It came from beyond our solar system. But the sun wasn’t happy with letting it go alone or in one piece.
Comet 2I / Borisov, a lump of dust and ice the size of an Eiffel Tower, plunged into our solar system last fall and exhaled steam as it hummed closest to our sun around Christmas. This alien visitor must have formed around a distant and unknown star.
It was slumbering as it crossed the frozen gulf of the interstellar space. But now the sleeper is suddenly awake and kicking. To the simultaneous joy and frustration of the world’s astronomers, Borisov has stripped off at least one fragment in the past few weeks.
These fireworks offer astronomers a unique glimpse into the exposed guts of this interstellar object, just the second humanity ever discovered. The The first visitor to another star system, 1I / Oumuamua 2017, behaved like a lazy boulder. “This has now broken open its sticky center and we can see what’s inside,” said Michele Bannister, a planetary astronomer at the University of Canterbury in New Zealand.
The astronomers had hoped, even predicted, that Borisov could leave this spring while returning from the solar system to be among the stars again. But the first signs of this were in early March when the coronavirus pandemic increased. At that moment, ground-based astronomers in Poland discovered that the comet suddenly became brighter, although the further it was from the sun it should have darkened.
Several competing teams of scientists had already booked coveted slots to study the comet with Hubble over the next few months. Inspired by the news from Poland, they hurried to improve their own observations in hopes of catching the comet.
The clincher came on March 30 when a group led by David Jewitt from the University of California at Los Angeles downloaded a new image from Hubble. Instead of just a circular blob that would show the comet’s nucleus, they saw an elongated shape, suggesting that a smaller fragment of the nucleus had split off and slowly drifted away from the main object. “It’s like a little wheel nut fell off your car,” said Dr. Yes.
Another team, led by Bryce Bolin at Caltech, said they found onen Earlier lump, which also broke off in Hubble pictures, possibly corresponding to a piece that could have caused Borisov’s sudden brightening in early March. “I hope this object will produce more fragments,” said Dr. Bolin, “but not completely, disastrously crumbled into a million pieces in a cloud of dust.”
In every normal month, giant mountaintop telescopes in Chile and Hawaii had already started to turn towards the comet, making the interstellar visitor subject to 24-hour surveillance of the astronomy world. With these telescopes, astronomers were able to track Borisov’s brightness from night to night and look for chemical elements that are now escaping from within.
Of course, the past month was not normal. Most observatories are now closed to protect workers from the pandemic.
“The classic phrase is that comets are like cats,” said Dr. Bannister. “You don’t do what you expect. Or what you want.”
Even with Hubble alone, observing how a fragment from Borisov splits and drifts should help astronomers understand the size of the comet’s original nucleus and how closely connected it was, and then compare these properties to bodies found in ours own solar system were formed.
Other research will focus on why Borisov hosted a show – and why now. A possible explanation for the comet’s breakup is that after months of sunlight, pockets of volatile ice buried on the surface had warmed to explode.
Another hypothesis is that gas is sprayed by the comet like the stubborn nozzle of a fire extinguisher and turns Borisov in space. As soon as the comet turned fast enough, it centrifuged into more than one piece that could escape the meager attraction of the original nucleus. Dr. Jewitt, who wants to prove this model, hopes that future observations will measure the speed of the spin.
Hubble pictures taken on April 3rd show that the piece that Dr. Jewitt has discovered that already appears to have faded, said Quanzhi Ye, an astronomer at the University of Maryland.
More fragments could fall off, said Dr. Ye. “If I had to say something, I would assume that it is not yet finished.”
Borisov’s timing has given astronomers everything from dismay to a welcome distraction. “It is somewhat reassuring that heavenly events are still taking place even when our lives on earth have changed,” said Dr. Bannister.