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C. B. Agnor (University of California Santa Cruz), D. P. Hamilton (University of Maryland)
The recent observational discovery of satellites orbiting asteroids and Kuiper belt objects has revealed a whole new constituency of solar system objects (see Merline et al. 2002 for a review). Binaries appear to be ubiquitous among the minor planets, representing ~16% of near-Earth asteroids (Margot et al. 2002), at least ~2% of asteroids in the main belt (Merline et al. 2002), and a growing number of Kuiper belt objects (currently estimated at ~14%, Noll 2005).
Given the existence and prevalence of binaries, 3-body encounters between a binary pair and a third body become an inevitable and frequent occurrence in the dynamical evolution of the solar system. This type of encounter has been studied previously in the context of stellar clusters where binary-single star encounters represent an important process in the evolution of binary star populations (see e.g. Hills 1975, Heggie & Hut 1993). The possible outcomes of these encounters include physical collisions, hardening, softening or disruption of the binary, and exchange reactions in which one member of a binary is expelled and its place taken by the third body. In the context of solar system dynamics, the significance of 3-body encounters is only beginning to be examined and appreciated (Funato et al. 2004).
We are studying exchange reactions during planet-binary encounters as a new pathway for the capture of irregular planetary satellites from initially heliocentric orbits. Satellite capture via exchange reaction requires gravitational disruption of the binary and retention of one member by the planet. In general, we find that exchange capture is efficient when binaries approach the planet to a distance less than the appropriate Roche radius of the system and with sufficiently low encounter velocities.
As application of this capture mechanism, we consider the possibility that Neptune's massive retrograde satellite Triton may have been captured during a gravitational encounter between Neptune and a binary composed of Triton and a third body. We find that Triton's capture can be realized from a variety of binaries, including ones resembling the Pluto-Charon pair (i.e. mass ratio mPluto/mCharon = 8 and semi-major axis to primary radius ratio aB/RPluto = 17.3). We will discuss the encounter dynamics required for capture via this mechanism, compare this mode of satellite capture to others suggested for Triton, and relate them to the dynamical environment of Neptune's accretion and putative migration.
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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.