To unveil the mystery of the 'dirty snowball' and welcome the return of Halley's Comet in the 20th century, astronomers made thorough preparations. An international organization of 900 astronomers from 50 countries was formed to systematically observe the comet and conduct the first-ever space field investigations. They launched five space probes to closely study the comet, with the 'Giotto' probe being the most notable. It ventured within just over 500 kilometers of the comet's nucleus, penetrating deep into the comet's coma and collecting a wealth of valuable data, allowing humanity to witness the true appearance of the comet's nucleus for the first time. This comet has a reflectivity of only 4%, making it darker than coal, and features unique jets that can reach thousands of kilometers. The surface of the nucleus is rough like coal and extremely dark. The outer layer of the nucleus is composed of a non-volatile, porous material, with temperatures ranging from 30°C to 130°C when close to the sun, while ice still exists inside at temperatures reaching -70°C. The surface is riddled with cracks and pits, from which gas and dust flow outward.
China has the earliest records of comet observations and studies. The 'Spring and Autumn Annals' notes: 'In the autumn of the 14th year of Duke Wen of Lu, a comet appeared in the Big Dipper.' This record predates foreign accounts by several centuries. Additionally, a silk manuscript unearthed from the Han tomb at Mawangdui contains a comet illustration dating back to 168 BC, earlier than the comet sighting over Jerusalem in 66 AD. These records are a valuable asset of Chinese astronomy, inspiring us to advance towards global standards.
Although the splendor of comets gradually fades and they eventually disintegrate, new comets appear from time to time, joining the ranks of those discovered by humanity. For instance, at the turn of the century, we witnessed the discovery of Comet Hyakutake and Comet Hale-Bopp, allowing people to continuously experience the allure of comets.
