The characterization of a new interstellar comet is reported in a paper published this week in Nature Astronomy. The comet, named 2I/Borisov, came from outer space, yet its colour and morphology are similar to any other active comet in the Solar System.

Although several interstellar objects are expected to pass through the Solar System in great numbers, they are usually too faint to be noticed. The first detected interstellar body, ‘Oumuamua, was only discovered in 2017 and surprised researchers with an absence of cometary activity and its elongated shape. Two years later, on 30 August 2019, amateur astronomer Gennady Borisov discovered a rapidly approaching comet, C/2019 Q4, whose highly open orbit was confirmed officially by the International Astronomical Union on 24 September and renamed 2I/Borisov.

Piotr Guzik and colleagues were alerted to the open trajectory by their data mining code and observed the comet on 10 September using the Gemini North Telescope at Mauna Kea, Hawai’i as well as the William Herschel Telescope (WHT) at La Palma, Spain. On 13 September, they observed it again with WHT. The authors identified an extended coma and short tail, and estimated a nucleus radius of roughly 1 km. They observed a coma uniform in colour and slightly reddish, but compatible with Solar System comets.

As 2I/Borisov was discovered on its approach towards the Sun (with the closest encounter expected on 8 December), and is brighter than ‘Oumuamua, astronomers will be able to acquire a more extensive, complete and precise set of data before the comet becomes too faint to observe in approximately a year from now.