2I/Borisov
First observed rogue comet and second interstellar interloper.
2I/Borisov, first spotted as C/2019 Q4 (Borisov), is the second known object from outside our solar system, after ʻOumuamua, and the first rogue comet ever observed. Amateur astronomer and telescope builder Gennadiy Borisov discovered it on 29 August 2019 UTC (30 August local time) at the MARGO Observatory. With a heliocentric orbital eccentricity of 3.36 and a hyperbolic excess velocity of 32 km/s, it is clearly unbound to the Sun—a genuine interstellar visitor.
The International Astronomical Union (IAU) officially named it "2I/Borisov," where "2I" marks it as the second interstellar object and "Borisov" follows the comet-naming tradition after its discoverer. Before this final designation, early orbit calculations initially placed it on the Minor Planet Center's Near-Earth Object Confirmation Page as gb00234. After thirteen days of observations, it was reclassified as a hyperbolic comet and labeled C/2019 Q4 (Borisov) on 11 September 2019. Astronomers including Davide Farnocchia, Bill Gray, and David Tholen confirmed its interstellar origin, and on 24 September 2019, the IAU's Working Group for Small Bodies Nomenclature formally announced it as 2I/Borisov.
Unlike ʻOumuamua, which looked like an asteroid, 2I/Borisov had a clear coma—a cloud of dust and gas surrounding its nucleus. Estimates of its nucleus diameter have ranged from 1.4 to 16 km, but it shrank noticeably during its solar system flyby, losing at least 0.4% of its mass before perihelion. Non-gravitational acceleration suggests an upper limit of 0.4 km for the nucleus, consistent with a Hubble Space Telescope limit of 0.5 km. It never came closer to Earth than about 300 million km (2.0 AU), ruling out radar measurements. An occultation of a star could determine its size and shape, but that would require precise orbit predictions and a network of small telescopes.
Hubble observations found no variation in its light curve, implying a rotational period longer than 10 hours. A study using Canada's NEOSSat satellite suggested a period of 13.2 ± 0.2 days, though that is unlikely to be the nucleus's spin. Monte Carlo simulations based on orbit data indicate its equatorial obliquity might be about 59 or 90 degrees, with the latter favored by the latest orbit determination.
David Jewitt and Jane Luu estimate the comet produced 2 kg/s of dust and lost 60 kg/s of water, based on its coma size.
Quick Facts
- Discoverer
- Gennadiy Borisov
- Discovery Date
- 29 August 2019
- Discovery Site
- MARGO Observatory, Crimea
- Mpc Name
- C/2019 Q4
- Designations
- gb00234
- Orbit Ref
- jpldata
- Epoch
- 5 January 2020 (JD 2458853.5)
- Observation Arc
- 964 days
- Obs
- 3,360 MPC-DB
- Earliest Precovery Date
- 13 December 2018
- Inclination
- 44.053°
- Eccentricity
- 3.3565
Facts from the source article.
Lore & Background
The comet was discovered by amateur astronomer Gennadiy Borisov on 30 August 2019 local time at his personal observatory MARGO in Crimea, using a 0.65 meter telescope he designed and built himself. Early orbit solutions suggested it could be a near-Earth object, listed as gb00234 on the NEOCP, but after thirteen days of observation it was clear the object was a hyperbolic comet, designated C/2019 Q4 (Borisov) on 11 September 2019. Astronomers including Davide Farnocchia, Bill Gray, and David Tholen concluded it was interstellar, and on 24 September 2019 the IAU formally announced the interstellar designation 2I/Borisov.
Unlike ʻOumuamua, which had an asteroidal appearance, 2I/Borisov's nucleus was surrounded by a coma of dust and gas. Its composition appears uncommon yet not unseen in Solar System comets, being relatively depleted in water and diatomic carbon (C2), but enriched in carbon monoxide and amines. The molar ratio of carbon monoxide to water in its tail is 35–105%, resembling the unusual blue-tailed comet C/2016 R2 (PanSTARRS). It also produced a minor amount of neutral nickel emission, with a nickel to iron abundance ratio similar to Solar System comets.
2I/Borisov entered the Solar System from the direction of Cassiopeia near the border with Perseus, indicating it originates from the galactic plane. It crossed the ecliptic plane on 26 October 2019 near the star Regulus, reached perihelion on 8 December 2019, and made its closest approach to Earth on 28 December 2019. It will leave the Solar System in the direction of Telescopium, taking roughly 9,000 years to travel a light-year relative to the Sun.
Reader's Guide
2I/Borisov is significant as the first observed rogue comet and only the second interstellar object detected passing through the Solar System, after ʻOumuamua. Its discovery by an amateur astronomer using a self-built telescope highlights the ongoing contributions of amateurs to professional astronomy. The comet's extremely hyperbolic trajectory, with an eccentricity of 3.36 and hyperbolic excess velocity of 32 km/s, provides strong evidence of its interstellar origin, much higher than the weakly hyperbolic comets or even ʻOumuamua. Its chemical makeup, enriched in carbon monoxide and amines but depleted in water and C2, offers a rare glimpse into the composition of material from another solar system. The comet's nucleus noticeably shrank during its flyby, losing at least 0.4% of its mass before perihelion, and its size is constrained to an upper limit of 0.5 km by Hubble Space Telescope observations. The comet's legacy includes demonstrating that interstellar comets can be active and chemically distinct, and that amateur astronomers remain vital in detecting such objects.
Did You Know?
- 2I/Borisov was discovered by amateur astronomer Gennadiy Borisov using a 0.65 meter telescope he built himself.
- The comet's tail was 14 times the size of Earth, as reported by Yale University astronomers in November 2019.
- Its molar ratio of carbon monoxide to water in the tail is 35–105%, far higher than the average 4% for Solar System comets.
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