[재미있는 과학] Halley’s Comet appeared in the Joseon night sky, see you again in 2061

The United Nations Educational, Scientific and Cultural Organization (UNESCO) selects cultures worth preserving around the world. Recently, the Korea Astronomical Research Institute, together with the Korean Astronomical Society, the Korean Space Sciences Association, and Yonsei University, announced that it would press for the registration of the Joseon Dynasty’s astronomical observation data, ‘Seongbyeon Geomyeonja’, on the UNESCO Memory of the World Register. What makes this document worth preserving internationally?

Comets appearing in old records

Seongbyeon Geukdanja is a report that records the changes of celestial bodies at Gwansanggam, an astronomy research institute in Joseon. He mainly recorded the changing of the stars. In particular, the records of comets remain very detailed. Records of comets appear in 1664, 1723, and 1759.

The 1759 comet record is of particular importance. This is because it is a record of the most famous comet among comets, ‘Halley’s Comet’. The record of Halley’s Comet by the asteroid is the oldest among the records of Halley’s Comet observations recorded by royal offices around the world.

Comet Halley is a comet predicted by the British astronomer Edmund Halley in 1705 with a cycle of 76 years. Halley looks through the records of the past and discovers that there are celestial bodies that appear repeatedly at certain times, and that the orbits of these celestial bodies also coincide. We also learn that this celestial body revolves around the sun and is different from a planet. And since the last time this object appeared was in 1682, it is expected to appear again around 1758-1759.

Halley died in 1742, but as he predicted, the comet appeared in 1758. And it was also recorded in Korea’s Seongbyeon-cheukhangja. FYI, the last approach of Halley’s comet was in 1986, and the next one to be observed is 2061.

Comets are one of the small celestial bodies in the solar system, and they orbit the sun in a highly distorted elliptical orbit with a longer period than planets or asteroids, or they wander in hyperbolic or parabolic orbits without a period They are different from meteors (meteoroids), which emit light as celestial bodies around the Earth are attracted by the Earth’s gravity and burn with frictional heat in the atmosphere.

comet formation process

Outside Neptune, the outermost planet in the solar system, is the ‘Kuiper Belt’, a collection of small celestial bodies that orbit the solar system. Starting at a distance of about 30 AU from the sun (1 AU is the distance between the sun and the earth, about 150 million km), it is known to be about 50 AU. There are relatively large exoplanets such as Pluto in the Kuiper Belt, but there are countless celestial objects smaller than a few tens of kilometers in diameter. These celestial bodies are materials that did not yet become planets when the solar system was created. There are rocks, dust and gas, methane or ammonia, and ice scattered around. When these celestial bodies are attracted by the sun or Jupiter’s gravity and begin to rotate, they develop into comets.

Comets with long periods (over 200 years) or non-periodic comets are formed in the ‘Oort Cloud’ which is further out than the Kuiper Belt. The Oort Cloud is an area about 50,000 AU from the Sun, and its main components are water, ammonia, and methane. Comets that are created in the Oort cloud are pushed into the solar system by the gravity of other stars or galaxies passing nearby, and only then are they estimated to be captured by the gravity and orbit of the sun

Comet too far, exploration and research is underway

The origin of the comet is too far away. Therefore, only recently have probes been sent to comets to obtain data directly. It must be estimated based on observations of the Kuiper belt and Oort cloud, but this celestial body is too small and does not emit light on its own, making it difficult to observe.

Is there any way to send probes directly to the Kuiper Belt and Oort Cloud? Voyager is the most distant spacecraft sent by mankind. It narrowly escaped the Kuiper Belt some time ago and is currently passing through the space between the Kuiper Belt and the Oort Cloud. If it goes according to plan, it will enter the beginning of the Oort Cloud around 2310.

In any case, comets that start as small chunks of the Kuiper Belt or Oort Cloud are made up of rock, dust and solid carbon dioxide. The nucleus of a comet is usually smaller than expected, ranging from 100 mi 30 km in diameter. As this nucleus approaches the Sun, it becomes brighter and larger. At the same time, the temperature also rises. At this time, solid components change to gas and create an atmosphere. This is called a ‘coma’. The coma is starting to become really visible as it reflects sunlight. In particular, the remnants of the coma that are pushed away by the solar wind are called ‘tails’. Of course, the closer you get to the sun, the longer your tail will get. The tail is formed in the opposite direction of the sun, regardless of the direction of travel, as the solar wind creates it.

Comets play an important role in studying the origin of the solar system. The reason for this is that the Kuiper Belt and Oort Cloud are believed to contain the material from when the solar system was formed. Unlike planets that have undergone planetary evolution, comets that have flown away are likely to have the same primordial materials. Because of this, scientists obtain various data by striking or landing a probe on a comet that is close to the sun.