BALANCED EARTH SATELLITE ORBITS

An analytic model for third-body perturbations and for the second zonal harmonic of the central body's gravitational field is presented. A simplified version of this model applied to the Earth-Moon-Sun system indicates the existence of high-altitude and highly-inclined orbits with their apsides in the equator plane, for which the apsidal as well as the nodal motion ceases. For special positions of the node, secular changes of eccentricity and inclination disappear too (''balanced'' orbits). For an ascending node at vernal equinox, the inclination of balanced orbits is 94.56-degrees, for a node at autumnal equinox 85.44-degrees, independent of the eccentricity of the orbit. For a node perpendicular to the equinox, there exist circular balanced orbits at 90-degrees inclination. By slightly adjusting the initial inclination as suggested by the simplified model, orbits can be found - calculated by the full model or by different methods - that show only minor variations in eccentricity, inclination, argument of perigee, and longitude of the ascending node for 10(5) revolutions and more. Orbits near the unstable equilibria at 94.56-degrees and 85.44-degrees inclination show very long periodic librations and oscillations between retrogade and prograde motion.