Rotation of cometary meteoroids

Abstract

The aim of this study is to estimate the rotational characteristics of meteoroids after their release from a comet during normal activity. The results can serve as initial conditions for further analyses of subsequent evolution of rotation in the interplanetary space. A sophisticated numerical model was applied to meteoroids ejected from 2P/Encke comet. The meteoroid shapes were approximated by polyhedrons with several thousands of surface elements, which have been determined by 3D laser scanning method of 36 terrestrial rock samples. These samples came from three distinct sets with different origin and shape characteristics. Two types of gas-meteoroid interactions (diffuse and specular reflection of gas molecules from the surface of meteoroid) and three gas ejection models (leading to very different ejection velocities) were assumed. The rotational characteristics of ejected meteoroid population were obtained by numerical integration of equations of motion with random initial conditions and random shape selection. It was proved, that the results do not depend on specific set of shape models and that they are applicable to (unknown) shapes of real meteoroids. A simple relationship between median of meteoroid spin frequencies $\bar{f}$ (Hz), ejection velocities $v_{\rm ej}$ (m s$^{-1}$) and sizes $D$ (m) was determined. For diffuse reflection of gas molecules from meteoroid's surface it is: $\bar{f}\simeq 2\times 10^{-3} v_{\rm ej} D^{-0.88}$, and for specular reflection of gas molecules from meteoroid's surface it is: $\bar{f}\simeq 5\times 10^{-3} v_{\rm ej} D^{-0.88}$. The distribution of spin frequencies is relatively wide; $2\sigma$-interval can be described as $\sim(0.1, 10)\times \bar{f}$. The results were determined for 2P/Encke comet, but it was shown that their validity is general.