Complex rotation with internal dissipation. Applications to cosmic-dust alignment and to wobbling comets and asteroids

TL;DR

This paper investigates how internal dissipation causes relaxation of freely spinning bodies like asteroids, comets, and cosmic dust, affecting their wobble damping, alignment, and rotational dynamics with applications to astrophysics and space observations.

Contribution

It provides a detailed analysis of internal dissipation effects on precessing bodies and introduces new insights into dust grain alignment and wobble damping timescales.

Findings

Internal dissipation leads to relaxation of precessing bodies towards principal axis rotation.

Wobble damping in comets can be observed within months to years with current spacecraft technology.

Precession damping influences cosmic dust grain alignment and flipping behavior.

Abstract

Neutron stars, asteroids, comets, cosmic-dust granules, spacecraft, as well as whatever other freely spinning body dissipate energy when they rotate about any axis different from principal. We discuss the internal-dissipation-caused relaxation of a freely precessing rotator towards its minimal-energy mode (mode that corresponds to the spin about the maximal-inertia axis). While the body nutates at some rate, the internal stresses and strains within the body oscillate at frequencies both higher and lower than this rate. The internal dissipation takes place mostly the second and higher harmonics. We discuss the application of our findings to asteroids. Regarding the comets, estimates show that the currently available angular resolution of spacecraft-based instruments makes it possible to observe wobble damping within year- or maybe even month-long spans of time. We also discuss cosmic-dust astrophysics; in particular, the role played by precession damping in the dust alignment. We show that this damping provides coupling of the grain's rotational and vibrational degrees of freedom; this entails occasional flipping of dust grains due to thermal fluctuations. During such a flip, grain preserves its angular momentum, but the direction of torques arising from H2 formation reverses. As a result, flipping grain will not rotate fast in spite of the action of uncompensated H2 formation torques. The grains get ``thermally trapped,'' and their alignment is marginal.