• DOI:10.1038/s41550-019-0931-8
• Corpus ID: 204509394

Initial characterization of interstellar comet 2I/Borisov

@article{Guzik2019InitialCO,
  title={Initial characterization of interstellar comet 2I/Borisov},
  author={Piotr Guzik and Michał Drahus and Krzysztof Rusek and Wacław Waniak and Giacomo Cannizzaro and In{\'e}s Pastor-Marazuela},
  journal={Nature Astronomy},
  year={2019},
  volume={4},
  pages={53-57}
}

• Piotr Guzik, M. Drahus, +3 authors I. Pastor-Marazuela
• Published 12 September 2019
• Physics, Geology
• Nature Astronomy

Interstellar comets penetrating through the Solar System had been anticipated for decades 1 , 2 . The discovery of asteroidal-looking ‘Oumuamua 3 , 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 objects 4 – 6 , challenging our view of interstellar minor bodies 7 , 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 ‘Oumuamua 9 and theoretical predictions 2 , 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. 2I/Borisov is the first interstellar comet we know to have visited the Solar System, and the second interstellar visitor after ‘Oumuamua. Initial observations with the William Herschel Telescope and Gemini North Telescope reveal cometary features indistinguishable from Solar System comets, apart from its hyperbolic orbit.