Comet 12P/Pons-Brooks, the brightest known cycle of the year, has returned after 71 years and has recently entered the best observation period! During this regression cycle, Comet 12P passed perihelion on April 21 and reached the closest position to the Sun, reaching its highest brightness in the constellation Taurus. Since it is only 22° away from the Sun, this means that the comet will soon set below the horizon as soon as the Sun goes down. In most cases, the comet is low in the sky, but due to its greater brightness, it is visible to the naked eye in excellent viewing conditions.
Figure 1: 12P/Pons-Brooks taken with a DSLR at Alibaba丨Photo by Chen Tao
12P/Pons-Brooks, whose name derives from two observers: the French astronomer Jean-Louis Pons and the American astronomer William Brooks. Comet 12P/Pons-Brooks has been recorded in China as early as 1385 and in Italy in 1457, a time difference of exactly 71 years. In July 1812, Pons discovered the comet, and astronomers calculated the comet's orbit based on Pons' observations, and found that the comet's orbital period was 65-75 years. It was later discovered by Brooks on the return of 12P in September 1883, during which four eruptions were observed.
In 2024, 12P returns again, and luckily we will have the opportunity to see the brightest 12P when it passes perihelion on April 21, when the comet is only 0.781 AU from the Sun and can reach a magnitude of around 4.0. Although 12P will make its closest approach to Earth on June 2, 2024, after perihelion, it will disappear from the night sky in the Northern Hemisphere and be much darker than perihelion, so the best time for us to observe it is in the next few weeks. If this opportunity is missed, the return of Comet 12P will not be observed again until 2095. So don't miss this rare opportunity to observe and have a wonderful encounter with this "visitor from the sky"!
Figure 2: Sketch of Comet 12P/Pons-Brooks by American astronomer Herbert Couper Wilson on January 21 and 22, 1884 丨Source: https://adsabs.harvard.edu/
01
Orbital properties of comets
The orbit of a comet is very different from that of a planet or asteroid, it is an extremely flattened ellipse that is capable of periodic return and is called a periodic comet. Periodic comets are divided into short-period comets, and long-period comets. Short-period comets orbit the Sun with a period shorter than 200 years, such as 12P/Pons-Brooks. Long-period comets orbit the Sun for more than 200 years, such as the Purple Mountain-Atlas Comet C/2023 A3 (Tsuchinshan-ATLAS), which has an orbital period of about 62,000 years. C/2023 A3 is the eighth comet discovered by the Purple Mountain Observatory and the sixth comet named after the Purple Mountain. The comet is expected to pass through the Earth's orbit in August 2024, and then get closer to the Sun, and its brightness will increase rapidly, and it is expected to exceed 12P in September 2024, becoming the most anticipated large comet visible to the naked eye this year.
12P/Pons-Brooks is a "Halley-type" short-period comet with an orbital period of 71 years. Its return period is not much different from that of Halley's Comet, which returns every 76.1 years, hence the name "Halley-type" comet (orbital period between 20 and 200 years). With an orbital inclination of about 74.2° and an eccentricity of 0.955, 12P has a perihelion of only about 0.781 AU and an aphelion of 33 AU, just beyond Neptune's orbit, as shown in Figure 3. The comet's orbit is relatively stable, and it appears to be unaffected by any of the major planets between 1740 and 2167. Three other comets—13P/Olbers, 23P/Brorsen–Metcalf, and the now-defunct 20D/Westphal (which quickly disappeared in 1913 and no return was found) are also Halley-type comets, also known as comets of the Neptune family, with 12P second only to Halley's comet in brightness.
Figure 3: 12P orbit, where the white curve represents the 12P orbit. 丨Source: Purple Mountain Observatory Drawn by Hu Shoucun
12P is also a "near-Earth comet" whose orbit is very close to the Sun and can pass through the Earth's orbit. Near-Earth comets are rarer than near-Earth asteroids, with only 122 known of them and about 35,000 near-Earth asteroids. At present, the continent is paying close attention to NEOs and has launched a near-Earth asteroid defense program to monitor the impact risk of any small objects. Table 1 shows that in 1884, 12P was closest to the Earth (0.63 AU), and the geocentric distance gradually increased to greater than 1.5 AU after the return of this near-Earth event.
Table 1 Comet 12P near-Earth events
02
Physical properties of comets
Like an ancient ice volcano that flies through space, the comet erupts from time to time, sending large amounts of ice and gas into space. They are about 4.6 billion years old and formed at the same time as the Sun, Earth, and other planets. The gravitational pull of large planets throws them from beyond Neptune's orbit toward inner planets (which include Mercury, Venus, Earth, and Mars), making them one of the most chaotic and interesting objects in the solar system.
Comets, unlike asteroids, usually do not have a fixed volume and consist of a comet nucleus as well as an observable comet and tail. The nucleus is the solid part that makes up a comet, usually between 1 and 50 kilometers in diameter, and has the structure of a dirty snowball, mainly made of ice and dust, among other things. When a comet approaches the Sun, due to heating, part of the water ice sublimates from a solid to a gas and drives the movement of dust, forming a comet and a comet tail. Comets and asteroid impacts may be the source of some of Earth's water, or they may be responsible for the storage of ice at the bottom of the moon's shadow crater.
Figure 4: American astronomer Dr. Whiple and his proposed "dirty snowball" model丨Source: https://www.daviddarling.info/
The distinguishing feature of comets is the tail of the comet, which has two types, dust tail and ionic (or plasma) tail. When the sun heats the water ice on the comet, it releases gas and dust into space, and the dust falls into the orbit around the comet, where it is affected by the sun's light pressure to form a fan-shaped comet tail. The gas in the comet is ionized by the solar wind plasma to form charged particles, which form a narrow ionic comet tail, which is pushed away from the direction of the sun by the solar wind. The tail of the comet is also responsible for the annual meteor shower that brings to Earth the beautiful meteor shower that we see as the Earth's gravitational pull as the Earth passes through the dust cloud left behind by the comet. Comet 12P/Pons-Brooks is thought to be the parent body of the December Draconid K meteor shower. This meteor shower is active from November 29 to December 13 each year and is expected to be the most abundant meteor shower associated with the comet.
Figure 5: Image of Comet 12P/Pons-Brooks taken by the 40 cm telescope of Ali Observatory 丨Source: Purple Mountain Observatory
Comet 12P/Pons-Brooks can reach a diameter of up to 30 kilometers, which is three times larger than Mount Everest. Comet 12P is also widely known as the "Demon Comet" because when Comet 12P erupted on July 20, 2023, it was observed that the comet formed a striking "angle" or "horseshoe" shaped comet tail (Figure 6-7), which quickly faded after the eruption. The formation of the "devil's horn" may be caused by the comet's uneven jet, it may be a shadow effect, where the comet's geometry causes its center to be occluded, or it may be a random and unpredictable outbreak event.
Figure 6-7: The scene at the outbreak of 12P/Pons-Brooks in July 2023, people compare this novel shape with the Millennium Falcon in the "Star Wars" series丨Source: https://starwalk.space/
03
Interesting comet
In the history of human society, comets have always been a fascinating celestial phenomenon. Its unique morphology and irregular movement are distinctly different from familiar planets and stars. Therefore, in ancient times, when there were no telescopes, comets were regarded as a symbol of disaster in the heavens. Aristotle's belief that comets are a kind of atmosphere lasted for thousands of years until the Renaissance. Astronomer Tycho observed C/1577 V1 and found that its distance was much greater than the Earth-Moon distance, demonstrating that the comet was an astronomical phenomenon.
By Newton's time, he observed two similar comets and speculated that they were affected by the sun's gravity. Based on this view, Newton proposed the theory of gravitation. Halley, on the other hand, used this theory to calculate the comet's orbit and predict its return time. When Halley's Comet returned at the scheduled time, the power of science prevailed over ancient superstitions.
From Newton's time to the early 20th century, advances in observation techniques and astronomical theories have led to a deeper understanding of comets. Dynamically, as more comets are discovered, comet orbits take on two forms, elliptical orbits and parabolic orbits, with the Jupiter family of comets located in elliptical orbits with a period of less than 20 years being the most familiar type of comet. These comets were shown to be "trapped" by Jupiter's gravity, a discovery that facilitated the development of the Sun-Jupiter-comet restrictive three-body problem in celestial mechanics.
In the early 50s of the 20th century, three important theories emerged in comet science, which explained the structure of comet nuclear matter, the origin of comets, and the interaction of comets with the solar wind. First, the "dirty snowball" model of ice-dust mixtures proposed by Whipple (1950) was the first to describe the properties of the comet nucleus and the source of the comet's activity. Then, Finson et al. (1968) proposed a kinetic model of coma gas and described the interaction of gas-dust near the comet nucleus, explaining the formation of comet tails and facilitating the development of the solar wind theory. At the same time that the comet nucleus model was proposed, Oort deduced that they may have originated at a distance of more than 200,000 AU from the Sun, called the Oort Cloud, based on the distribution of long-period comet orbits. Kuiper also suggested that there may also be a small celestial belt outside Neptune's orbit, called the Kuiper Belt; Fernandz proved that short-period comets may have originated in the Kuiper Belt.
Comets originate from the edge of the solar system, the Oort cloud and the Kuiper belt, they carry the most primitive material at the beginning of the formation of the solar system, by studying the material composition of the comet, we can infer the elemental composition of the origin of the solar system, and understand the research mechanism of the solar system.
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Detection of comets
With the maturity of deep space exploration technology, people are no longer satisfied with ground observation, and have carried out a number of space exploration missions for such small celestial bodies. Table 2 lists the comet missions that have been carried out so far. The first comet probe was ESA's Giotto, which flew past Halley's Comet in 1986 and obtained the first high-resolution image of a comet nucleus in human history and found evidence of the presence of organic matter on the comet. This was followed in 1992 by a flight over 26P/Grigg-Skjellerup, just 200 km from the comet nucleus. Since the 21st century, many more comets have been detected, images of comets with different shapes have been obtained, and shape models with high spatial resolution have been constructed based on space optical data. The Deep Impact mission to 9P/Tempel 1 achieved an impact on a comet, detecting for the first time relatively fresh material below the comet's nucleus. The Stardust mission returned to Earth with dust particles from the coma 81P/Wild 2, giving us a new understanding of the physical properties of the comet's nuclear fallout. The Rosetta mission is the first companion comet exploration mission, and during the two-year companion flight, the whole process of 67P passing through perihelion in 2015 was clearly observed, and the related phenomena of comet activity were intuitively presented, which is the most detailed observation of comet in human history.
Table 2: Major space exploration missions targeting comets
In the journey of space exploration, each exploration has revealed new mysteries of the universe for us and provided scientists with valuable data and observation opportunities. In 2025, the mainland will launch the Tianwen-2 probe, which will bring back samples from the near-Earth asteroid 2016 HO3, the first time China has retrieved samples from an interplanetary space. In addition, Tianwen-2 will also go to the active asteroid 311P/PANSTARRS for companion exploration. 311P is a special class of small objects with comet-like activity but asteroid orbits. This mission not only helps to understand the physical properties of asteroids and comets, such as density, size, and thermal conductivity, but also provides new insights or evidence for the formation of the solar system and the origin of life.
Source: Purple Mountain Observatory, Chinese Academy of Sciences
Edit: Snow Shadow
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