At about 128km (80 miles) across, it’s about 50 times bigger than any other and could have a mass of as much as 500 trillion tonnes. That is the assessment of researchers at a state laboratory in Macau who say they have confirmed the size of the largest comet nucleus astronomers have found. When the comet was first discovered from archived observation data in June 2021, “it caught our attention immediately”, said Man-To Hui of the Macau University of Science and Technology, Taipa Macau, who is lead author of a paper published in Astrophysical Journal Letters last week. “Comets are believed to be the most pristine objects in our solar system and still have abundant information regarding the earliest history and evolution of our solar system,” he said. The colossal comet – C/2014 UN271 or Bernardinelli-Bernstein – is believed to have arrived from a bubblelike, dark, icy region called the Oort Cloud at the edge of our solar system. It follows a highly elongated path and completes one orbit of the sun every few million years. In contrast, Halley’s comet was formed in the Kuiper Belt, a ring-shaped region just beyond the orbit of Neptune, and comes back to Earth every 76 years. What a blast: Nasa’s space telescope launches ‘a new era’ Scientists took the first images of C/2014 UN271 in 2014. In January, Hui and his colleagues used the Hubble Space Telescope to take five high-resolution photos of the comet and used them to estimate the size of the body. Technically, the size of a comet nucleus is hard to determine, because it is far from Earth and usually surrounded by a flurry cloud called coma, made up of materials escaping from the nucleus, including water, carbon monoxide, carbon dioxide and dust, according to Shi Jianchun from the Purple Mountain Observatory, Chinese Academy of Sciences in Nanjing, who is not involved in the research. The coma could be hundreds or thousands of times thicker than the nucleus itself, and Shi said the researchers used a “really neat” model to separate the nucleus and the coma based on their surface brightness. As C/2014 UN271 keeps approaching the sun, the best time to have an up-close look will be January 2031, when it comes closest to Saturn’s orbit. Hui said that although its diameter was 100 times larger than that of the Halley’s comet, it would not be nearly as spectacular because of its great distance from both the sun and Earth. Scientists have been using space and ground-based telescopes to study comets at various wavelengths across the electromagnetic spectrum. In 2014, the European Space Agency’s Rosetta mission landed a craft on a Jupiter-family comet 67P/Churyumov-Gerasimenko and found water with a different composition to water found on Earth. In addition to Hubble, the newly launched James Webb Space Telescope will be a powerful tool to study comets using infrared light. The Chinese Space Station Telescope will have a similar aperture to Hubble but 300 times the field of view. It is under development and expected to go into orbit in 2023. There are more than 250 state key laboratories under the Ministry of Science and Technology of China, including 16 in Hong Kong and four in Macau. ***