148 research outputs found
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
Yarkovsky-evolved asteroid dynamical families: a correlation between their present properties and the impact geometry?
No full textThe original orbital elements of the members of a dynamical family are subject to changes, some connected to the overall dynamical evolution, and others due to the Yarkovsky effect (mainly the diurnal effect). While the former term may be analyzed in terms of proper elements, the latter one introduces a sort of ageing of proper elements. However, the Yarkovsky-driven mobility depends on the spin vector of a fragment- after the collisional and, possibly, the reaccumulation processes; the spin vector is not -in general -randomly oriented, but connected to the impact geometry and to the physical properties of the collision. Thus the spin vector and the initial ejection velocity may be strongly correlated. In this paper we try, on the basis of simplified assumptions, to discuss how the original distribution of family members in the space of orbital elements evolves in time, due to the Yarkovsky effect, and how far this evolution may differ according to different impact geometries and physical assumptions
The Role of Big Events in the Overall Collisional Evolution of Asteroids
No full textA new numerical model of asteroid collisional evolution and a few preliminary results are presented. The algorithm allows to put into evidence the role of rare major collisional events, those whose probability of occurrence throughout the whole history of the Solar System is about -or less than- unity. It is discussed how these events affect the following collisional evolution and how their long time effects are observable in the mass distribution and in other properties. The method allows estimates of how accurately we are able to reconstruct the initial properties of the Belt as well as to ascertain the occurrence of big events in a recent past (in astronomical sense, i.e. compared to the age of the Solar System)
La maschera fittile della tomba 1188 della necropoli di Valle Trebba. Forme di ritualità di passaggio nella Spina tarda.
No full textIl contributo prende in considerazione la nota maschera fittile della tomba 1188 VT di Spina, riesaminata alla luce del contesto di ritrovamento. Oltre ad affrontare criticamente l'associazione della maschera alla produzione punica, si affronta il significato che l'oggetto assume nel quadro della ritualità funeraria, anche grazie all'analisi di altri ambiti culturali, primo fra tutti quello greco. Ne emerge una ritualità incentrata sul passaggio di status di bambini verso la fase adolescenziale e adulta, molto coerente con i dati della tomba di Spina, appartenente ad un individuo sub-adulto
I costi dell'illegalità,Camorra ed estorsioni in Campania
No full textL'attenzione al fenomeno estorsivo si rivolge alla Campania, e in particolare alle province di Napoli e Caserta: due aree caratterizzate dalla densa presenza delle organizzazioni camorristiche, e nelle quali il fenomeno estorsivo è estremamente, e capillarmente, diffuso. Il volume presenta una quantificazione, rigorosa e trasparente, dei costi economici dell'estorsione, elaborati attraverso un database, e si avvale di dati ricavati dagli atti giudiziari e da fonti di prima mano: interviste a magistrati, esponenti delle forze dell'ordine, imprenditori, vittime del racket. La base empirica utilizzata consente la più accurata analisi degli effetti economici e sociali dell'attività estorsiva. E si rivela ulteriormente utile a diagnosticare peculiarità e recenti trasformazioni delle organizzazioni camorristiche, delle loro modalità operative e strutture organizzative. Il volume presenta un'interessante novità: l'analisi testuale, anche attraverso appositi software, dei "discorsi" intorno alla camorra. E non trascura un elemento di speranza: il recente avvio dell'associazionismo antiracket, anche in comparazione con l'esperienza siciliana
Distribution of spin vectors: differences between Main Belt and Near Earth Asteroids longitudes.
No full textPole anisotropy and spin statistics
No full textThe assumption of pole isotropy entails a "Maxwellian" distribution of the angular velocity of asteroids. Nevertheless, it is becoming more and more apparent that the isotropy assumption may not hold, while, on the other hand, the observed spin distribution cannot be always represented by a single maxwellian. It is interesting to check whether these difficulties may be related. On the basis of the existing data on poles, we plot possible multivariate distributions statistically consistent with the data, looking for the resulting non-maxwellian spin distributions. It comes out that the distributions fitting the pole data exhibit only a slight excess at large ω values. More severe anisotropies (which one might find to affect particular samples, such as dynamical families, and so on) give spectacular deviations. The extension of the observational efforts to assign asteroid poles is required
Retrograde spins of near-Earth asteroids from the Yarkovsky effect
No full textDynamical resonances in the asteroid belt are the gateway for the production of near-Earth asteroids(1) (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(1-3), but difficulties with that idea have led to the suggestion that orbital drift arising from the Yarkovsky effect(4-7) dominates the injection process(8-10). ( 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(2) 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
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