2021/05/11 by David Jewitt · 1 voice · 1 citation
Physics and Astronomy · Earth and Planetary Sciences · #Astro and Planetary Science #High-pressure geophysics and materials #Planetary Science and Exploration
paper · pdf · doi:10.3847/1538-3881/abf09c
openalex publication_date 2021/05/11 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Abstract Anisotropic outgassing from comets exerts a torque sufficient to rapidly change the angular momentum of the nucleus, potentially leading to rotational instability. Here, we use empirical measures of spin changes in a sample of comets to characterize the torques, and to compare them with expectations from a simple model. Both the data and the model show that the characteristic spin-up timescale, τ s , is a strong function of nucleus radius, r n . Empirically, we find that the timescale for comets (most with perihelion 1–2 au and eccentricity ∼0.5) varies as , where r n is expressed in kilometers, and τ s is in years. The fraction of the nucleus surface that is active varies as . We find that the median value of the dimensionless moment arm of the torque is k T = 0.007 (i.e., ∼0.7% of the escaping momentum torques the nucleus), with weak (<3 σ ) evidence for a size dependence . Sub-kilometer nuclei have spin-up timescales comparable to their orbital periods, confirming that outgassing torques are quickly capable of driving small nuclei toward rotational disruption. Torque-induced rotational instability likely accounts for the paucity of sub-kilometer short-period cometary nuclei, and for the pre-perihelion destruction of sungrazing comets. Torques from sustained outgassing on small active asteroids can rival YORP torques, even for very small (≲1 g s −1 ) mass-loss rates. Finally, we highlight the important role played by observational biases in the measured distributions of τ s , f A , and k T .