NASA’s Hubble Space Telescope has worked out the size of a huge comet nucleus spotted by astronomers last year - and it’s the biggest ever seen.
The comet’s coal-black nucleus is thought to be 80 miles across, almost as wide as Wales, and it weighs in at a colossal 500 trillion tons.
The vast object has been falling towards our sun for more than a million years - but NASA experts say there's no risk it will go anywhere near Earth.
The comet was spotted by astronomers last year in archival images from telescopes.
It’s a hundred thousand times the mass of a ‘normal’ comet we see near the sun, NASA says.
David Jewitt, a professor of planetary science and astronomy at the University of California, Los Angeles (UCLA) said, "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.
"We've always suspected this comet had to be big because it is so bright at such a large distance. Now we confirm it is."
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The vast comet, C/2014 UN271 (Bernardinelli-Bernstein) is flying towards the inner solar system at 22,000 miles per hour from the edge of the solar system.
But NASA’s experts say there’s no reason to worry: it will never get closer than 1 billion miles away from the sun, which is slightly farther than the distance of the planet Saturn.
It will reach its nearest point in the year 2031.
"This is an amazing object, given how active it is when it's still so far from the Sun," said the paper's lead author Man-To Hui of the Macau University of Science and Technology, Taipa, Macau.
"We guessed the comet might be pretty big, but we needed the best data to confirm this."
Hui’s team used Hubble to take five photos of the comet on 8 January 2022 - assessing the size of the nucleus using its brightness (by ‘subtracting’ the effect of the dusty coma surrounding it).
The 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 found in an image from November 2010, when it was three billion miles from the sun, which is nearly the average distance to Neptune.
Since then, it has been intensively studied by ground and space-based telescopes.
The new Hubble measurements are close to the earlier size estimates from ALMA, but convincingly suggest a darker nucleus surface than previously thought.
"It's big and it's blacker than coal," said Jewitt.
The previous record holder is comet C/2002 VQ94, with a nucleus estimated to be 60 miles across.
It was discovered in 2002 by the Lincoln Near-Earth Asteroid Research (LINEAR) project.
It is coming from the hypothesised nesting ground of trillions of comets, called the Oort Cloud.
The diffuse cloud is thought to have an inner edge at 2,000 to 5,000 times the distance between the sun and the Earth.
Its outer edge might extend at least a quarter of the way out to the distance of the nearest stars to our sun, the Alpha Centauri system.
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