The Hubble Telescope is approaching the largest comet ever detected

Last year, scientists announced that they had discovered a colossal comet that remained just inside the orbit of Neptune. They estimated that its icy core was between 62 and 125 miles long, depending on its brightness. If the estimates were accurate, this would be the largest comet ever discovered.

But scientists wanted to make sure the superlative got stuck, so in January they aimed the Hubble Space Telescope at the comet and accurately measured its core. As reported this week in The Astrophysical Journal Letters, the comet’s core could be up to 85 miles wide, more than twice the width of the state of Rhode Island. It also has a mass of 500 trillion tons, equivalent to approximately 2,800 Mount Everest.

“It’s 100 times larger than the typical comets we’ve been studying all these years,” said David Jewitt, an astronomer and planetary scientist at the University of California, Los Angeles and author of the new study.

Despite its impressive size, this comet — named C / 2014 UN271 (Bernardinelli-Bernstein) in honor of its two discoverers — will only be visible to the naked eye for a short time. It goes to the sun at 22,000 miles per hour. But in its closest approach, in 2031, it will only reach a billion miles from the sun, just behind Saturn’s orbit, where it will appear as a faint glow in the night sky before coming out into the shadows again.

With the help of Hubble, however, astronomers can see and study this effervescent alien visitor in all its glory, almost as if he wanted to be by his side: a spectral blue mist that envelops a seemingly bright white heart. “The picture they have is beautiful,” said Pedro Bernardinelli, co-discoverer of the comet, an astrophysicist at the University of Washington who did not participate in the study.

Despite its weight, measuring the size of this comet’s core proved difficult. Although far from the sun, only a ray of sunlight is enough to vaporize the volatile carbon monoxide gels in the core, creating a dusty atmosphere known as a coma.

Hubble could not clearly see the comet’s core through this fog. But by taking high-resolution images of the comet with the space telescope, Dr. Jewitt and his colleagues were able to make a computer model of the coma, which allowed them to digitally remove it from the images. With only the core remaining, sizing it was a breeze.

His analysis also revealed that his icy core is blacker than coal. This may be partly due to being “cooked by cosmic rays,” Dr. Jewitt said. High-energy cosmic rays have been bombarding the core, breaking the chemical bonds on its surface. This allowed some of the lighter elements, such as hydrogen, to escape into space, leaving dark carbon behind, making the core look a bit like a burnt slice of toast.

This dark core suggests that this comet, despite its large size, is not too different from the others. “Comet nuclei are almost always super dark,” said Teddy Kareta, a planetary scientist at the Lowell Observatory in Flagstaff, Arizona, who did not participate in the study. He suggests comparing kites with piles of snow on the road. “While it’s still mostly ice, just adding a little dirt and grime can make a lot of snow look nasty and dark.”

More secrets of the comet will be revealed as it approaches Saturn’s orbit. But in 2031, when the return of its three-million-year-old solar circuit begins, astronomers will not know much about its origin, probably in the Oort cloud, a hypothetical and currently unobservable bubble around the solar system. full of primitive ice fragments of different shapes and sizes.

C / 2014 UN271 is a welcome preview of what is hidden inside this bubble. But “finding this thing is a reminder of how little we know about the outer solar system,” Dr. Jewitt said. “There are a lot of things we haven’t seen, and a lot of things we haven’t even imagined.”

He added: “Who knows what the hell is going on out there.”

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