Comet 41P/Tuttle-Giacobini-Kresak, a small icy body orbiting Jupiter’s neighborhood, did something astronomers rarely witness: after swinging past the sun in April 2017, it slowed its spin to a near-halt, then started rotating in the opposite direction. A new preprint by UCLA astronomer David Jewitt, drawing on archival Hubble Space Telescope images, presents the strongest evidence yet that the comet fully reversed its spin, raising fresh questions about how solar heating reshapes these fragile objects.
A Comet That Slammed the Brakes
The story begins with the comet’s unusually close 2017 pass. On April 1 of that year, 41P swept within roughly 0.14 astronomical units of Earth, giving ground-based and space-based telescopes a rare, detailed look at its behavior. At the time, multiple teams were already tracking its spin. A peer-reviewed study published in Nature documented that between March and May 2017, the comet’s apparent rotation period ballooned from roughly 20 hours to more than 46 hours, meaning it was spinning far more slowly by late spring.
That dramatic slowdown was not subtle. An independent 47-night imaging campaign at Lowell Observatory, running from mid-February through early July 2017, used cyanogen gas-jet morphology to produce 78 separate rotation-period measurements. Those data showed the spin-down was accelerating: the period was around 24 hours in late March and had climbed past 48 hours by late April. NASA’s Swift spacecraft, observing the comet with its ultraviolet and optical telescope on May 7 through 9, 2017, confirmed the rotation period had more than doubled since early March. Water-production estimates from the same Swift observations helped quantify the intense outgassing driving this change.
Gas Jets Acting as an Invisible Brake
The explanation for such rapid deceleration centers on torque from asymmetric gas jets. As the comet neared the sun, solar heating vaporized ices on its surface, producing jets of gas and dust. When those jets happen to fire from locations that create a net rotational force opposite to the spin direction, they act like a brake. The Nature study attributed the spin-down to a fortuitous alignment of gas emission that generated an anomalously strong torque on the small nucleus. Because 41P is a relatively tiny comet, even modest outgassing can exert outsized rotational effects compared to a larger body with more angular momentum to shed.
What made this case unusual was the sheer speed of the change. Cometary nuclei are known to alter their rotation rates near perihelion, but most adjustments are gradual, playing out over multiple orbits. Comet 41P managed to more than double its period in roughly two months. The Lowell Observatory team’s 78 measurements captured this acceleration in real time, offering one of the most finely sampled records of a comet spinning down ever assembled.
Hubble Catches the Reversal
The real surprise came months later. After perihelion in April 2017, the comet moved to a part of its orbit where it was difficult to observe from Earth. But by December 2017, it had emerged back into view, and the Hubble Space Telescope captured images of its bare nucleus. Jewitt, examining those archival Hubble frames recently, derived a nucleus lightcurve consistent with a two-peaked period of 0.60 plus or minus 0.01 day, equivalent to approximately 14.4 hours. That is far shorter than the 46-plus-hour period recorded in May, and it points to a comet that had not merely stopped spinning but had picked up speed again in the opposite direction.
The inference is striking: sometime around mid-2017, the comet’s rotation likely ground to a halt, and then the same outgassing forces that had been slowing it began pushing it the other way. Because the gas jets did not shut off the instant the spin reached zero, the residual torque would have started accelerating the nucleus in reverse. By December 2017, the comet was spinning with a 14.4-hour period, a pace not far from its original pre-perihelion rate but in the opposite sense. This sequence, as reported by Phys.org, represents the clearest documented case of a cometary spin reversal.
Why Standard Models Fall Short
Most existing models of cometary spin evolution treat outgassing torques as relatively stable forces that nudge rotation rates up or down over many orbits. The 41P episode challenges that assumption. The comet did not simply slow down and settle into a new, gentler spin state. It passed through zero angular momentum and came out the other side, all within roughly eight months. That timeline demands a torque geometry that remained effective even as the spin direction flipped, which is not a trivial condition. If the active regions on the nucleus shifted or shut down at the wrong moment, the reversal would not have completed.
One open question is whether the comet’s internal structure played a role. A loosely bound rubble pile, for instance, might redistribute mass as rotation slowed, altering the moment of inertia and the effective torque arm. No direct evidence for such restructuring exists in the current data, but the possibility is important because similar processes could push other small bodies toward rotational fission. Jewitt’s preprint notes that even small changes in the pattern of active areas could dramatically change the net torque, implying that 41P’s reversal may have required a delicate balance of surface activity and internal cohesion that is not yet captured in standard numerical models.
What 41P Reveals About Comets—and About Data
For planetary scientists, 41P is a reminder that comets are not passive snowballs but evolving worlds whose spins, shapes, and even internal structures can change over a single orbit. The dramatic reversal suggests that outgassing can, under the right conditions, completely reorient a nucleus in less than a year. That has implications for how sunlight falls on the surface during subsequent orbits, which regions become active, and how quickly the comet erodes. Over many perihelion passages, repeated episodes of strong torque could randomize spin axes or even break apart weaker nuclei, potentially explaining why some short-period comets fragment unexpectedly.
Equally notable is the role of open archives and preprint culture in stitching together 41P’s story. The spin-down was first quantified in a peer-reviewed journal article, while the Lowell team’s dense timeline of rotation periods appeared in a separate observational study. Jewitt’s analysis of the spin reversal itself is circulating as a preprint on arXiv, where astronomers routinely share new results before formal publication. That ecosystem, supported by a network of institutional members and individual donations, enables rapid comparison of independent datasets and theories. Clear submission guidance and curation standards also help ensure that preprints like Jewitt’s can be critically evaluated and built upon quickly, accelerating the scientific process that turned an odd, slowing comet into the first well-documented case of a full spin reversal.
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*This article was researched with the help of AI, with human editors creating the final content.
