V. Zappala

Rotational properties of outer belt asteroids

Abstract We present new photometric observations of 13 asteroids (153, 190, 225, 279, 334, 420, 483, 528, 588, 692, 721, 940, 1583) orbiting in the outer part of the belt, beyond the Hecuba gap at 3.3 AU from the Sun. The resulting periods and amplitudes are then used to compare the properties of a main-belt asteroid sample and an outer-belt sample at similar sizes. The outer-belt sample includes objects which are likely to have been affected less by collisional evolution and to have retained a better record of their “primordial” features. The distributions of rotational periods are not different at a statistically significant level; however, the outer-belt sample does not include the “population” of show rotators which make the main-belt distribution markedly non-Maxwellian. As already pointed out by L. M. French (1987 Icarus 72,325) and W. K. Hartmann and coworkers (1988, Icarus 73, 487) for Trojans and Hildas, amplitudes in the outer belt are clearly higher than in the main belt. We suggest that elongated shapes might have been frequent among planetesimals in the outer belt, as a consequence of smaller impact velocities and lower densities than for the present main-belt asteroids.

Asteroid collisional evolution: I. Angular momentum splash: Despinning asteroids through catastrophic collisions

Abstract When an asteroid suffers a catastrophic impact, with only a fraction of the initial mass reaccumulating into a “rubble pile,” a significant amount of angular momentum is carried away by the escaping material. The reaccumulated core will have a lower rotation rate than it would without this effect, and may even have a net spin-down, relative to its preimpact spin rate, due largely to the preferential escape of high-angular-momentum fragments. We call this effect angular momentum splash in analogy with the angular momentum drain mechanism studied by A.R. Dobrovolskis and J.A. Burns (1984, Icarus 57, 464–476) for smaller cratering impacts. It is quantitatively assessed using the model for catastrophic fragmentation events developed by P. Paolicchi, A. Cellino, P. Farinella, and V. Zappala (1989, Icarus 77, 187–212). The splash mechanism is most effective at intermediate asteroids sizes (∼100 km), where the ejection velocities of fragments are of the same order as the escape velocities of the targets; the corresponding relative spin-down is found to be of the order of the core/target mass ratio. The splash-related spin-down is in competition with the spin-up due to angular momentum transfer from the projectile in noncentral impacts. Although the outcome depends on several collisional parameters, net spin-down prevails in a range of target sizes around 100 km, while larger objects are more often spun up by shattering impacts. This result holds for a wide range of collisional parameters and is in good agreement with the observed distribution of asteroid rotation rates with size.

Photoelectric photometry of 17 asteroids

Abstract Seventeen asteroids were observed photoelectrically in the V band at the Torino Observatory in 1983–1984 as part of the coordinated campaign for pole determinations. The obtained lightcurves allowed us to deduce new pole coordinates of four objects, while for the other three a check of previous results was possible. Additionally, the ambiguity for the rotation period of 776 Berbericia was solved in favor of a shorter value, and the importance of this problem was evidenced once more after analyzing the lightcurves of 69 Hesperia and 349 Dembowska. A new possible value of the period of 121 Hermione was also suggested.

Asteroids observations with the Hubble Space Telescope ? FGS I. Observing strategy, and data analysis and modeling process

Five main belt asteroids and one Trojan - selected mainly on the basis of their possible binary nature as deduced from light curve morphology - have been observed with the Fine Guidance Sensors (FGS#3 and FGSR#1) of the Hubble Space Telescope (HST). In this first paper we present the selection and observation strategy, data reduction and analysis. A careful analysis of the precision for derived parameters is also given. The HST/FGS proves to be valuable in determining asteroid sizes, shapes and spin axis orientations, and also to identify nearly-contact binary systems.

On the rotation of cometary nuclei and small asteroids

The spin rate distributions of comet nuclei and small asteroids are compared, and it is shown that Whipple’s (1982) finding of a faster average rotation for the asteroid sample was due to observational biases. In fact, the presently available rotational data do not exhibit any clear differentiation among comet nuclei and asteroids, except possibly for a higher abundance of short rotational periods among the Apollo-Amor objects

Asteroid Collisional Evolution: Outcomes of Catastrophic Impacts

We discuss how catastrophic collisional events have influenced some outstanding properties of asteroids, like their structure, rotation, shape and family membership. The ultimate outcomes of the collisional evolution are found to be strongly dependent on the asteroid size, because of the different importance of self-gravitational forces and of the different probability of impacts with high projectile-to-target mass ratio.

Asteroid rotation rates - Comparison between theory and observations

Harris’ (1979) theory on the collisional evolution of the asteroid spin rates is compared with the observational evidence provided by the statistics of photometric data. The resulting discrepancies can be qualitatively explained as due to different physical processes, occurred in the frame of the asteroid collisional history and connected in particular with the outcomes of catastrophic impact events.

Commission 15: Physical Study of Comets, Minor Planets, and Meteorites (L’Etude Physique des Cometes, des Petites Planetes et des Meteorites)

The present report of Commission 15 has been, as usual, prepared primarily by the chairpersons of the two working groups. E. Tedesco wrote the section about Asteroids and Meteorites, with the assistance of A. Cellino, G. Consolmagno and C.-I. Lagerkvist. W. F. Huebner prepared the section about Comets, with the assistance of J. Benkhoff, H. Boehnhardt, J. Brandt, M. T. Capria, A. Cochran, G. Cremonese, M. Duncan, W. Huntress, H. Levison, and G. P. Tozzi. Moreover, the whole document has been assembled by K. Muinonen, who did the final editing, to merge the two reports and fit the document into the allotted space. Material taken from both major areas regarding the relationship between comets and asteroids has been combined into a single section.

Division III: Planetary Systems Sciences (Sciences Des Systemes Planetaires)

The Division III activity in the triennial period since the time of GA XXIV in Manchester run in various aspects involving both scientific and business matters. The business of IAU Division III is normally carried out by its President and a Board, which consists of the current Presidents of the Commissions in the Division plus additional individuals for a total of 12 members. It is understood that the Commission Presidents may consult the members or organizing committees of their Commissions on divisional matters as they deem appropriate. Meetings of the entire membership of the Division are held at each General Assembly. The first such meeting was held at GA XXIV in Manchester and it approved the current plan of organization. The current Board was the first to be formed under the plan of organization adopted at the 24th GA. It was elected by Division III during GA XXIV. In the scientific area, the focus was placed in organizing through the respective divisional Commissions and supported bodies symposia and colloquia in the years 2000 through 2002, as well as planning scientific events during GA XXV. In the business area, there were discussions of some current issues with an intention to be better prepared for more thorough discussions and (when necessary) making decisions during the forthcoming Commission/Division business meetings which are scheduled to be held during GA XXV. Divisional Working Groups (CBSN, WGPSN, and WGESP) continuously performed business in their respective fields of small bodies and distant bodies naming, names assignment to the new discovered planetary satellites and planetary surface features, and collecting data on the new discovered extrasolar planets. The Board evaluated in detailed the Commissions proposals on meetings to be organized before requesting EC endorsement to have it as an IAU event, working in permanent contact with the IAU GS, AGS, and Secretariat.