Why the mega comet is so fascinating — and not a threat to Earth

A colossal comet, maybe the largest ever discovered, is hurtling through our solar system. The newly found icy object, thankfully, is on a path that won’t threaten us, or any planet. Yet at some 60 miles across, or perhaps even larger, the comet — a mass of ices, dust, and rocks — is a fascinating and atypical visitor to our neighborhood.

Astronomers recently announced the comet’s existence, first dubbed 2014 UN271 but now called Comet Bernardinelli-Bernstein (after the astronomers who discovered it, Pedro Bernardinelli and Gary Bernstein). During a survey of galaxies in the deep cosmos, a big telescope in the high Chilean desert also picked up hundreds of icy bodies in the outskirts of our solar system, beyond the far-off planet Neptune. The galactic survey actually detected the giant object in 2014, but it took years and the help of intensive computing for scientists to sift through loads of observations. Over half a decade later, they identified the remote object.

Now that the comet is known, astronomers are certain it came from a place called the Oort cloud, a sphere of ancient, icy objects surrounding the solar system. It’s normal for comets to get ejected from the Oort cloud and swoop through our solar system. At its closest to Earth, this one will pass near Saturn’s orbit, more than 1 billion miles from the sun, in 2031.

Yet this isn’t an everyday transient. Astronomers think it came from some 3.7 trillion miles away, hasn’t visited in perhaps millions of years, and crucially, is unusually giant. Most comets are one kilometer (0.6 of a mile) in size or smaller, and are often only noticeable to us because they start melting when they travel closer to the sun, leaving an iconic “tail” or coma.

But this one is some 60 miles wide — 10 times larger than the asteroid that exterminated the dinosaurs.

“The physical size is what’s really impressive about this comet,” said Samantha Lawler, an astronomer at the University of Regina who researches deep objects in our solar system. “This comet is so big that it was able to be discovered way before it got anywhere near the sun.”

“It is by far the largest comet ever discovered in modern times,” she added.

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Where the giant comet is headed

Comet Bernardinelli-Bernstein is currently 1.8 billion miles from the sun. The icy rock is traveling “under” us, meaning it’s coming from below Earth’s orbit around the sun. It’ll swing upward near where Saturn orbits, and then zoom back out to the deep, deep solar system. (This trajectory is shown below.)

But how do astronomers know where, exactly, it’s headed?

Dozens of sightings over several years reveal the comet’s path, explained Jean Creighton, an astronomer and the director of the Manfred Olson Planetarium at the University of Wisconsin–Milwaukee. To determine where it is and how fast it’s moving, astronomers measure how the comet moves against the cosmic backdrop of stars, somewhat like watching close and distant trees from a moving car. Then, they factor in how the powerful gravitational pull of the sun (in addition to gravity from the planets) will tug at the comet, and influence its path.

“Combining the current distance, the current speed, and the gravity of the sun and planets, we can make a very accurate prediction of where they will be in the future,” explained Lawler. “We know from observations of this new comet that it will have its closest approach to the sun near [Saturn’s orbit], after which its high speed will take it zooming back out of the solar system.”

Fortunately, Comet Bernardinelli-Bernstein’s path won’t bring it anywhere near Earth, or any planet for that matter. “One hundred kilometers [or 60 miles] is extremely large. It’s very, very, dangerous,” noted Creighton. “We’re pleased it’s not coming close to us.”

But what if it were to hit Earth, or another planet?

In 1994, Comet Shoemaker-Levy 9, about 1.2 miles (or 2 km) across, collided with Jupiter. Before the collision, however, it broke into around a dozen pieces. They still had a big impact. “As each piece impacted Jupiter, it created a mushroom cloud the size of the Earth, and left weird dark bruises on the surface of Jupiter for months afterward,” said Lawler. “So a 100-kilometer comet colliding with Saturn would be pretty spectacular.”

“We’re pleased it’s not coming close to us.”

Indeed. But such a collision would be catastrophic to life on Earth. That’s why knowing what’s out there — specifically what objects are threats and what aren’t — is critical. “We want to monitor if there’s something coming close to us,” said Creighton. Fortunately, NASA has a Planetary Defense Coordination Office to survey the skies for hazardous objects, and potentially warn of impending impacts. One day, humanity might develop spacecraft capable of deflecting dangerous asteroids or comets away from Earth. (That would be a wise investment.)

While astronomers have a solid grasp on where the comet is headed, exactly what propelled this icy body to hurtle through the heart of the solar system remains unknown, and likely will remain a mystery. Something, in deep space, perhaps another giant object passing by, could have given the comet a gravitational nudge that sent it toward the sun.

“Whatever happened, probably happened many millions of years ago,” explained Creighton.

Even a perturbation from a (relatively) nearby star could have nudged the comet, explained Peter Veres, an astronomer who researches comets and related space objects at The Center for Astrophysics-Harvard & Smithsonian, a collaborative research group between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. Stars are constantly on the move through the galaxy, and they throw their mass around.

Fascinating object

Astronomers would relish getting a good look at this comet.

“I wish there was a spacecraft that could fly there and tell us what this object looked like,” said Veres.

Alas, that ship probably sailed. It takes the better part of a decade just to send a spacecraft into Saturn’s realm.

But that doesn’t mean astronomers can’t still learn from this colossal object. “We can get loads of interesting information,” emphasized Lawler.

As the comet journeys through space, astronomers can more precisely measure its orbit. Ultimately, this allows them to model, via computer simulations, when the comet last approached the sun — if ever. If so, perhaps much of its icy surface has already melted away, leaving a more rocky, dusty core. That means advanced telescopes can glean the composition of the comet. This is valuable: Knowing where this ancient object likely came from, and what it’s made of, gives rare insight into the early years of our solar system, from some 4 billion years ago.

A new, giant telescope can peer at the comet. The Vera Rubin Observatory in Chile, with a mirror the width of a tennis court (the larger the mirror the better the resolution), will be in excellent position to observe Comet Bernardinelli-Bernstein, noted Lawler. The telescope should go online in the next year or two. (When the comet is closer, it should also be visible through larger amateur telescopes.)

Vera Rubin may find other icy behemoths hurtling through our solar system, too. “It will almost certainly find more huge comets like this that we haven’t discovered before because they never get quite close enough to the sun,” noted Lawler.

Over the next decade, both astronomers and the public have at least one giant comet to ponder, and learn more about. It’s a non-threatening object that invites intrigue.

“Any object in the sky that makes people interested and look up is a good object in my opinion,” said Creighton.