1 Introduction

In the early morning of 30th June 1908, a powerful explosion (10-15 Mton of energy) over the basin of the Podkamennaya Tunguska river flattened $2150\pm 50$ km² of Siberian taigà. For more than ninety years, Tunguska was, and still is a conundrum, although many scientists around the world have written essays on the subject and proposed their solutions. Shapley (1930) was the first to suggest that the Tunguska event was caused by the impact of a comet. Kulik (1939, 1940) subsequently proposed the first asteroidal hypothesis (iron body), followed shortly afterwards by Fesenkov (1949), who hypothesised a stony meteorite of at least some millions tons. Fesenkov (1961) later worked out a definite model of an impact between a comet and the Earth's atmosphere. From that time, the majority of Russian scientists followed the cometary hypothesis, while many western scientists preferred an asteroidal model (see, e.g., Sekanina 1983; Chyba et al. 1993). For many reasons, these two "schools'' practically ignored each other until the international workshop Tunguska96, held in Bologna (Italy) from 15th-17th July 1996 (see the special issue of Planetary and Space Science, vol. 46, n. 2/3, 1996, ed. M. Di Martino, P. Farinella, & G. Longo). There is no reason to review here what is known about the Tunguska event and is reviewed in Krinov (1966), Trayner (1997), Vasilyev (1998), Bronshten (2000c).

Despite great efforts, the main question, i.e. the nature of the Tunguska Cosmic Body (TCB), which caused the explosion, is still open. Although almost every year there is an expedition to Tunguska, so far no typical material has permitted a certain discrimination to be made between an asteroidal or cometary nature of the TCB. Neither the chemical and isotopic analyses of peat (see, e.g., Kolesnikov et al. 1998), nor studies on iridium in the impact site (e.g. Rasmussen et al. 1999), nor the search of TCB microremnants in tree resin (Longo et al. 1994) were sufficient to prove definitely the nature of the TCB.

In July 1999, an Italian Scientific Expedition, organized by the University of Bologna with the collaboration of researchers from the Turin Astronomical Observatory and the CNR Institute of Marine Geology, went to Siberia in order to collect more data and samples (Longo et al. 1999; Amaroli et al. 2000). The many samples collected during the expedition are still under examination. This field research should be strengthened by theoretical studies and modelling and the present paper is a step in that direction. In this paper, we first construct a sample of possible TCB orbits, then we use a dynamic model to compute the most probable source of the TCB if placed on each of these orbits, thus obtaining the corresponding probabilities for an asteroidal or a cometary origin.

Our paper is divided as follows: in Sect. 2, we discuss the choice of the different TCB parameters, which are used to compute the possible orbits. This data includes the physical parameters of the explosion, the speed values, and the radiant coordinates. In Sect. 3, using the chosen set of parameters we first compute the lower and upper boundaries of the dynamic elements of the heliocentric orbits, then we deduce the dynamic geocentric parameters (Sect. 4). We can thus build up a sample of possible TCB orbits and calculate their respective initial osculating elements (Sect. 5). In Sect. 6, first of all, we briefly recall the dynamic method, which allows us to identify the principal sources of small bodies, then we estimate for a fictitious TCB on each orbit with orbital elements (a, e, i) the probabilities of its coming from the different sources, and discuss the results. In Sect. 7, we present a sample of numerical integrations over a long timespan of the orbital evolution of fictitious TCB coming from each source according to our probability computations. Such numerical integrations allow us to identify the various dynamic mechanisms at work and to compare their orbital behaviour. The conclusions are presented in Sect. 8.