This Comet Mysteriously Reversed Its Spin After Passing The Sun, But Why?

20 Feb. 2026, By M. STARR

https://www.sciencealert.com/this-comet-mysteriously-reversed-its-spin-after-passing-the-sun-but-why

Comet 41P photographed in March 2017. 

(Kees Scherer/Flickr, CC0)

A comet whizzing through the Solar System has astonished scientists by doing something they had never seen before.

In early 2017, comet 41P/Tuttle-Giacobini-Kresák made its 5.4-year close approach to the Sun, or perihelion.

As it did so, its spin appeared to slow down to a complete halt, before likely starting up again in the opposite direction, according to astronomer David Jewitt of the University of California, Los Angeles.

The reversal itself isn't the crazy part; changes in cometary spin are known to happen sometimes as these icy objects draw near to the Sun. Rather, it's how quickly and dramatically the reversal occurred.

https://www.youtube.com/watch?v=oe-d9RxLLaM&t=3s

"The previous record for a comet spindown went to 103P/Hartley 2, which slowed its rotation from 17 to 19 hours over 90 days," said astronomer Dennis Bodewits of the University of Maryland, describing the slowdown phase in 2018.

"By contrast, 41P spun down by more than 10 times as much in just 60 days, so both the extent and the rate of this change is something we've never seen before."

The sequence of events is as follows. Observations taken in March 2017 showed that 41P's rotation period was about 20 hours. By May, the rotation had slowed to more than double that length, with one rotation every 53 hours or so.

By December, however, something really weird had happened. The rotation period of the comet had shortened to 14.4 hours – a change, Jewitt believes, that can best be explained if its rotation had slowed to a complete stop sometime around June 2017 and then changed direction.

This is actually fairly easy to explain, in theory. Comets are clumpy agglomerations of rock and ice that spend most of their orbital periods just trucking along. However, as they get closer to the Sun, the ice in their bodies starts to transition directly into gas, a process called sublimation.

Jets and geysers burst forth, spraying vapor out into space. Each of these jets imparts a torque on the cometary nucleus. This is why so many comets change their spin as they go around the Sun, some spinning up to such high speeds that they fall apart completely.

In addition, the spin of a smaller comet changes more readily than the spin of a larger one. At roughly a kilometer wide – about the length of 10 football fields laid end to end – 41P is small enough for those gas jets to have an outsized effect.

If the Sun heated it unevenly, or if the distribution of its ice content was lopsided, its rapid reversal is relatively easy to explain mathematically.

Now, there's still a little bit of a question mark. The light-curve measurements taken of 41P can give its spin rate, but not its spin direction.

Jewitt arrived at his conclusions by plotting the lightcurves along with new estimates of the comet's size, calculated from archival Hubble Space Telescope data. He could only make them line up smoothly if the comet's spin had slowed to zero and then flipped.

"The observed, rapid changes are natural consequences of torques from outgassed volatiles acting on the very small nucleus," Jewitt writes in his preprint, which has not yet been peer-reviewed.

If 41P's spin continues evolving at the rate seen in 2017, it could spin itself apart within a few short decades, Jewitt found. We don't know yet if that's the case. There are no published spin rates from its September 2022 perihelion. The next opportunity to measure its spin rate will be its 2028 perihelion.

Comets are among the more fascinating relics of the early Solar System. They're fragile and change rapidly, but somehow they're still here, 4.5 billion years after the Solar System formed.

The changes displayed by 41P over the course of 2017, and the decades prior, suggest that it may be the remnant of a much larger comet that has been gradually whittled away by its long, slow dance with the Sun.

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