All known groups of fireballs contain bodies which may reach the Earths surface as meteorites and are of planetary origin. Since some fireballs have cometary orbits in the Solar System, then comets are probably of planetary origin. The possible explosion
of a large planet is discussed again. This time on the basis of gravitationally decelerated expansion of an originally superdense embryo.
For small meteoric bodies, terminating high in the atmosphere (i.e.
under free molecular conditions) it was possible to neglect the effect of thermal motions of air particles. As regards fireballs, bodies with masses in excess of about 0.1 kg, penetrating deep into the atnosphere, the ambient atmosphere has the properties of a continuous medium. Under these conditions, the thermal motions of atmospheric particles behind ťhe shock wave become
the decisive factor for momentum and energy transfer to the meteoroid. F'or fireballs these thermal motions practically replace the effect of direct impacts of particles of hie undisturbed atmosphere, considered earlier under free molecular conditions. The form of the equations, describing the motion and ablation of a large body (fireball) will remain the same as for small bodies, but the coefficients occurring in the equations will have a different
physical meaning.
In a previous paper (Padevet, 1984), the author pointed out the decisive role of thermal motions of air particles in the continuous medium behind a shock wave for fireballs. Nevertheless, the change in quality of the mechanism of momentum and energy transfer had practically no effect on the values of the transferred quantities. It was found that the role of thernal motions of particles of meteoric material becomes distinctly manifest only if the interaction of the atmosphee with the ablated meteoric material is considered. This may explain some of the observed phenomena, e.g. the "mass paradox", or the photographed fragmentation of fireballs. It also appears probable that all four groups of fireballs, I, II, IIIA, IIIB, determined earlier, can be assigned to known types of meteorites, in particular to chondrites of four known types, viz H, L, CM and CI. However, the theory of fireballs in a continuous medium should be further developed in several directions, as pointed out by the author in the discussion.