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A brief history of research concerning the risk of impacts by asteroids or comets onto the Earth is presented with attention to the role played by the IAU. Special focus is placed on the events that occurred about 20 years ago, which caused the IAU to become seriously involved in dealing with the impact hazard and to take a leading role in international coordination of these activities.
A total of 16 among the new IAU members have asked to join Commission 20; they are: Jerome Berthier, Nicholas J. Cooper, Marco Delbò, Romina P. Di Sisto, Michael W. Evans, Tetsuharu Fuse, Ludmila Hudkova, Yurij N. Krugly, Elena N. Polyakhova, Zhanna Pozhalova, Alessandro Rossi, Qi Rui, Jonathan D. Shanklin, Slawomira E. Szutowicz, Gino Tuccari and Hong-Suh Yim. Moreover, two requests to join the Commission have been received by astronomers that are already IAU members: Peter De Cat and Ricardo A. Gil-Hutton.
The observing program of the Nordic Near-Earth-Object Network (NEON) accrues knowledge about the physical and dynamical properties of near-Earth objects (NEOs) using state-of-the-art inverse methods. Photometric and astrometric observations are being carried out at the Nordic Optical Telescope. Here, the NEON observations from June 2004–September 2006 are reviewed. Statistical orbital inversion is illustrated by the so-called Volume-of-Variation method. Statistical inversion for spins and shapes is carried using a simple triaxial shape model yielding analytical disk-integrated brightnesses for both Lommel-Seeliger and Lambert scattering laws. The novel approach allows spin-shape error analyses with the help of large numbers of sample solutions. Currently, such spin-shape solutions have been derived for 2002 FF12, 2003 MS2, 2003 RX7, and 2004 HW. For (1862) Apollo, an unambiguous spin-shape solution has been obtained using the conventional, convex inversion method and, for (1685) Toro and (1981) Midas, the conventional method has been applied repeatedly to map the regime of possible solutions.
Recent years have seen a revolution in the possibility to understand cometary capture, i.e., the origin of the cometary population that moves in orbits confined to the inner Solar System. This is due to the discovery of the major source populations: the Edgeworth-Kuiper belt, and the scattered disk. We review the current understanding of the links between the distant sources, including the Oort cloud, and the observed, short-period population, and the problems that remain. Some highlights of present research in this field will serve to illustrate recent progress and major issues that are currently arising.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html
Because the division consists of many very active commissions, most activities are included in the reports of the individual commissions. This report highlights a small subset of the major achievements that are covered in detail in the reports by the commissions. Some administrative activities of the division and reports of the divisional working groups and committees are also included as subsequent sections of this divisional report.
After a brief summary of current concepts on cometary nuclei, resulting from recent advances in observational techniques and laboratory experiments on cometary analogue materials, some of the outstanding problems facing theoretical modelling and interpretations of the new data is described. Emphasis is placed on the outgassing of species more volatile than water over the history of cometary nuclei, the structure of the nuclei in terms of porosity and tensile strength on a variety of different scales, and the volumetric importance and distribution of the refractory “dust” component. Near-surface interactions of dust grains and outflowing gas may be associated with the formation of large dust aggregates and mantling of the nuclei.
Cometary nuclei are often considered to be the most primitive bodies of the solar system. Thus it is particularly important to find out what structural changes may have been brought about as a result of their thermal evolution. Is there reason to believe that the bulk composition of the nucleus of a present-day short-period comet may differ from that of the original planetesimal in the solar nebula? Apart from the development of a non-volatile surface layer (‘dust mantle’), what further depth-dependent differentiation can we expect in such a nucleus? These are the ultimate questions addressed in this paper, and attention is focused on the two most active stages of thermal evolution: the early planetesimal stage with internal heating basically due to radioactive decay, and the recent or present cometary phase with strong external heating due to insolation of the surface.
Various aspects of comet/asteroid distinctions and interrelations are reviewed with emphasis on recent work and paying special attention to the following problems: characteristics of cometary activity at large heliocentric distance and uniqueness of comet P/Schwassmann-Wachmann 1 with respect to physical properties, the rôle of Trojans and other small bodies in the outer planetary system concerning comet/asteroid classification, possibilities for physical evolution of comets into asteroids, orbital and dynamical overlap of the comet and asteroid populations, and the cometary versus asteroidal origin of Earth-approaching asteroids. With regard to these latter questions it is argued that recent discoveries indicate a more substantial probability for Jupiter family comets to develop into asteroidal objects than earlier believed, and several examples of cometary association for newly discovered Apollo-Amor asteroids are also referred to. However, the fractional cometary contribution to the traditional Apollo-Amor asteroid population (aphelia far inside Jupiter’s orbit) apparently can not yet be reliably estimated.
Our investigation demonstrates that the asteroidal objects 1983 SA and 1983 XF can be considered as temporary visitors of the 4/3 and 2/1 resonance with Jupiter, respectively. Evidence is given that both objects are first rank candidates for a cometary origin. The case of 1983 SA is remarkable in so far as (279) Thule is up to now the only known asteroidal 4/3 librator.
The possible effects of non-gravitationnal forces on the motion of the comet P/Boethin are investigated for various values of the orbital period. A time interval of 2000 years backward and forward is treated. The authors find in all cases that the comet librates temporarily around the 1/1 resonance with Jupiter as a remote jovian satellite during at least two centuries.
In 1975 a remarkably large number of short-period comets were discovered - only the most recent number from 1977 is comparable. While the average discovery rate has been 0.8-1.0 new short-period comets per year (Kresák 1974), in 1975 there were six discoveries. In five of the cases IAU Circulars soon afterwards contained indications that close encounters with Jupiter had recently taken place (Marsden 1975, Kastel' 1975). For two of the comets, P/Kohoutek and P/Smirnova-Chernykh, also pre-encounter orbital elements were outlined, suggesting that substantial reductions of the perihelion distances had occurred.
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