Once in 50,000 years! A rare green comet will "approach" the Earth on February 2. How to observe it?

The green comet, which occurs once every 50,000 years, is approaching the Earth and is expected to pass the perigee on February 2. If the observation conditions are good, its shadow may be clearly visible to the naked eye. Observation Guide >>

Written by reporter Duan Ran, Photo and text editor Chen Yongjie

New Media Editor/Lv Bingxin

Interview experts:

Kou Wen (Senior Engineer, Beijing Planetarium)

In mid-to-late January 2023, when people were celebrating the Lunar New Year, an uninvited guest quietly broke into the night sky. A large number of astronomy enthusiasts began to move, carrying various equipment in the biting cold wind to various observation sites, locking their horizons in the estimated area and searching carefully, hoping to catch a glimpse of its shadow in the vast starry sky. They were looking for a "unprecedented" comet, and its name was just a line of monotonous characters: C/2022 E3 (ZTF).

▲The running position of C/2022E3(ZTF) in the starry sky on January 29 (Photo source: theskylive.com)

I searched for him everywhere

The photos released so far show that the comet is emerald green, with a bright and wide tail. Researchers analyzed that it emits green light because the dust cloud (coma) around the nucleus contains carbon. Its nucleus is about 1.6 kilometers in diameter, and its tail stretches for millions of kilometers. According to astronomers, the C/2020 E3 (ZTF) comet that visited the night sky this time has an orbital period of about 50,000 years-of course, this is just a rough estimate based on current observation data. But in any case, as a long-period comet, we are destined to see it only once in this lifetime. According to public observation data, the comet passed the perihelion on January 12 and will reach the perigee on February 2, 42 million kilometers (about 0.28 astronomical units) from the earth.

▲The orbit of C/2022E3 (ZTF) from January 28 to February 4 calculated using Stellarium software (Image source: Stellarium)

So how bright will the comet be then?

People use "magnitude" to measure the brightness of celestial bodies. The lower the value, the brighter it is. Halley's Comet, which returned in 1986, reached a magnitude of 3.5 at its brightest. In comparison, the brightness of Comet C/2022E3 (ZTF) is much inferior. "The brightness of this comet is between magnitude 5 and 6. It can be seen as a hazy cloud with binoculars and other equipment." Kou Wen, a senior engineer at the Beijing Planetarium, introduced. But he believes that the observation conditions for this comet are still relatively optimistic: "Many comets appear early in the morning and late in the evening, close to sunrise and sunset, and the observation window is very short. At present, this comet is near the North Star, and the time for observation at night is very long, so observation activities can be arranged calmly."

Comet C/2022 E3 (ZTF) Observation Tips Best Observation Dates

The comet's brightness will be around magnitude 5 between January 28 and February 4, Beijing time. The comet will reach its perigee on February 2, Beijing time, and will be most suitable for observation from February 1 to February 2.

Observation time recommendations

LHAASO detected gamma photons with an energy of 1.4 peta-electron volts (petaelectron volts) from the Milky Way, which is the highest energy photon ever observed by humans. To observe comets, it is also necessary to avoid interference from sunlight and moonlight, and it is best to choose after the moon sets. For example, the moon sets at around 4 a.m. on February 1 in Beijing, and the sunrise time on February 2 is around 7:30 a.m. The best observation effect is during this time period.

Observation site selection

Try to avoid remote suburban areas where light pollution from the city is less likely to affect you. In Beijing, the Siling area of ​​the Ming Tombs, Huahai in Yanqing, and the Dazhuangke Great Wall are all ideal observation sites.

Observation tool recommendations

Depending on your own conditions, an 8-10x telescope can meet general observation needs. If you want to take photos, it is best to use a SLR + telephoto lens. Due to strong winds on the day of observation, it is recommended to use a tripod or other equipment to fix the observation equipment.

Other considerations

From January 31 to February 1, there will be strong winds and cooling weather in Beijing, and the suburbs will be generally cold. Please pay attention to keep warm during observation.

Comet C/2022 E2(ZTF) has aroused great enthusiasm among astronomy enthusiasts. Mr. Lin, an amateur astronomical observer in Beijing, made careful preparations for this observation activity. Mr. Lin recalled that what impressed him most was the Hale-Bopp Comet that visited the night sky in 1997. When it passed the perihelion, its magnitude reached -1.8. "Basically, except for the moon, this comet was the brightest in the night sky at that time, and it lasted for several months." Mr. Lin said excitedly, "The brightest comet since then is Comet Neowise in 2020. I was on the Dazhuang Great Wall at that time and observed this comet at dawn. I was able to clearly capture its shadow with an ordinary telephoto lens." For astronomy enthusiasts who want to see this comet, Kou Wen suggested: "First, you have to get away from the interference of city lights. It is difficult to see stars of magnitude 2-3 in a brightly lit city. This time the comet is only magnitude 5-6, and the observation point must be in a place with less light pollution." Secondly, Kou Wen believes that it is necessary to avoid the interference of moonlight on observation as much as possible. "Therefore, try to choose to observe at the time of moonset in the suburbs to achieve better observation results." Kou Wen emphasized. In order to observe Comet C/2022 E2(ZTF), Mr. Lin chose the Siling area of ​​the Ming Tombs as the best observation point, which is far away from the light interference of the urban area and is an ideal observation point. He arrived at the Siling Tomb in the early morning of January 29 for the first observation. "Currently, the comet is moving between the Ursa Minor and the Draco. I can roughly see the blurry image of the comet with ordinary binoculars. If you use a 20x telescope, you can see a layer of fog on the comet. This is basically the limit of ordinary civilian observation equipment." Mr. Lin said.

▲Comet Neowise photographed at the Dazhuang Great Wall on July 4, 2020 (Photo credit: provided by Mr. Lin)

Although C/2022 E2(ZTF) is much dimmer than the 2020 Comet Neowise, Mr. Lin, an astronomy enthusiast, is still very excited. "Comets that can be seen by ordinary observation equipment are rare, and it is very exciting to be able to capture one," said Mr. Lin. Currently, the comet is approaching the North Star at a very fast speed, and will reach its brightest moment around February 2. Although its appearance is not outstanding, as a look back in ten thousand years, this encounter between heaven and earth is still worth looking forward to by the majority of astronomy enthusiasts in the cold night.

A rare visitor to the sky

Most people have only seen the exact appearance of comets in books, on TV and on the Internet, and there are not many opportunities to see them in the night sky with their own eyes. Compared with the stars in the sky, the appearance of comets is a rare opportunity. Comets are generally not large in size. The core diameter of a small comet is only a few hundred meters, and the largest is only tens of kilometers. In 2022, NASA discovered a giant comet, whose nucleus is only 129 kilometers. "There are a lot of comets in the entire solar system, but the number of comets discovered by humans is still very small!" said Kou Wen. Scientists estimate that there are as many as trillions of comets in the solar system! Compared with the "protagonists" such as the eight major planets in the solar system, comets can only be regarded as "group performances" at best. But according to data released by the Small Body Center (MPC) of the International Astronomical Union (IAU): As of January 27, 2023, humans have discovered a total of 4,446 comets, which is only the tip of the iceberg compared to the total.

▲Image of Comet C/2022E3(ZTF) taken by the ZTF device in California on December 24, 2022 (Photo credit: NASA)

How are comets named ?

In the past, people generally named newly discovered comets after their discoverers. For example, Comet Hale-Bopp, which appeared in 1998, was named after Alan Hale and Thomas Popp. However, as the number of comets discovered increased, this naming method became outdated. It was not until 1994 that the International Astronomical Union formulated a new comet naming system, using a string of letters and numbers to identify comets. "From the comet number, we can see important information such as the year and month of discovery and type." Kou Wen introduced to reporters, "For example, the C/2022 E3 (ZTF) that appeared this time, 2022 means it was discovered in 2022; at the same time, each month is divided into the upper and lower tenths, represented by 24 letters, such as A for early January, B for late January, so E means it was discovered in early March, and 3 represents the number of the discovery. The initial C represents a long-period or non-periodic comet, and if it is the letter P, it represents a short-period comet." "In addition, comets will be named after the discoverer or the discovering organization. The C/2022 E3 (ZTF) that appeared this time, the ZTF in brackets is the abbreviation of its discovering organization "Zwicky Transient Facility". "Ko Wen further added. According to this numbering system, all discovered and confirmed comets have their own ID numbers, which greatly facilitates the collation of comet information. For example, the first confirmed short-period comet, Halley's Comet, is officially numbered 1P/1682 Q1 in this system, which means it was confirmed in late August 1682. "Comets orbit the sun in different periods, ranging from a few years to tens of thousands of years. Generally speaking, comets with an orbit period of less than 200 years or that are discovered twice after passing the perihelion are considered short-period comets, and those with an orbit period of more than 200 years are long-period comets," Kou Wen explained.

What kind of comets can we see?

The operating period of short-period comets generally ranges from several years to several decades, with the shortest being only three years. These comets mainly come from the Kuiper Belt of the solar system. They are "permanent residents" of the solar system. Long-period comets beginning with "C" are generally believed to come from the more distant Oort Cloud. The period of such comets orbiting the sun ranges from hundreds of years to tens of millions of years. Some simply break into the solar system with hyperbolic or parabolic trajectories, and are thrown into deep space and never return after passing the perihelion. They really become "waving sleeves and leaving without a cloud"! "Comets have very special orbits, most of which are flat elliptical shapes. For most of their operation, they are far away from the sun, very dim, and difficult to observe. The number of comets that can be seen each year is quite limited, and it takes about 8-10 years on average for a relatively bright comet to appear." Kou Wen said. "Besides that, most of the comets that appear every year are just a small dark spot." In fact, except for the emission of material under the influence of radiation when approaching the sun, which makes the brightness increase, comets are "hermits" in the solar system for most of the time, and the visibility is very low. Most comets are destined to be out of our sight. After entering the solar system, these little guys not only have to go through a long journey, but also need to dodge and maneuver in the cracks of the planet's gravity and survive in danger. This leads to a considerable number of comets, whose orbits are difficult to accurately predict. Even for short-period comets, due to various objective factors, it is difficult for people to judge when they will return and whether they can be seen by us. For example, the Brosen Comet discovered in 1846 originally had an operating period of 5 years, but it disappeared in the second half of the 19th century. Another example is the Comet Schwarzmann-Wachmann 3 discovered in 1930. When it was discovered, it had an operating period of 5 years, but it began to disintegrate into several pieces in 1995 and gradually died out. The comets that can truly overcome numerous dangers and difficulties and enter our field of vision can really be described as just a drop in the ocean!

"Evil star"? "Lucky star"!

Comets have often appeared in people's vision since ancient times when humans began to observe celestial phenomena in a planned manner. Humans' understanding of comets has gone through a long process.

2,600 years ago, during the Spring and Autumn Period, the historian of the State of Lu wrote down the words "In the seventh month of autumn, a comet entered the Big Dipper", thus completing the earliest record of comets by humans to date. However, in an era lacking scientific rationality, the ancients saw comets as ominous things that dragged their tails across the starry sky and were unpredictable, so they naturally regarded them as a sign of impending disaster. In ancient Chinese books, there is a saying that "when a comet appears, disaster will arise". In the West, comets are also often seen as a sign of God's wrath or even the end of the world, and are rooted in Western popular culture as an evil star.

Ironically, comets, which were originally considered "evil stars", have strongly promoted the development of human science in modern times. In 1682, a huge comet broke into the sky. This spectacular astronomical phenomenon caused widespread social panic in Europe, but in this atmosphere, Newton and his friend Halley were very interested in discussing the movement of the comet. Newton explained to Halley in detail how he used the law of universal gravitation he discovered to calculate the elliptical orbit of the comet. This method surprised Halley so much that he later funded Newton to write his theory into the book "Mathematical Principles of Natural Philosophy". As a "comet enthusiast", Halley also examined the comet records of the past three hundred years and used Newton's law of gravity to speculate that the comet that appeared in 1682 should be the same comet as the comets in 1531 and 1607, and its operation period is about 76 years. He also boldly predicted that 76 years later, the comet will reappear in the sky! 76 years later, in 1758, Halley's prediction came true, but Newton and Halley were no longer among the crowd looking up at the comet. And this comet was none other than Halley's Comet, a frequent visitor in the history of human comet observations.

The return of Halley's Comet not only verified Halley's scientific prediction of the trajectory of comets, but also provided strong evidence for Newton's classical mechanics. The appearance of Halley's Comet did not bring disaster to the world, but indirectly pushed the development of modern astrophysics forward by a key step. From this perspective, this comet is actually more like a lucky star for human civilization.

▲European woodcut: The great comet that appeared over Prague in 1577 (Photo credit: Zurich Central Library)

In today's modern scientific world, human beings have made considerable progress in their exploration of comets. From ground-based survey telescopes capturing the ghostly shadows of these comets to directly launching probes to observe the structures of comets at close range, the true face of comets is now clearly visible to human beings through scientific means. Halley's Comet continues to contribute its residual heat to human exploration as an observation sample: when Halley's Comet returned again in 1986, ESA's Giotto, the former Soviet Union's Vega 1 and 2, Japan's Comet and Pioneer all rushed to fly over Halley's Comet at close range and collected first-hand information on the comet's nucleus. The researchers found that this is a small peanut-shaped body with a length of 15 kilometers and a width of 7-10 kilometers. The ejected material that forms the coma and tail is mainly a mixture of water, carbon monoxide, methane and other substances. These substances are ejected from the seven ejecta holes of the comet's nucleus under the action of the solar wind. The comet, which has been regarded as an ominous star by Eastern and Western cultures for thousands of years, is just an ordinary small body composed of water ice, rocks and frozen gases.

▲Halley's Comet returning in 1986 (Photo source: britannica.com)

In addition to Halley's Comet, in the following decades, humans launched probes such as Deep Space 1, Deep Impact, Stardust and Rosetta to study other comets through close detection, impact and landing. Human understanding of comets is constantly improving.

Produced by: Science Central Kitchen

Produced by: Beijing Science and Technology News | Beijing Science and Technology Media

Welcome to share to your circle of friends

Reprinted by public accounts, newspapers, etc.

Please send an email to [email protected]

Reproduction without authorization is prohibited, and offenders will be prosecuted