ABSTRACT
The main satellites of the Solar System are listed in Tables 7.1 through 7.5, and an examination of their orbital characteristics shows that they fall into two main types. One type is exemplified by the satellites of Jupiter out to Callisto. All these satellites have direct orbits very close to circular and almost in the planet’s equatorial plane. Such satellites are referred to as regular satellites; the satellites of Saturn out to Titan and all the satellites of Uranus, except the last two, listed in Table 7.4 fall into this category, although Miranda’s orbit is inclined by 4.2° to the equatorial plane. The six small innermost satellites of Neptune also have the characteristics of regular satellites. It is a common assumption that regular satellites are somehow connected with the formation of the parent planet. The idea behind this assumption is that the spin axis of the planet and the rotation axis of the satellites’ orbits are both aligned with the net angular momentum vector of the original body of material that formed both the planet and its satellites. Although this assumption is probably true, it must be treated with some caution. The planets with regular satellites all spin rapidly and are therefore quite oblate. If the regular satellites were somehow placed in orbit around the planet not too long after its formation, then the combination of the gravitational pull of the equatorial bulge plus energy dissipation due to a residual resisting medium would pull the satellite orbit towards the equatorial plane.
