A Comet That Hit the Brakes, Then Reversed
The story of 41P’s spin change unfolded in stages. In March 2017, astronomers using Lowell Observatory’s Discovery Channel Telescope measured the comet rotating once roughly every 20 hours. That was already a relatively slow spin for a small comet, but what happened next was far more striking. By May 2017, images from NASA’s Swift mission showed the comet spinning about three times slower than it had just two months earlier. Swift measured the rotation period at 46 to 60 hours, a rate of deceleration NASA described as unprecedented among observed comets. Related peer-reviewed research in Nature discussed the comet’s rapid spindown and linked the slowdown to torque generated by gas emissions as the comet neared the Sun. That finding alone was exceptional. But the real surprise came months later, when Hubble turned its optics on 41P in December 2017, well after the comet had passed its closest approach to the Sun. The rotation period Hubble recorded was substantially different from the values measured earlier that year, and the data pointed to something no one had seen before: the comet’s spin had reversed direction.How Gas Jets Act Like Tiny Thrusters
The mechanism behind this reversal is outgassing torque. As a comet approaches the Sun, solar heating causes ices beneath the surface to sublimate, producing jets of gas and dust that stream away from the nucleus. These jets do not fire symmetrically. Depending on where active vents sit relative to the comet’s spin axis, the escaping gas can push the nucleus like an off-center rocket thruster, speeding up or slowing down its rotation. “Jets of gas streaming off the surface can act like small thrusters,” said David Jewitt of the University of California, the paper’s author, in NASA’s announcement of the discovery. The smaller the comet, the more susceptible it is to these forces. At just 0.6 miles across, 41P’s nucleus has very little rotational inertia, making it especially vulnerable to torque from even modest outgassing. This is the physical explanation researchers propose for why 41P did not simply slow down and stop. In that interpretation, jets continued to apply torque even as the rotation slowed dramatically, allowing the nucleus to transition into rotation in the opposite direction. Think of it as pressing the brakes on a spinning top so hard that the top begins turning the other way.Why a 2017 Event Matters in 2026
The observations that captured this reversal were taken in 2017, so a fair question is why the finding is only being reported now. The answer lies in the difficulty of the measurement and the time required to analyze it. Hubble’s December 2017 data had to be carefully compared against the earlier Swift and ground-based observations, and the team needed to rule out alternative explanations for the changed rotation period. The study, titled “Reversal of Spin: Comet 41P/Tuttle-Giacobini-Kresak,” was posted to arXiv on February 6, 2026. The formal NASA announcement followed on March 26, 2026. The gap between observation and publication is not unusual in planetary science, where complex datasets often require years of reduction and peer scrutiny before results can be stated with confidence. The earlier spindown research, which established the baseline that made the reversal detectable, went through a similar timeline. The original open-access version of that study documented the dramatic 2017 slowdown and laid the groundwork for the follow-up Hubble analysis.What This Changes for Comet Science
Before this result, spin reversals in comets were theoretical. Models predicted that outgassing could, in principle, flip a comet’s rotation, but no one had caught it happening. Confirming the phenomenon in a real object forces a rethink of how small comets evolve over repeated orbits around the Sun. Each time a Jupiter-family comet like 41P passes through the inner solar system, it loses material to outgassing. If that process can also reverse the spin, the structural stresses on the nucleus become far more complex than a simple gradual slowdown would produce. A spin reversal means the comet’s internal structure must withstand forces pulling in the opposite rotational direction, raising the question of how many small comets fracture or disintegrate partly because of these torque-driven flips. The finding also has practical implications for predicting comet behavior. Dust and gas production rates depend on which parts of a comet’s surface face the Sun and for how long, both of which change if the spin direction shifts. For astronomers trying to forecast a comet’s brightness or tail structure during future apparitions, knowing that spin reversals are possible, and that they can happen within a single orbit, adds a significant variable to their models. More from Morning Overview*This article was researched with the help of AI, with human editors creating the final content.
