Modern Celestial Mechanics deals with the study of the motion of celestial bodies, natural or man-made, thus representing a discipline whose methods and applications are developed by mathematicians, physicists and engineers. Since the early eclipse records of the ancient Chaldeans to the work of Henri Poincare' (whose intuitions on the ubiquity of chaos still represent an enlightening source of inspiration), until the space age, the number of objects orbiting in space, either natural or man made, has been dramatically increasing in time. In recent years the widespread diffusion of fast computers, the advances in microelectronics applied to astronomical observations and the routine access to near Earth and interplanetary space has greatly contributed to widen the horizons of Celestial Mechanics. As soon as it was possible to extend consistently the time span of numerical integrations, chaos became the subject of experiment; more recently, the confirmation by observation of the existence of the Edgeworth-Kuiper belt called for a major reassessment of the outer solar system dynamical evolution. At the same time space flight dynamics grew as a self-standing field of study, mapping the space ways for man-made celestial objects to travel. This in turn allowed observations from short range of planetary systems, and resulted in further dicoveries of peculiar orbital configurations among celestial bodies.

From this cultural context comes the idea of organising a national Italian meeting on Celestial Mechanics, to establish a common ground among people working in this field and belonging to different scientific communities such as astrophysics, planetary and space sciences, dynamical systems. Thus, in the spring of 1993, the First Italian Meeting on Celestial Mechanics was hosted by the Department of Mathematics of the University of L'Aquila - a pleasant town located in the mountains of central Italy.

Following the success of this early experience, the Second Italian Meeting on Celestial Mechanics (organised by the editors in collaboration with G. B. Valsecchi, IAS-CNR, Roma, Italy) took place from 21 to 24 April 1997. The meeting was attended by more than 60 scientists and engineers belonging to Italian universities, research centers, astronomical observatories and aerospace industries. The welcomed participation of foreign guest scientists allowed to place the contribution of the Italian community in an international context.

The organisation of the papers published in this special issue of Planetary and Space Science follows the three main topics discussed in the meeting, namely Perturbation Theories, Solar System and Stellar Systems, Spaceflight Dynamics.

Since the master works of Leverrier, Delaunay, Tisserand, Poincare' and Birkhoff, perturbation theories have been developed to investigate the dynamics and stability of planetary systems. More recently, new techniques have been introduced by A.N. Kolmogorov, V.I. Arnold and J. Moser. In the framework of nearly-integrable Hamiltonian systems, KAM theory provides a powerful technique to give an explicit estimate on the perturbing parameter, ensuring the existence of invariant surfaces. When dealing with low-dimensional systems, KAM theory provides important stability results in the sense of confinement in phase space. An alternative technique, due to N.N Nekhoroshev, allows to confine the motions in an open set of initial data for exponentially long times. Both KAM and Nekhoroshev theories have been widely investigated in recent years by Italian researchers. New techniques have been developed, which provide interesting results on physically significant applications to Celestial Mechanics.

Among most important topics in the current research in Solar System Dynamics are: planet-crossing orbits, with the recent progress in understanding the meteorite transport route and the intensified effort to discover most objects with potentially Earth-impacting orbits; the study of the transneptunian belt of small planets, predicted by Edgeworth and Kuiper and only recently confirmed by observations; and the in depth study of the properties of 4-body systems, extending the accumulated knowledge on 3-body systems. For Stellar Dynamics, the global chaoticity of the n-body systems with large n requires to seek methods to describe the collective behaviour, as much as possible in geometric terms. The direct application of the known properties of low dimensional chaos is feasible for simplified models with smooth potentials.

The section devoted to Spaceflight Dynamics exhibits a wide variety of topics and targets, triggered by the renewed interest of the major space agencies in low-cost missions for solar system exploration, e.g. to the Moon and Mars. The presentations witnessed also the experience developed in Italy on satellite laser ranging data analysis for geodetic purposes and the successful transfer of knowledge and methods from the asteroid collisional evolution and the dynamics of meteors to the space debris problem. A panel discussion on Small Satellites and Scientific Mission proved to be of good auspices as a few months later the Italian Space Agency (ASI) started a national program for small scientific satellites.

In conclusion, our hope is to have succeeded in giving an overall picture of the activities carried out in Italy in close connection with the international community, and in stimulating our audience to find connections among the various fields of study involving Celestial Mechanics.

Many institutions and people have helped us in the not easy task of merging different scientific communities. The "Scuola Superiore G. Reiss-Romoli" did much more than hosting the participants in its beautifully placed headquarters on the hills nearby L'Aquila: logistics, a state-of-the-art conference room, program graphic assistance, printing facilities and entertainment have been fully appreciated by everyone. In particular the time and the work dedicated by Mrs. S. Cerone to us has been of invaluable help. The "Gruppo Nazionale per la Fisica Matematica" of CNR, the University of L'Aquila and its Department of Mathematics, Telespazio (the Italian company for space communications) and Regione Abruzzo, have given us a strong support, providing also the basic funding scenario for the realisation of the meeting. We acknowledge also the support of Azienda Autonoma di Soggiorno.

Personal contributions have also been highly appreciated: our foreign colleagues who joined us, agreeing to be surrounded by Italians all day long, and Andrea Boattini, who has brought us to visit the Campo Imperatore Observing Station, located just below the Gran Sasso peak (2912 meters). Last but not least, we thank Planetary and Space Science, and especially its editor in chief, M. Coradini, for accepting to publish the papers presented at the meeting, after a refereeing procedure equal to the one used for ordinary papers submitted to the journal has been carried out.

The proceedings of the Second Italian Meeting on Celestial Mechanics are dedicated to the memory of Giovanni Paladin, distinguished scientist at the University of L'Aquila, who died in a mountain accident on June 1996. Giovanni Paladin made fundamental works on Statistical Mechanics and Dynamical Systems. He was always puzzled by the mysteries of Celestial Mechanics and he contributed to the organisation of our meeting not only with his enthusiasm, but also giving us many valuable suggestions and practical support. We sadly miss our friend and colleague.

Dipartimento di Matematica Pura e Applicata, Universita' di L'Aquila (Italy)

Dipartimento di Matematica, Universita' di Pisa (Italy)

Servizi e Sistemi Spaziali, Telespazio, Roma (Italy)

* CELMEC * Program * Proceedings *