First interstellar comet

The first known interstellar comet is 2I/Borisov, a roughly 2–3 km (1.24–1.86 mi) comet discovered by amateur astronomer Gennady Borisov (UKR/RUS) on 30 August 2019.

Gennady logged his initial findings with the Minor Planet Center (MPC), which is an international organization that collects observational data on objects such as asteroids and comets. The comet was given the provisional designation C/2019 Q4 and its coordinates were published to the MPC's website.

Intrigued by this new report, large observatories all over the world turned their telescopes towards the object. This included the massive 10.4-m Gran Telescopio Canarias in the Canary Islands and the Pan-STARRS observatory in Hawaii (which discovered the first interstellar visitor, the asteroid 1I/'Oumuamua). These observations allowed the astronomical community to get a better view of the comet and to calculate its trajectory with greater precision.

We now know that 2I/Borisov (this name was approved by the Minor Planet Center after it was confirmed to be interstellar) approached our Solar System from the direction of the constellation Cassiopeia at a speed of around 41 kilometres per second. It passed just outside the orbit of Mars (a distance of around 300 million kilometres) on 8 December 2019, with its interaction with the Sun's gravity deflecting its onward path by about 40 degrees (in astronomy terms, it has an orbital eccentricity of 3.4). It is now travelling towards the constellation Telescopium.

Unlike 1I/'Oumuamua, which was only spotted after it had made its closest approach to the Sun, 2I/Borisov was still two months away from perihelion when it was discovered. This means that the astronomical community has had more time to study the object while it's still fairly close. What they've learned is that, exotic origins aside, 2I/Borisov is a surprisingly normal comet. It measures 2–3 km (1.24–1.86 mi) in diameter, and is composed primarily of water ice. As it got closer to the sun, it developed a pronounced "tail", made from sublimating water ice and other molecules, just like more familiar solar system comets.