Comet Zijinshan-Atlas has a reverse tail? What's going on?

Image caption: Comet Purple Mountain-Atlas in the night sky of Puerto Rico after sunset on October 13, with an abnormal tail faintly visible on the other side of the bright tail. Photo by YarelisMedina

Comet Purple Mountain-Atlas has undoubtedly become the focus of attention on the starry sky in the evening recently. Since October 12, it has quietly appeared in the short night after sunset, attracting the attention of astronomy enthusiasts around the world. What is particularly striking is that in the shots captured by some enthusiasts, the comet actually showed a strange "tail" pointing towards the sun. Usually, the comet tail we know extends away from the sun, so what exactly is this "tail" pointing towards the sun?

How is the tail of a comet formed? A comet is a celestial body composed of rocks, dust, ice, and various frozen gases. Astronomers often jokingly call them "dirty snowballs." When a comet approaches the sun, it is exposed to strong sunlight and gradually heats up.

As they heat up, the various frozen gases that make up a comet can change directly from a solid to a gaseous state in a process called sublimation. The gases and dust particles released during sublimation can form a cloud around the comet, called a coma.

The gas and dust in the coma can form two distinct comet tails under the interaction of the solar wind. One of them is composed of gas and is called the ion tail . Solar radiation strips electrons from the gas in the coma, making them positively charged and becoming ions.

The solar wind then pushes these charged gas particles away from the sun, creating a blue tail. The blue color comes from the large number of carbon monoxide ions in it, which shine brightly in the sunlight.

The other tail is called the dust tail , which is composed of dust particles released during the sublimation process. These dust particles are pushed away from the sun under the pressure of sunlight, forming a curved tail that looks like a broom. For this reason, comets are sometimes called "broom stars."

The closer a comet is to the sun, the longer and brighter its tail will be, and its length can even far exceed the nucleus itself, reaching hundreds of thousands of kilometers.

Image caption: On October 14, Comet Purple Mountain-Atlas appeared over Austria. A long and thin beam of light appeared on the right side of the comet, which is an abnormal comet tail.

What about the reverse "tail"?

After understanding the reasons for the formation of comet tails, you will find that comet tails are actually "blown" by the solar wind, so they always extend away from the sun. So what is the matter with the tail of Comet Zijinshan-Atlas pointing to the sun?

In fact, this extra tail is often called an anomalous comet tail or anti-tail. Unlike the commonly known comet tail, it is not composed of debris directly ejected from the comet itself, but is composed of dust particles left behind by the comet in its orbit.

These dust particles are relatively large and less affected by the solar wind, so they stay roughly in the orbital plane of the comet. When the Earth happens to cross this orbital plane, the sunlight reflected by these dust particles can be seen by people on Earth, creating the illusion of an abnormal comet tail.

Image description: Schematic diagram of the relationship between the abnormal comet tail, dust tail, and ion tail

However, Comet Zijinshan-Atlas is not the only comet with an abnormal comet tail. Comet Hale-Bopp in 1997, the green comet C/2022E3 (ZTF) that appeared in January 2023, and the "devil comet" 12P/Pons-Brooks that passed by the Earth in June this year all appeared in the night sky with abnormal comet tails.

Comet Zijinshan-Atlas is moving away from us

Comet Purple Mountain-Atlas, officially designated C/2023A3, was first discovered by China's Purple Mountain Observatory on January 9, 2023, and was initially thought to be an asteroid. It was not until February 22 that its true identity was revealed with the help of the Asteroid Terrestrial Alert System (ATLAS) telescope in South Africa - it turned out to be a comet.

At the end of September, this space visitor was getting closer to the sun, and its brightness increased significantly, quickly becoming the focus of major media platforms. On October 12, Comet Purple Mountain-Atlas passed the perigee, and in the following two weeks, every time the sun set, it appeared in the low western sky with a long comet tail.

As the comet gradually moves away from the Earth and the Sun, its brightness is rapidly decreasing. If you want to witness the charm of this comet, you must seize the nearest observation window. The next time it visits the Earth may be tens of thousands of years later.

References

https://earthsky.org/space/comet-a3-has-an-anti-tail-can-you-see-it/

https://theconversation.com/comets-101-everything-you-need-to-know-about-the-snow-cones-of-space-213342

Compiled by: Wen Xing

Reviewed by: Li Xin, Research Librarian of Beijing Planetarium