Comet C/2019 Q4 (Borisov) will reach perihelion—its closest point to the Sun—on 8 December 2019. To learn more about this comet and other interstellar objects, I spoke with Karl Battams, an astrophysicist and computational scientist at the Naval Research Laboratory in Washington, D.C. Battams operates the NASA-funded Sungrazer Project, which uses space-based solar telescopes to look for comets flying close to the Sun.

Why are objects from outside our solar system so important to study? What can we learn about the cosmos and our own origins by studying them?

I think what it boils down to is that one of the big questions we have in all of astronomy, or even beyond astronomy, is, “Are we alone?” Is our solar system and our planet completely unique? Is there something about us that's completely different from the rest of our galaxy and the universe? It's really an impossible question to fully answer, but what's nice about an interstellar object is we can see how similar or dissimilar it is to objects in our own solar system. If it turns out that this comet is like a typical comet that we see in our own solar system, then we’ll think, "Well, okay. Maybe we're not so unique in composition." Or maybe it's going to have some truly bizarre chemistry and we'll say, "Wow! Things are wild out there!"

This object definitely appears to be a comet, whereas 'Oumuamua had traits of both asteroids and comets. If this is truly our first interstellar comet, how big of a deal will that be for astronomers? What sort of things will we try to learn from it, and how?

The nice thing about comets is that they’re active. The reason comets have the diffuse coma around them and the tail trailing behind them is because sunlight is hitting all of the volatile materials on the surface and basically causing them to vaporize. As they lift away from the surface, they bring other material up as well. In other words, you're boiling the surface away.

Whereas an asteroid is an inert rock, and all you can do is look at how light is reflecting off of it, with a comet there are a lot of chemical processes that are going on. It's a lot more complex, so you get to understand a little bit more of the gases and dust being released from the comet, and hopefully get a much better idea of its composition.

Also, comets are slightly easier targets to see. 'Oumuamua was very small, faint and stellar in appearance, whereas this comet, for people with a half-decent telescope, is going to be easier to see—though still not bright by any means!

What does a comet look like up close?