NASA's Hubble Space Telescope has determined the size of the nucleus of the largest icy comet ever seen by astronomers. Its estimated diameter is approximately 130 km wide, almost as long, for example, as the straight-line distance between Madrid and Valladolid. The nucleus is about 50 times larger than that found at the heart of most known comets, and its mass of 500 billion tons is one hundred thousand times greater than that of a typical comet much closer to the Sun. .
The giant comet, C/2014 UN271 (Bernardinelli-Bernstein) is heading towards Earth at 35,400 km per hour from the edge of the solar system. Fortunately, it will not approach more than 1.6 billion km from the Sun, a little further than the distance of the planet Saturn, something that will happen in the year 2031.
The new comet "is literally the tip of the iceberg of many thousands of comets that are too faint to see in the most distant parts of the solar system," says David Jewitt, professor of planetary science and astronomy at the University of California, Los Angeles ( UCLA), and co-author of a new study published in 'The Astrophysical Journal Letters'. «We have always suspected that this comet had to be large because it is very bright at that distance. Now we confirm that it is," he says. The previous record holder is comet C/2002 VQ94, with a nucleus estimated at 96 km in diameter.
Comet C/2014 UN271 was discovered by astronomers Pedro Bernardinelli and Gary Bernstein in archival images from the Dark Energy Survey at the Cerro Tololo Inter-American Observatory in Chile. It was first observed by chance in November 2010, when it was 4.8 billion km from the Sun, which is almost the average distance to Neptune. Since then, it has been intensively studied by ground-based and space-based telescopes.
"This is an amazing object, given how active it is when it is still so far from the Sun," says Man-To Hui, of the Macau University of Science and Technology, Taipa, Macau, and lead author of the study. "We assumed it could be quite large, but we needed the best data to confirm it." So his team used Hubble to take five photos of the comet on January 8, 2022.
Comparison of the size of the solid, icy nucleus of Bernardinelli-Bernstein with other comets
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NASA, ESA, Zena Levy (STScI)
Blacker than coal
The great challenge was to differentiate the solid core from the enormous dusty coma that surrounded it. The comet is currently too far away for Hubble to visually resolve its nucleus, so researchers had to use a computational model of the surrounding coma to 'separate' it from the nucleus.
The new Hubble measurements are close to previous size estimates from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, but they strongly suggest a darker core surface than previously thought. "It's big and blacker than coal," says Jewitt.
The Bernardinelli-Bernstein has been falling toward the Sun for more than 1 million years. It comes like trillions of comets from the hypothetical Oort Cloud and follows an elliptical orbit of 3 million years, taking it as far from the Sun as about half a light year. As it stands, it is warm enough for carbon monoxide to rise from the surface and produce the dusty coma.