https://www.scientificamerican.com/article/covid-19-shutdown-may-obscure-mysteries-of-cracked-interstellar-comet/ > About four months ago, in December 2019, the interstellar comet known as 2I/Borisov made its closest approach to our sun. After its initial discovery by Crimean amateur astronomer Gennady Borisov in August 2019, astronomers raced to observe the object—only the second known visitor from another star since the asteroidlike ‘Oumuamua in 2017—before it drifted out of view. But aside from merely watching 2I/Borisov, they were hoping for something else: that the warmth of our sun would crack the comet apart, releasing material from its innards that was scarcely, if at all, altered after forming billions of years ago in an alien star system. > In late March those hopes were fulfilled. Observations from two teams of astronomers using the Hubble Space Telescope have confirmed that a large chunk of debris up to 100 meters in size has broken off from the comet’s solid icy core, known as the nucleus, which is itself up to 500 meters across. This fragment was moving away from the comet at about 0.5 meter per second and was seen more than 180 kilometers from its nucleus. “A small fragment of the primary nucleus has come off,” says David Jewitt of the University of California, Los Angeles, who leads one of the teams. “Something came out.” Later images from Hubble revealed the fragment had since disintegrated—but the comet could well continue to cast off debris.