Orbital Behavior of Captured Satellites: The Effect of Solar Gravity on Triton's Post-Capture Orbit

Abstract

The effects of solar perturbations on the postcapture orbital behavior of satellites are investigated in the context of the restricted, circular three-body problem as applied to Neptune, Triton, and the Sun. Highly eccentric and inclined satellite orbits are considered; thus a numerical, phenomenological approach is taken to describe variations of the satellite's orbital elements. We focus on harmonic variations in specific orbital angular momentum h, and thus pericenter distance q, eccentricity e , semimajor axis a, and inclination to Neptune's orbital plane i . From prograde and retrograde simulations over a range of eccentricities and semimajor axes, a momentum oscillation is found with a period of half a Neptune year and an amplitude proportional to a2e2 cos i. Inclined orbits also experience a longer period, secular-torque-driven variation in h associated with orbital precession and nutation, upon which the semiannual oscillation is superimposed. The amplitude of the longer period variation can exceed and dominate the semiannual variation, and the two can combine to produce much larger variations in the elements q, e, and i than is possible for noninclined orbits, leading in some circumstances to "Neptune,grazing." Consequently, if Triton was temporarily gravitationally captured, solar perturbations could have increased e and reduced h sufficiently to drive the pericenter close to Neptune. There, interactions with a gaseous protoplanetary nebula or a collision with an existing satellite could have dissipated enough orbital energy to make capture permanent. It is more likely, though, that Triton was promptly captured by collision or gas drag into a lower q state to begin with. In either situation, capture at lower q ensures that further orbital variation does not bring Triton dangerously close to Neptune. Repeated close flybys following permanent capture are likely (and could also occur in the less likely event of an extended temporary capture). Multiple close flybys increase the probability of further satellite collisions and multiply the effects of gas drag. For gas-drag capture, possibly recurring larger pericenters associated with the longer period inclination oscillation imply longer gas-drag evolution time scales, because Triton's passages through the nebula are reduced in frequency. Similarly, for tidal evolution, pericenter oscillations could have delayed the onset of large-scale melting within Triton's interior and extended Triton's tidally driven postcapture orbital contraction if large inclination variations were present. The behavior described here may have influenced other satellite captures and can be identified in integrations of the present motion of the outer jovian satellites.