Comet Borisov

11 December 2019 

There’s a comet around, although it’s a bit hard to see.

I’m often a bit skeptical when it comes to comets. You hear that there’s one coming, and that astronomers hope it’ll be bright, but so often, particularly in the city, you see a dull fuzzy blob that may just as well be a smear on your lens.

So, I’m always trying to manage expectations down, not up.

Comet 2I/Borisov approached the solar system from the North, crossing the equator a few months back. Its closest pass to the Earth is on the 28th of December.

As to brightness, it's been dim so far, but there’s the possibility of an outbreak of gas as the comet warms close to the Sun. Experts guess it might get as bright as magnitude 15.

That’s not bright. You’ll need a big aperture - something like a 10-inch Dobsonian ( to see that.

The interesting thing about Borisov is that it’s an interstellar comet.

In fact, Borisov is only the second interstellar object known to have passed through our solar system – although clearly there must have been more. The previous object was last year’s Oumuamua – easy for you to say. This one which defied classification, being a thin lump of rock that shot through the solar system. You could maybe call it a comet at a stretch.

How do they know Borisov is interstellar? From the shape of the orbit. Most comets have orbits that are oval-shaped, meaning eventually they stop and come back. Borisov’s trajectory is hyperbolic, suggesting that it’s just passing through.

How do you see it? The best way is photographically. Take a photo, and then take a second one later. Comets move against the background stars (see my sequence for 46P/Virtanen).

But there's a final thing about a hyperbolic trajectory. It’s moving pretty much directly away from us, so your photos won’t show a lot of movement.

Borisov is the most frustrating comet EVER!