It got beyond our solar system. But the sun did not content to leave it alone or in one piece.
Comet 2I / Borisov, an Eiffel tower size of dust and ice, plunged into our solar system last fall and exhaled steam as it buzzed closest to our sun around Christmas. This strange visitor must have been formed around a distant and unknown star.
It slumbered as it crossed the frozen bay into interstellar space. But now suddenly the sleeping person is awake and kicking. To the simultaneous joy and frustration of the astronomers of the world, Borisov has released at least one fragment in recent weeks.
These fireworks offer astronomers a unique glimpse of the exposed intestines of this interstellar object, just like any other humanity has ever seen. The the first visitor from another star system, 2017’s 1 / Oumuamua, behaved like an inert rock ball. “This one has now broken open its center and we can see what’s inside,” said Michele Bannister, a planetary astronomer at the University of Canterbury in New Zealand.
Astronomers had hoped, even predicted, that Borisov would be able to crack this spring when they were on their way back out of the solar system to re-stay among the stars. But the first signs of a stir came in early March, just as the coronavirus pandemic was rising. That was when ground-based astronomers in Poland suddenly discovered the comet to be brighter, although it should have disappeared when it came further from the sun.
Several competing research groups had already booked coveted places to study the comet in the coming months with Hubble. Due to the news from Poland, they rushed to move up their own observations in hopes of catching the comet’s appearance.
The health care came on March 30, when a group led by David Jewitt at the University of California, Los Angeles, downloaded a new image taken by Hubble. Instead of just a circular lump that would show the comet’s core, they saw an elongated shape, suggesting that a smaller fragment of the nucleus had split and slowly drifted away from the main object. “It’s like a little tapping nut dropped your car,” Dr. Jewitt.
Another team, led by Bryce Bolin at Caltech, said they have seen onen earlier lump that burst into Hubble images as well, possibly representing a piece that could have caused Borisov’s sudden brightening in early March. “I hope this item will produce more fragments,” Dr. Bolin, “but not quite, disastrously broken up into a million pieces in a cloud of dust.”
During all the normal months, huge mountain telescopes in Chile and Hawaii would have already begun to swing at the comet, placing the interstellar visitor under the world equivalent of 24-hour astronomy. These telescopes would allow astronomers to track Borisov’s brightness from night to night and search for chemical elements that now track from the inside.
Of course, the last month was not normal. Most observatories are now closed to protect employees from the pandemic.
“The classic phrase is that comets are like cats,” Dr. Bannister. “They don’t do what you expect. Or what you want.”
Even with Hubble alone, watching a fragment fragmented and drifting from Borisov should help astronomers understand the size of the comet’s original nucleus and how tightly it was bound together and then compare these properties with bodies formed in our own solar system.
Other research will focus on why Borisov presented a show – and why now. One possible explanation for the comet’s eruption is that after months of sunlight on the surface, buried pockets of volatile ice had warmed enough to suddenly explode.
Another hypothesis claims that gas that was ejected from the comet as a real nozzle of a fire extinguisher, which spins Borisov in space. When the comet rotated fast enough, it centrifuged itself in more than one piece that could escape the lean core’s gravitational pull. Dr. Jewitt, who is trying to prove this model, hopes that future observations will clock up the speed of the spin.
Hubble pictures taken on April 3 show that the bit that Dr. Jewitt discovered seems to have disappeared, says Quanzhi Ye, an astronomer at the University of Maryland.
More fragments may fall off, Dr. Ye. “If I have to say something, I would guess it’s not done yet.”
Borisov’s time has offered astronomers everything from consternation to a welcome distraction. “It is somewhat comforting that celestial events are still happening even though our lives on earth have been appreciated,” Dr. Bannister.