Small bodies · Space Codex
2I/Borisov
Found by amateur astronomer Gennadiy Borisov in August 2019, it looked like an ordinary comet, but carried unusually large amounts of carbon monoxide.
The first interstellar comet
At a glance
Key points
- Discovered by an amateur astronomer
- Unusually rich in carbon monoxide
- Second known interstellar object
Ordinary, then not
Its normal comet appearance was the surprise: planet formation elsewhere seems to make comets much like ours. Its coma, though, held far more carbon monoxide than any comet previously seen near the Sun, hinting at a very cold birthplace.
How it was found
Amateur astronomer Gennadiy Borisov spotted it from Crimea on 30 August 2019, using a 0.65 m telescope he had designed and built himself. Unlike ʻOumuamua it was caught on the way in, more than three months before perihelion, so astronomers had time to plan campaigns with Hubble, ALMA and ESO's Very Large Telescope. Its orbit, far too open to be bound to the Sun, confirmed it as the second known interstellar object.
Reading its chemistry
Hubble ultraviolet spectra and ALMA millimetre-wave measurements, taken between December 2019 and January 2020 when the comet was about 2 AU from the Sun, found carbon monoxide unusually abundant compared with water. The Hubble team suggested it formed in the cold outskirts of a planetary system around a red dwarf, the commonest type of star. The ALMA team raised another possibility: a CO-rich fragment chipped from a dwarf planet.
A pristine visitor
Polarisation measurements with the Very Large Telescope, published in 2021, showed its dust scattered light differently from almost every Solar System comet. The one close match was Hale–Bopp, regarded as one of the least altered comets known. The team concluded that 2I/Borisov had probably never passed close to a star before 2019, making it an unusually untouched sample of the cloud in which it formed.
What we still don't know
Which star system it came from. Its path has not been traced back to a convincing parent star, and the size of its nucleus is known only within broad limits, because the coma hid it from view.
Missions
- 1990–
Hubble Space Telescope
NASA / ESAActiveUltraviolet to near-infrared imaging from low Earth orbit.
Related entries
- 1I/ʻOumuamuaThe first known interstellar visitor
- 3I/ATLASThe third interstellar object, and the fastest
Sources
- Bodewits et al. 2020, Nature Astronomy (opens in a new tab)
- Bagnulo et al. 2021, Nature Communications (opens in a new tab)
- NASA Science: Interstellar comet Borisov reveals its chemistry (opens in a new tab)
- JPL Small-Body Database: 2I/Borisov (opens in a new tab)
- Jewitt et al. 2020, The nucleus of interstellar comet 2I/Borisov, ApJL (arXiv) (opens in a new tab)
- Cordiner et al. 2020, Nature Astronomy (opens in a new tab)
- ESO: First interstellar comet may be the most pristine ever found (eso2106) (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.