It came from beyond our solar system. But the sun wasn’t content to let it leave in peace, or in one piece.
Comet 2I/Borisov, an Eiffel Tower-sized clod of dust and ice, plunged into our solar system last fall, exhaling vapor as it buzzed nearest to our sun around Christmas. This alien visitor must have formed around a distant and unknown star.
It slumbered as it crossed the frozen gulf of interstellar space. But now, suddenly, the sleeper is awake and kicking. To the simultaneous delight and frustration of the world’s astronomers, Borisov has sloughed off at least one fragment over the last few weeks.
The action began last month — March 2020, of all times — when the Hubble telescope spotted at least one chunk of the comet breaking off like a calving iceberg. That clump has since fizzed away into nothingness.
These fireworks offer astronomers a unique glimpse at the exposed guts of this interstellar object, just the second humanity has ever spotted. The first visitor from another star system, 2017’s 1I/Oumuamua, behaved like an inert hunk of rock. “This one has now cracked open its gooey center and we can see what’s inside,” said Michelle Bannister, a planetary astronomer at Queen’s University Belfast in New Zealand.
Astronomers had hoped, even predicted, that Borisov might crack up this spring while heading back out of the solar system to once again sojourn among the stars. But the first signs it was stirring came in early March, right as the coronavirus pandemic ramped up. That’s when ground-based astronomers in Poland spotted the comet suddenly brighten, even though it should’ve been dimming as it got farther from the sun.
Several competing teams of scientists had already booked coveted slots to study the comet over the next few months with Hubble. Spurred by the news out of Poland, they rushed to move up their own observations, hoping to catch the comet acting up.
The clincher came on March 30, when a group led by David Jewitt at the University of California, Los Angeles, downloaded a fresh image taken by Hubble. Instead of just a circular blob that would show the comet’s nucleus, they saw an elongated shape, suggesting a smaller fragment of the nucleus had split off and was slowly drifting away from the main object. “It’s like a little lug nut dropped off your car,” Dr. Jewitt said.
Another team, led by Bryce Bolin at Caltech, said they’ve spotted an earlier clump breaking off in Hubble images, too, possibly corresponding to a piece that could have caused Borisov’s sudden brightening in early March. “I’m hoping that this object is going to be producing more fragments,” Dr. Bolin said, “but not completely, catastrophically break up into a million pieces in a cloud of dust.”
In any normal month, huge mountaintop telescopes in Chile and Hawaii would have already begun swiveling toward the comet, putting the interstellar visitor under the astronomy world’s equivalent of 24-hour surveillance. Those telescopes would let astronomers track Borisov’s brightness from night to night and scan for chemical elements now spewing from its insides.
Of course, the last month wasn’t normal. Most observatories are now shuttered to protect employees from the pandemic.
“The classic phrase is that comets are like cats,” Dr. Bannister said. “They don’t do what you expect. Or what you want.”
Even with Hubble alone, watching a fragment split off and drift from Borisov should help astronomers understand the size of the comet’s original nucleus and how tightly it was bound together, and then compare those properties with bodies formed in our own solar system.
Other research will focus on why Borisov put on a show — and why now. One possible explanation for the comet’s breakup is that after months of sunlight on the surface, buried pockets of volatile ice had warmed enough to suddenly explode.
Another hypothesis holds that gas sprayed off the comet like the wayward nozzle of a fire extinguisher, spinning Borisov in space. Once the comet was rotating fast enough, it centrifuged itself into more than one piece that could escape the original nucleus’ meager gravitational pull. Dr. Jewitt, seeking to prove this model, is hoping future observations will clock the speed of the spin.
Hubble images taken on April 3 show that the chunk Dr. Jewitt spotted seems to have already faded away, said Quanzhi Ye, an astronomer at the University of Maryland.
More fragments might fall off, Dr. Ye said. “If I have to say anything, I’d guess that it’s not done yet.”
Borisov’s timing has offered astronomers everything from consternation to a welcome distraction. “There’s something comforting, in a way, that celestial events still continue to happen even as our lives on Earth have been upended,” Dr. Bannister said.