2I/Borisov — the interstellar comet
The second interstellar visitor and the first interstellar comet — born around another star, yet strikingly like the comets of our own solar system.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
On 30 August 2019 Gennadiy Borisov, an amateur astronomer in Crimea, found an unfamiliar comet in the dawn sky with a 65 cm telescope he had designed and built himself. Its orbit came out with an eccentricity of 3.36. Any value above 1 means an object is not held by the Sun and passes only once, and 3.36 was far above that. It was the second interstellar object after ‘Oumuamua and the first confirmed interstellar comet.
The comet made its closest pass by the Sun on 8 December 2019, at about 2 AU. Unlike ‘Oumuamua, Borisov behaved like a perfectly ordinary comet. It had a clear cloud of gas and dust, called a coma, and a tail, and water and other gases were seen streaming off it. That meant a comet formed around another star was made of much the same material as our own comets.
There was one striking difference: it held a great deal of carbon monoxide. Compared with water, it had roughly ten times more carbon monoxide than a typical comet in our solar system. Carbon monoxide stays frozen only in very cold places, so scientists think Borisov formed far from its parent star, in a very cold region. Its core is probably less than 1 km across, and the star it came from has never been identified.
‘Oumuamua was already leaving when it was found and vanished from view within weeks. Borisov, by contrast, was discovered while still approaching and was watched for close to a year. In March 2020 the Hubble Space Telescope photographed a small piece breaking away from its core. It is now leaving toward the constellation Telescopium at about 32 km/s, and the orbit view replays its open path crossing the solar system at an angle.
When and where to see it
Borisov can no longer be seen with any telescope. It is now tens of AU from the Sun, heading away toward Telescopium in the southern sky. Its closest approach to Earth came on 28 December 2019, but at 1.9 AU it was farther away than Mars’s orbit, so it was never visible to the naked eye.
Choose Borisov in the orbit view’s event list to replay its closest pass by the Sun on 8 December 2019 and its path crossing the solar system at about 44 degrees to Earth’s orbital plane. Placed beside ‘Oumuamua’s track, the two visitors can be seen entering from different directions and leaving in different directions, which shows that interstellar objects do not all come from one side.
See this for yourself — replay the moment in the orbit view
Past returns and history
Gennadiy Borisov was an experienced amateur who had found several comets, and by day he worked as an engineer at the Crimean Astrophysical Observatory. When he found the comet it was about 3 AU from the Sun and shone at magnitude 18, roughly 60,000 times fainter than the dimmest star the naked eye can see.
In February and March 2020 Borisov had several sudden outbursts of brightness, and in March a fragment was seen breaking away. Later analysis showed that the piece was tiny, only about 0.1 percent of the comet’s total mass. Scientists think part of the core’s outer layer, warmed by the Sun, crumbled and broke off.
A system for naming interstellar objects took shape around this time. In 2017 the International Astronomical Union gave ‘Oumuamua the first interstellar number, “1I”, and Borisov, following the comet tradition of adding the finder’s name, became “2I/Borisov”. The third object, found in 2025, was named “3I/ATLAS” after the survey telescope that spotted it. 3I/ATLAS swept inside the orbit of Mars as it rounded the Sun in late October 2025.
Why it matters
Finding that a comet is still a comet wherever it forms is quiet but important evidence. It showed, through a real object rather than a theory, that ice and dust clump together in the disk around another star in the same way as around our Sun. That supports the idea that planets and comets form in similar ways in many parts of the universe.
Comparing it with ‘Oumuamua is just as valuable. The first visitor sped up oddly with no tail, while the second was a textbook comet. With only two examples, it was already clear that interstellar objects are not all of one kind. 3I/ATLAS, found in 2025, became the third sample and gave even more to compare.
It also matters that an amateur found it. Even in an age when large survey telescopes scan the sky automatically, an individual observer can get there first in places those surveys watch less closely, such as the sky near the Sun just before dawn. Borisov found this comet by patiently watching exactly that kind of dawn sky.
FAQ
How is it different from ‘Oumuamua?
Borisov behaved like an ordinary comet with a coma and a tail and was watched for close to a year; ‘Oumuamua had no tail and faded from view within weeks.
Could it be seen with the naked eye?
No. Even at its closest it was 1.9 AU from Earth, so it could only be seen through a telescope.
Which star did it come from?
Nobody knows. Its large amount of carbon monoxide only suggests that it formed somewhere very cold.
Who discovered it?
Gennadiy Borisov, an amateur astronomer in Crimea, found it on 30 August 2019 with a 65 cm telescope he built himself.
Related
- 'Oumuamua — first interstellar visitor
- Halley's Comet perihelion (1986)
- Comet NEOWISE — the great comet of 2020
- The Sun — object index
- Mars — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.