2I/Borisov — the interstellar comet
The second interstellar visitor and the first interstellar comet — strikingly like our own.
The moment of 2I/Borisov — the interstellar comet: https://nowsky.org/e/borisov — opens at the right place and time, replayable, no login.
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 built himself. The orbit came out with eccentricity 3.36 — a value the Sun could not hold. It was the second interstellar visitor after 'Oumuamua and the first interstellar comet.
Rounding perihelion (2.0 AU) on 8 December, Borisov behaved, unlike 'Oumuamua, like a thoroughly ordinary comet: it had a coma and a tail, water and cyanogen outgassing were detected, and its brightness followed predictions. A comet formed around another star had been made from the same chemistry as ours.
The interesting difference was carbon monoxide — its ratio to water was more than three times the solar-system average, which means it formed somewhere very cold, far out in its parent disk. The nucleus is about 1 km across, and the home star was never identified.
Where 'Oumuamua vanished within weeks, Borisov was found while still inbound and was observed for more than half a year. In March 2020 Hubble caught its nucleus splitting in two, and it is now leaving toward Telescopium at 32 km/s. The orbit view replays the hyperbolic track.
Why it matters
The confirmation that a comet is a comet anywhere is quiet evidence that planet formation is a universal process. That the same ices and dust clumped the same way in the disk of another star was shown not by theory but by one physical object.
The contrast with 'Oumuamua is valuable. The first visitor accelerated without a tail; the second was a textbook comet. Two objects were enough to show that interstellar bodies are not one kind, and 3I/ATLAS in 2025 became the third sample.
That an amateur found it is symbolic too. Even in the age of large survey telescopes sweeping the sky, Borisov proved that regions the automated surveys watch poorly — near the Sun at dawn — can be claimed first by an individual observer.
How to watch
Borisov is already beyond every telescope, tens of AU from the Sun and leaving toward Telescopium in the southern sky.
Choose Borisov in the orbit view's event list to replay perihelion on 8 December 2019 and the hyperbolic track crossing the solar system at 44° to the ecliptic; closest approach to Earth (28 December, 1.9 AU) was beyond Mars's orbit, so it was never a naked-eye object.
Placed beside 'Oumuamua's track, the two visitors can be seen entering from different directions and leaving in different directions — interstellar objects do not come from one place.
See this for yourself — replay the moment in the orbit view
Background
Borisov was a veteran amateur who had found nine comets since 2013 and worked by day as an engineer at the Crimean Astrophysical Observatory. At discovery the comet was 3 AU from the Sun at magnitude 18.
The fragmentation Hubble photographed in March 2020 was a piece of a few percent of the nucleus breaking away; the two pieces dispersed within days, probably from thermal stress after perihelion.
The third interstellar object, 3I/ATLAS, found in July 2025, is the largest of the three at several kilometres across and came inside Mars's orbit around its October perihelion. Interstellar visitors are now being caught every few years.
The naming convention for interstellar objects was settled with Borisov. The International Astronomical Union gave ‘Oumuamua the first interstellar number, “1I”, in 2017, and Borisov, showing cometary activity, received a comet-style designation, “2I/Borisov” — the first case combining the discoverer-name tradition of comets with an interstellar number. 3I/ATLAS in 2025 took the name of the discovering survey. Future interstellar objects will continue in the same way as 4I, 5I and so on.
FAQ
How does it differ from 'Oumuamua?
Borisov behaved like an ordinary comet with a coma and tail and was observed for over half a year; 'Oumuamua had no tail and vanished within weeks.
Was it visible to the naked eye?
No — it came only within 1.9 AU of Earth and was a telescope object throughout.
Which star did it come from?
It was never identified; its high carbon monoxide content shows only that it formed somewhere very cold.
Who discovered it?
Gennadiy Borisov, an amateur in Crimea, with a telescope he built himself, on 30 August 2019.
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.