An interstellar comet on its way to visit us has been officially named after the amateur astronomer who discovered it.
2I/Borisov was previously known as C/2019 Q4 Borisov. The 'I' stands for 'interstellar' and the '2' because it's only the second ever seen, behind 2017's 1I/'Oumuamua.
Its recognition as an interstellar object is no surprise, with its distant origins obvious within days after Ukrainian skywatcher Gennady Borisov saw it in August on his home-made telescope.
"After a week of observations by amateur and professional astronomers all over the world, the Minor Planet Center was able to compute a preliminary orbit, which suggested this object was interstellar," the International Astronomical Union said.
"The orbit is now sufficiently well-known, and the object is unambiguously interstellar in origin; it has received its final designation as the second interstellar object, 2I. In this case, the IAU has decided to follow the tradition of naming cometary objects after their discoverers, so the object has been named 2I/Borisov."
Its orbit is hyperbolic, meaning it has enough speed to escape the sun's gravitational pull. Its origins are unknown, but a paper by Polish astronomers suggests it may have been flung at us by a binary star, Kruger 60, 13 light-years away.
"When searching for a home system of an interstellar body one should look for a past close proximity at a very small relative velocity of order of just a few kilometres per second," they wrote.
"We found that a double-star Kruger 60 is a good candidate for a home system of this comet."
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Unlike 'Oumuamua, which was only seen on its way out, Borisov is yet to arrive. It'll make its closest approach to the sun on December 7, about twice the distance from Earth to the sun.
It'll be easiest to spot from Earth in December and January, but only from the southern hemisphere, according to space.com. It's not known whether it'll be visible to the naked eye, but is already starting to form a tail.
Its size is estimated at a few kilometres across, and its speed at 150,000km/h.