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arXiv · 2602.06403

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

David Jewitt
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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.

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David Jewitt. 2026-02-28. Reversal of Spin: Comet 41P/Tuttle-Giacobini-Kresak. https://arxiv.org/abs/2602.06403

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