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Astro Bob: Hubble confirms giant comet nucleus, biggest ever seen

At approximately 80 miles across the core of Comet Bernardinelli-Bernstein is bigger than the state of Rhode Island. We also have an aurora alert for Weds. night April 13.

Comet C/2014 UN271
Comet Bernardinelli-Bernstein (C/2014 UN271) was around 2 billion miles (3.2 billion km) away when the Hubble Space Telescope took this photo on Jan. 8, 2022. The charcoal-black nucleus is estimated to be as large as 85 miles in diameter, but it's so far away Hubble can't discern its shape.
Contributed / NASA, ESA, Man-To Hui (Macau University of Science and Technology), David Jewitt (UCLA)
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DULUTH — Clear the deck for Comet Bernardinelli-Bernstein ! This giant ball of dirty ice is hurtling sunward at 22,000 miles per hour from the edge of the solar system. Most comets are small, fragile objects typically a mile (1.6 km) or less across. Comet B-B is some 50 times larger with a diameter of about 80 miles (129 km). Despite its impressive speed and girth, the chances of it hitting Earth are a big, fat zero. The object won't get any closer than 1 billion miles from the sun, just beyond the orbit of Saturn. And that won't occur until January 2031.

Astronomers Pedro Bernardinelli and Gary Bernstein discovered their comet in archival images from the Dark Energy Survey serendipitously in November 2010, when it stood some 3 billion miles (4.8 billion km) away, farther than the planet Neptune. Comets are coal-dark, so to see it at all at such an incredible distance indicated it had to be large.

Comet nuclei sizes
This diagram compares the size of the icy, solid nucleus of Comet Bernardinelli-Bernstein (C/2014 UN271) to several other comets. Most comet nuclei observed are smaller than Halley’s comet — just a mile across or less. Comet C/2014 UN271 is currently the record-holder for big comets.
Contributed / Space Telescope Science Institute, NASA, ESA, Zena Levy

"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 astronomer David Jewitt (UCLA), co-author of the new study .

Team leader Man-To Hui (Macau University of Science and Technology) tapped the Hubble to take five photos of the comet on January 8, 2022, to confirm its size. Currently 1.8 billion miles (2.9 billion km) from Earth, the object's solid nucleus can't be resolved by the orbiting telescope. It's just a bright spike of light enveloped by a large, dusty coma, the cloud of debris that forms as the sun vaporizes the comet's ices.

Comet C/2014 UN271 Nucleus
The Hubble photo of the comet appears at left, with a model of its coma in the middle panel. Astronomers then subtracted the coma, unveiling the point-like glow from the nucleus. Combined with radio telescope data, astronomers arrived at a precise measurement of the nucleus size.
Contributed / NASA, ESA, Man-To Hui (Macau University of Science and Technology), David Jewitt (UCLA)

Hui's team tackled the problem by making a computer model of the surrounding coma and adjusting it to fit the Hubble images. They then subtracted the glow of the coma to leave behind the star-like nucleus and compared its brightness with earlier radio observations (made in the light of radio waves) from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.


2942551+ALMA ESO etc.jpg
Several antennas of the Atacama Large Millimeter Array (ALMA) are silhouetted against the beauty of the southern Milky Way. ALMA and Hubble data were combined to arrive at the size of Comet Bernardinelli-Bernstein. Contributed / ESO, B. Tafreshi, TWAN (

Combining the data they were able to narrow down the diameter and brightness of the nucleus to around 80 miles, roughly the distance between Chicago and Milwaukee. The new measurements are close to the radio values from ALMA and suggest that the comet's core is even darker than they thought.

The moon, which reflects 12 percent of the light it receives from the sun, is as dark as a freshly-laid asphalt road. Comet Bernardinelli-Bernstein is blacker yet. With a reflectivity of just 4.4 percent it's identical to the charcoal we use for grilling.

The comet has been falling toward the sun for well over a million years and originates from a hypothesized spherical swarm of trillions of comets called the Oort Cloud. Scientists place its inner edge at 2,000 to 5,000 times the distance between the Earth and the sun. Its outer edge extends extends for at least a light-year, or a quarter of the way to the Alpha Centauri star system. NASA's Voyager spacecraft won't reach the inner realm of the Oort Cloud for about another 300 years and spend some 30,000 years passing through it. The bigness of space sometimes hurts my head.

Oort cloud
This theoretical Oort Cloud is the biggest structure in the solar system but so far away we can only infer its existence. Astronomers do this by observing that distant comets drop into the inner solar system at every imaginable angle from both above and below. This implies they reside in a sphere.
Contributed / Pablo Carlos Budass. CC BY-SA 4.0

Far-flung comets only leave the Cloud and drop back in toward the sun when they're disturbed by the gravitational tug of a passing star, which NASA poetically describes as "like shaking apples out of a tree."

Comet B-B takes some 3 million years to orbit the sun. When farthest it's roughly half a light-year away. At its current 1.8-billion-mile distance the temperature hovers around -348° Fahrenheit (-211° C). While shockingly cold, that's still warm enough for the sun to vaporize carbon monoxide ice and create a coma.

When closest in 2031 the comet will be too faint to see with the unaided eye but will likely be visible in 10-inch or larger amateur telescopes from a dark sky.

In other news, there's a good chance for a moderate G2 geomagnetic storm on Wednesday night, April 13 between the hours of 10 p.m. and 1 a.m. Central Time. The moon will be bright, but the aurora may still punch through.

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"Astro" Bob King is a freelance writer for the Duluth News Tribune.

"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune.
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