Two years ago ʻOumuamua was the first interstellar object to be detected passing through our solar system. But the second interstellar object, the comet 2I/Borisov, is passing through right now.
I remember reading about ʻOumuamua in 2017 with a lot of excitement. A weird elongated object from some far-away star? It sounded like the beginning of a sci-fi first-contact movie, and I loved it. By the time anyone on Earth knew about ʻOumuamua it was already on its way out, but it gave us a good idea of what to look for next time.
Next time was just over two months ago. An amateur astronomer named Gennadiy Borisov spotted a comet through a telescope that he built himself. This comet was coming in on a very strange angle, and at a very high speed. Subsequent investigations confirmed that it is definitely from outside of our solar system. It’s the second interstellar object, and the first interstellar comet.
There’s a small chance that 2I/Borisov will disintegrate as it passes by the sun, but if it survives it has a sufficiently hyperbolic trajectory to pop out the other side and back out into the wider universe. And this is all happening right now: next month, on December 6, it will be roughly between the Sun and the Earth.
Now, it’s the second time we’ve actually spotted an interstellar object, but it is believed that they are pretty common – perhaps 10,000 a day between the Sun and Neptune. I don’t know how scientists came to that estimate, given how few we’ve spotted, but it’s a good indication that 2I/Borisov will not be the last interstellar object we encounter.
[Thanks to Kottke.org, for introducing me to this subject.]