Relating Changes in Cometary Rotation to Activity: Current Status and Applications to Comet C/2012 S1 (ISON)

TL;DR

This paper introduces a parameter X to predict cometary rotational changes, analyzes its consistency across different comets, and applies it to comet C/2012 S1 (ISON) to forecast potential rotational alterations during perihelion.

Contribution

The paper proposes a new parameter X for predicting cometary rotational changes and demonstrates its near-constancy across multiple comets, applying it to ISON for perihelion activity predictions.

Findings

Parameter X remains nearly constant across studied comets.

Active fractions vary widely but do not affect X significantly.

ISON is expected to undergo observable rotational changes near perihelion.

Abstract

We introduce a parameter, X, to predict the changes in the rotational period of a comet in terms of the rotational period itself, the nuclear radius, and the orbital characteristics. We show that X should be a constant if the bulk densities and shapes of nuclei are nearly identical and the activity patterns are similar for all comets. For four nuclei for which rotational changes are well documented, despite the nearly factor 30 variation observed among the effective active fractions of these comets, X is constant to within a factor two. We present an analysis for the sungrazing comet C/2012 S1 (ISON) to explore what rotational changes it could undergo during the upcoming perihelion passage where its perihelion distance will be ~2.7 solar radii. When close to the sun, barring a catastrophic disruption of the nucleus, the activity of ISON will be sufficiently strong to put the nucleus into a non-principal-axis rotational state and observable changes to the rotational period should also occur. Additional causes for rotational state changes near perihelion for ISON are tidal torques caused by the sun and the significant mass loss due to a number of mechanisms resulting in alterations to the moments of inertia of the nucleus.