As is evident from Director Dan Green's detailed report below, the Central Bureau for Astronomical Telegrams (CBAT) continues to function in a very reliable and efficient way. The electronic telegrams (CBETs) introduced in 2002 have proved to be a great success; they allow for very rapid dissemination of more extensive information than there is room for on printed Circulars, and they are frequently cited in the astronomical literature. The workload on the Director seems to be forever increasing. It is therefore gratifying that an Assistant Director, Gareth Williams, has been appointed. Since he also serves as Associate Director of the Minor Planet Center (MPC) and performs many of the computations on which the CBETs and the Circulars are based, the ties and mutual benefits between the CBAT and the MPC have been made even stronger.

Since the IAU General Assembly in Sydney in July 2003, the WGPSN has conducted its business through numerous e-mail exchanges between the members. A nomenclature workshop was held at Hardingasete, western Norway on September 1–3, 2005. That meeting was attended by eight members from the WG and two from the Task Groups (TG) for the small bodies and for the outer solar system. Input to the meeting had also been received by e-mail from other members.

The past triennium has continued to see a huge influx of astrometric positions of small solar system bodies provided by near-Earth object (NEO) surveys. As a result, the size of the orbital databases of all populations of small solar system bodies continues to increase dramatically, and this in turn allows finer and finer analyses of the types of motion in various regions of the orbital elements space.

This Comm. 20 triennial report is prepared as a short summary of the scientific progress in the fields of interest, where, by necessity, only a few highlights are to be mentioned. On the Comm. 20 webpage, the interested reader may find a more complete list of recent references to the relevant literature. Its URL at the time of writing (October 1999) is www.astro.uu.se/IAU/c20/, but mirror sites are expected to have been set up at the time of appearance of these Transactions.

Since July 1996, 815 new names on features on bodies in the Solar System have been assigned by the WGPSN and approved at the IAU General Assembly in Kyoto in 1997. Of these names, 666 were for Venus, 17 for Mars, 3 for the Moon, 125 for the Galilean satellites, 3 for the Uranian satellite Miranda, and 1 for the minor planet Ida. 71 additional names mostly on Venus have been selected and have been given or are awaiting provisional approval by the IAU Executive Committee (EC). These names are up for final approval at the next IAU General Assembly.

Somewhat more than a century after its introduction for the purpose of discovering minor planets, photography is now rapidly giving way to the CCD as the technology of choice for observing these bodies. A CCD has been used in scanning mode in the University of Arizona's ‘Spacewatch’ program for the discovery of minor planets since as long ago as 1984 (Gehrels 1984, Gehrels et al. 1986), while a CCD in stare mode was first applied as a matter of routine to an established observing program for astrometric follow-up in 1989—that at the Oak Ridge Observatory in Massachusetts (McCrosky 1990). After its initial 1984–1986 success, Spacewatch was modified with the help of a larger CCD and improved computer software and with the adoption of the particular mission of searching for NEOs, or minor planets (and comets) that pass close to the earth (Rabinowitz 1991, Scotti 1994). The Oak Ridge program utilizes a 1.5-m reflector, and the first CCD observations were reduced using the Astrographic Catalogue, the mainstay of the Oak Ridge photographic program back to its inauguration in 1972, as well as of other older photographic programs in which the fields observed were significantly less than 1° across. Within months, the availability, on CD-ROMs, of the STScI Guide Star Catalogue (Villard 1989) effectively consigned the venerable AC to the scrap-heap, and the rapid development of ready-made and relatively inexpensive CCD systems (e.g., di Cicco 1992) has recently increased the volume of CCD astrometry considerably, allowing it to be conveniently and reliably carried out, even by amateur astronomers. At the present time, very nearly 50 percent of the astrometric observations, typically 6000, published each month in the Minor Planet Center's Minor Planet Circulars are obtained by means of a CCD.

Past surveys are described in the logical sequence of (1) comets visually, (2) asteroids visually, (3) asteroids photographically and (4) comets photographically. Plots show the evolution of asteroid surveys in terms of visual discovery magnitude and ecliptic latitude, and similarities and differences between surveys for the different types of body are discussed. The paper ends with a brief discussion of more recent discovery methods and some thoughts on the future.

The recent and current situation with regard to our analysis and understanding of the nongravitational effects in cometary motions is reviewed. Comets can be categorized according to the different physical situations that may exist in their nuclei. Further experimentation with theoretical models and empirical fits to observations is encouraged.

The Commission has been fortunate in being able to continue its work during the last triennium with a minimum of effort, and thanks for this should go largely to the Director of the Bureau, Dr B.G. Marsden, for his untiring dedication, as well as to the Smithsonian Astrophysical Observatory for its general support. If any question has arisen, it was where the line should be drawn between paid and unpaid contributions to the Circulars. This involves a somewhat delicate balance of interests between, on the one hand, the encouragement given to certain types of observations which have traditionally been published in the Circulars, (e.g. accurate positions of comets), and, on the other, the income on whi h the continued successful operation of the Bureau depends. The position will therefore have to be kept under review at all times. At present all evidence points to general satisfaction with current policy.

One of the most extensive programs of systematic observations of minor planets currently being conducted is that at the Crimean Astrophysical Observatory. The observed positions, together with the residuals from the predictions in the annual Ephemeris volumes, have been listed in the Minor Planet Circulars for several years now; and inspection of the residuals yields information about the accuracy of minor planet ephemerides as a whole.

The observations are made using a 40 cmf/4 double astrograph, the limiting magnitude being about 18. Between June 1968 and October 1969, corresponding to one synodic period of an average minor planet, about 40 percent of all the numbered objects were observed. These observations may be regarded as reasonably representative of all the planets. Observations were made of 60 of the objects with numbers in the 100’s and only 29 of those in the 1100’s, but in general the distribution was surprisingly uniform.

There has long been speculation as to whether comets evolve into asteroidal objects. On the one hand, in the original version of the Oort (1950) hypothesis, the cometary cloud was supposed to have formed initially from the same material that produced the minor planets; and an obvious corollary was that the main physical difference between comets and minor planets would be that the latter had long since lost their icy surfaces on account of persistent exposure to strong solar radiation (Öpik, 1963). However, following a suggestion by Kuiper (1951), it is now quite widely believed that, whereas the terrestrial planets and minor planets condensed in the inner regions of the primordial solar nebula, icy objects such as comets would have formed more naturally in the outer parts, perhaps even beyond the orbit of Neptune (Cameron, 1962; Whipple, 1964a). Furthermore, recent studies of the evolution of the short-period comets indicate that it is not possible to produce the observed orbital distribution from the Oort cloud, even when multiple encounters with Jupiter are considered (Havnes, 1970). We must now seriously entertain the possibility that most of the short-period orbits evolved directly from low-inclination, low-eccentricity orbits with perihelia initially in the region between, say, the orbits of Saturn and Neptune, and that these comets have never been in the traditional cloud at great distances from the Sun.