It is not difficult to observe with the naked eye, and the photos taken with mobile phones are very good. The big comet discovered by my country is back!

Produced by: Science Popularization China

Author: Serial No.

Producer: China Science Expo

According to the latest observation data on October 10, 2024, the maximum brightness of Comet Purple Mountain-Atlas reached -4.9, exceeding the brightness of Venus and becoming the brightest comet in the Northern Hemisphere in nearly 30 years.

As the relative positions of the Earth, the comet and the Sun change, the comet has slowly moved to the east of the Sun as seen from the Earth. This means that it has changed from a morning star that originally rose before sunrise to an evening star that sets a little later after sunset.

It is expected that in the two weeks starting from October 12, after the sun sets, this bright comet will appear in the low western sky with a beautiful and spectacular comet tail. It is not only an important research target for astronomers, but also a star captured by countless star lovers and photographers. It may also leave an unforgettable observation experience for everyone who looks up at the starry sky.

The comet's origin story is full of twists and turns

1. Twist and turn 1: Found and lost, it got a combined name

As can be seen from the naming of Comet Purple Mountain-Atlas, it is related to two "discoverers".

The 1.2-meter telescope at the Xuyi Observatory of the Purple Mountain Observatory in my country first discovered it on January 9, 2023, and it was considered to be a 19th magnitude asteroid at the time. However, in the nearly one month after it was reported to the Minor Planet Center (MPC), no other observatories or observers observed it, so it was removed from the list of pending confirmations.

On February 22, the 50cm telescope of the Asteroid Terrestrial-impact Last Alert System (ATLAS) installed at the South African Astronomical Observatory also captured an object with comet characteristics.

Combining more observational data and the retrieval of historical archived data, it was eventually confirmed that the images taken by Purple Mountain and ATLAS were of the same celestial body, thus confirming the existence of this new comet.

In addition to being named after its discoverer, it also has an official number, C/2023 A3, where C stands for a comet with a period of more than 200 years, 2023 for the year of discovery, and A3 for the third comet discovered in January.

Figure 1 The 1.2-meter telescope at Xuyi Observatory (left) and the 0.5-meter telescope at Atlas (right)

(Image source: Purple Mountain Observatory, Henry Weiland)

2. Twist and turn 2: Brightening stagnated, it was suspected to have disintegrated midway

Through subsequent observations, the comet's orbit was determined more accurately, and the time when the comet was closest to the sun (perihelion) and the earth (perigee) was also determined. Combining the observed brightness of the comet, its morphological changes, spectral information, etc., scientists speculated through models that the brightness of this comet near perihelion is about 0 magnitude - equivalent to the brightness of Vega visible to the naked eye. In the future, it may be possible that the brightness will increase significantly by about 4 magnitudes before and after passing the perigee due to a special "forward scattering" effect.

Comet that is extremely bright and visible to the naked eye is often called a "big comet". Obviously, Comet Purple Mountain -Atlas has the potential to become a big comet. The brighter the comet, the more data and information the telescope can observe. Therefore, in the more than one year since it was discovered, telescopes around the world have been closely monitoring the changes in its brightness.

At first, its brightness matched the model predictions very well, and it gradually brightened as it flew closer to the sun. However, starting in mid-April 2024, the brightness of the comet has been hovering around magnitude 10.5 for three months, and it has not brightened as it gets closer to the sun. This has caused scientists to worry about whether it is still intact and whether it can reach the expected brightness of a "big comet."

The bright nucleus of a comet is a "dirty snowball" composed of water ice, rocks, frozen gas, etc. It is not a solid body structure and is easily affected by the sun or other massive celestial bodies and disintegrated, resulting in a decrease in brightness. However, the telescope did not observe any signs of the comet's disintegration. Therefore, some scientists have proposed that the lack of brightness increase as expected is due to the phase effect - the positional relationship between the comet, the sun and the earth.

Fortunately, after mid-July, the comet's brightness gradually increased, slowly breaking through magnitude 9, and the first concern about its disintegration also dissipated.

Figure 2 Curve of comet brightness changing over time (solid line is model prediction value, solid point is observation value)

(Image source: astro.vanbuitenen)

3. Twist and turn three: Heart-wrenching moment, the test from perihelion

As the comet gets closer and closer to the sun, the moment that worries more people - perihelion - has arrived. The closest comet to the sun is 0.4 astronomical units, which is close to the distance between Mercury and the sun. At this time, the radiation pressure and heat of the sun will bring the most severe test to the stability of the comet's nucleus structure. If the comet nucleus is "strong" enough to survive the perihelion and then slowly move away from the sun, the possibility of splitting or disintegrating later will be greatly reduced.

Good news!

On September 28, the comet safely passed perihelion without breaking up .

Astronomy enthusiasts from all over the world began to compete to take photos, recording its beautiful figure in the morning light as its comet tail grew longer every day.

Figure 3 Comet Zijinshan-Atlas in the morning light

(Photo credit: Yuri Beletsky photographed in Chile on September 29)

A visual feast! You can enjoy it with your naked eyes!

On October 12, 2024, Comet Purple Mountain-Atlas will pass through perigee.

According to recent observation data, the brightness of the comet has surpassed that of Venus due to the "forward scattering" effect. This effect can only occur when the comet meets certain conditions, thereby greatly increasing the brightness of the comet and bringing us on Earth the luck of appreciating the comet with our naked eyes.

When a comet happens to be almost between the Earth and the Sun, the phase angle of the comet is close to 180 degrees. This means that part of the light from the Sun to the comet will pass through the comet to the Earth. If the relatively loose coma and tail around the comet nucleus contain a lot of dust, the sunlight will be scattered when passing through the dust, making the brightness of the comet significantly increased, which is the origin of the name "forward scattering".

This effect is like when you breathe on a cold morning and are illuminated by sunlight from behind, the matter you exhale will also be passed through and scattered by the sunlight, and you will see a bright white cloud. However, if the sun rises and is not shining from behind you, it will be difficult to see the bright white cloud you exhaled.

In short, the closer the comet's phase angle is to 180 degrees, and the more dust there is around the comet nucleus, the more "forward scattering" effect will occur.

Figure 4: The effect of seeing breathing “white clouds” in the early morning with your back to the sun

(Image credit: Nathanael Callon)

In the curve of comet brightness changing over time in Figure 2, the green solid line corresponding to around October 12 is the brightness after the "forward scattering" effect is superimposed, and the red solid line is the brightness value without considering the forward scattering effect.

**In addition to its brightness,** another feature of Comet Purple Mountain-Atlas is the frequent changes in its tail. The direction and shape of a comet's tail are mainly affected by the sun. According to its orbit, before passing the perigee, from the perspective of the earth, its direction in the sky will make a "sharp turn" relative to the sun, and the comet's tail will also "swing" accordingly. The shape of the comet's tail that we see every day will change with the comet's sharp turn.

Figure 5 Simulation of the change of the comet tail over time

(Photo credit: Nicolas Lefaudeux)

Such a bright and colorful comet Purple Mountain-Atlas will slowly move away from the sun and appear in the low western sky after sunset in the two weeks starting from October 12.

How to seize the opportunity to appreciate the comet?

To appreciate comets, you first need good weather. If it is sunny where you are, you can set out before sunset and find an open and unobstructed place in the west to try to observe . When looking for comets, you can look for them in the low sky facing the west, and use star map software or by checking the relative positions of Venus and the comet to lock on the comet. The altitude of the comet will increase as the date goes by. If the light pollution in your area is serious, you need to try to use binoculars or a camera to "see" the comet.

Figure 6 Schematic diagram of the position of the comet seen in Beijing at 18:30 every day.

(Image source: Stellarium)

Not only is it beautiful, it also brings a scientific research carnival to astronomers

Comet Purple Mountain-Atlas, as a large comet that flies close to the Earth, has also provided astronomers with a rare sample for comet research. Many telescopes, large and small, around the world are trying their best to obtain all kinds of data on the comet.

1. Studying comet morphology changes by imaging

The changes in comet morphology can be analyzed and studied by taking photos and imaging. The Solar and Heliospheric Observatory (SOHO), which is continuously observing the sun, captured a picture of a comet approaching the sun. Not only can the changes in the comet's morphology be analyzed in detail, but the sun also happened to have a coronal mass ejection (CME) at this time , which can be understood as the sun sneezing a few times . The impact of CME events on comets is also worth further study.

Figure 7 Comet image recorded by SOHO satellite

(Image credit: SOHO/NASA)

2. Take a look at comet spectra and learn about the early solar system

In addition to directly studying the morphological changes of comets using images, it is also possible to study the ratio of dust to gas in the coma.

Spectra are also a powerful tool for exploring the mysteries of comets. By photographing comet spectra, we can understand the material composition and proportion of comets. Comets come from the distant outer solar system, which is more empty and extremely cold, allowing the comets themselves to retain the material nuclear state of the early formation of the solar system. Therefore, the material composition reflected by the comet spectrum is, in a sense, equivalent to a "time capsule" of the solar system, which can help us grasp more information about the early formation of the solar system. Through long-term continuous observation, we can also understand the evolution of comet activity and discover possible gas bursts.

3. Purple Mountain-Atlas’ unique research perspective on Comet.

According to predictions by Samuel Grant of University College London and Geraint Jones of the European Space Agency, the Earth is likely to intersect with the comet's ion tail, meaning that our planet will pass directly through its ion tail. The ion tail of a comet is composed of charged molecules that are pushed away from the comet by the solar wind. Although comet tails often stretch for hundreds of millions of kilometers, they are only a few million kilometers wide in the vast universe, so the chances of the Earth encountering them directly are very low, and such a comet only appears once every few decades. This time, scientists no longer use telescopes to take images and spectra of the ion tail from a distance, but can directly enter the ion tail and "be in it" to directly detect the composition and content of the ion tail with a space probe.

Go outdoors and have a romantic encounter with the magnificent comet!

Comet Purple Mountain-Atlas is a comet with a near-parabolic orbit and a period of tens of thousands of years. It only encounters us once in our lifetime. Its arrival is a blessing for astronomers, enthusiasts and even every one of us ordinary people.

Seize the opportunity, go outdoors, admire its magnificent comet tail, and experience this once-in-a-lifetime romantic experience!