 They extrapolate that it became active in June 2019 when it was between 4 and 5 au from the Sun.A search of image archives found precovery observations of 2I/Borisov as early as 13 December 2018, but not on 21 November 2018, indicating it became active between these dates.2I/Borisov's 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 (NH2).