Have you ever looked up at a night sky full of stars? It’s probably not easy to see because of the bright city lights. However, if you do, you will see many celestials such as comets, moons and stars shining bright. The majority of people can't figure out what kinds of stars they are seeing. Even if you don't know much about the universe, you’d better to look at the sky carefully in May because a comet will be approaching Earth and this is a rare opportunity for you to see one. A comet is a small celestial body belonging to the solar system that orbits the sun or planets in a circular motion. It has a specific cycle, due to its fixed orbit. However, the cycle length varies widely from a few years to hundreds of thousands of years. Comets are mainly produced from the Kuiper belt or Ort cloud and gravitate towards the solar system.
Comets are largely divided into three parts: the nucleus, coma and tail. The nucleus is the center of the comet, consisting of rock, iced water vapor, iced methane and iced ammonia. Usually, the diameter of a comet's core is several kilometers long. There is also active evaporation taking place on the surface of nucleus. During this process, gases and dust are released. This atmosphere of the comet is called a coma. A coma looks clearer as it gets closer to the sun. Also, the comet's tail is divided into two parts, the Dust-tail and Ion-tail, depending on its composition. The tail forms on the opposite side of the direction of the comet's progress, and become longer as it nears the sun. However, comets are so small that we don't have many opportunities to observe them with our bare eyes, but there have been exceptions. When they occur, the comets involved are referred to as 'the great comets'. For instance, the IKEYA-SEKI comet was discovered in 1965 and is a representative of the great comets. When the comet approached Earth, its apparent magnitude was -7 and appeared as bright as crescent moon. The comet inspired the animation movie 'Your Name' released in Japan.
Another great comet is expected to emerge in the 21st century. Its full name name is Asteroid Terrestrial-impact Last Alert System (ATLAS) comet (C1219 Y4) and was first observed in December, 2019. The comet's orbital cycle is about 6,000 years and it is now passing between Earth and Mars. It can be seen near the Camelopardus with high-performance telescopes. It is estimated that the ATLAS Comet will be closest to Earth at about 0.78AU on May 23rd, 2020 and is expected to be closest to the sun around May 31st, 2020.
When it was first discovered, it was expected to have an apprent brightness of -6.7, so we should be have been able to see it with our naked eyes when it comes closest to the earth. However, according to recent observations reported, ATLAS won’t be as bright as expected. On April 6th, 2020, Quanzhi Ye (University of Maryland) and Qicheng Zhang(Caltech) reported the possibility of the comet ATLAS’s fragmentation.
Based on the images of the comet from April 2nd to 5th, they said its form changed from starlike and compact to elongated and fuzzy. This change often means that the comet is heading for fragmentation. Because of its change in form, on April 19th, the apparent brightness of ATLAS was downgraded to -4.95 when it will be closest to earth. This means that its brightness will have decreased by about five times. However, on April 29, Korea Astronomy & Space Science Institue reported that ATLAS’s flagmentation has been confrimed. Using Optical Wide-field patroL Network, which is an astronomical observation facility, they observed the ATLAS’s nucleus splited into several pices and some of them fall off. They also said, the more they approach to the sun, the more its collapse gets worse.
At first, experts didn’t expect that the ATLAS comet would get further split after it had progressed to some fragment. Unfortunately, however, the ATALAS comet is collapsing even at this moment. If you can’t be seen this comet, you probably won't be able to see the Great Comet in our lifetime. Therefore, this makes us feel sorry not only for astronomers but also for us.
|▲ The comet has fragmented into more than a dozen pieces. (Photo from NASA)|
 Apparent magnitude (brightness): Rating of the brightness of the celestial body measured on Earth. The smaller number, the brighter it is. A difference of 1.0 in magnitude corresponds to a brightness ratio of , or about 2.512.
 Camelopardus: The constellation of Giraffe.
 1AU (Astronomical Unit) : One of the units that measures distance in astronomy, which is about 149.6 million kilometers between Earth and the sun.
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