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Stunning Comet Spiral Offers Glimpse of Icy Snowball at Its Core

Stunning Comet Spiral Offers Glimpse of Icy Snowball at Its Core

Comet 12P/Pons-Brooks is hiding a strange spiral in its icy heart—and it may tell scientists about the comet’s innards

Photo of comet 12P/Pons-Brooks and its rotating core with a backdrop of stars

An image of Comet 12P/Pons-Brooks and its rotating core taken by Jan Erik Vallestad.


Jan Erik Vallestad

Comet 12P/Pons-Brooks is a shapeshifter: in the summer of 2023 it sported wings like the Millennium Falcon, the iconic Star Wars ship. By autumn it had been dubbed the “Devil comet” for its horned appearance.

Now, for astrophotographers with the right equipment, Comet 12P appears to hide a perfect spiral—and the stunning sight could tell scientists more about this particular ice ball, which is one of the brightest comets on record. Still, it can be tricky to observe and photograph. Comet 12P is trekking toward its closest approach to the sun, set to occur in late April, so currently, it never rises high above the horizon and competes with the dregs of sunlight.

“I didn’t have any high expectations at all because when I started the sessions each day, you could still see the sunlight on the horizon,” says Jan Erik Vallestad, an amateur astrophotographer based in Norway. “I thought I would probably get just the coma—a blob.” (A coma is the fuzzy-looking cloud surrounding the icy nucleus, or core, of a comet and is created by gas and dust lifted off its surface.)

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But Vallestad got much more than a blob: in an image taken on March 9, he captured not only the comet’s long wispy tail streaking across the sky but also a spiral feature that he was able to highlight in editing.

As bizarre as the phenomenon looks, it’s real, says Quanzhi Ye, a planetary astronomer at the University of Maryland, who noted that he and his colleagues have also seen the feature in Comet 12P.

The spiral is far from unprecedented: astronomers had begun noticing that the hearts of certain comets contained a spiral as early as 1858. And its explanation is surprisingly simple, Ye says. “We know that comets release gas and dust into space, and the nucleus is also rotating just like any celestial object,” he says. The shed gas and dust become the coma, which reflects sunlight and gives a comet’s core its characteristic blurry appearance. When different parts of the comet’s surface lose material at different rates, the coma can become uneven. And because the comet’s nucleus is spinning, the brighter and fainter parts of the coma twist into a spiral.

That formation story means the spiral isn’t just pretty—scientists can work back in time, starting from the visual pattern, to learn more about the comet. “We can use spiral features like this to try to get a sense of how fast and in what direction the nucleus is rotating,” Ye says. “It tells us a whole lot about the comet itself, which is pretty amazing.”

Comet 12P is an exciting comet to learn more about, he says. It’s particularly prone to outbursts of gas and dust, which can make it appear bright in the sky. Although the spiral pattern isn’t triggered by an outburst, Ye says, skywatchers hope the comet might undergo an outburst in the next few weeks that could make it bright enough to see during the total solar eclipse that will cross North America on April 8.

In addition, this object hails from an especially interesting cohort of comets, dubbed Halley-type comets after their most famous member. These comets swing through the solar system once every 20 to 200 years—Comet 12P clocks in at a 71-year orbit. Short-period comets with orbits that are less than 20 years long come from the Kuiper belt region beyond Neptune. Long-period comets with orbits that are more than 200 years long come from the spherical Oort cloud in the cold outer reaches far beyond. Being able to study more Halley-type comets could help scientists understand the area between these two comet-laden regions, Ye says.

Comet 12P is also a witness to the ways skywatching has grown and changed since its last pass through the inner solar system, which occurred from 1953 to 1954, Ye says. “Last time it was around, we didn’t have so many telescopes; we didn’t have modern computers to give us this information,” he says. This time, humans have many more observatories, and a particularly powerful one should catch Comet 12P’s retreat: the Vera C. Rubin Observatory in Chile should begin its sky survey next year. And even when the comet lurks too close to the horizon, where science facilities struggle to observe it, amateur astronomers around the globe are eager to monitor the ice ball and share what they see.

“That’s kind of the beauty of it all, I think,” Vallestad says of astrophotography, which he began doing in earnest about two years ago. “Not only do you get to create images that can be aesthetically pretty to look at, but you can also bring forth scientific information as well.”

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