Astronomers studying Comet 41P/Tuttle-Giacobini-Kresak have reported evidence that the small icy body may have done something never before observed: stopped spinning, and reversed its rotation direction. The finding, based on NASA Hubble Space Telescope images from late 2017, caps a remarkable sequence of observations that tracked the comet’s spin slowing dramatically over just a few months. If confirmed, the reversal would represent a first in cometary science and force researchers to reconsider how outgassing reshapes the physical behavior of these objects as they approach the Sun.
What is verified so far
The story begins with a well-documented slowdown. In March 2017, multiple observing teams measured the comet’s rotation period at roughly once every 20 hours. That baseline was established through independent efforts, including data from NASA’s Neil Gehrels Swift Observatory and a sustained ground-based campaign at Lowell Observatory that spanned 47 nights from mid-February through early July 2017. The Lowell team tracked changes in jets of gas streaming from the comet’s surface, and their morphology analysis showed the spin period climbing smoothly, passing roughly 24 hours by late March and reaching approximately 48 hours by late April.
Swift observations conducted between May 6 and May 9, 2017, confirmed the trend from space. Ultraviolet and optical photometry from the Swift/UVOT instrument indicated a rotation period of at least 46 hours. A peer-reviewed study published in Nature attributed the slowdown to outgassing torque, the asymmetric force created when jets of sublimating ice push unevenly against a comet’s surface as it warms near the Sun. That Nature paper documented the period increasing from roughly 20 hours to more than 46 hours between March and May 2017, calling it the largest abrupt rotational change recorded for any comet at the time.
NASA’s Goddard Space Flight Center described the event in similar terms, noting that the Swift data showed the period had more than doubled and that continued torque could slow the comet to extremely long rotation periods. That prediction appears to have been borne out, and then some. Hubble images taken in December 2017 showed the comet spinning with a period of roughly 14 hours, a figure that only makes physical sense if the rotation first slowed to a near-stop, then reversed direction, speeding back up under the same outgassing forces now pushing the opposite way.
The interpretation, detailed in a study published in The Astronomical Journal with DOI 10.3847/1538-3881/ae4355, is attributed to the Space Telescope Science Institute. The STScI analysis places the Swift-era rotation period at roughly 46 to 60 hours and argues that continued torque likely slowed the comet to a virtual standstill before reversing its spin. In this reading, the December 2017 Hubble measurements capture a comet that has already passed through zero rotation and is now accelerating in the opposite direction, driven by the same jets that once acted as a brake.
What remains uncertain
The spin-reversal claim rests on a chain of inference rather than continuous direct observation. No telescope watched the comet in real time as it decelerated through zero rotation and began turning the other way. Instead, researchers are connecting two data points separated by months: the extremely slow spin measured by Swift in May 2017 and the faster but oppositely directed spin detected by Hubble in December 2017. The gap between those observations leaves room for alternative explanations, including the possibility that the comet entered a complex tumbling state rather than a clean reversal.
The Nature study itself raised the prospect of rotational instability and tumbling, a chaotic condition in which a comet wobbles around multiple axes rather than spinning smoothly around one. Whether the December 2017 Hubble measurement reflects a stable new spin state or a snapshot of ongoing tumbling has not been definitively resolved in the available literature. The Lowell team’s ground-based campaign ended in July 2017, leaving a five-month observational gap before the Hubble data. During that interval, seasonal changes on the comet’s surface or evolving jet patterns could have altered the torque in ways that are not directly constrained by observations.
No primary observational data from subsequent apparitions of Comet 41P have been cited to confirm whether the reversed rotation persisted or stabilized over time. The comet orbits the Sun roughly every 5.4 years, meaning additional close approaches have occurred, but publicly available follow-up results addressing the spin state are absent from the current reporting. Without that confirmation, the reversal should be treated as the leading interpretation rather than settled fact, a hypothesis that best fits the available snapshots but still awaits long-term verification.
The mechanism itself, outgassing torque, is well established in cometary physics. But the specific conditions that would cause a torque to persist through zero rotation and then accelerate the body in the opposite direction are not fully modeled for an object this small. Comet 41P is estimated at roughly 0.4 miles across, making it particularly susceptible to rotational disruption from even modest jet activity. Whether the same process could occur on larger comets or whether 41P’s small size makes it a special case is an open question. Detailed numerical models that couple gas flow, nucleus shape, and surface composition will be needed to test whether a smooth reversal is dynamically plausible under realistic conditions.
How to read the evidence
The strongest evidence in this story comes from direct observational measurements published in peer-reviewed journals. The March-to-May 2017 slowdown is supported by at least three independent data streams: the photometric analysis associated with the Nature study, the Lowell Observatory jet morphology campaign, and the Swift/UVOT ultraviolet observations. When three separate methods using different instruments and techniques converge on the same trend, the conclusion that the comet’s rotation slowed dramatically over those months is on solid ground.
The December 2017 Hubble result is also a direct measurement, but it stands alone in time. It tells researchers how fast the comet was spinning and in what direction at that specific moment, but not how it got there. The inference of a full stop and reversal is therefore a model-dependent step: scientists assume that the same outgassing torques identified earlier continued to act, and that no other major processes, such as fragmentation or a violent outburst, intervened to reset the spin. The available reports do not present evidence for such catastrophic events, but the lack of continuous monitoring means they cannot be fully ruled out.
For non-specialists, one way to interpret the current state of knowledge is to separate what is firmly measured from what is inferred. It is firmly measured that Comet 41P’s rotation period more than doubled over a few months in 2017, reaching tens of hours, and that later in the year it was rotating again with a much shorter period and an opposite sense of spin. It is inferred, but not directly observed, that the comet must have passed through a near-zero spin state in between, and that outgassing alone can account for this behavior.
The strength of the reversal hypothesis lies in its parsimony: a single physical mechanism, acting over an extended period, explains both the slowdown and the eventual fast spin in the opposite direction. Its weakness is the reliance on an unobserved transition phase, during which complex tumbling, changing jet geometry, or internal structural shifts could have played a role. Future work, including more sophisticated models and targeted observations during upcoming apparitions, will be needed to distinguish among these scenarios.
Access to the underlying research also shapes how the evidence is weighed. The key Swift and Hubble analyses appear in journals that are often read through institutional subscriptions or individual access, such as those available via Nature’s services. For broader transparency, some teams also circulate preprints and technical summaries, which allow other researchers to scrutinize methods and assumptions more closely. As additional modeling and follow-up data emerge, this open exchange will be crucial for testing whether 41P truly executed the first documented spin reversal, or whether an even more complex tale of cometary dynamics is waiting to be told.
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*This article was researched with the help of AI, with human editors creating the final content.
