The idea that the Solar System possesses a primordial isotropic cloud of comets has been with us in quantitative form for about thirty years (Oort 1950). A considerable edifice has been built on this proposition (e.g. Weissmann 1982), which has proved durable despite indications that the cloud may in fact be significantly non-thermal and display Galactic alignments (Tyror 1957, Richter 1963, Oja 1975, Yabushita et al. 1979, Radzievsky 1981). However with the discovery in recent years of a system of massive molecular clouds in the Galactic disc, it has become apparent that the environment in which the Oort cloud has to survive is very different from that envisaged in 1950; a re-appraisal of the standard picture is therefore called for.

One corollary of the discovery of the molecular clouds is that the traditional objections to an interstellar comet cosmogony, such as the lack of observed hyperbolic comets and the difficulty of growing and capturing them, may no longer apply (see for example the recent review by Clube & Napier 1982a, hereinafter CN). Another consequence, reviewed here, is that tidal effects due to molecular clouds may be so large that the Oort cloud is frequently disrupted (CN; Napier & Staniucha 1982). In this situation frequent replenishment is implied and it is therefore necessary to discriminate between the primordial and the observed Oort cloud and to consider possible alternative sources for the latter. Such an enquiry would provide constraints on the as yet unsolved problem of comet formation.

The dynamical evolution of comets in the Oort cloud under the influence of stellar perturbations has been modeled using Monte Carlo techniques. It is shown that the cloud has been depleted over the history of the solar system. Comets are lost from the cloud by direct ejection due to close stellar encounters, diffusion of aphelia to distances beyond the sun’s sphere of influence, or diffusion of perihelia into the planetary region where Jupiter and Saturn perturbations either eject them on hyperbolic trajectories or capture them to short-period orbits. The population of the cloud is estimated to be 1.0 – 1.5 × 1012 comets and the total mass is on the order of 1.9 earth masses. In addition to random passing stars, less frequent encounters with giant molecular clouds may play a significant role in randomizing the orbits of comets in the cloud and reducing the effective radius of the sun’s sphere of influence.

There is a difference in rotational properties between asteroids of various taxonomic types. The M and CMEU asteroids increase their spin with diameter and rotate much faster than asteroids of types C and S of the same diameter do. There is a difference in spin between the C and S types but the faster spin for the S asteroids larger than 150 km in diameter is probably not statistically significant. For both C and S asteroids the spin increases with diameter, but in a much less pronounced fashion than for types M and CMEU. The curve for the S asteroids is remarkably flat for asteroids smaller than 100 km.

The CMEU asteroids are more irregular than other types of asteroids. Both C and S asteroids have larger amplitudes for diameters about 100 km.

The family asteroids smaller than 100 km in diameter have a faster spin than non-family main-belt asteroids of the same size. Family asteroids show an increase of spin and lightcurve amplitude with decreasing diameter.

Population of the solar system by different types of interplanetary objects is reviewed. Their sources, evolution and interrelations are discussed, in particular in terms of their degree of stability, potential evolutionary paths and survival times.

If we investigate the interrelations among different classes of small solar system bodies, we must be aware that we deal with nonrepresentative and practically noncomparable data samples consisting of bodies differing either in size, or in orbits, or in their nature. So, practically, all our knowledge about small bodies - from kilometer-sized Apollo asteroids (including Atens) and Amor asteroids (designated below as AA bodies) to fine dust particles - refer to bodies on orbits intersecting the ecliptic plane at a heliocentric distance ≃ 1 A.U. To solve the problem of origin of this ensemble it would be advantageous if analogous data could be obtained at least for one more heliocentric distance. Since planetary atmospheres are excellent detectors for small cosmic objects, the necessary data could be obtained with the aid of artificial satellites orbiting a planet, e.g. Mars or Venus. Thus the source for the ensemble of Earth-crossing objects may not be correctly pictured. Are comets or asteroids the general supplier? We know the answer to this question only in two cases: (1) meteor streams such as the Leonids originate from comets, (2) meteorites originate from asteroids. But these two categories represent a minute part of the considered ensemble by mass. As to its other members- the selection of the source is a matter of tradition rather than a logic conclusion.

Camera networks for the study of bright fireballs now have a history approaching two decades• It was hoped that the networks would produce a statistically significant group of recovered meteorites with accurate orbits. Due to the great difficulty in locating the meteorites from a photographed event, there are still only three meteorites with orbits determined from suitable photographs; Pribram, Lost City and Innisfree (Ceplecha I96I, McCrosky et al. 1971, Halliday et al. 1978, respectively). Networks do, however, provide an alternative approach to the problem. Instead of determining approximate orbits from visual observations of recovered meteorite falls, it is now preferable to use reliable orbits from the camera networks for fireballs which are believed to have dropped meteorites that could not be located, or, that are believed to have been physically identical to meteorites, although no appreciable mass survived the atmospheric flight. This paper will review current knowledge based on this approach to the problem.

A detailed summary of the progress of meteor spectroscopy over more than a century has been published recently (Millman 1980), and only a few historical facts will be noted here. Serious research in this field was initiated by Alexander Stewart Herschel, a grandson of Sir William Herschel, in 1863 (Herschel 1865). Originally, the observational data were entirely visual records but, in the first decade of the twentieth century, a brief program for the photography of meteor spectra was carried out at the Moscow Observatory by S. Blajko (1907). Interest in this type of observation developed slowly and further programs were not attempted until the thirties. At the outbreak of World War II some 60 meteor spectra had been photographed. In the post-war period a general interest in the upper atmosphere led to the development of more efficient meteor cameras which employed replica gratings, and later electronic image-intensification systems recording on video tape (Hemenway et al. 1971; Clifton et al. 1979). As a result several thousand meteor spectra are now available for study.

It is shown that a number of features of meteor showers such as the correlation of orbital semimajor axes and longitudes of the ascending nodes with meteor magnitudes, the restriction of visibility periods and the displacement of maximum activity dates is explained by the joint influence of planetary perturbations, the Poynting-Robertson effect and its corpuscular analogue, light pressure and ejection velocities of different mass meteoroids from cometary nuclei.

A brief discussion is presented on the use of zodiacal light observations to derive information on the optical and physical properties of interplanetary dust and its spatial distribution.

This contribution will attempt to give a coherent and up-to-date view of the interplanetary dust cloud as well as to delineate some of the current outstanding problems. It is largely based on studies carried out at Laboratoire d’Astronomie Spatiale during the past years.

The formation of planetary rings is discussed in the context of formation theories of the gaseous planets. The subsequent evolution of Saturn’s ring system, both dynamically and mechanically, is described, and the consequences are compared with observations.

I have been invited to present a brief review of optical, infrared and ultraviolet observations of active nuclei. I will only describe a small sub-set of these observations, in particular those which I believe are most likely to be important in elucidating the properties of the central energy source to which we attribute the origin of all high energy phenomena. I will therefore restrict my remarks to certain aspects of BL Lac objects, optically violently variable quasars, quasars, Seyfert I galaxies and broad line radio galaxies. If we could understand these systems, we should be able to account for most other types of active nuclei, the Seyfert II galaxies, narrow line radio galaxies, all other types of Markarian galaxy, compact blue galaxy, etc. In my view, the recent observations of NGC 4151 provide us with a benchmark against which we should measure all other observations of active nuclei.

The systematical statement of the concept of the activity of the nuclei of galaxies was first made public by Ambartsumian about 25 years ago (Ambartsumian 1958, 1962, 1965). Since 1965 he has been advancing and developing the idea of the basic role of the nuclei of galaxies in their life and evolution.

According to Ambartsumian the “activity” of the nuclei of galaxies manifests itself mainly in the following forms:

1. 1. Outflow of ordinary gas matter (in form of jets or clouds) from the nuclear region at the velocity of up to hundreds of kilometres per second.

2. 2. Continuous emission of the flux of relativistic particles or other agents, producing high energy particles, as a result of which a radio halo may form around the nucleus.

3. 3. Eruptive ejections of gas matter (M82 type).

4. 4. Eruptive ejections of concentrations of relativistic plasma (NGC 4486, 5128, etc.)

I should like to emphasize on the form of activity which says:

• 5. Ejection of compact blue condensations with an absolute magnitude of the order of luminosity of dward galaxies (NGC 3561, IC 1182). Here the division of the nucleus into two or more comparable componentsis also presumed; initiating the formation of multiple galaxies.

A substantial fraction of active galaxies contain linear radio sources with sizes of a few hundreds or thousands of parsecs. Such sources are found in essentially all classes of active galactic nuclei, including Seyfert galaxies of both types, X-ray selected active nuclei, radio galaxies and quasars. The radio emission is clearly energised by the active nucleus, probably in the form of a jet. A number of observable consequences of the interaction of the jet with the interstellar medium of the galaxy are discussed. These processes include jet disruption by instabilities, acceleration of cosmic rays by shocks or turbulence, ionization and radial acceleration of interstellar clouds, creation of a hot thermal component through the agency of shock waves and bending of the jet by the ram pressure of a rotating interstellar medium.

For the purpose of this survey we shall use the term radio active nucleus to refer to compact radio sources which are found in galactic nuclei and which show activity either in the form of flux density variations or outward flow of material from the nucleus. There is great similarity between the observed properties of radio active nuclei with those of radio active quasars, and it is widely supposed that the quasar phenomena represents an extreme case of an active galaxy. In this paper however, we shall concentrate on compact radio sources found in the nuclei of identifiable galaxies, and quasars are discussed only to the extent that they help to understand the origin and evolution of radio active galactic nuclei.

Since radio active nuclei are typically in extent, they can be mapped only with VLBI techniques. Detailed maps are available for only a few of the strongest nuclei, but a much larger number can be identified from conventional interferometry with a resolution ~ 1”. In the few radio galaxies where most of the observed radio flux comes from the active nucleus, activity is observed directly on a time scale of months or even weeks. In others, it may be inferred from arguments based on synchrotron lifetime.

Recent X-ray observations of active galactic nuclei and Seyfert galaxies in particular are briefly reviewed. The application of the efficiency limit to rapidly varying luminous sources such as NGC 6814 is discussed. It is argued that the variability and probable MeV spectral turnover imply that most of the electrons which radiate the observed flux are only mildly relativistic. A possible link between the steep soft X-ray spectra and featureless optical continua of BL Lac objects is considered.

X-ray variability of active galactic nuclei is commonplace on scales from days to years. It also occurs, although rarely, on time scales as short as 200 seconds. Both these statements must be strongly qualified by the irregularity and insensitivity of the available observations. In the X-rays we expect that we are seeing deep within the active nucleus, near what is usually taken to be a massive black hole. The X-ray variability time scales may then give us the fundamental structural length scales. We would like to review data on active galaxies, present our new data on Einstein observations of BL Lac objects, and discuss all these in terms of a statistical quantification of the observations.

Gamma-ray observations from active galactic nuclei (AGN) are important in trying to understand the nature of their central sources. A handful of mechanisms can give rise to γ-rays either from nonthermal or from thermal processes. Hot accretion disks around massive black holes in the centers of AGN could provide the required thermal electrons, pions and relativistic electrons and positrons to explain both the X-ray and γ-ray emission.

The phenomena associated with Active Galactic Nuclei raise three main astrophysical problems: (1) the nature of the primary source of energy; (2) the physical conditions within the radiation source; (3) the nature of the population evolution over cosmological time-scales.

I shall outline the links between (1) and (2), (1) and (3), that briefly go as follows. The Prime Mover is very likely to be a converter of gravitational energy in a very compact mass configuration. The associated radiation source, if it is also very compact, is so efficient and loss-dominated as to require specific conditions for the power transport and supply to the radiating particles: collisionless, effected by electromagnetic fields coherent on scales collective or macroscopic, to the point of producing at times anisotropic bulk motions. Very compact Prime Movers working at high regimes need also a compact mass supply; the output from these compound engines undergoes a characteristic change that accounts for the type of population evolutions of the associated sources.

We discuss observations of the intense, time variable and very narrow 0.511 MeV positron-electron annihilation line from the Galactic Center, and the implications of these and other observations on the source of the positrons and their annihilation site.