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It came from outside of our solar system and now it is dissolving



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.

The campaign started last month – of all things in March 2020 – when the Hubble telescope discovered at least part of the comet that broke off like a calving iceberg. This lump has bubbled into nothing ever since.

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.


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