  An attempt to determine spatial location of the main source of short-period
comet nuclei was made. There were carried out numerical calculations for orbit
evolution of Jupiter family comets, comets with middle-period orbits and bodies
of Centaur group. On the basis of the calculations it was shown, that orbital
evolution of the solar system small bodies is mainly going in the direction of
the semi-major axes increase. It belongs to the bodies which can undergo
approaches the planets, and orbital evolution of which is mainly going due to
the gravitational forces. Such result is confirmed by qualitative analysis of
changes of small body semi-major axes under approaches the planets. The
conclusion was drawn that the main source of nuclei of Jupiter family comets is
apparently situated at distances from the Sun not more than 6 AU.
