Formation of asteroid pairs by rotational fission (1009.2770v1)

Abstract: Asteroid pairs sharing similar heliocentric orbits were found recently. Backward integrations of their orbits indicated that they separated gently with low relative velocities, but did not provide additional insight into their formation mechanism. A previously hypothesized rotational fission process4 may explain their formation - critical predictions are that the mass ratios are less than about 0.2 and, as the mass ratio approaches this upper limit, the spin period of the larger body becomes long. Here we report photometric observations of a sample of asteroid pairs revealing that primaries of pairs with mass ratios much less than 0.2 rotate rapidly, near their critical fission frequency. As the mass ratio approaches 0.2, the primary period grows long. This occurs as the total energy of the system approaches zero requiring the asteroid pair to extract an increasing fraction of energy from the primary's spin in order to escape. We do not find asteroid pairs with mass ratios larger than 0.2. Rotationally fissioned systems beyond this limit have insufficient energy to disrupt. We conclude that asteroid pairs are formed by the rotational fission of a parent asteroid into a proto-binary system which subsequently disrupts under its own internal system dynamics soon after formation.

Analysis of Asteroid Pair Formation via Rotational Fission

The paper investigates the formation mechanisms of asteroid pairs, with a specific focus on rotational fission as a primary process. Asteroids that share similar heliocentric orbits and separated gently are identified as pairs. The paper emphasizes that these separations usually occur with low relative velocities, suggesting rotationally induced disruptions of the original parent asteroids.

Key Findings

The paper utilizes a robust sample of 35 asteroid pairs, employing photometric observations and backward orbit integrations to gather data. These observations demonstrate that asteroid pairs with mass ratios under approximately 0.2 exhibit rapid primary rotation, nearing their critical fission frequency. As these mass ratios approach 0.2, primary rotation periods increase, indicating a critical energy dynamic where higher mass ratios prevent sufficient energy for complete disruption.

1. Mass Ratio and Spin Correlation: The paper asserts the absence of asteroid pairs with mass ratios above 0.2, where rotationally fissioned systems do not possess adequate energies for disruption. The primary rotation period correlates significantly with the mass ratio, leading to slower rotation as the ratio nears the critical threshold of 0.2.
2. System Energy Dynamics: The paper provides a quantitative representation of the free energy within these systems. It finds that systems with mass ratios below 0.2 have positive free energy, facilitating eventual separation. Conversely, a negative free energy inhibits separation due to insufficient system energy extraction during proto-binary formation.
3. Consistency and Variability in Observed Data: The paper's data aligns with theoretical models, strengthening the hypothesis that asteroid pairs emerge from proto-binaries formed by the rotational fission of critically spinning asteroids. The observed rotation periods and inferred model parameters, such as angular momentum and energy conservation, provide critical insights into the mechanisms driving asteroid pair formation.

The research suggests that asteroid shape and elongation do not significantly alter these general energy dynamics, though variability in shape ratios can affect precise mass ratio thresholds for disruption. Such findings point toward mechanical processes rather than mineralogical composition driving these systems.

Implications and Future Research Directions

Practically, this paper enhances the understanding of how asteroid pairs form and evolve, hinting at a broader applicability across the main belt and Hungarian asteroids. This understanding helps delineate pathways for assessing the evolution of near-Earth objects and potential threats to Earth.

Theoretically, the paper expands on existing models of asteroid disruption and rotational dynamics, offering a clear framework for the evaluation of asteroid pair formation via rotational fission. This research sets the stage for further studies on the diverse processes yielding multilayer asteroid systems and their long-term stability and evolution under rotational stress.

Future research can refine mass ratio estimates with improved observational techniques, particularly focusing on albedo and density variations between primaries and secondaries. Additionally, a deeper examination of how non-mechanical factors, such as thermal properties and particle cohesion, influence rotational fission processes could extend the findings. Comprehensive simulations integrating these variables would clarify discrepancies and contribute to the robustness of asteroid evolutionary theories.

In conclusion, the paper provides substantial evidence supporting rotational fission as a viable mechanism for asteroid pair formation. Its contributions lay a foundation for ongoing exploration into dynamics of small planetary bodies within our solar system.