According to our results, it appears that the TCB has a greater probability
of coming from an asteroidal source. More precisely, we have found that about 
(739/886) of the orbits can be reached from a main belt object and
thus only 
can be reached from a cometary source. These results
were obtained with a parameter choice extremely favourable for the cometary
hypothesis. Moreover, by using a more greater range of velocity and
inclination parameters (e.g.
V = 10-35 kms-1 and
)
the asteroidal hypothesis would be strengthened.
One may argue that our set of orbits corresponds to only one cometary
source, i.e. the Jupiter family comets with 2< T <3,
while the TCB may be a fragment of a Halley type comet (HTC) (with an orbital
period P<200 yr and a Tisserand parameter T<2), or even a fragment of a
long period comet (LPC); these two types of comets are supposed to come from
the Oort cloud, and form the nearly-isotropic comets
population (NIC). However, the size of the NIC is not well known,
and the ratio of extinct comets to new comets has not yet been determined, Bottke
et al. (2001) concluded that it is not yet possible to estimate the
contribution of the NIC to the NEO population. However it should be noted
that our results giving a 17% chance of JFC origin for the TCB is in the
range of the 
of the Earth craters estimated to result from impacts of NICs
(Shoemaker 1983). Thus, even if we include these estimates for the
contribution of the whole comet populations (say 30%), an asteroidal origin is
still the most probable.
Furthermore, numerical integrations showed that particles starting from the OB region (with a>2.8 AU) result in an orbital distribution in the NEO region that is not clearly different from the one provided by the JFC. Most of the asteroids from the outer belt that enter in the NEO region are then pushed onto Jupiter-crossing orbits and are subsequently ejected from the inner solar system. Thus, the JFC and outer belt sources are degenerated. Even adding the contribution of the OB source to the JFC to define our cometary source and thus consider the maximum possible role of a cometary source, we find that an asteroidal origin of the TCB is the most probable.
However we know that some asteroids are on comet-like orbits
and also that some comets behave as asteroids (Yeomans 2000). Thus
three objects have presently received a dual designation (Yeomans 2000),
in particular the asteroid 1979 VA , which has been a comet
discovered by Wilson-Harrington 30 years ago and is now known as
107 P/Wilson-Harrington = (4015) Wilson-Harrington. On the
other hand some C-type asteroids may have a very low bulk density like
Mathilde (
1300 kgm-3, just higher than water), which
suggests that they are porous bodies. They might thus be eventually pulverised
when impacting the Earth (cf. Foschini 1998).
Our work agrees with that of Andreev (1990) who, after an analysis carried out with different methods of a large set of orbits obtained from testimonies, inferred an asteroidal origin for the TCB (an Apollo asteroid). On the other hand, Bronshten (1999a) performed a similar investigation on data from eyewitnesses and from forest devastation. He too obtained a small set of orbits consistent with the cometary hypothesis, while all radiants for geocentric speed smaller than 30 kms-1 correspond to Apollo-like asteroids. However, Bronshten concludes that the stony hypothesis is not reliable, because neither macroscopic remnants nor craters were found. The main novelties of our work with respect to Andreev's and Bronshten's papers are that we considered a much larger and statistically significant sample of orbits and we estimated the relative probability that the TCB came from one of four particular sources.
The key problem regarding the Tunguska event still is to explain how a stony object could completely disintegrate in the Earth's atmosphere. In this work, we found that, the recent model for atmospheric fragmentation suggests a predominance of solutions for bodies with a high mechanical strength, which appears to be consistent with findings of interplanetary dynamics.
Presently, taking into account that our sample of possible TCB orbits is much larger and statistically more robust than previous ones, we can conclude that our study based on combining interplanetary and atmospheric dynamic considerations gives as the most probable an asteroidal origin for the Tunguska cosmic body of June 30th, 1908.
Acknowledgements
This work has been partially supported by MURST Cofinanziamento 2000 and has made use of NASA's Astrophysics Data System Abstract Service. We like to acknowledge P. A. Dybczynski for helping us with the JPL DE405 ephemeris.
Copyright ESO 2001