The Hubble Space Telescope is approaching the largest comet ever seen

Last year, scientists announced that they had discovered a colossal comet that lingered in Neptune’s orbit. They estimated its ice core to be between 62 and 125 miles long, based on its brightness. If the estimates were correct, it would be the largest comet ever discovered.

But scientists wanted to make sure the superlative held, so in January they aimed the Hubble Space Telescope at the comet and measured its nucleus accurately. As reported this week in The Astrophysical Journal Letters, the comet’s nucleus can be up to 85 miles in diameter, making it more than twice as wide as Rhode Island. It also has a mass of 500 trillion tons, which is equivalent to approximately 2,800 Mount Everest.

“It’s 100 times bigger than the typical comets we’ve been studying all these years,” said David Juit, 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) after its two discoverers – will be visible to the naked eye for only a short time. It travels to the sun at 22,000 miles per hour. But at its closest approach, in 2031, it will reach only one billion miles from the sun – just behind Saturn’s orbit – where it will appear as a faint glow in the night sky before returning to the shadows.

However, with the help of Hubble, astronomers can see and study this blazing alien visitor in all its splendor, almost as if flying right next to it – a spectral fog of blue, enveloping a seemingly bright, white heart. “The image they have is beautiful,” said comet co-discoverer Pedro Bernardinelli, an astrophysicist at the University of Washington who was not involved in the study.

Despite its weight, measuring the size of this comet’s nucleus proved difficult. Although far from the sun, just a trickle of sunlight is enough to evaporate the volatile carbon monoxide of the nucleus, creating a confusing dusty atmosphere known as a coma.

Hubble could not clearly see the comet’s nucleus through this fog. But by making such high-resolution images of the comet with the space telescope, Dr. Juit 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 left, it was easy to resize.

Their analysis also revealed that its ice core is blacker than coal. This may be due in part to being “prepared from cosmic rays,” Dr. Juitt said. High-energy cosmic rays bombard the nucleus, destroying the chemical bonds on its surface. This allowed some of the lighter elements, such as hydrogen, to escape into space, leaving behind dark carbon – making the core a bit like a burnt slice of toast.

This dark nucleus suggests that this comet – despite its oversize – is not too different from the others. “Comet nuclei are almost always super-dark,” said Teddy Carretta, a planetary scientist at the Lowell Observatory in Flagstaff, Arizona, who was not involved in the study. He suggests comparing comets to piles of roadside snow. “Even though it’s still mostly ice, just adding a little dirt and grime can make a pile of snow just look nasty and dark.”

More of the comet’s secrets will be revealed as it approaches Saturn’s orbit. But in 2031, when the reverse phase of the three-millionth orbit of the Sun begins, astronomers will not know much more about its origins, probably in the Oort Cloud, a hypothetical and currently unobservable bubble around the solar system filled with primitive icebergs of various shapes. and dimensions.

C / 2014 UN271 is a welcome brief overview of what lies behind this bubble. But “finding this thing is a reminder of how little we know about the solar system,” said Dr. Juit. “There are a huge number of objects we haven’t seen and a huge number of things we haven’t even imagined.”

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

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