Fe in Draconids (Giacobinids) with corresponding data for carbonaceous chondrites of type CI, CII, CIIIV, and CIIIO and ordinary chondrites of type H, L and LL. The correlation of values Fe/Mg, Na/Mg and Ca/Mg in Draconids agrees with their successive change in carbonaceous chondrites from CIIIV to CI. By its composition, meteoric substance is more distant from CIII than from CI. This result agrees with the idea that cometary matter condensed farther away from the sun and contains lighter volatile components, than CI chondrites.