186,640 research outputs found
Collisional evolution of asteroids constrained by their present rotational properties
No full textThe abundance of asteroids having unusually large angular momenta of rotation in the diameter range 200 - 300 km implies that, if these angular momenta have a collisional origin, the asteroid population was of the order of a few times the present one at the time when the orbits were stirred up and the disruptive collisional process begun. The meaning of this result and the issue of the collisional vs. "primordial" origin of the currently observed asteroid rotations are shortly discussed
Updated collisional probabilities of minor body populations
No full textThe consistent increase in the discovery rate of new asteroids and Trans Neptunian Objects (TNOs) in these last years has urged an update of the values of intrinsic probability of collision and impact velocity for some minor body populations. With the statistical method of Dell'Oro & Paolicchi (\cite{delloro98b}), we have recomputed the values of impact probability and velocity for Hilda asteroids, for Trojans vs. Short Period Comets (SPC), and for TNOs. The algorithm of Dell'Oro and Paolicchi is particularly suited for the task since it can account for resonant behaviour (Dell'Oro et al. \cite{delloro98a}) and for the clustering of the perihelion longitude of Main Belt asteroids and Hildas, caused by the presence of a forced component in the eccentricity. The Hilda population turns out to be well sampled in the orbital parameter space since no significant changes are found for the collision frequency among Hildas, and of Hildas with Main Belt asteroids, although a much larger sample of orbits has been used in our computations (232 objects) vs. the smaller group used in previous computation by Dahlgren (\cite{dahlgren}) (40 objects). We also computed the impact rate of SPCs vs. Trojans that turned out to be an order of magnitude lower respect to the Trojans vs. Trojans impact rate. The relative velocity is instead about 30% higher. Using reasonable estimates of SPC and Trojan number densities, we find that approximately 1 every 100 collisions involving Trojans may be with an SPC. In the case of TNOs there is a consistent discrepancy between our values of the collision probability and impact speed, and those computed by Davis & Farinella (\cite{davis}). The consistent increase in the number of known TNOs (186 at present, only 16 at the time of the Davis and Farinella's work) has led to a better knowledge of their distribution in the phase space and, consequently, to more reliable estimates of the collisional probability and impact velocity
Physical and Dynamical Properties of Asteroid Families
No full textThe availability of a number of statistically reliable asteroid families and the independent confirmation of their likely collisional origin from dedicated spectroscopic campaigns has been a major breakthrough, making it possible to develop detailed studies of the physical properties of these groupings. Having been produced in energetic collisional events, families are an invaluable source of information on the physics governing these phenomena. In particular, they provide information about the size distribution of the fragments, and on the overall properties of the original ejection velocity fields. Important results have been obtained during the last 10 years on these subjects, with important implications for the general understanding of the collisional history of the asteroid main belt, and the origin of near-Earth asteroids. Some important problems have been raised from these studies and are currently debated. In particular, it has been difficult so far to reconcile the inferred properties of family-forming events with current understanding of the physics of catastrophic collisional breakup. Moreover, the contribution of families to the overall asteroid inventory, mainly at small sizes, is currently controversial. Recent investigations are also aimed at understanding which kind of dynamical evolution might have affected family members since the time of their formation. In addition to potential consequences on the interpretation of current data, there is some speculative possibility of obtaining some estimate of the ages of these groupings. Physical characterization of families will likely represent a prerequisite for further advancement in understanding the properties and history of the asteroid population
Catastrophic fragmentation as a stochastic process: sizes and shapes of fragments
No full textIt is rather difficult to understand theoretically and to analyse the experimental data concerning the mass and shape distributions of fragments created by catastrophic collisions. The fragmentation process is discussed as being a purely stochastical phenomenon; the size and shape distributions obtained in this way are compared with the results of laboratory experiments. The results are presented of some computer simulations of random volume fragmentation processes; they are a 3-D generalization of the numerical experiments described in Grady and Kipp (J. Appl. Phys. 58(3), 1210-1222, 1985). The features of the size distribution are discussed, comparing it with the expectations of the Mott-Linfoot and Grady-Kipp theories. In the literature the shape of fragments is defined in terms of the ratios B/A and C/A, where A, B, C are defined as the sizes of a fragment along three orthogonal axes. The definition of the shape of a fragment cannot be considered unique, since it is not obvious in which order to define the three axes when the fragments are not ellipsoidal. A few possible methods are introduced explicitly, and the resulting differences are discussed. In this light, the shape results (the mean values and the distribution of the axial ratios) obtained in recent laboratory experiments are rediscussed and critically reviewed. For what concerns the stochastical modelling, the results of various simulations, corresponding to different assumptions regarding fragmentation properties are presented. It is shown that the main features of the shape distributions from laboratory experiments cannot be satisfactorily reproduced. Comparison of the results with the outcomes of the semiempirical fragmentation model by Paolicchi et al. (Icarus 121, 126-157, 1996), as well as with some results coming out from hydrodynamical simulations, shows how only a ``global'' and physical model, not a purely statistical one (neither global nor ``local''), can afford to reproduce the observed data
Trojan collision probability: a statistical approach
No full textWe study the long term evolution of the collision probability and of the impact velocity vimp in the two Trojan asteroid swarms. The new mathematical formalism by Dell'Oro and Paolicchi (1998) has been used since, in the calculation of the collision probability, it allows to account for the dynamical links among the Trojans and Jupiter orbital angles, due to the 1:1 resonance. This statistical method permits to compute both and vimp over a long timespan (we considered 1 Myr) without making use of heavy numerical integrations. Moreover, it allows to easily update the values of and vimp anytime more complete samples of Trojan orbits are available. The values of and vimp over a short timescale have been compared to those of Marzari et al. (1997) and a good agreement has been observed. Over a long timescale the influence of the secular frequency g5-g6 is clearly visible in . The large oscillations due to the secular frequency are wider for L4 than for L5. We have considered two different initial samples of orbits. The first is the same sample used by Marzari et al. (1997) and includes the orbits of 114 Trojans. The second, more complete, includes 223 objects. We observe an increase of in both the swarms when the more complete sample of Trojan orbits (223) is used. The vimp, instead, slightly decreases compared to the vimp found by Marzari et al. (1997) from the sample of 114 Trojans
An Improved Semi-Empirical Model of Catastrophic Impact Processes I-Theory and Laboratory Experiments
No full textSeveral improvements to the semi-empirical approach to the physics of catastrophic breakup events (see P. Paolicchi, A. Cellino, P. Farinella, and V. Zappalà,Icarus77, 187–212, 1989) have been recently developed and are described in the present paper. The main new features of the model consist of the derivation of a set of realistic, non-overlapping fragments, as well as of a better treatment of the role played by gravitational effects. The main physical results obtained by means of the improved model in situations similar to those encountered in laboratory experiments are discussed, and compared with the experimental evidence and with the outcomes of hydrodynamical simulations, as well as with the analogous results found in the previous version of the model. The present model appears as being able to fit, also quantitatively, the experiments, and to enlight hidden interrelations among various observed properties, in spite of its simplified physics. The problems related to the possibility of deriving reliable fragment mass distributions are pointed out and extensively discussed. The systematic extension of the present model to the cases in which gravitational effects are dominating will be postponed to a forthcoming paper
Planet formation process as a phase transition. I - Elementary model
No full textA model for the condensation of ring-shaped nebular matter into a protoplanet is described. The condensation is considered as a phase transition between a ring phase and a planetary phase. A constant nonzero temperature is assumed. The model is used to study the effect of physical parameters on the critical ring width permitting transition. The parameters considered are mass and temperature of the nebular matter
The period distribution of eclipsing and spectroscopic binary systems. I
No full textThe period distribution of eclipsing and spectroscopic binary systems was analysed by various methods to take into account selection effects on observational data, with the purpose of deriving the actual distribution curve. The discussion of results shows the presence of some secondary maxima in the distribution which are probably of a nonstatistical character. They could be regarded as an indirect clue to the discrete nature of the star formation process, according to the spiral density wave theory of galactic evolution by Lin et al. (1969)
The non-gravitational effects on the dynamical evolution and on the rotational properties of the asteroids
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