Organic chemicals — compounds that contain carbon — are the substance of life and pervade the universe. Is there a connection between comets, which are rich in prebiotic organics, and the origin of life? Current concepts of biomolecular evolution are first reviewed, including the important paradigm of catalytic RNA. At the very least, impacting comets appear to have supplied a substantial fraction of the volatile elements required for life shortly after the Earth formed. Some impacting material may even have survived chemically intact to directly provide necessary complex prebiotic organic chemicals. For life to originate and evolve in comets themselves, liquid H2O would be absolutely required: arguments for and against 26Al radiogenic melting of cometary cores are presented. Cometary panspermia, if theoretically possible, is not necessary to explain the origin of life on Earth. The Halley spacecraft provide evidence against Earth-type microorganisms in this comet’s dust.

Dynamical studies during the past decade have resulted in an almost explosive increase in our understanding of the Oort cloud of comets, which surrounds the solar system. Cometary orbits in the cloud evolve under the complex interaction of stellar, galactic, and giant molecular cloud perturbations, as well as planetary and nongravitational perturbations when the orbits re-enter the planetary region. Evidence has continued to build for a dense, inner Oort cloud of comets which acts as a reservoir to replenish the outer cloud as comets there are stripped away. A ring of comets beyond the orbit of Neptune, which may be the source of the short-period comets, is also likely. Both the estimated number and mass of comets in the Oort cloud have grown such that the total mass may be comparable to the mass of the planets. Temporal variations in the flux of comets from the Oort cloud into the planetary region by a factor of 50% are typical, and by factors of 20 to 200 are possible. The most intense cometary “showers” may have serious implications for biological extinction events on Earth as well as for the impact history of planets and satellite systems. Comets in the Oort cloud are processed by galactic cosmic rays, heated by nearby supernovae, eroded by interstellar dust impacts, and disrupted by mutual collisions (in the inner cloud). A detailed estimate of the Oort cloud’s dynamical history is not possible because of the inability to reconstruct the Sun’s varying galactic motion over the history of the solar system, and because of uncertainty over where comets actually formed. However, it is likely that a substantial fraction of the original Oort cloud population has been lost to interstellar space. We are approaching the time when Oort clouds around other stars may be detectable, though searches to date have so far been negative.

The observed frequency of passages of Earth-crossing long-period (LP) comets (P > 200 yr) is about three per year for comets brighter than absolute magnitude H10 ∼ 10.5. About one out of six LP comets is estimated to be new, i.e., making its first passage through the inner planetary region. The sample of observed LP comets shows an excess of retrograde orbits that may be accounted for by the shorter dynamical lifetimes of comets on direct orbits due to planetary perturbations. The original semimajor axes of new comets concentrate in the range 7 × 103 ≳ aorig ≳ 4 × 104 AU, which tells us about the region of the Oort cloud where forces other than planetary perturbations act with the greatest efficiency. Yet the distribution of original semimajor axes cannot tell us anything about the existence of a dense inner core of the Oort cloud. Besides planetary perturbations, passing stars, molecular clouds and the galactic tidal force also influence the dynamical evolution of Oort cloud comets. The observed distribution of the aphelion points of near-parabolic comets shows such a dependence on the galactic latitude. Molecular clouds and stars penetrating very deeply in the Oort cloud are found to give rise to major enhancements in the influx rate of new comets, known as comet showers, at average intervals of a few 107 yr.

An important issue to solve concerns how the frequency of comet passages varies with time, in particular as regards to the current level of comet appearances. Should we be passing through a highly intense phase, most aphelia of the incoming Oort comets would concentrate on the sky area where the strong perturber exerted its greatest effect. By contrast, the observed galactic latitude dependence of the aphelia suggests a dominant influence of the vertical galactic tidal force as compared with random strong perturbers. This seems to indicate that the frequency of comet passages is currently at, or near, its quiescent level. Whether intense comet showers are reflected in the impact cratering record is still a debatable issue. A periodicity of ∼ 26-30 Myr in the impact cratering rate is quite uncertain, owing to the small size of the sample of well-dated craters and the noise from background impact craters from asteroids.

The family of short-period (SP) comets (orbital periods P < 20 yr) has long been regarded as the dynamical end-state of new comets on low-inclination orbits captured by Jupiter. However, if SP comets came from a spherical population of comets (e.g., incoming new comets), we should expect to find a percentage of them on retrograde orbits, which contradicts the observations. An alternative hypothesis for the origin of most SP comets is that they come from a trans-Neptunian comet belt. Extensive searches aimed at detecting faint slow-moving objects are required to assess the size of the comet population in the outer planetary region. Modeling of the transfer rate of comets from an outer belt to SP orbits gives transient populations between Saturn and Neptune on the order of 106 – 107 bodies. This is roughly comparable to the upper limit set by the most recent searches of outer solar system bodies.

The impact crater production rate of comets, at the present time, can be estimated to be on the order of 10% of the value corresponding to asteroidal impacts. These estimates, however, are subject to large uncertainties in the brightness-mass relation of comets and crater scaling law. The Earth could have received about 2 × 1020 g of cometary material over the last 4 billion years — if the injection rate of new comets remained constant in the time interval. Within the context of H2O inventory, the cometary influx should have rather minor effects. On the other hand, because of the paucity of H2O content in the atmospheres of Venus and Mars, cometary impact could strongly modulate their water contents.

Because there is no necessary connection between the time required to remove the volatile component of a cometary nucleus by solar heating (physical lifetime) and the dynamical lifetime of a comet, it is possible that a comet may evolve into an observable object of asteroidal appearance. Almost all comets have dynamical lifetimes much shorter than their physical lifetimes and in these cases complete loss of volatiles will not occur. Mechanisms do exist, however, whereby a small but significant fraction of comets will have longer dynamical lifetimes, permitting them to evolve first into Jupiter-family short period comets and then into comets with relatively safe decoupled orbits interior to Jupiter’s orbit. Observed Jupiter-family objects of asteroidal appearance (e.g., 1983SA) are much more likely to be of cometary rather than asteroidal origin. “Decoupling” is facilitated by several mechanisms: perturbations by the terrestrial planets, perturbations by Jupiter and the other giant planets (including resonant perturbations) and non-gravitational orbital changes caused by the loss of gas and dust from the comet. The dynamical time scale for decoupling is probably 105–106 years and almost all decoupled comets are likely to be of asteroidal appearance. Once decoupled, the orbits of the resulting Apollo-Amor objects will evolve on a longer (107–108 year) time scale, and the orbital evidence for these objects having originally been comets rather than asteroids will nearly disappear. Statistically, however, a large fraction of the bodies in deep Earth-crossing orbits with semi-major axes ≳ 2.2 AU are likely to be cometary objects in orbits that have not yet diffused into the steady state distribution. For plausible values of the relevant parameters, estimates can be made of the number of cometary Apollo-Amor “asteroids,” the observed number of Earthcrossing active and inactive short period comets, and the production rate of short period comets. These estimates are compatible with other theoretical and observational inferences that suggest the presence of a significant population of Apollo objects that formerly were active comets.

The evolution of meteor streams is controlled basically by: (a) the initial velocities with which the particles were ejected from the parent body; (b) gravitational perturbations by the planets; (c) radiation forces; and (d) collisions. This review focuses mainly on recent numerical modelling dealing with (b) and (a).

Ejection velocities spread the particles around the orbit, closing the ring in a few tens of revolutions. The greater ejection velocities of smaller particles cause more rapid dispersion both around the orbit and in the cross section.

A determination of the effects of gravitational perturbations must take into account the distributed properties of the stream. The stream’s evolution is dependent on the short-term impulse nature of planetary perturbations, as well as on long-term secular effects. The combined effects produce complex stream cross-sections as in the ribbon-like form of the Halley stream (Orionid and η Aquarid showers) or as in the changes in the annual position of peak shower activity shown by the Quadrantids. Perturbations may cause the orbit of a parent body to sweep rapidly across the orbit of the Earth. But the associated particle stream may not be lost as a meteor shower because it tends to become dispersed in a manner that ensures a continuing supply of particles in Earth-crossing orbits. The nodes of the observed meteoroid orbits may show very little motion compared with the rapid motion of the nodes of the orbit of the parent object.

Radiation effects contribute to size separation of particles. Very small particles are blown out of the stream or spiral in toward the sun because of Poynting–Robertson drag. Older meteor streams usually show a predominance of large particles.

Cometary dust particles, larger grains, and fragments as products of the disintegration processes are subjected to gravitational and nongravitational forces, causing their dynamical and physical evolution. Critical analysis of some fairly large differences in the observational data obtained for the mass productions, dust/gas ratios, cut-off masses, particle size/mass distributions, erosion factors, bulk densities, and other physical properties of particles leads to a more complex view of the cometary matter than is assumed in some of the current models. This view allows reasonable limits for the dynamical and physical interrelations between the debris and its parent bodies to be obtained.

Observational evidence for the aging processes in periodic comets is reviewed. This includes progressive changes of the comets’ absolute brightness, sudden destructive events (outbursts and splitting of cometary nuclei), temporary intermissions in activity, total disappearance, and existence of asteroidal objects moving in cometary orbits. Indirect statistical evidence is provided by the equilibrium between the aging rate and the net injection of comets into the inner planetary system. All of this information is consistent with typical active lifetimes of 200 to 500 revolutions at small perihelion distances (q < 3 AU). The active lifetimes are sometimes intermitted by dormant periods, which tend to occur especially during the latest phases of evolution. Splits and outbursts do not seem to have a decisive statistical effect on the survival time. Some periodic comets evolve into inactive asteroid-like objects, but the question of whether these still contain some supply of volatiles, and thus can renew their activity, remains open.

Recent evidence suggests that comets formed at low temperatures (≤ 25 K) and that, while the interiors have not been considerably altered since formation, the outer layers have undergone substantial modification. Comets exhibit a wide range of physical characteristics, some of which may be attributed to systematic physical differences between comets making their first close approach to the Sun from the Oort cloud (new comets) and those having made many approaches (old comets). These differences may reflect either primordial differences between two populations or the differences may be a manifestation of aging processes. There are many processes that might be responsible for causing aging in comets. These include: (i) radiation damage in the upper layers of the nucleus during the long residences in the Oort cloud, (ii) processing from heating and collisions within the Oort cloud, (iii) loss of highly volatile species from the nucleus on the first passage through the inner Solar System, (iv) buildup of a dusty mantle, which can eventually prohibit further sublimation, and (v) a change in the porosity, and hence the thermal properties of the nucleus. Although Oort’s (1950) original work on the comet cloud required that new comets fade after their first close passage, past searches for evidence of aging in comets have produced conflicting results, partly due to a lack of systematic data sets. An understanding of the evolutionary processes of comet nuclei that give rise to compositional or physical differences between ‘fresh’ Oort cloud comets and thermally processed periodic comets will improve our knowledge of the possibly primordial comet composition and therefore conditions in the early Solar System. Recent observations suggest that there are distinct differences between the two groups with respect to intrinsic brightness and rate of change of activity as a function of distance.