Largest-ever comet cometh! But have no fear, it won’t come near

Astronomers have detected a comet hurtling toward Earth at around 10 kilometres a second. And it’s the largest comet ever observed, weighing in at 500 trillion tonnes. But don’t lock yourselves away in your bunkers just yet. This comet will only get as close as 1.6 billion kilometres from the Sun in 2031, so we’re in no danger. This time.

Comets are the icy remnants of the period of planet formation in the early solar system. They are now thought to exist in the hypothesised Oort Cloud – a vast reservoir of comets flung billions of kilometres beyond the farthest planets encircling the Sun. When observed, comets have a characteristic tail, sometimes stretching millions of kilometres, made of ice and gas. But at the head of every comet is a ball of ice and dust.

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The gargantuan comet headed our way is called Comet C/2014 UN271. The four billion-year-old comet has been falling toward the Sun for well over one million years. It was discovered by accident in 2010 by astronomers Pedro Bernardinelli and Gary Bernstein when it was nearly five billion kilometres from the Sun; they came across it when studying archival images from the Dark Energy Survey at the Cerro Tololo Inter-American Observatory in Chile.

A new study of the comet, published in the Astrophysical Journal Letters, was co-authored by Professor David Jewitt of the University of California Los Angeles (UCLA), US. “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,” Jewitt says. “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.”

“This is an amazing object, given how active it is when it’s still so far from the Sun,” says lead author Man-To Hui from the Macau University of Science and Technology. “We guessed the comet might be pretty big, but we needed the best data to confirm this.”

NASA Hubble Space Telescope observations were needed to see the solid “nucleus” of the comet behind the huge dusty shield enveloping it. The researchers found the comet is at least 130km in diameter.

Hui and Jewitt’s team used Hubble to take five photos of the comet on January 8, 2022. Though too far away to be seen directly, the team identified the bright spot in the images as the comet’s nucleus. Using a computer model, Hui and this team were able to compensate for the surrounding dust to resolve the solid nucleus.

Combining their data with earlier observations by the Atacama Large Millimetre/submillimetre Array in Chile, the researchers narrowed down the diameter and reflectivity of the comet’s nucleus. In addition to its massive size – 50 times larger than any other known comet – the comet is darker than predicted. “It’s big and it’s blacker than coal,” says Jewitt.

The Oort Cloud is thought to contain trillions of comets, with its inner edge expected to lie somewhere between 2,000 and 5,000 times the distance from the Sun to Earth. Its outer edge may stretch to a quarter the distance from our Sun to the Alpha Centauri system, the nearest stars to us.

Other deep sky surveys and observations of comets like Comet C/2014 UN217 will help astronomers understand the role that the Oort Cloud plays in the evolution of the solar system.