Most comets orbit round the Sun, so the arrival of an interstellar comet, C/2019 Q4 (Borisov) is a highly unusual event
A new interstellar object has been discovered passing through the solar system. Discovered by Ukrainian amateur astronomer Gennady Borisov on 30 August, 2019 [Crimea skyMap]. The veteran comet hunter pointed the pointed the Crimean Astrophysical Observatory low on the northeastern horizon to Gemini perilously close to the Sun’s glare. Now the object has been designated Comet C/2019 Q4 (Borisov). At the time, the comet was approaching the inner solar system from beyond the orbit of Mars.
So I dusted off my home-made orbital computer simulation model, got hold of Borisov’s state vectors from NASA Horizons, and plotted its path.
First the view looking down onto the plane of the solar system shows that Borisov is approaching just outside the orbit of Mars:
Looking along the plane of the solar system, Borisov is coming almost vertically down onto the plane of the solar system, and is initially moving in the galactic plane (blue line).
These two views correspond to a plan and elevation of the inner solar system.
It’s going to take months to pass through the solar system. In my model it passes through the plane of the ecliptic at the end of October 2019. It doesn’t get anywhere near any planet, but since it passes through the asteroid belt between Mars aand Jupiter, it might dislodge an asteroid, depending on how big it is
This is only the second such object to be discovered and tracked entering our solar system from interstellar space. The first being Oumuamua (1I/2017 U1) discovered in October 2017.
Initial observations indicate the nucleus is between 2 and 16 kilometres (1.2 and 10 miles) in diameter.
P.S. Apparent motion of Comet Borisov through Gemini 30 July 2019 to 13 Oct 2019:
Apparent motion of Comet Borisov 30 July 2019 to 2 Jan 2020