Largest comet ever seen has a heart 'blacker than coal,' and it's headed this way

Astronomers have measured the icy heart of one of the largest comets ever discovered — a gargantuan, 4 billion-year-old rock that's currently barreling toward Earth at 22,000 mph (35,000 km/h).

Don't worry: The enormous, icy rock — named C/2014 UN271, or Bernardinelli-Bernstein (BB) after its discoverers — is on course to miss our planet by about 1 billion miles when it makes its closest approach in 2031, Live Science previously reported. For comparison, that's greater than the average distance between Saturn and the sun — and far enough away that stargazers won't be able to see BB's flyby with the naked eye.

However, as BB zooms ever closer, astronomers are taking the opportunity to study it in ever greater detail. Previous research showed that the icy space rock measures more than 80 miles (128 km) across — about twice the width of Rhode Island — and is about 100 thousand times more massive than a typical comet. BB is so large that it was once mistaken for a dwarf planet; more recent observations showed that the rock sports a glowing tail, or coma, which is a clear indicator of an icy comet soaring through the relatively warm inner solar system.

Now, astronomers have used the Hubble Space Telescope to peer through the rock's blazing coma and focus directly on its icy heart. While BB is still too far away to image in clear detail, the Hubble observations allowed researchers to identify a bright spot of light corresponding to the comet's heart, or nucleus, according to research published April 12 in The Astrophysical Journal Letters.

The team then used a computer model to digitally remove the glow of the comet's bright coma, leaving behind just the nucleus. The resulting data shows that the comet's nucleus is about 50 times larger than typical comets observed in the inner solar system — the single largest nucleus astronomers have ever detected.

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The team's analysis also revealed the color of the comet's icy nucleus.

"It's big and it's blacker than coal," study co-author David Jewitt, a planetary science professor at UCLA, said in a statement.

Still roughly 2 billion miles (3.2 billion kilometers) from Earth, BB has plenty of space to cover before its close-up in 2031. Researchers reported in a study published in November 2021 in The Astrophysical Journal Letters that the comet made its last close approach to Earth 3.5 million years ago, when it came within about 1.6 billion miles (2.6 billion km) of the sun. 

In the meantime, BB has been swooping through the Oort cloud — a vast scrapyard of icy rocks that encircles our solar system, potentially stretching for billions of miles into space.

Originally published on Live Science.