We present a study of the influence of tidal splitting In order to clarify the role of comets as Earth impactors and bringers of organics to the young planet. Based on low values for the density and material strength, as indicated by Comet Shoemaker-Levy 9, we have modeled the evolution of a comet in an orbit leading to Earth impact. We find that cometary nuclei impacting the Earth with radii ≥ 0.1 - 0.3 km will split just before doing so. Nuclei of several km radius will split when passing within a distance ≤, 5 - 8 Earth radii. Cometary nuclei with radii ≥ 1km stand a much larger chance of being tidally split than of hitting the Earth. Consequently, streams of fragments and debris form in Earth-crossing orbits more often than large nuclei cause impacts. The mass distribution of impacting material is shifted toward sub-km objects, whereby the survivability of organics may be significantly increased. If the cometary impactors move in orbits where ≥ 5 passages within Earth’s Roche zone are expected, we estimate that most of the material reaching Earth’s surface will appear in the form of split fragments.

Developing countries have their own particular political, economic and cultural circumstances. There are, therefore, no unique solutions. However there are some factors which are common to all or most of them, namely the lack of human and financial resources and in most cases an absence of a political commitment to the advancement of science. Such situations are in a sense not peculiar to developing nations because in some developed countries astronomy has a ‘developing’ status. Even in countries where astronomy is well established, the commitment and allocations required to maintain the status quo need to be continuously addressed. Hence strategies for fighting this “constant battle” are relevant to all astronomers of the world, while being especially vital to those in the developing world.

Short lasting flashes, called as First Precursor (PC1), were observed by some ground-based near-infrared observations for the impacts of large-sized fragments of comet Shoemaker-Levy 9 (SL9) in July 1994. The impact detections by the spacecraft Galileo [2, 7] about 10 seconds after the PC1 detections by ground based telescopes, combined with the far-side impacts of SL9 fragments as viewed from Earth, suggest that the source of the PC1 should be located in the Jovian upper atmosphere above the limb, at which the atmospheric pressure is extremely low. Thus, an important problem on the PC1 is how does the falling cometary fragment, which is a huge meteorite, emit near-infrared in the extremely thin atmosphere. The ablation model, which is usually used for an impact bolide, can only estimate flux from the bolide in a dense atmosphere at visible wavelength. In this paper, we assume that the PC1s are thermal radiation from the fragments and attempt quantitative estimations of the PCI fluxes using a simple entry flash model.

Proposals on future astronomy facilities have been surveyed in China recent years. Among the others, the Large Multi-Object Spectroscopy Telescope (LAMOST) has been cosidered to have higher priority by the astronomical community because of its optimum fit of large aperture and large field of view (POV) in the design. The LAMOST has been approved as a national project this year.

Astronomy in Vietnam is still heavily underdeveloped. The few telescopes of the Observatory of Phu Lien (Hai Phong) were moved during the wars and become unusable. Most of the astronomers who graduated from the universities in the former Soviet Union are now working in other fields. Astronomy and astrophysics are among the basic sciences that Vietnam is willing to promote.

This section overviews the papers which were presented at the oral and poster sessions but are not published individually in these Proceedings. The papers can be topically grouped in five blocks and they will be reviewed within these blocks, as follows: 1. Comets and Asteroids: Observations and Models; 2. Accretion and Formation Processes; 3. Dust Particles and Fluxes; 4. Near-Earth Asteroids: Observations and Dynamics; 5. Comet P/Shoemaker-Levy 9 and Related Phenomena. Abstracts of most of these papers were published in the IAU GA Abstract Book, Kyoto, August 17-30, 1997, pp. 24-31.

Modern astronomy came to India in tow with the Europeans and was institutionalized more than 200 years ago by the (English) East India Company with the establishment in 1790 of Madras Observatory for assistance in navigational and geographical surveys. One hundred years later, in 1899, it was replaced by a solar observatory at Kodaikanal set up by the government to meet the European scientists’ demand for sunny skies and in the hope that a study of the Sun would help predict the failure of monsoons, the key factor then as now in Indian economy. It is mildly interesting to note that, when the scientific agenda was laid down by the Royal Society, no mention was made of climate or rains [1].

F.W. Argelander proposed to a meeting of the Astronomischen Gessellschaft in 1867 that an accurate catalogue of all stars down to 9th magnitude should be compiled. The sky was divided into zones of declination and observations were undertaken with transit circles at a number of observatories in an international collaboration. The importance of fitting these zonal observations to one global reference frame was realized, and for this purpose A. Auwers produced a Fundamental Catalog (FC) for the Northern and Southern hemispheres in 1879 and 1883, respectively. The accuracy of the positions was about half of an arcsecond. This was the first international celestial reference frame.

This reference frame was improved over the next hundred years by continued international cooperation which was later coordinated through the IAU. These efforts culminated in the FK5 which was issued in 1988. The progress in accuracy from the FC to the FK5 is shown in Figure 1. Meanwhile, radio astronomy through the technique of Very Long Baseline Interferometry (VLBI) began to produce positions of extragalactic sources with an accuracy of a milliarcsecond (mas), which is almost two orders of magnitude better than that of the optical positions of the stars in the FK5.

The Soviet Union developed science (including astronomy) to a high level, although largely isolated from the rest of the world. In 1990 about 2% of GDP, i.e. tenths of a billion dollars per year, were invested in science. The USSR occupied fourth place in the world in respect of its investment in science. More than a million persons worked in scientific research, with about 3,000 of those in astronomy.

We aim to help the scientifically lonely astronomer in small astronomical institutions. Criterion: Any country or university the IAU helps to develop astronomy must contribute significantly.

The goal of the work described here is to create the definitive catalogue for the new International Celestial Reference Frame (ICRF) using the best data and methods available at the time the work was done. This work is the joint cooperative effort of a subgroup of the IAU Working Group on Reference Frames which was formed expressly for this purpose in February 1995. The authors of this report constituted the subgroup. A fuller account of this report can be found in the introduction to the ICRF catalog (IERS 1997).

The ICRF of 608 sources presented here is based on essentially all the VLBI observations accu-mulated over about 15 years in several worldwide programs. Dual frequency Mark III data have both geodetic and astrometric applications. Most of the data (95% of nearly 2 million observations) were acquired primarily for geodetic purposes. The major geodetic programs include: NASA’s Crustal Dynamics Project/Space Geodesy Program and USNO’s NAVEX sessions for the terrestrial reference frame, as well as IRIS, NAVNET and NEOS sessions for monitoring Earth rotation. The geodetic programs have used the brightest radio sources, gradually concentrating on the most com-pact as sensitivity improved. These geodetic sources were also the foundation of astrometric work because of the large number of observations for the ~150 most commonly used. The astrometric programs which densify the sky include the Radio-Optical Reference Frame sessions done by US Naval Research Laboratory (NRL) and USNO and the space navigation efforts of Jet Propulsion Laboratory (JPL).

The Hipparcos reference frame is defined by the positions and proper motions of ~105 stars given in the Hipparcos Catalogue (ESA, 1997). It is the optical counterpart of the International Celestial Reference Frame defined by the radio positions of extragalactic sources. The linking of the Hipparcos frame to the extragalactic frame is described by Kovalevsky et al. (1997) and Kovalevsky (1997). In the present review we consider only the intrinsic properties of the Hipparcos reference frame.

A complete description of the contents of the Hipparcos Catalogue is contained in Volume 1 of ESA (1997). Recent overviews have been given by Perryman et al. (1997) and Perryman (1997). The principal astrometric data provided are the barycentric coordinates (α, δ) of the stars, the trigonometric parallaxes (π) and the proper motion components (μα* = μα cos δ, μδ). These parameters are given in the main catalogue for 117955 entries along with estimated standard errors and correlation coefficients, all data being referred to the common reference epoch To = J1991.25 on the Terrestrial Time (TT) scale. Typical ranges of the astrometric standard errors and some other characteristics of the catalogue are shown in Table 1.

The IAU program on Exchange of Astronomers is directed by Commission 38. The details of the program and the way it is carried out are decribed in the IAU Information Bulletins under the heading “Guidelines for Grants”. Here I only give some basic facts about the program.

First of all it is a general program. All astronomers can apply and until now Commission 38 has been able to offer a grant to those who fulfill the conditions in the “Guidelines for Grants”.

The IAU has as its task (Statute 1):

1. (a) to develop astronomy through international cooperation;

2. (b) to promote the study and development of astronomy in all aspects;

3. (c) to further and safeguard the interests of astronomy.

The Union to carry out its mandate is organized into 11 Divisions representing major branches of astronomical activity. The Divisions are composed of specialist Commissions -out of 39 Commissions, 31 operate within the divisional structure: of the 8 Commissions which do not, 6 may be regarded as representing “Services”. Additionally there are 4 Working Groups of the Executive Committee. Table 1 lists the Divisions and their constituent Commissions.

The Tycho Catalogue contains astrometric and photometric data for the one million brightest stars on the sky. About 900000 stars are high quality astrometric reference stars. Photometry in a blue and visual band is provided.

Two extensions of the catalogue are under way. One is the extension to one magnitude fainter stars by a second reduction of the original Tycho observations, resulting in a catalogue with a total of about three million stars. The other extension is the derivation of more accurate proper motions by means of the Astrographic Catalogue positions as first epoch. This latter work will be carried out first for the one million stars, and later for all the three million, resulting in the Tycho Reference Catalogue (TRC).

The sky-limited, large-area photographic surveys which are being used for the construction of the second generation Guide Star Catalog (GSC-II) will be for many years to come the largest astronomical database available. The survey plates are typically placed on a 5-degree grid, with each plate area of about 42 square degrees, for a total of more than 1700 plates covering the whole sky. The expected data volume is about 2 Tbytes per bandpass. The final product will consist of an all-sky catalog of positions, proper motions, magnitudes and at least one color to a minimum of V=18. Among the scientific motivations behind such an endeavor let us mention operation of ground-based telescopes and future space missions, observation planning, statistical investigations (see, e.g., Lasker et al. 1995, Cannon 1997)

Of the 185 countries that are Member States of the United Nations, nearly 100 have professional or amateur astronomical organizations. Only about 60 of these countries, however, are sufficiently involved in astronomy to belong to the International Astronomical Union. Only about 20 countries, representing 15% of the world’s population, have access to the full range of astronomical facilities and information.

Education is a necessary part of the development of astronomy, both to train astronomers and teachers, and to increase public understanding and appreciation of our science. In all countries (and particularly the developing ones), astronomy can attract young people to science and technology. Much is known about how to teach astronomy effectively, at all levels, both in and out of school. The problem is to publicize and implement this knowledge.

I will consider here some practical astrometric requirements, from the perspective of an optical astronomer working on objects much fainter than the limits of the primary catalogues, i.e., from 15 mag down to 20 mag and fainter. I take ‘high density’ to mean surface densities of order 100 or more stars per square degree; these two criteria are roughly equivalent (Bahcall & Soneira, 1980). The positions of such objects are routinely available to an accuracy of around 0.5-1.0 arcsec, e.g. from various digitised versions of the Schmidt Telescope photographic sky surveys. However, these sky surveys themselves readily yield internal accuracies at the 0.1 arcsec level, as demonstrated by repeated measures of the same plates. The problem is that the discrepancies can be ten times larger than this when different machines or plates are used, when a wide magnitude range is covered, when relative positions are compared over distances of a degree or more, or when absolute positions are needed for comparison with, say, radio catalogues. The challenge now is to improve this accuracy; we should to be able to specify the absolute positions of all well-defined optical objects to better than 250 mas, anywhere in the sky, at any epoch and regardless of magnitude or colour.