Initial characterization of interstellar comet 2I/Borisov

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Interstellar comets penetrating through the Solar System had been anticipated for decades1,2. The discovery of asteroidal-looking ‘Oumuamua3,4 was thus a huge surprise and a puzzle. Furthermore, the physical properties of the ‘first scout’ turned out to be impossible to reconcile with Solar System objects4,5,6, challenging our view of interstellar minor bodies7,8. Here, we report the identification and early characterization of a new interstellar object, which has an evidently cometary appearance. The body was discovered by Gennady Borisov on 30 August 2019 ut and subsequently identified as hyperbolic by our data mining code in publicly available astrometric data. The initial orbital solution implies a very high hyperbolic excess speed of ~32 km s−1, consistent with ‘Oumuamua9 and theoretical predictions2,7. Images taken on 10 and 13 September 2019 ut with the William Herschel Telescope and Gemini North Telescope show an extended coma and a faint, broad tail. We measure a slightly reddish colour with a g′–r′ colour index of 0.66 ± 0.01 mag, compatible with Solar System comets. The observed morphology is also unremarkable and best explained by dust with a power-law size-distribution index of –3.7 ± 1.8 and a low ejection speed (44 ± 14 m s−1 for β = 1 particles, where β is the ratio of the solar gravitational attraction to the solar radiation pressure). The nucleus is probably ~1 km in radius, again a common value among Solar System comets, and has a negligible chance of experiencing rotational disruption. Based on these early characteristics, and putting its hyperbolic orbit aside, 2I/Borisov appears indistinguishable from the native Solar System comets.

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Fig. 1: Astrometric residuals of 2I/Borisov calculated for three different orbital solutions.
Fig. 2: Median-stacked images of 2I/Borisov from Gemini North.
Fig. 3: Colour of 2I/Borisov in the context of Solar System comets.

Data availability

The ACAM data are available from the corresponding authors upon reasonable request. The GMOS-N raw data will be available in the Gemini Observatory archive at after the expiration of the 12 month proprietary period.


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Based in part on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). The William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. We thank J. Blakeslee for rapid evaluation and approval of our Gemini North director’s discretionary time request and P. Jonker for sharing time on the William Herschel Telescope. We also thank the staff of both observatories for assistance and vital contributions to making these observations possible. M.D. and P.G. are grateful for support from the National Science Centre of Poland through SONATA BIS grant no. 2016/22/E/ST9/00109 and Polish Ministry of Science and Higher Education grant no. DIR/WK/2018/12. G.C. acknowledges support from European Research Council Consolidator Grant 647208. I.P.-M. acknowledges funding from the Netherlands Research School for Astronomy (grant no. NOVA5-NW3-

Author information

K.R. and P.G. developed Interstellar Crusher. P.G. and M.D. designed the observations, wrote the telescope time proposal, performed photometry, estimated the size of the nucleus and wrote the paper. P.G. computed the orbit. M.D. compared the colour to Solar System comets and estimated the probability of rotational disruption. W.W. reduced raw images and performed Monte Carlo dust modelling. G.C. and I.P.-M. obtained data at the William Herschel Telescope.

Correspondence to Piotr Guzik or Michał Drahus.

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Guzik, P., Drahus, M., Rusek, K. et al. Initial characterization of interstellar comet 2I/Borisov. Nat Astron (2019) doi:10.1038/s41550-019-0931-8

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