Planetary Close Encounters: An Investigation on Temporary Satellite-Capture Phenomena

Abstract

This research is part of a wider one (Carusi & Pozzi, 1978a, b) concerning a detailed study of the dynamics of close encounters between a giant planet and a minor object. A special result of that investigation was the recognization of some satellite-capture events, already found by Everhart (1973). An important remark about this previous work is that all satellite-captures occurred with low inclination objects which orbits were initially near-tangent to the Jupiter's one. Starting from this consideration, a hundred fictitious orbits have been generated in order to study the phenomenon in greater detail. Their initial distribution is shown in fig. 1. The initial angular parameters i, ω, Ω, were chosen to be equal to those of the most interesting case of the previous research. Eccentricities were selected regularly in the range .01-.5, with a step of .01, giving the same value to an object of the upper band and to the next of the lower. The semimajor axes were chosen at random between limits computed so that the aphelion for the lower band, or the perihelion for the upper band, would lie within a distance of 108km from Jupiter's orbit. As the orbital planes do not coincide with that of Jupiter, the minimum distance point between the two orbits does never coincide with the object perihelion or aphelion, but is always close to them. Fig. 2 shows the final situation of this population: we note that in no case a permanent binding occurred. We can do some remarks on this picture. First of all we note that 56% of objects experienced a temporary binding to the planet. Secondly, 67% of objects, bounded or not, had a final orbit lying, on the a-e diagram, on the opposite band with respect to their initial one. This kind of transition is especially significant if compared with the case of observed comets, because it gives a simple mechanism to transform long-period comets in short-period ones, and to transfer comets from one family to another. Actually, on the basis of the computations of Kazimirchak-Polonskaya (1972), we can say that a similar process has been experienced, for example, by comets Whipple, Oterma, Brooks 2, Lexell, Kearns-Kwee and others. A third remarkis that, between the initial and final situations, small eccentricities are increased, as a consequence of the encounter, whilst the great ones are decreased. This phenomenon leads to a clustering of final orbits in the eccentricity range .1-.2.