To start things off, a limerick:

My dear old friend ‘Oumuamua

I asked her — What’s up? What is new-ah?

“I’ve been thinking of

That guy Borisov

The interstellar number Two-ah!”

Figuring out what’s up with a new population of astronomical objects is like going to a party without knowing the dress code.

Here’s what’s happening with interstellar objects.



The first person we met was wearing a clown suit with lightning bolt earrings and no shoes. That’s ‘Oumuamua, with its tumbling rotation, its non-gravitational acceleration, and its lack of cometary activity despite looking like a cometary nucleus.

The second person we met looks perfectly normal (so far). It’s wearing a button-down shirt and jeans, with zero surprises. That’s Borisov. It’s almost too normal-looking….

But after meeting only two people, we still don’t know what kind of party this is. Is ‘Oumuamua the unusual one? Or is Borisov the weirdo who didn’t dress up for the costume party?

Once we’ve found ten or twenty interstellar objects we’ll have a better idea. For now, we’re in the pleasantly confusing situation of not really knowing what’s going on. (I say “pleasantly” because this is where science really happens: when there is a taste of something new but you’re still don’t know what’s going on.)

Borisov the interstellar object is named for its discoverer, Gennadiy Borisov, a Russian telescope maker and amateur astronomer.

You might be wondering: how could an amateur make such a big-deal discovery?

First of all, the telescope Borisov used was not a puny old thing — it was 65 cm in diameter. Not too shabby.

But the main reason that Borisov (the man) found Borisov (the object) was because of his search strategy.

Big telescopes don’t point too close to the Sun. They don’t want to fry their mirrors or cameras. It makes sense — we humans aren’t supposed to look directly at the Sun either. These telescopes stay at least 45 degrees or so away from the Sun. (It’s not dark enough to observe until the Sun is about 15 degrees below the horizon).

Because he was using his own telescope, Borisov could search closer to the Sun. He found Borisov just 38 degrees away from the Sun.

There are more detectable moving objects the closer you look to the Sun.

This is true for interstellar objects as well as other classes of objects, like Near-Earth Objects (aka, asteroids that could bash into the Earth). Here is a map of how easy it is to detect Near-Earth Objects in different parts of the sky:

There are two main reasons that there are more objects to be found in the direction of the Sun.

The key thing to know is that by looking in the direction of the Sun, the objects that we can find are mostly on the opposite side of the Sun.

Now, since detectable objects in the direction of the Sun are farther away, there are more per square degree. One image with our telescope will see many more objects than if we point far from the Sun.

Another factor is that, when we see an object on the far side of the Sun, we see its fully illuminated side. It’s like seeing the full Moon instead of a thin crescent. This makes objects appear brighter.

Borisov found Borisov by pointing his telescope right where most interstellar objects are waiting to be found, and where the big telescopes avoid.

What do we know about Borisov so far?

Like ‘Oumuamua, Borisov is not bound to the Sun. Its orbit is hyperbolic (see its Minor Planet Center page for the latest). Borisov’s closest passage to the Sun will be in December 2019. It will be within the reach of our telescopes for months to come.

Unlike ‘Oumuamua, Borisov is active. It has a detectable coma and is clearly outgassing.

Borisov’s nucleus is probably much bigger than ‘Oumuamua. Estimates put Borisov at about 1 km in diameter, about 5-10 times the size of ‘Oumuamua. That makes Borisov hundreds of times more massive!

Maybe the most interesting thing about Borisov is how boring it looks.

It looks just like a normal old comet in every measurement to date.

‘Oumuamua was weird. It was a struggle to understand its seemingly contradictory properties. It was very stretched out. It was tumbling. It looked like a comet or water-rich asteroid and had a small non-gravitational acceleration but no detectable outgassing.

‘Oumuamua really got astronomers’ brain-juices flowing. My own personal idea was that ‘Oumuamua started off as a comet-like object (a “planetesimal”) that was torn to pieces and then lightly toasted before being tossed into interstellar space.

An appealing feature of this model is that Borisov fits in seamlessly. Borisov would simply be a comet-like planetesimal that was not toasted before being launched into interstellar space. Both outcomes happen at about the same rate in my simulations, so both “extinct comet-like” (‘Oumuamua) and “normal comet-like” (Borisov) interstellar objects should exist.

But let’s wait and see what the next one looks like.

Or maybe, when it hits the dance floor we’ll find out that Borisov will finally show that it’s not as boring as it looks….

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