The Effect of Gravitational Encounters on Binary Planetesimals

Sammanfattning: Binary planetesimals in the Kuiper belt can experience close encounters with other bodies. These encounters can disturb the orbital dynamics of the binary in several ways. Energy can be added to the system by the impactor body and lead to the disruption of the binary. Less energetic encounters can alter the orbital elements of the mutual orbit of the binary. The impactor could swap with one of the components of the binary creating a new binary. These interactions are interesting because determining how frequently they occur and what the results of the interactions are, can reveal information about the dynamical evolution of the Kuiper belt, and about planet formation and migration. Contact binaries in the Kuiper belt may be formed by this process. In this project we investigate encounters involving binaries and similarly sized bodies in the cold classical region of the Kuiper belt. We carry out several experiments; we investigate certain combinations of binary and impactor with small random changes to the initial conditions, and analyse outcome probabilities as a result of these changes; we subject different kinds of binaries to a parameter space of impactors, investigating on a large scale how binaries respond to many different encounter situations; and finally, we follow a single binary in the Kuiper belt as it evolves over the course of many encounters, with the bodies it encounters drawn from a distribution that is representative of the environment there. We find that the mutual orbital characteristics of a binary are critical to its ability to survive encounters. Wide and eccentric orbits are more susceptible to disruption during encounters. Encounters frequently result in the formation of new binaries composed of one of the original components and the impactor. These new binaries tend to have very high eccentricities. The encounter timescale is an important factor in determining outcomes, with destructive encounters requiring a large encounter timescale. This requires a slow relative encounter velocity, which in turn implies dynamically similar orbits, with low eccentricities and inclinations. Binaries generally evolve over their lifetime to have larger eccentricities as a result of encounters. Around 80% of wide binaries survive 105 encounters. Collisions that occur as a result of these kinds of encounters typically have speeds similar to the escape speed of the bodies.