Alessandro Cordelli

Information and dynamical systems: a concrete measurement on sporadic dynamics

Abstract We present a method for the study of dynamical systems based on the notion of quantity of information. Measuring the quantity of information of a string by using data compression algorithms, it is possible to give a notion of orbit complexity of dynamical systems. In compact ergodic dynamical systems, entropy is almost everywhere equal to orbit complexity. We have introduced a new compression algorithm called CASToRe which allows a direct estimation of the information content of the orbits in the 0-entropy case. The method is applied to a sporadic dynamical system (Manneville map).

The proliferation of orbiting fragments: A simple mathematical model

We present and discuss a simple mathematical model (two coupled, non‐linear, first‐order differential equations) for the future proliferation in low earth orbit of space debris that is created by high‐velocity destructive collisions of small objects with artificial satellites or other “big” orbiting bodies. The model predicts that such collisional generation of fragments will be the dominant source of debris in a few decades. Subsequently, if the satellite launch rate remains comparable with the current one, a quasi‐exponential growth of potential projectiles will cause the number of satellites to reach a peak (of the order of 104) in about 150 years and then to rapidly decline by about a factor of 10. Some alternative choices of model parameters (for example, to account for uncertainty in the projectile‐to‐target mass ratio required for breakup and the rate of future injection of material into orbit and intentional generation of debris) show that this evolution may be anticipated or delayed, but not qual...

Simulation of the geomagnetic field experienced by the International Space Station in its revolution around the Earth: effects on psychophysiological responses to affective picture viewing.

There is evidence suggesting that exposure to an abnormal magnetic environment may produce psychophysiological effects related to abnormalities in responses to stress. This may be of relevance for space medicine where astronauts are exposed to a magnetic field different from that exerted by the Earth. Aim of this study was to assess how the exposure of the head to a magnetic field simulating the one encountered by the International Space Station (ISS) during a single orbit (90 min) around the Earth affects the cardiovascular and psychophysiological parameters. Twenty-four human volunteers were studied double blindly in random order under sham and magnetic exposure. During exposure, the persons were shown a set of pictures of different emotional content while subjective self-rating, skin conductance (SC), blood pressure (BP), and heart rate (HR) were measured. In addition, BP, HR, and tooth pain threshold were assessed before and after exposure. While subjects were under magnetic exposure, skin conductance was strongly differentiated (F(2,36)=22.927; p=0.0001), being high during emotionally involving (positive and negative) pictures and low during neutral pictures. Conversely, when subjects were under sham exposure, no significant differences were observed. There was, however, a trend for higher heart rate during picture viewing under magnetic exposure as compared to sham exposure. No effects were found for the other variables. These results suggest that an abnormal magnetic field that simulates the one encountered by ISS orbiting around the Earth may enhance autonomic response to emotional stimuli.

Interaction of the Satellite Constellations with the Low Earth Orbit Debris Environment

The effect on the orbital debris environment of several satellite constellations, to be launched and maintained in LEO over the next decades, has been analyzed with the SDM software system, including updated initial conditions and traffic model scenarios.

Investigations of a simulated geomagnetic field experienced by the international space station on attentional performance

We have previously reported that the exposure to an abnormal magnetic field simulating the one encountered by the International Space Station (ISS) orbiting around the Earth may enhance autonomic response to emotional stimuli. Here we report the results of the second part of that study which tested whether this field also affects cognitive functions. Twenty-four volunteers participated in the study, 12 exposed to the natural geomagnetic field and 12 to the magnetic field encountered by ISS. The test protocol consisted of a set of eight tests chosen from a computerized test battery for the assessment of attentional performance. The duration of exposure was 90 min. No effect of exposure to ISS magnetic field was observed on attentional performance.

Physical and numerical aspects in Lanczos and modified Lanczos calculations

Abstract The use of recurrence relations in Lanczos procedure is often limited in practice by loss of numerical stability. In many applications this fact severely limits the numerical reliability of the results and hence the interpretation of the physical models proposed. In this paper we focus on the effects of finite precision arithmetic in the Lanczos algorithm and examine some methods for assessing and testing the reliability of the tridiagonal matrix generated. In addition to the residual vector test we propose an initial vector test, and an orthogonalization test to assess the quality of the eigenvalues. We also implement the use of multiple precision to obtain a significant improvement in the tridiagonal chain transformation. As a final resort, we consider the modified Lanczos procedure to obtain one by one, energies and wavefunctions of a system, within any desired energy range and with any desired accuracy.

A NEW MODEL TO SIMULATE IMPACT BREAKUP

Abstract We have developed a preliminary version of a new type of code to simulate the outcomes of impacts between solid bodies, which we plan to further refine for application to both asteroid science and space debris studies. In the current code, colliding objects are modeled as two-dimensional arrays of finite elements, which can interact with each other in both an elastic and a shock-wave regime. The finite elements are hard spheres with a given value for mass and radius. When two of them come into contact the laws of inelastic scattering are applied, thus giving rise to the propagation of shock waves. Moreover each spherical element interacts elastically with its nearest neighbours. The interaction force corresponds to that of a spring having an equilibrium length equal to the lattice spacing, and results into the propagation of elastic waves in the lattice. Dissipation effects are modeled by means of a dissipative force term proportional to the relative velocity, with a given characteristic time of decay. The possible occurrence of fractures in the material is modeled by assuming that when the distance of two neighbouring elements exceeds a threshold value, the binding force between them disappears for ever. This model requires finding a plausible correspondence between the input parameters appearing in the equations of motion, and the physical properties of real solid materials. Some of the required links are quite obvious (e.g., the relationship between mass of the elements and elastic constant on one side, and material density and sound velocity on the other side), some others a priori are unclear, and additional hypotheses on them must be made (e.g., on the restitution coefficient of inelastic scattering). Despite the preliminary character of the model, we have obtained some interesting results, which appear to mimic in a realistic way the outcomes of actual impacts. For instance, we have observed the formation of craters and fractures, and (for high impact energies) the occurrence of catastrophic breakup. The masses and velocities of the fragments resemble those found in laboratory impact experiments.

Artificial life and speciation, a case study: heterochromatin and speciation in the Microtus savii Group (Rodentia-Arvicolinae).

Artificial life is a tool which is used for simulation of peculiar cases of evolutionary events. The main characteristic of artificial life is that with this technique it is possible to simulate for a high number of generations the evolution of a population of individuals. Each individual is characterised by a small number of parameters, but each individual has its own evolutive story. So far it is possible to simulate the evolution of a population of some thousands specimens, for a high number of generations. The realistic aspect of the simulation is that each specimen is taken individually. In our opinion this instrument is very useful to simulate the evolution of the hybrids barrier during speciation. For this reason it is applied to a peculiar case of speciation, that of the Savi pine vole (Microtus savii) whose experimental data were recently investigated.

Geophysiological Modeling: New Ideas on Modeling the Evolution of Ecosystems.

Living organisms evolve within ecological associations (from ecosystems to the biosphere) that are constituted by a biological component and a physico-chemical component. It is generally supposed that interactions such as competition and predation between the biological components of ecosystems are the main cause for the observed organization of ecosystems. We believe that in the search for a more comprehensive theory of evolution a much greater attention should be paid to the ways in which living organisms interact with the physico-chemical environment. To test our ideas, we develop a mathematical model to study the evolution of ecosystems, and we apply it to the study of hydrothermal vents. The model proposed is still a qualitative model. It tries to study, in a first approximation, the behaviours of the biological and chemical components. In the following we hope to develop and to improve it so to give a more realistic model.

Augmented space formalism and renormalization for ground and excited states of magnetic systems

The augmented space formalism has been used so far for the study of disordered crystals. We stress here that the topological structure of the augmented space of a binary alloy is formally equivalent to the set of configurations of an Ising system. In particular in both cases a very useful binary representation for labelling the vectors of the space can be done. Starting from this observation we have developed a very efficient description of the Ising hamiltonian, and we have shown that it can be succesfully applied both in the direct diagonalization approach and in conjunction with the renormalization scheme. Within this last case we have proposed a generalization of the techniques commonly used in the literature, obtaining very good results.