The interstellar interloper

With confirmation of its interstellar origin, C/2019 Q4 (Borisov) has now been given the alternative designation of (with the ‘I’ meaning interstellar). The comet has, not surprisingly, attracted a good deal of attention, but probably the most surprising thing about it is the lack of surprises! Essentially, it is indistinguishable from any small Solar System comet. Intrinsically quite faint, 2I is estimated to have a nucleus of several hundred metres diameter made up of the same material as typical comets belonging to the Sun’s family. The only ‘oddities’ appear to be some greater similarities with dynamically evolved solar comets than with typical first arrivals from the Oort Cloud.