Nucleusunder ≈ 0.5 kmupper limits from Hubble and from its motion[1]
Carbon monoxide35–105 % of waterSolar System comets average about 4 %[1]
About the image. Drawn as a comet with a bright coma and tails pointing away from the Sun. Its nucleus, at most about half a kilometre across, was never resolved; the coma and tail were well observed.
Gennadiy Borisov is an amateur astronomer in Crimea who builds his own telescopes. On the night of 30 August 2019, with a 0.65-metre instrument at his MARGO Observatory in Nauchnij, he found a faint, fuzzy object. Its path soon showed that it was not bound to the Sun. On 24 September it was given the name 2I/Borisov: the second known interstellar object, two years after ʻOumuamua.
A comet that looked familiar
ʻOumuamua had been a puzzle: no coma, no tail and a strange shape. Borisov was the opposite. It had a coma and a tail and behaved like an ordinary comet, the first confirmed comet from another star. That made it a rare chance to compare material from another planetary system with our own.
It never came close. It passed the Sun on 8 December 2019 at 2.0 AU, outside Mars’s orbit, and was closest to Earth on 28 December at about 1.9 AU. Hubble followed it from October 2019 to September 2020.
The carbon monoxide
The difference showed up in its gas. Cyanide was the first gas ever detected from an interstellar object. Then measurements in 2020 found far more carbon monoxide than expected: between 35 and 105 percent of the amount of water in its tail, against an average of about 4 percent for comets of our Solar System. Carbon monoxide freezes only at very low temperatures, so Borisov probably formed in a very cold place, far from its own star.
Not quite a breakup
In February and March 2020 the comet flared, and images from late March seemed to show a second piece. Later analysis found that only about 0.1 percent of its mass had been thrown off. It was a large outburst, not a breakup, and the nucleus carried on, at most about half a kilometre across.
It arrived from the direction of Cassiopeia, near Perseus, so it most likely came from the plane of the Milky Way. At about 32 kilometres per second it needs some 9 000 years to cross a single light-year. No spacecraft could have caught it.
Gallery
A glowing pale-gold comet head with a thin, curving tail against black space, an impression of the interstellar comet 2I/Borisov. AI-generated painting.A white comet with a long, streaming tail crossing dark space diagonally, an impression of 2I/Borisov. AI-generated painting.2I/Borisov, 3D artist's impression, rendered from the interactive model.2I/Borisov, 3D artist's impression, from another angle, rendered from the interactive model.2I/Borisov, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
2018Already visible in images from 13 December, found later.
2019Discovered by Gennadiy Borisov on 30 August; named 2I/Borisov on 24 September.
2019Closest to the Sun on 8 December and to Earth on 28 December.
2020A large outburst in March is briefly mistaken for a breakup.
2020Hubble's observations end in September.
Questions people ask
Who discovered 2I/Borisov?
Gennadiy Borisov, an amateur astronomer in Crimea, with a 0.65-metre telescope he built himself.
How is 2I/Borisov different from ʻOumuamua?
Borisov had a coma and a tail like a normal comet. ʻOumuamua showed neither.
Did 2I/Borisov break apart?
No. In March 2020 it lost about 0.1% of its mass in an outburst, but the nucleus survived.