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After 3.5 million-year hiatus, the largest comet ever discovered is headed our way

An enormous comet — possibly the largest one ever detected — is barreling toward the inner solar system with an estimated arrival time of 10 years from now, according to new research published on the preprint server arXiv.org.

The comet, known as the Bernardinelli-Bernstein comet (or C/2014 UN271, in astro-speak), is at least 62 miles (100 kilometers) across — about 1,000 times more massive than a typical comet. It's so large that astronomers previously mistook it for a dwarf planet, according to a statement announcing the comet’s discovery in June 2021.

But a closer analysis of the object revealed that it was moving rapidly through the Oort cloud — a vast scrapyard of icy rocks, billions of miles from Earth. The object appeared to be headed our way, and it even had a glowing tail, or "coma", behind it — a clear indication of an icy comet approaching the relatively warm inner solar system.

Now, researchers have studied the massive comet in more detail, and they have new estimates about its journey toward the sun.

For starters, the enormous rock poses no threat to Earth. Right now, Bernardinelli-Bernstein (BB) is cruising through the Oort cloud at about 29 times the distance between Earth and the sun, or 29 astronomical units (AU). The comet's closest approach to Earth will occur sometime in the year 2031, when scientists predict the comet will swoop within 10.97 AU of the sun — putting it just outside of Saturn's orbit, according to the researchers.

While that's far enough from Earth that humans won't be able to see the comet without telescopes, it's considerably closer than the rock's last visit to our part of the solar system. After modeling the comet's trajectory, the study authors calculated that comet BB made its last approach 3.5 million years ago, coming within 18 AU of the sun.

Since then, the comet traveled as far as 40,000 AU away, deep into the mysterious Oort cloud, according to the researchers.

"We conclude that BB is a 'new' comet in the sense that there is no evidence for [a] previous approach closer than 18 AU," the researchers wrote in their study; in other words, humans have never laid eyes on it before.

We owe our current view of the large, distant comet to the Dark Energy Survey (DES) — a project to study the expansion of the universe, which ran between August 2013 and January 2019. During the survey, astronomers mapped 300 million galaxies in the southern sky, discovering more than 800 previously unknown objects beyond the orbit of Neptune. The Bernardinelli-Bernstein comet was one of those objects.

Researchers have plenty of time to study the massive comet as it soars ever closer to Earth over the next decade. Getting a closer look at the rock could help scientists understand a bit more about the chemical composition of the early solar system, as comets from deep in the Oort cloud are thought to be relatively unchanged since they were booted away from the sun billions of years ago. With millions of years separating the comet's next close approach from its following one, it'll be a once-in-a-lifetime brush with the early solar system. 

Reference: Space.com: Brandon Specktor