It Looks Like Interstellar Comet Borisov Really Is Breaking Apart

Comet Borisov in December at its closest approach to the Sun, over 300 million kilometers away. NASA/ESA/D. Jewitt

Observations of Comet 2I/Borisov in early March suggested that the interstellar visitor might be breaking apart on its way out of our Solar System. Now, new data from the Hubble Space Telescope may have caught a piece of the comet’s nucleus being flung into space.

The first observations from March 4-12 by the ground-based observatory OGLE (Optical Gravitational Lensing Experiment) detected two separate increases in brightness consistent with nuclear fragmentation. After the passage through the inner Solar System last December, when it made its closest pass to the Sun, the comet may now be breaking apart.

A different team performed observations using Hubble on March 23 without initially seeing any specific change to the structure of the comet, but observations carried out on March 28 and March 30 showed an extended core (in yellow), the strongest evidence yet that Borisov has now broken down into pieces.

You can see how the nucleus appears very small on March 23, but it is extended in the following observations. NASA/ESA/D. Jewitt

“Images from March 23 show a single inner brightness core, like that observed in all previous HST images of 2I/Borisov (Jewitt et al. (2020) ApJLetters, 888:L23(8pp)). In contrast, images from March 30 show a clearly non-stellar core, consistent with two unresolved components separated by 0.1 arcseconds (180 km at the distance of the comet) and aligned with the main axis of the larger dust coma,” the team wrote in a post on The Astronomer's Telegram.

Additional Hubble observations are already in the works. The journey through the Solar System might have proved fateful for this interstellar traveler, but its fragmentation is an incredible chance for astronomers to learn as much as possible about the composition of comet Borisov.

Comets are "dirty snowballs", small icy bodies made up of frozen gases, rock, and dust, that – in our Solar System at least – orbit the Sun. When a comet's orbit takes it close to the Sun, the heat melts the ice and it fragments, spewing out gas and dust, creating a gaseous tail as the comet continues on its journey. Many comets in the Solar System experience fragmentation events on a regular basis, before merging back together. Observations have shown plenty of similarities between this comet and the ones that exist in the Solar System, suggesting that comets all form in similar ways. The fragmentation of Borisov will provide more data supporting or denying this hypothesis.

Comet Borisov is the second interstellar object we've discovered passing through the Solar System. The first one, asteroid ‘Oumuamua, visited us in 2017. Statistical estimations suggest that at any one time, there are thousands of interstellar objects within the orbit of Neptune, although spotting even a fraction of them might be beyond our current capabilities.



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