This Comet Is Slowing Down Like A Reverse Pinwheel Firework

Artist's impression of the comet and its jets. NASA Goddard

Astronomers have spotted a comet doing something never seen before – slowing its rotation significantly as it makes its way around the Sun.

Known as comet 41P/Tuttle-Giacobini-Kresák, or just 41P, images of the comet in May 2017 from NASA’s SWIFT spacecraft showed that it was rotating three times slower than it was in March. This is more than any comet we’ve seen before. A paper on the findings is published in Nature.

"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 lead author Dennis Bodewits from the University of Maryland in a statement, who presented the findings yesterday at the American Astronomical Society (AAS) meeting in Washington.

"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."

Jets fired from the surface of the comet are thought to be the cause, which push against the comet like a reverse pinwheel firework (terrible video quality but hopefully you get what we mean), producing a torque against its rotation. These jets are the result of the Sun heating the surface of the comet, with its surface ice then explosively changing into plumes of gas.

Comet 41P takes 5.4 years to orbit the Sun, during which time it moves from the orbit of Jupiter to the orbit of Earth. In April 2017, it passed just 21.2 million kilometers (13.2 million miles) from our planet, before making its closest approach to the Sun eight days later.

Swift was able to follow the comet as it made its journey, snapping images in May to study its rotation. These showed its rotation had doubled to between 46 and 60 hours. By now, it’s thought this may have continued to slow to about 100 hours.

The reason Comet 41P may be slowing dramatically more than other comets may be in part due to its small size. It’s thought to be less than 1.4 kilometers (0.9 miles) across, and coupled with what’s thought to be a large amount of water production, half of the comet’s surface may be covered in jets. Most comets only have jets on about 3 percent of their surface.

And the slowing rotation may ultimately cause the comet to become completely unstable, meaning it will spin all over the place with no fixed axis. This will cause more of the surface to become heated, in turn producing more and more jets and causing further erratic behavior.

"The behavior of comet 41P/Tuttle-Giacobini-Kresák suggests that it is in a distinct evolutionary state and that its rotation may be approaching the point of instability," the team wrote in their paper.

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