NASA’s Hubble Space Telescope has taken a picture of the largest icy comet ever seen by astronomers. It’s enormous, measuring about 80 miles across, which is 50 times larger than most comets we know of, and it’s on a collision course with Earth.
The enormous comet, also known as Comet Bernardinelli-Bernstein or C/2014 UN271, is racing toward Earth at an incredible speed of 22,000 miles per hour, all the way from the outer reaches of our solar system. NASA explains that even though it’s coming our way, it will never come closer to the Sun than 1 billion miles, which is just a bit farther than the gap between Earth and Saturn. Additionally, it won’t reach its nearest point to Earth until the year 2031.
Scientists found the center part of Comet C/2014 UN271, which is called the nucleus. This was tricky because the nucleus was surrounded by a big cloud of dust and gas. They used a special camera on the NASA Hubble Space Telescope to take a picture of the comet on January 8, 2022. Then, they made a model of the cloud around the nucleus, called the coma, and took it out of the picture. This revealed the small, bright dot that is the nucleus.
They also used data from radio telescopes to measure how big the nucleus is. This was a big achievement because the comet is about 2 billion miles away. Even though the nucleus might be as wide as 85 miles, Hubble couldn’t see it clearly. They figured out its size by looking at how much light it reflects.
Surprisingly, the nucleus is very dark, like charcoal. They learned about the nucleus’s area from radio observations.
The Largest Comet Ever Observed
This new comet, with its size of around 80 miles wide, beats the previous record holder, C/2022 VQ94, which was found in 2022 by the Lincoln Near-Earth Asteroid Research (LINEAR) project and had a nucleus about 60 miles wide. Just to give you an idea, Comet Neowise is only about three miles wide, and Halley’s Comet is seven miles wide.
Astronomers Pedro Bernardinelli and Gary Bernstein found Comet C/2014 UN271 in old pictures from the Dark Energy Survey at the Cerro Tololo Inter-American Observatory in Chile. They first saw this comet in November 2010 when it was incredibly far away from the Sun, about as far as Neptune is from Earth, which is about three billion miles. Since then, scientists have been looking at it using telescopes on the ground and in space. Even though they spotted it more than ten years ago, they only recently figured out how big it really is.
In the picture, you can see a comparison of the size of the hard, icy core of Comet C/2014 UN271 (also called Bernardinelli-Bernstein) with a few other comets. According to NASA, most comet cores that scientists have seen are smaller than Halley’s comet and are usually just a mile wide or even smaller.
The lead author of the paper, Man-To Hui from the Macau University of Science and Technology in Macau, Taipa, is amazed by this object. He says, “This is an incredible thing, especially considering how active it is even though it’s very far from the Sun. We thought the comet might be quite large, but we needed the most accurate data to be sure.”
Separating the Nucleus from the Coma.
NASA explains that figuring out the real size of a comet can be tricky because it’s often hard to tell the solid core of the comet from the huge cloud of dust and gas around it. Currently, only the Hubble Space Telescope has the capability to provide a precise estimate of the nucleus’s size. However, even Hubble’s powerful cameras can’t fully see it clearly because the comet is still quite distant.
Hui and his team employed the Hubble Space Telescope to take five pictures of the comet on January 8, 2022. The remarkable telescope displayed a strong point of light in the middle of what it could see, signaling the nucleus’s position. Afterward, they created a computer model of the coma around it and tweaked it to match the fresh Hubble images. After these adjustments, they removed it from the Hubble images to reveal a picture of the nucleus.
The team then compared this outcome with how bright the nucleus appeared when observed using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. This helped them get a more precise measurement of its width and how much light it reflects. What they found is that the comet’s size is similar to what they had estimated using ALMA, but it’s also an extremely dark comet—darker than they had initially thought. So, not only is it large, but it’s also darker than coal.
David Jewitt, a professor of planetary science and astronomy at the University of California, Los Angeles (UCLA), and co-author of the new study in The Astrophysical Journal Letters, says, “This comet represents just a small part of a much larger group of comets in the farther reaches of the solar system. There are many thousands of comets out there that are too faint to see.”
“We’ve had a hunch that this comet had to be sizable because it shines so brightly even from such a great distance. Now, we have the confirmation,” stated the researchers.