Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

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

David Jewitt

TL;DR

Archival HST data from 2017 December show that comet 41P/Tuttle-Giacobini-Kresak has a sub-kilometer nucleus with radius $r_n \approx 0.50$ km and a rotation period of $P \approx 0.599$ d in a two-peaked lightcurve ($a/b \gtrsim 1.4$). The study derives a dimensionless moment arm for outgassing torques of $k_T \approx 0.013$ and finds the active fraction declined from $f_A \approx 2.4$ in 2001 to $f_A \approx 0.14$ in 2017, indicating secular surface evolution. The rapid spin evolution is likely driven by asymmetric outgassing, with a spin-up timescale of a few decades ($\tau_s$) that is short relative to the dynamical lifetime ($\tau_d \sim 10^3$ yr), suggesting TGK either experiences unusually strong activity presently or is a remnant of a larger body. These results illuminate how small comet nuclei can undergo dramatic rotational changes on orbital timescales due to outgassing, with implications for nucleus stability and surface evolution.

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.

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

TL;DR

Archival HST data from 2017 December show that comet 41P/Tuttle-Giacobini-Kresak has a sub-kilometer nucleus with radius km and a rotation period of d in a two-peaked lightcurve (). The study derives a dimensionless moment arm for outgassing torques of and finds the active fraction declined from in 2001 to in 2017, indicating secular surface evolution. The rapid spin evolution is likely driven by asymmetric outgassing, with a spin-up timescale of a few decades () that is short relative to the dynamical lifetime ( yr), suggesting TGK either experiences unusually strong activity presently or is a remnant of a larger body. These results illuminate how small comet nuclei can undergo dramatic rotational changes on orbital timescales due to outgassing, with implications for nucleus stability and surface evolution.

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.
Paper Structure (9 sections, 6 equations, 4 figures)

This paper contains 9 sections, 6 equations, 4 figures.

Table of Contents

  1. INTRODUCTION
  2. OBSERVATIONS
  3. DISCUSSION
  4. Size of the Nucleus
  5. Nucleus Rotation
  6. Secular Variations
  7. Active Fraction
  8. Lifetime
  9. SUMMARY

Figures (4)

  • Figure 2: (Left panel:) Composite 3840 s integration on TGK. A 1$^{\prime\prime}$ scale bar and the marked cardinal directions apply to both panels. (Right panel:) Same image contoured to emphasize the near-nucleus coma. Direction arrow show the antisolar direction (-S) and the projected negative heliocentric velocity vector (-V).
  • Figure 3: Phase dispersion minimization plot showing the minimization parameter as a function of the assumed rotational frequency. Minima at $\Omega_1$ = 1.669 day$^{-1}$ and $\Omega_2$ = 3.339 day$^{-1}$ correspond to rotational periods at 0.599 day and 0.299 day, respectively. $\Omega_1$ and $\Omega_2$ give two-peaked and one peaked lightcurves.
  • Figure 4: Two-peaked lightcurve computed for the period $P$ = 0.599 day (14.4 hour), corresponding to $\Omega_1$ in Figure \ref{['PDM']}. Data are distinguished by the UT 2017 December date on which they were obtained, as marked.
  • Figure 5: Rotational frequency as a function of time expressed as Day of Year in 2017. The yellow-filled circles show data from Bod18 and Sch19. Points A (black-filled circle) and B (grey diamond) show the two solutions for prograde and retrograde rotation deduced from the HST lightcurve. The solid black line is a parabola added to guide the eye. Frequencies above(below) the dashed horizontal line are prograde(retrograde). The date of perihelion is indicated by a dashed vertical line.