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 Michele Bannister, a planetary astronomer at the University of Canterbury 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.