P. Paolicchi

Retrograde spins of near-Earth asteroids from the Yarkovsky effect

Dynamical resonances in the asteroid belt are the gateway for the production of near-Earth asteroids (NEAs). To generate the observed number of NEAs, however, requires the injection of many asteroids into those resonant regions. Collisional processes have long been claimed as a possible source, but difficulties with that idea have led to the suggestion that orbital drift arising from the Yarkovsky effect dominates the injection process. (The Yarkovsky effect is a force arising from differential heating—the ‘afternoon’ side of an asteroid is warmer than the ‘morning’ side.) The two models predict different rotational properties of NEAs: the usual collisional theories are consistent with a nearly isotropic distribution of rotation vectors, whereas the ‘Yarkovsky model’ predicts an excess of retrograde rotations. Here we report that the spin vectors of NEAs show a strong and statistically significant excess of retrograde rotations, quantitatively consistent with the theoretical expectations of the Yarkovsky model.

The interpretation of laboratory experiments in the framework of an improved semi-empirical model

Abstract The semi-empirical model of catastrophic breakup events developed by Paolicchi et al. (Icarus77, 187–212, 1989) has recently been improved by means of new algorithms allowing the generation of sets of nonoverlapping fragments, and to take into account gravitational effects. In this paper we give the results of simulations aimed specifically at reproducing laboratory experiments. A comparison with both the experimental evidence and the results of the previous version of the model is presented, and particular attention is devoted to the problem of the shape distribution of the fragments. The results seem encouraging, and allow us to undertake more detailed investigations in order to analyse in detail the capability of the model for reproducing both the laboratory results and the properties exhibited by the asteroidal population, in particular, asteroid families.

CATASTROPHIC FRAGMENTATION AS A STOCHASTIC PROCESS : SIZES AND SHAPES OF FRAGMENTS

Abstract It 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 explicity, 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.

A new way to estimate the distribution of encounter velocity among the asteroids

Abstract The statistics of asteroid mutual encounters has been studied by several authors, mainly with the purpose of estimating collisional rates (and thus mean collisional lifetimes) and the distribution of encounter velocities. A new approach to this problem, very simple and fast in terms of computer time is presented. Not-withstanding these properties, and in spite of its heuristic nature, the method proved to give results similar to those obtained by means of more sophisticated and time-consuming techniques. Moreover, the method is particularly suitable for undertaking more detailed analyses, including the distribution of impact directions for a given target, and an evaluation of the quantitative relevance of a number of possible effects, not taken into account in previous studies.