Reversal of Spin: Comet 41P/Tuttle-Giacobini-Kresak

TL;DR

This study analyzes the dramatic rotational changes of comet 41P/Tuttle-Giacobini-Kresak post-perihelion, revealing a small nucleus likely undergoing spin reversal due to intense outgassing, with implications for its physical lifetime and surface evolution.

Contribution

It provides new measurements of the comet's nucleus size, rotation period, and evidence of spin reversal, highlighting the effects of outgassing torques on small cometary nuclei.

Findings

Nucleus radius approximately 500 meters.

Rotation period of about 0.60 days.

Evidence of spin reversal between perihelion and December 2017.

Abstract

The rotations of cometary nuclei are known to change in response to outgassing torques. The nucleus of comet 41P/Tuttle-Giacobini-Kresak exhibited particularly dramatic rotational changes when near perihelion in 2017 April. Here, we use archival Hubble Space Telescope observations from 2017 December to study the post-perihelion lightcurve of the nucleus and to assess the nucleus size. From both Hubble photometry and non-gravitational acceleration measurements we find a diminutive nucleus with effective radius r = 500+/-100 m. Systematic optical variations are consistent with a two-peaked (i.e., rotationally symmetric) lightcurve with period 0.60+/-0.01 days, substantially different from periods measured earlier in 2017. The spin of the nucleus likely reversed between perihelion in 2017 April and December as a result of the strong outgassing torque. We infer a dimensionless moment arm k = 0.013, about twice the median value in short-period comets. The lightcurve range of 0.4 magnitudes indicates a projected nucleus axis ratio greater than 1.4:1, while the active fraction of the nucleus decreased from 2.4 in 2001 (suggesting augmentation of the gas production by sublimating coma ice grains) to 0.14 in 2017, a result of long-term modification of the surface. We find that the physical lifetime of this small nucleus to spin-up is short compared to the reported 1500 year dynamical time spent in the current orbit. Two limiting reconciliations of this inequality are suggested. The nucleus could be in a state of unusually strong activity, leading us to over-estimate the average mass loss rate and outgassing torque and so to under-estimate the physical lifetime. Alternatively, the nucleus could be the surviving remnant of a once larger body for which outgassing torques were less effective in changing the spin.