Astronomers Watched a “Near-Sun” Comet Disintegrate as it Flew too Close to the Sun

Comets that venture close to the Sun can transform into something beautiful, but sometimes they encounter incineration if they get too close. Of the various types of comets that orbit close to the Sun, astronomers had never seen the destruction of the type classified as “near-Sun” comets. But thanks to a variety of telescopes on summit of Mauna Kea in Hawai?i, scientists have now captured images of a periodic rocky near-Sun comet breaking apart. They say the disintegration of this comet could help explain the scarcity of such periodic near-Sun comets.

Astronomers have certainly witnessed other types of comets disintegrating in the solar corona. For example, early this year — after putting on a spectacular show — Comet Leonard broke apart after perihelion (its closest point to the Sun) in January, 2022. Comet Leonard was considered a Sun-grazing comet, the most commonly known comets that make close passes of the Sun. The Solar and Heliospheric Observatory (SOHO) spacecraft has detected about 4,000 Sun-grazers since it launched in 1995.

But there are also near-Sun, Sun-skirters and Sun-divers. However, the nomenclature and difference between them all is not clearly defined, says Karl Battams, from the US Naval Research Laboratory in Washington DC. Battams tracks Sun-grazing comets with data from SOHO, particularly those that belong to the ‘Kreutz’ family of comets, named after Heinrich Kreutz, a nineteenth-century astronomer who calculated many of their orbits.

It’s hard to make out, but the dot in the cross-hairs is a comet streaming toward the Sun. This image is from 2015, and the comet is the 3,000th one discovered by SOHO since it was launched. Image: SOHO/ESA/NASA

“There’s no clearly defined separation between what we would classify as a near-Sun versus Sun-grazing comet,” Battams said via email. But he was part of a group of astronomers that drafted some guidelines in a 2018 survey paper.

They proposed clear definitions for comets, based on how close they get to the Sun: near-Sun comets should encompass all objects that come within the orbit of Mercury, about .3 AU. Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s center, equal to half of Mercury’s perihelion distance, and the commonly-used phrase Sun-grazers to be objects that reach perihelion within 3.45 solar radii. Finally, comets with orbits that intersect the solar photosphere are termed Sun-divers.

But, says Battams, the definitions can still be confusing because all Sun-grazing comets are near-Sun comets, but not all near-Sun comets are Sun-grazing comets.

The near-Sun comet that was recently observed while breaking apart, named 323P/SOHO, is a short-period ‘near-Sun’ (but NOT Sun-grazing) comet that experienced a different environment to the Kreutz Sun-grazers.

Near-Sun object 323P/SOHO observed by the Subaru Telescope on December 21, 2020 (left) and CFHT on February 11, 2021 (right). 323P/SOHO on its way to perihelion is seen as a point source in the center of the left image; after the perihelion, the comet has developed a long narrow tail as seen in the right image. (Credit: Subaru Telescope/CFHT/Man-To Hui/David Tholen)

“Instead of plunging deep into the corona and getting ripped apart or vaporized,” said Battams, “it has been getting slowly baked with repeated passes in a toasty environment, leading to a slow-but-steady demise. Probably many of the non-Kreutz that SOHO sees are undergoing this process, but this is the first time we’ve been able to definitely say that this is what’s happening.”

Battams, who was not involved in the ground-based study of this comet, said this first- of-its-kind observations for this class of comet is an important result for telling us about the evolution and end stage of comets in that “near-Sun” region. An outstanding question is that there are far fewer observed near-Sun objects than models show there should be.

Comet 323P/SOHO was discovered in 1999 with SOHO, and is a relatively small comet, with a nucleus of only about 172 meters (560 ft) in diameter. It rotates quickly, at 0.522 hr., the most rapid rotation of every known comet in the Solar System. Its orbital period is just over four years, so it has come close to the Sun several times. Plus, on its final pass, it came perilously close to the Sun, with a perihelion of only 0.04 astronomical units. While a larger comet could perhaps survive such a close approach, smaller comets that come that close are skirting with eventual doom.  

Because comets like 323P pass so close to the Sun, they are difficult to spot and observe. To study this object, an international group of astronomers from Macau, the US, Germany, Taiwan, and Canada used multiple telescopes including the Subaru Telescope, the Canada France Hawaii Telescope (CFHT), the Gemini North telescope, Lowell’s Discovery Telescope, as well as the Hubble Space Telescope.

Convex shape model of the nucleus of 323P/SOHO in three orthogonal views. The left and middle panels show edge-on views of the nucleus that are 90° apart from each other, with its spin axis (the Z-axis) pointing upwards, while the nucleus is viewed in a pole-on configuration (the line of sight along the negative Z direction) in the right panel. Credit: Man-To Hui/David Tholen)

Observations with the Surbaru telescope, which has a wide field of view, marked the first time 323P/SOHO was captured by a ground-based telescope. With this data, the researchers were able to better constrain the orbit, and they knew where to point the other telescopes when 323P/SOHO started to move away from the Sun again.

To their surprise the researchers found that 323P/SOHO had changed remarkably during perihelion. In the Subaru Telescope data, 323P/SOHO was just a dot.

“However, in our post-perihelion observations, it developed a long, narrow tail mimicking a disintegrating cometary debris cloud,” the authors wrote in their paper.

They also found that 323P/SOHO’s color is unlike anything else in the Solar System. They says observations of other near-Sun comets are needed to see if they also share these traits.

The researchers said they believe that the intense radiation from the Sun caused parts of the comet to break off due to thermal fracturing, similar to how ice cubes crack when you pour a hot drink over them. This mass loss mechanism could help explain what happens to near-Sun comets and why there are so few of them.

The researchers hope to spot whatever remains of the comet on its next perihelion in order to learn more about it. But it will likely plunge into the Sun on one of the next close passes.  

Paper: The Lingering Death of Periodic Near-Sun Comet 323P/SOHO.
NAOJ Press release
Email exchange with Karl Battams