A celestial wanderer traversing our Solar System has presented the scientific community with an unprecedented observation.
In the early part of 2017, comet 41P/Tuttle-Giacobini-Kresák achieved its closest orbital proximity to the Sun, a point known as perihelion, occurring every 5.4 years.
During this close passage, its rotational motion reportedly decelerated to a complete standstill, subsequently reinitiating in the inverse direction, according to David Jewitt, an astrophysicist at the University of California, Los Angeles.
The reversal in rotation is not the perplexing aspect; alterations in cometary spin are documented occurrences, particularly when these icy bodies approach the Sun. The extraordinary element, however, is the remarkable swiftness and magnitude of this rotational transformation.
“Prior to this, the most significant observed spin-down in a comet was attributed to 103P/Hartley 2, which experienced a deceleration of its rotation from 17 to 19 hours over a 90-day period,” stated astronomer Dennis Bodewits of the University of Maryland, detailing the slowdown phase observed in 2018.
“In stark contrast, 41P underwent a rotational slowdown exceeding tenfold that rate within a mere 60 days, indicating that both the extent and velocity of this change are entirely novel phenomena.”
The progression of events is as follows: Observations conducted in March of 2017 indicated that 41P’s rotational period approximated 20 hours. By May, its rotation had diminished significantly, extending to over double that duration, with a single rotation requiring approximately 53 hours.
However, by December, a truly anomalous event had transpired. The comet’s rotational period had contracted to 14.4 hours – a modification that, in Jewitt’s estimation, can be most plausibly accounted for by its rotation reaching zero and subsequently reversing its direction around June 2017.
This phenomenon is theoretically quite straightforward to elucidate. Comets are essentially heterogeneous aggregates of rock and ice that maintain a steady trajectory for the majority of their orbital journeys. Yet, as they draw nearer to the Sun, the ice comprising these bodies initiates a direct transition into a gaseous state, a process termed sublimation.
This sublimation results in the expulsion of vapor in directed streams and ejections, which are propelled into space. Each of these energetic outflows exerts a torque upon the cometary nucleus. This mechanism explains the common alteration of cometary spin as they orbit the Sun, with some experiencing such accelerations that they disintegrate entirely.
Furthermore, the rotational characteristics of smaller comets are more susceptible to change than those of larger ones. With a diameter of approximately one kilometer – equivalent to the length of ten football fields placed end-to-end – 41P possesses a size that permits those vaporous jets to exert a disproportionately significant influence.
Should the Sun’s heating have been non-uniform, or if the distribution of its icy constituents were asymmetrical, the rapid rotational reversal can be readily substantiated through mathematical modeling.
Currently, a degree of uncertainty persists. While photometric measurements of 41P can ascertain its rotational velocity, they do not provide information regarding its spin orientation.
Jewitt’s deductions were derived from the analysis of light curves, integrated with revised estimations of the comet’s dimensions, which were computed using historical data from the Hubble Space Telescope. The smooth alignment of these data points was only achievable under the assumption that the comet’s rotation had ceased before reversing its trajectory.
“The observed, accelerated transformations are logical outcomes of torques generated by effervescent volatile substances acting upon the exceptionally diminutive nucleus,” Jewitt asserts in his unreviewed manuscript.
If 41P’s rotational evolution continues at the pace observed in 2017, it could potentially fragment within a few decades, according to Jewitt’s projections. It remains undetermined whether this trajectory will be followed. No published spin rates are available from its September 2022 perihelion passage. The subsequent opportunity to assess its rotational velocity will be during its 2028 perihelion.
Comets stand as among the most captivating remnants from the nascent stages of the Solar System. They are inherently fragile and undergo rapid modifications, yet they persist, having survived for 4.5 billion years since the Solar System’s formation.
The dynamic changes exhibited by 41P throughout 2017, and indeed in the preceding decades, suggest that it might represent the fragmented core of a significantly larger progenitor comet that has been progressively diminished through its prolonged, gradual interaction with the Sun.
These findings have been made publicly accessible on arXiv.
