The Role of Kozai Cycles in Near-Earth Binary Asteroids

TL;DR

This study examines the Kozai mechanism's influence on near-Earth binary asteroids, finding that primary oblateness suppresses Kozai cycles in most cases, but they may affect wider binaries and contribute to contact binary formation.

Contribution

It demonstrates that Kozai cycles are generally suppressed in close near-Earth binaries due to primary oblateness, and explores their potential role in wide binary evolution and contact binary formation.

Findings

Kozai cycles are suppressed in most close near-Earth binaries.

Wide binaries can experience Kozai-induced eccentricity and inclination changes.

Kozai effect may contribute to the formation of contact binaries in wide systems.

Abstract

We investigate the Kozai mechanism in the context of near-Earth binaries and the Sun. The Kozai effect can lead to changes in eccentricity and inclination of the binary orbit, but it can be weakened or completely suppressed by other sources of pericenter precession, such as the oblateness of the primary body. Through numerical integrations including primary oblateness and 3 bodies (the two binary components and the Sun), we show that Kozai cycles cannot occur for the closely-separated near-Earth binaries in our sample. We demonstrate that this is due to pericenter precession around the oblate primary, even for very small oblateness values. Since the majority of observed near-Earth binaries are not well-separated, we predict that Kozai cycles do not play an important role in the orbital evolution of most near-Earth binaries. For a hypothetical wide binary modeled after 1998 ST27, the separation is large at 16 primary radii and so the orbital effects of primary oblateness are lessened. For this wide binary, we illustrate the possible excursions in eccentricity and inclination due to Kozai cycles as well as depict stable orientations for the binary's orbital plane. Unstable orientations lead to collisions between binary components, and we suggest that the Kozai effect acting in wide binaries may be a route to the formation of near-Earth contact binaries.