On the starry sky stage in the evening recently, comet Purple Mountain-Atlas has undoubtedly become the center of attention. Since October 12, it has begun to appear quietly in the short night after sunset, attracting the attention of astronomy enthusiasts around the world. What is particularly striking is that in the footage captured by some enthusiasts, the comet actually shows a strange "tail" facing the sun.
Usually, the tail of a comet in our understanding extends in the direction away from the sun, so where is this "tail" pointing towards the sun? Next, Mr. Tadpole will reveal this mystery for you.
How comet tails are formed
Comets are celestial bodies composed of rock, dust, ice, and various frozen gases. Astronomers often jokingly call them "dirty snowballs." When a comet approaches the sun, it is subject to intense sunlight and gradually heats up. As the comet heats up, the various frozen gases that make up the comet change directly from solid to gas, a process called sublimation. The gas and dust particles released during the sublimation process 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 made of gas and is called the ion tail. Solar radiation strips electrons from the gas in the coma, giving them a positive charge and becoming ions. The solar wind then pushes these charged gas particles away from the sun, creating a blue tail whose blue color comes from the abundance of carbon monoxide ions inside, which sparkle in sunlight.
The other tail, called the dust tail, is made up of dust particles released during the sublimation process. These dust particles are pushed away from the sun by the pressure of sunlight, forming a curved tail that resembles 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 comet nucleus itself, reaching hundreds of thousands of kilometers.
What's up with the reverse "tail"?
After understanding the reasons why comet tails are formed, you will find that comet tails are actually "blown" by the solar wind, so they always extend away from the sun. So what’s going on with Comet Purple Mountain-Atlas’ tail pointing towards the sun?
In fact, this extra tail, often called an anomalous comet tail or anti-tail, is different from the usual comet tail in that it is not made up of debris ejected directly from the comet body, but is made up of dust particles left behind by the comet in its orbit. These dust particles are relatively large and are less affected by the solar wind, so they remain roughly in the plane of the comet's orbit. When the Earth happens to cross this orbital plane, the sunlight reflected by this dust can be seen by people on Earth, creating the illusion of an abnormal comet tail.
Image description: Schematic diagram of the positional relationship between the anomalous comet tail, dust tail, and ion tail
However, Comet Purple Mountain-Atlas is not the only comet with an unusual tail. The Hale-Bopp comet in 1997, the green comet C/2022E3 (ZTF) that appeared in January 2023, and the "devil comet" 12P/Pons-Brooks that passed the earth in June this year have all appeared in the night sky with abnormal comet tails.
Comet Purple Mountain-Atlas is moving away from us
Comet Purple Mountain-Atlas, officially designated C/2023A3, was first discovered by the Purple Mountain Observatory in China on January 9, 2023. It was initially thought to be an asteroid. It was not until February 22 that with the help of the Asteroid Impact Continuous Alert System (ATLAS) telescope in South Africa, its true identity was revealed - it turned out to be a comet.
At the end of September, this space visitor was getting closer to the sun, its brightness increased significantly, and it quickly occupied the focus of major media platforms. On October 12, comet Purple Mountain-Atlas passed perigee. In the next two weeks, every time after the sun set, it appeared low in the western sky with a long tail.
As the comet gradually moves away from the earth and the sun, its brightness is also rapidly weakening. If you want to see the comet with your own eyes, you must seize the nearest observation window. The next time it visits the earth again may be tens of thousands of years later. (Compiled by: Wen Xing)
Reference information
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
Planning: Liu Kun, Li Peiyuan, Zhang Chao and Yangliu
Reviewed by: Li Xin, Research Librarian of Beijing Planetarium
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