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Last year, news broke that scientists had discovered a massive incoming comet called Bernardinelli-Bernstein. It was the farthest comet we had ever discovered. and it was huge, so great scientists originally thought it was a minor planet. But it got to the point where all our photos were still blurry. So we waited. Now NASA confirms that the Hubble Space Telescope (HST) has picked up the object — and it absolutely destroys all of our predictions. Comet Bernardinelli-Bernstein, or “BB” for short, is the largest comet ever recorded.

The arXiv preview of the comet’s discovery gave a low estimate of two trillion tons. For scale, that’s 10 times the mass of Hale-Bopp, and Hale-Bopp became known as the Great Comet of 1997. But comet Bernardinelli-Bernstein is an absolute unit. The thing has broken our previous predictions. NASA estimates the mass of comet Bernardinelli-Bernstein 500 trillion tons

And that’s not the only place we’re wrong. When we reported on Comet BB last year, we said it was twice the size of Phobos and Deimos, elongated from end to end. But BB made a liar out of all of us there too. To be taller† The previous largest comet ever measured was 80 kilometers wide. Comet BB is eighty miles wide.

‘dirty atmosphere’

Despite its distance from Earth, Comet BB has already started putting on a show. When we picked it up last year, it threw off enough debris that we called it a “garbage sphere.” Now it is close enough to the sun that the comet begins to form a coma. But that haze of dirt and reflected light makes the work of astronomers more difficult.

“This is an astonishing object given how active it is when it is this far from the sun,” said Man-To Hui, lead author of a report detailing these more recent observations. “We suspected that the comet could be quite large, but we needed the best data to confirm this.” So Hui and colleagues waited until the comet was close enough for Hubble to take its picture.

The comet is still too far away for Hubble to visually resolve its nucleus. Instead, Hubble’s CCD just shows us a “bright peak of light” where the nucleus should be. So Hui and his team created a computer model of the surrounding coma and then adapted it to the Hubble images. Then they pulled the glow of the coma, leaving only the “star-like” core.

“This comet is literally the tip of the iceberg for many thousands of comets that are too faint to see in the more distant parts of the solar system,” said study co-author David Jewitt, a professor of planetary science and astronomy at UCLA. I always suspected that this comet must be big,” Jewitt said, “because it’s so bright at such a great distance. Now we confirm it.”

Initial brightness measurements confirmed

Measurements of the comet’s composition were found to be correct. Prof. dr. Gary Bernstein — one of Comet BB’s namesakes — said, “The measurement Hui et al reported confirms our previous ground-based measurements of this body’s brightness.”

“We measured the brightness before there was a coma around it,” Bernstein told ExtremeTech. “The new result creates an image with HST after the coma has formed, and tries to pull the coma light away to isolate the light from the comet nucleus.”

Comet BB is a long-period comet, with an orbit lasting about three million years. It probably comes from the Oort cloud, which in itself is cool, because we don’t know much about the Oort cloud. But this comet is also by far the largest Oort cloud object ever identified. That makes this a unique opportunity to check our assumptions about what’s happening in the silent outer reaches of the solar system. Unfortunately, not even Voyager 1 or 2 can reach the Oort Cloud before they run out of fuel.

Far from home

The heliopause is where the solar wind loses its outgoing momentum. Voyager 1 flew through the heliopause in 2012, and Voyager 2 crossed the same boundary in 2018. Far beyond the heliopause, the Oort cloud is a spherical shell of ice, rock and dust that surrounds the entire solar system. It doesn’t have a good edge, because that’s where the sun’s gravity ends. Instead, like the glow of headlights in deep fog, they both fade into the vast emptiness of the interstellar medium. Their porous outer boundary marks where the sun’s gravity gives way to the galactic tide.

We think other stars may have their own distant clouds of comets. Alpha Centauri AB and Proxima Centauri will be closest to us in about 30,000 years. When they do, their Oort cloud is separated from ours by an opening only as wide as our Oort cloud itself.

The comets in the Oort cloud probably didn’t quite form there. Instead, NASA explains, “billions of years ago, they were thrown out of the solar system by a gravitational ‘pinball game’ between the massive outer planets, when the orbits of Jupiter and Saturn were still developing.”

But the planets aren’t the only things that can disrupt that distant cloud of comets. When Gliese 710 zooms into our solar system in about 1.3 million years, it is expected to come within 20,000 AU of the sun. That’s close enough to stomp through the Oort cloud. It won’t disturb Pluto, nor planets to the sun, but it can send us many more long-period comets.

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